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Chapter 19: Drones

Karang is a small island village in Loktak Lake, in Manipur. To reach it from the district hospital at Bishnupur, a health worker must travel by road to the lake shore and then cross the water by boat. In normal times that is inconvenient. In the middle of a vaccination drive, when doses must stay cold and arrive on a fixed day, it is a serious problem.

In October 2021, a research team flying under the Indian Council of Medical Research tried something different. A small four-rotor aircraft, carrying a padded box of vaccine doses under its frame, lifted off from the hospital ground and flew across the lake on its own. It followed a route programmed into it before take-off, held its height and heading over open water where there were no landmarks, and landed at Karang about fifteen kilometres away roughly a quarter of an hour later. No pilot sat inside it, and for most of the flight no pilot could even see it — which is exactly why the flight needed special permission from the aviation regulator before it could happen at all.

Photograph

Fig. 19.0-1 – a small quadcopter in level flight over open water with a rural shoreline behind it, carrying a small payload box slung beneath its central frame; all four rotors, the landing gear and the payload mount must be clearly visible in the frame; sourcing note – a stock or agency image of a civilian delivery or survey quadcopter is acceptable, but images of military drones, images with a visible manufacturer logo dominating the frame, and close crops that hide the rotor arms are unusable

NoteCuriosity Corner

Q1. A drone has no wings like an aeroplane, and nothing pushes it from behind. So what actually holds it up in the air? Q2. The four propellers of a quadcopter all point straight up and cannot be tilted. How, then, does it move forward, sideways, or turn to face a new direction? Q3. Why does hanging a heavier box under a drone shorten the flight as well as slow it down? Q4. How does a drone find its way to a village it has never visited, and how does it get home if the radio link to its operator is lost? Q5. Who decides how high a drone may fly in India, and which places it may not fly over at all?

This chapter answers all five questions.

19.1 Drones as Aerial Robots

19.1.1 What a Drone Is

You have probably watched a small flying machine hovering above a wedding ground or a cricket match, holding itself almost perfectly still while a camera under it swings slowly across the crowd. Standing still in the air is much harder than it looks. A helium balloon drifts, a kite needs a string and a steady wind, and a paper plane cannot stop where you want it to. The machine over the ground is doing something none of these can do: it is actively holding a position it has been told to hold.

That ability is what makes it worth studying in a robotics course. A drone is an aircraft that flies with no pilot on board, controlled either remotely by an operator on the ground or by its own onboard computers and sensors. The same machine is also called an unmanned aerial vehicle (UAV), and the two names mean the same thing.

NoteDefinition

A drone, or unmanned aerial vehicle (UAV), is an aircraft that carries no pilot on board and is flown either by remote control from the ground or autonomously by onboard computers acting on sensor data.

In regulation and in industry, the aircraft is rarely spoken of by itself. The drone together with its ground control station, its radio link and its supporting equipment is called an unmanned aircraft system (UAS). The distinction matters because the rules you will meet later in this chapter apply to the whole system, not only to the object in the air.

Why most drones use rotors

Most of the drones you will see carry several small propellers on arms spread around a central body. Because their lift comes from rotating propellers rather than from fixed wings, they belong to a family called multirotor aircraft. Each propeller, together with the motor that spins it, is called a rotor.

A multirotor design has one enormous advantage for robotics: it can take off vertically from a small patch of ground, hover over a chosen point, and move in any direction without needing a runway or a forward speed. That is exactly what a camera, a sprayer or a survey scanner usually needs.

It also has an honest cost. Holding a multirotor in the air means spending energy continuously just to stay up, because nothing but the downward push of air is supporting it. A fixed-wing unmanned aircraft glides on its wings and can stay aloft far longer on the same battery, but it cannot hover, and it needs space to launch and land. Typical small multirotors therefore fly for tens of minutes at a time, not hours, and almost every design decision in the rest of this chapter is shaped by that limit.

19.1.2 The Sense–Plan–Act Loop in the Air

A wheeled robot that stops computing simply stops moving. A multirotor that stops computing falls out of the sky.

This is not an exaggeration. Four motors mounted on arms cannot balance a body by themselves. The slightest gust, an uneven weight distribution, or a difference between two motors will start the machine tipping, and once it tips the lift no longer points straight up, so the tipping grows. Left alone, a multirotor is unstable in the way that a broomstick balanced upright on your palm is unstable. It stays up only because something keeps correcting it.

That correcting is done by the flight controller, the small circuit board at the centre of the drone that reads the motion sensors and sets the speed of every motor. You will meet it in detail in section 19.4.3. What matters here is the pattern it follows, which is the same pattern every robot follows, only very much faster.

The sense–plan–act loop Statement: A robot repeatedly senses the state of itself and its surroundings, decides what action that state calls for, and acts through its actuators; the result of the action changes the state, which is sensed again. In words: Measure, decide, move, repeat — with each round starting from what the last round produced. Why it holds: A robot has no direct knowledge of the world. Every fact it acts on came from a sensor, and every effect it has came from an actuator, so behaviour can only be produced by joining the two through a decision and repeating. Used for: Stabilising a hovering drone, holding a heading, following a programmed route, avoiding an obstacle, keeping a spraying height above a crop. Watch out: The loop is only as fast as its slowest part. If sensing or deciding takes too long, the drone acts on a picture of the world that has already changed, and the correction can arrive late enough to make matters worse.

For a hovering drone the loop closes hundreds of times every second, as shown in Fig. 19.1-1. The motion sensors report how the body is tilting and turning, the flight controller compares that with the attitude it is supposed to hold, and it trims the speed of individual motors up or down. Those new speeds change the tilt, the sensors report the new tilt, and the cycle begins again.

Diagram

Fig. 19.1-1 – flow: motion and position sensors → flight controller compares measured attitude with commanded attitude → decision on speed change for each motor → electronic speed controllers → motors and propellers → change in the drone’s attitude and position, with the last arrow returning to the sensor block to close the loop; a separate arrow enters the flight controller block from outside labelled “pilot command or programmed route”

Q. If the operator is holding the control sticks, what is left for the drone to decide?

A. A great deal. The operator asks for motion in human terms — lean forward, turn left, climb. Nothing on the drone can obey that directly, because there is no forward motor and no turning motor. The flight controller must work out, many times a second, what combination of four or six individual motor speeds will produce that motion and still keep the machine level. The loop runs whether or not a human is touching anything.

19.1.3 Levels of Autonomy

Everything so far is true of every drone, whether a hobbyist is flying it by hand or it is crossing a lake by itself. What separates one drone from another is how much of the deciding is done on board. That is a question of autonomy.

NoteDefinition

Autonomy is the extent to which a machine can decide its own actions from its own sensor data, without an instruction supplied by a human operator at the time of acting.

Autonomy is a scale, not a switch, and drones are commonly grouped into three bands along it.

A manual drone is flown entirely by the operator through a handheld controller. The flight controller still stabilises the body — that never stops — but every choice about where the machine goes comes from a human hand on a stick. Racing and training drones are usually of this kind.

A semi-autonomous drone takes over the tasks a human does badly. It holds its altitude, holds a position against a light wind using satellite navigation, and returns level when the sticks are released. The route, however, is still the operator’s business. Most camera drones sold today work this way, which is why a beginner can hover one steadily on the first afternoon.

An autonomous drone is given a mission rather than a set of movements — a list of points to visit, a field to cover, a height to hold — and executes it without further input, navigating by satellite position and onboard sensors and returning home by itself if the radio link is lost. The Manipur vaccine flight described at the start of this chapter was of this kind.

Table 19.1 – Levels of autonomy in drones | Feature | Manual | Semi-autonomous | Autonomous | |—|—|—|—| | Who chooses the path | Operator, continuously | Operator, continuously | Planned before flight, executed on board | | What the onboard system supplies | Stabilisation only | Stabilisation, altitude hold, position hold | Stabilisation, navigation, route following, obstacle response | | Operator attention needed | Constant | Frequent | Occasional monitoring | | Behaviour if the radio link is lost | Control is lost | Usually hovers or returns, if the feature is fitted | Continues the mission or returns to the launch point | | Typical use | Racing, flight training | Photography, short surveys | Mapping, spraying, delivery over long routes |

Autonomous is not the same as automatic

Students often use these two words as if they were interchangeable, and in ordinary speech they nearly are. In engineering they are not. An automatic machine repeats a fixed sequence: a washing machine runs the same cycle whatever is inside the drum. An autonomous machine chooses its action from what it senses at that moment, so two flights over the same field on two different days need not look the same, because the wind, the satellite reception and the obstacles were not the same.

Greater autonomy is not automatically better. An autonomous drone following a programmed route across a village has no idea that the school ground below it has filled with children since the route was drawn. Its judgement extends only as far as its sensors and its programming reach, and everything outside that boundary remains the responsibility of the person who planned the flight. This is also why the law treats a flight beyond the operator’s sight quite differently from one flown in plain view, as section 19.6 explains.

19.2 The Physics of Flight

19.2.1 Lift, Weight, Thrust and Drag

A drone in the air is acted on by four quantities, and every manoeuvre in this chapter is a change in the balance between them.

The weight of the drone is the downward pull of gravity on its mass, including its battery and whatever it is carrying. For a machine of mass \(m\), the weight is \(W = mg\), where \(g\) is the acceleration due to gravity, taken as \(9.8 \text{ m/s}^2\). Weight never helps and never rests; it acts at every instant of the flight.

Lift is the upward force that opposes weight. In a multirotor drone the lift comes from nothing but the rotors, and the force each rotor produces along its own axis is called its thrust. This is worth stating plainly, because it differs from the aeroplane you may have studied in physics. An aeroplane has wings that generate lift and engines that generate thrust, and the two are separate. A multirotor has no wings at all. Its rotor thrust is its lift, and when the machine tilts, the same single force does both jobs at once.

Drag is the resistance of the air to the drone’s motion through it. It acts opposite to the direction of travel and grows rapidly as speed rises, which is one reason a drone flying fast into a headwind empties its battery far quicker than one hovering in still air.

Fig. 19.2-1 shows these forces on a drone that is hovering: four upward rotor thrusts adding to a total that exactly matches the weight, with no motion and therefore no drag.

Diagram

Fig. 19.2-1 – free-body diagram of a hovering four-rotor drone seen from the side; four short upward arrows labelled T1 to T4 at the rotors, one long downward arrow at the centre labelled W = mg; a note beside the figure stating that the sum of the four thrusts equals the weight; no horizontal arrows, with a label stating that drag is zero because the drone is not moving

19.2.2 How a Propeller Produces Lift

Hold your palm flat out of the window of a moving bus and tilt it slightly, and you feel the air push your hand upward. You are deflecting a stream of air downward, and the air is pushing back.

A propeller does exactly this, continuously and in a circle. Each blade is a small wing, twisted so that as it sweeps round it meets the air at an angle and drives that air downwards. Fig. 19.2-2 shows the result: air is drawn in from above the rotor and thrown down beneath it as a fast-moving column. The drone is not resting on the air below it. It is being pushed up by the reaction to the air it is throwing down.

Diagram

Fig. 19.2-2 – airflow through a single rotor disc drawn in side view; converging arrows above the disc labelled “air drawn in from above”, a faster narrower column of arrows below the disc labelled “air pushed down”, and a single upward arrow on the rotor shaft labelled “thrust on the drone (reaction)”; the rotor disc itself shown edge-on with the direction of rotation marked

Rotor thrust by Newton’s third law Statement: A rotor produces thrust by accelerating a mass of air downward; the air exerts an equal and opposite force upward on the rotor and hence on the drone. In words: Push air down hard enough, and the air pushes you up. Why it holds: Forces occur in pairs. The rotor can only change the momentum of the air by exerting a downward force on it, and the air must exert the same force upward on the rotor. Used for: All vertical take-off, hovering and altitude control in a multirotor; it also explains why thrust depends on how much air is moved and how fast. Watch out: Thrust depends on the density of the air. The same rotor at the same speed produces less thrust on a hot day, at high altitude, or in thin air, because each revolution moves less mass.

Two consequences follow, and both matter in practice.

First, thrust rises very steeply with rotor speed. For a given propeller, thrust is approximately proportional to the square of the rotational speed, so a small trim in motor speed produces a comparatively large change in force. That is what allows a flight controller to make fine corrections hundreds of times a second without any moving control surface.

Second, a larger propeller turning slowly moves a wide column of air gently, while a small propeller must turn very fast to move the same mass of air. The larger, slower arrangement is more efficient, which is why long-endurance drones carry big propellers on long arms, and small racing machines do not.

The dependence on air density is why flight on Mars is so difficult. NASA’s Ingenuity helicopter, which first flew there in April 2021, weighed only about 1.8 kg and yet needed counter-rotating rotors about 1.2 m across, spinning at roughly 2,400 rpm, because the Martian atmosphere is only about one per cent as dense as air at sea level on Earth.

Q1. A drone has no wings like an aeroplane, and nothing pushes it from behind. So what actually holds it up in the air?

A. Its own rotors do, by throwing air downward. Each spinning propeller drives a column of air down through the rotor disc, and by Newton’s third law the air pushes the rotor upward with an equal force. That upward force is the drone’s thrust, and in a multirotor it is also its lift — there is no wing and no separate engine. When the four thrusts together equal the weight, the drone neither rises nor falls.

19.2.3 Hovering, Climbing and Descending

The whole of vertical flight follows from one comparison: total thrust against weight.

When the total thrust equals the weight, the net vertical force is zero and the drone hovers, holding its height. When total thrust exceeds weight, there is a net upward force and the drone accelerates upward. When thrust falls below weight, it descends. Notice that the drone does not need extra thrust to stay at a height, only to change it — but it does need full hover thrust continuously, which is why a hovering drone drains its battery almost as fast as a climbing one.

NoteDefinition

A drone hovers when the total thrust produced by its rotors is equal in magnitude to its weight, so that the net vertical force on it is zero and it maintains a constant height.

Thrust each motor must produce to hover

Given: A four-rotor drone of total mass \(m = 1.2 \text{ kg}\), including battery and camera. Acceleration due to gravity \(g = 9.8 \text{ m/s}^2\). All four rotors share the load equally.

Find: The thrust each rotor must produce for the drone to hover.

Method: In a hover the total thrust equals the weight, so \(4T = mg\), giving \(T = mg/4\). The rotors are identical and symmetrically placed, so an equal share is a fair assumption.

\[W = mg = 1.2 \text{ kg} \times 9.8 \text{ m/s}^2 = 11.76 \text{ N}\]

\[T = \frac{W}{4} = \frac{11.76 \text{ N}}{4} = 2.94 \text{ N}\]

Each rotor must produce about \(2.9 \text{ N}\) of thrust.

Check: Motor and propeller combinations are usually rated in grams of thrust. Dividing by \(g\), \(2.94 \text{ N}\) is about \(300 \text{ g}\) of thrust per motor — an ordinary figure for a small camera drone, so the magnitude is sensible.

Thrust-to-weight ratio

A drone that can only just lift itself cannot climb, cannot fight a gust, and cannot recover if it starts to tip. Designers therefore compare the maximum thrust the propulsion system can produce with the weight it must carry. This is the thrust-to-weight ratio, and a value of about 2 is a common target for a general-purpose machine: enough spare force to manoeuvre briskly, without motors so oversized that they waste weight and energy.

Climb acceleration at full throttle

Given: The same drone, \(m = 1.2 \text{ kg}\), weight \(W = 11.76 \text{ N}\). Each of its four motor-and-propeller units can produce a maximum thrust of \(600 \text{ g}\), that is \(0.6 \text{ kg} \times 9.8 \text{ m/s}^2 = 5.88 \text{ N}\).

Find: The thrust-to-weight ratio, and the initial upward acceleration at full throttle.

Method: Total maximum thrust is four times the thrust of one unit. The ratio is that total divided by the weight. The net upward force is total thrust minus weight, and Newton’s second law gives \(a = F_{net}/m\).

\[T_{max} = 4 \times 5.88 \text{ N} = 23.52 \text{ N}\]

\[\text{thrust-to-weight ratio} = \frac{23.52 \text{ N}}{11.76 \text{ N}} = 2.0\]

\[F_{net} = 23.52 \text{ N} - 11.76 \text{ N} = 11.76 \text{ N}\]

\[a = \frac{F_{net}}{m} = \frac{11.76 \text{ N}}{1.2 \text{ kg}} = 9.8 \text{ m/s}^2\]

The thrust-to-weight ratio is \(2.0\), and the initial upward acceleration is about \(9.8 \text{ m/s}^2\).

Check: A ratio of exactly 2 leaves a spare upward force equal to the weight, so the acceleration must come out equal to \(g\). The arithmetic agrees, which is a useful sign that no factor has been dropped. In practice the climb is gentler, because a pilot rarely holds full throttle and drag grows quickly as the drone gains speed.

A propeller spinning at flight speed will cut skin and eyes, and a motor can spin up unexpectedly the moment a battery is connected. Never connect a flight battery with propellers fitted unless the drone is on the ground and everyone is clear of it, and never hold a powered drone in your hands to test motors. Remove the propellers first for any bench test.

Rework the hover calculation for the same drone after a \(250 \text{ g}\) sprayer tank is bolted underneath it. Find the new thrust needed from each motor, then use the \(600 \text{ g}\) per-motor limit to work out what the thrust-to-weight ratio has fallen to.

19.2.4 Pitch, Roll and Yaw by Differential Thrust

A multirotor has a problem an aeroplane does not. It has no rudder, no ailerons and no tilting propellers. Every rotor points stiffly upward, and the only quantity the flight controller can change is how fast each one spins. Everything the drone does must therefore be produced by making some rotors spin faster than others — by differential thrust.

Start with the three ways a body in the air can turn, together called its attitude. Pitch is the nose tipping up or down, roll is one side dipping relative to the other, and yaw is the whole body turning to face a new direction while staying level. Fig. 19.2-3 shows the three axes on a four-rotor drone.

Diagram

Fig. 19.2-3 – a four-rotor drone drawn from above in an X layout, with the three rotation axes marked: the pitch axis running left to right through the centre, the roll axis running front to back, and the yaw axis vertical through the centre; each axis drawn with a curved arrow showing the direction of rotation; the front of the drone clearly marked; the two diagonally opposite rotors spinning clockwise marked CW and the other diagonal pair marked CCW

Pitch and roll

To move forward, the flight controller speeds up the two rear rotors slightly and slows the two front rotors by the same amount. The rear of the machine rises, the nose drops, and the whole body tilts forward. Because the rotors are fixed to that body, the total thrust now points forward and upward instead of straight up, as Fig. 19.2-4 shows.

Diagram

Fig. 19.2-4 – a four-rotor drone in side view tilted nose-down by an angle theta; a single arrow along the tilted rotor axis labelled total thrust T; dashed construction lines resolving it into a vertical component labelled T cos theta balancing the weight W, and a horizontal component labelled T sin theta driving the drone forward; a small arrow opposing the motion labelled drag

Resolving that tilted thrust explains the whole manoeuvre. The vertical part, \(T\cos\theta\), must still support the weight, so \(T\cos\theta = mg\). The horizontal part, \(T\sin\theta\), is what drives the drone forward against drag. Because \(\cos\theta\) is less than one for any tilt, the total thrust must be increased as the drone leans, or it will lose height while it accelerates. A drone that sags slightly as a beginner pushes the stick forward is showing exactly this effect.

Rolling works identically, using the left-hand and right-hand pairs instead of the front and rear pairs. This is why a multirotor can slide sideways without turning at all — something no aeroplane can do.

Yaw

Yaw is the interesting one, because turning to face a new direction is not done with thrust at all. It is done with torque.

A spinning rotor pushes air round as well as down, and by Newton’s third law the air pushes back on the frame, trying to turn the whole drone in the direction opposite to that rotor’s spin. To cancel this out, the rotors are arranged in counter-rotating pairs: on a four-rotor drone, one diagonal pair spins clockwise and the other diagonal pair spins anticlockwise. When all four run at the same speed, their reaction torques cancel and the drone holds its heading.

To yaw, the controller deliberately unbalances them. It speeds up the clockwise pair and slows the anticlockwise pair by a matching amount. The total thrust stays roughly the same, so the height does not change, but the reaction torques no longer cancel, and the body turns anticlockwise. Reversing the trim turns it the other way.

Q. Why do the two clockwise rotors sit diagonally opposite each other rather than side by side?

A. Because the pairs must cancel each other’s torque without also unbalancing pitch or roll. Diagonally opposite rotors are symmetric about both the pitch axis and the roll axis, so speeding one pair up and slowing the other leaves the tilt of the machine untouched and changes only the net torque about the vertical axis. If the two clockwise rotors were both at the front, the same trim would tip the nose as well as turn the drone.

Q2. The four propellers of a quadcopter all point straight up and cannot be tilted. How, then, does it move forward, sideways, or turn to face a new direction?

A. By spinning some rotors faster than others. Speeding up the rear pair and slowing the front pair tilts the whole body forward, so the fixed rotor thrust now has a horizontal component that drives the drone ahead; using the left and right pairs instead makes it slide sideways. Turning is different again: the rotors are arranged as two counter-rotating pairs whose reaction torques normally cancel, and speeding up one pair while slowing the other leaves a net torque that swivels the drone about its vertical axis without changing its height.

19.3 The Structure of a Drone

Everything in section 19.2 assumed that the four rotors stay in a fixed geometry: the same distance from the centre, the same distance apart, all pointing the same way. That assumption is the job of the drone’s structure. If an arm flexes in flight, the thrust from that rotor is no longer where the flight controller thinks it is, and the corrections it computes are wrong.

A drone’s structure is therefore not just a container for the electronics. It is what makes the mathematics of the previous section true. Fig. 19.3-1 names the parts, and the subsections that follow take them in turn.

Diagram

Fig. 19.3-1 – anatomy of a four-rotor drone drawn in three-quarter view, with these parts labelled by leader lines: central frame (carrying the flight controller and battery), one of the four arms, a motor mount at the end of an arm, the motor and propeller on that mount, the landing gear beneath the frame, and the payload mounting platform under the frame carrying a camera on a gimbal; the front of the drone marked

19.3.1 The Frame

At the centre of every drone is the frame, the structure that holds every other part in a fixed relationship. The flight controller, the battery, the power wiring and the payload mount all attach to it, and the arms extend from it.

A frame has two demands pulling against each other. It must be stiff, because flex means the rotors are not where they are assumed to be. It must also be light, because every gram of frame is a gram that must be lifted for the whole flight and is not available as payload or battery.

Three materials are in common use, and the choice is a judgement rather than a rule.

Table 19.2 – Common frame materials for drones | Feature | Carbon fibre | Aluminum | Moulded plastic composite | |—|—|—|—| | Stiffness for its weight | Highest | Moderate | Lowest | | Vibration behaviour | Damps vibration well | Transmits vibration; can ring | Damps reasonably, but flexes | | Behaviour in a crash | Cracks or splinters, usually unrepairable | Bends, often repairable | Bends or cracks; cheap to replace | | Cost | High | Moderate | Low | | Typical use | Professional survey, spraying and cinematography machines | Structural fittings, motor mounts, larger utility frames | Small consumer and training drones |

Carbon fibre dominates professional machines because it is stiff, light and quiet under vibration, and its cost is small beside the equipment it carries. A school or training drone is usually moulded plastic, for the honest reason that it will be crashed and must be cheap to replace. Aluminum appears most often in fittings and mounts, where a part must take a bolted, repeated load.

The shape of the frame follows the number of rotors: an X or cross layout for four rotors, a Y for three, and a radial or circular hub for six or eight arms. Whatever the layout, it must be symmetric about the centre, because the flight control of section 19.2.4 assumes that opposite rotors have equal leverage about the axis they turn.

19.3.2 The Arms and Motor Mounts

The arms carry the rotors out from the centre. Their length sets how much leverage each rotor has about the pitch and roll axes, and that leverage decides how sharply the drone can be tilted and how quickly it settles again.

Longer arms allow larger propellers, and section 19.2.2 explained why larger, slower propellers are more efficient. They also give a steadier machine, because a correction applied far from the centre acts gently over a long moment arm. Shorter arms give a compact, agile drone that changes attitude fast, which is what a racing pilot wants and a survey camera does not.

At the end of each arm is a motor mount, the plate or bracket the motor bolts to. Two things are asked of it. It must hold the motor shaft accurately perpendicular to the frame, because a rotor tilted even slightly by a bent mount produces a small sideways force that the flight controller must fight for the whole flight. It must also isolate vibration, since a spinning propeller shakes the arm at its rotation frequency, and that shaking travels straight to the accelerometer on the flight controller.

Many designs therefore use rubber grommets or a soft mounting layer, and larger machines use quick-release mounts so that a damaged motor or propeller can be swapped in the field without unbolting the arm.

Q. Why should a small vibration matter to a machine that is producing several newtons of thrust?

A. Because the flight controller measures acceleration, and it cannot tell the difference between the drone genuinely accelerating and the sensor being shaken. Propeller vibration adds a rapidly alternating signal to every reading, and the controller responds to it with equally rapid motor corrections. The result is a machine that wobbles, drains its battery faster, and produces blurred images — all from a fault in a mounting bracket rather than in the electronics.

19.3.3 Landing Gear

The landing gear is the structure beneath the frame that takes the shock of touchdown and holds the drone clear of the ground when it is parked.

On a small training drone this may be nothing more than a pair of moulded plastic skids. On a professional machine it is often taller and sometimes retractable, folding upward once the drone is airborne so that the camera can see all the way round without a leg in the picture.

Its real purpose is protection. The battery and the payload sit low on the airframe, and they are the two most expensive things on a drone. Ground clearance must be enough that a sensor or a gimbal never touches the surface, and enough that the propellers do not throw dust and loose grit up into the machine on take-off from a field.

19.3.4 Payload Mounting Platforms

The equipment a drone carries for its mission is mounted on a dedicated platform, usually on the underside of the frame and sometimes on the front. Section 19.5 deals with what that equipment does; here the question is only how it is attached.

Three requirements govern the mounting. It must be near the centre of the frame, so the load does not unbalance the machine. It must be isolated from vibration, or a camera will record a rippling image and a scanner will return blurred data. And on a machine that changes payloads between missions, it should be quick to release, because a spraying tank and a mapping camera are rarely wanted on the same day.

For camera work, the mount is usually a gimbal: a pivoting cradle, driven by small motors, that holds the camera pointing steadily at its subject while the drone tilts and turns around it. This is what allows a smooth aerial shot from a machine that is constantly making the attitude corrections described in section 19.2.4.

Photograph

Fig. 19.3-2 – close view of a camera mounted on a three-axis gimbal beneath the frame of a civilian drone, with the pivot points and the vibration-damping mounts between the gimbal and the frame clearly visible; sourcing note – the gimbal arms and damping balls must be in focus and unobscured; images where the gimbal is hidden by a cover, or where a manufacturer logo dominates the frame, are unusable

Mounting a payload wherever it fits What students do: Bolt an added camera, tank or battery to one side of the frame or well behind the centre, because that is where there is room. What happens: The centre of gravity shifts away from the centre of thrust. The flight controller compensates by permanently running the rotors on one side harder than the others, so the drone leans, drifts steadily in one direction when the sticks are released, and reaches the thrust limit of the loaded motors long before the others. The fix: Mount the load as close as possible to the centre of the frame, and where the shape prevents that, balance it with a matching mass on the opposite side. A drone should hang level when balanced on a finger under its centre.

19.3.5 Rotor Configurations

Multirotor drones are named by how many rotors they carry, and the count is a real engineering choice rather than a style. It decides how much the machine can lift, how steady it is, and — most importantly — what happens when something fails.

Fig. 19.3-3 shows the four common layouts with their rotation directions marked.

Diagram

Fig. 19.3-3 – four top-view layouts side by side, labelled tricopter, quadcopter, hexacopter and octocopter; each drawn with its arms from a central hub, each rotor marked CW or CCW; the tricopter shown with a Y frame and its tail motor drawn on a tilting mount with a curved arrow showing the tilt; the front direction marked on each

A tricopter has three rotors on a Y-shaped frame. An odd number of rotors cannot be arranged in counter-rotating pairs, so the reaction torques described in section 19.2.4 do not cancel by themselves. A tricopter solves this by mounting its tail motor on a small tilting servo, which swings the rotor sideways to control yaw. It is light and uses fewer motors, but that moving joint is one more thing to wear and to fail, which is why the design is uncommon.

A quadcopter has four rotors in two counter-rotating pairs. It is the simplest arrangement that balances its own torque with no moving parts at all, and it is by a wide margin the most common drone in the world.

A hexacopter has six rotors and an octocopter has eight. Both lift more, and both are steadier in wind, because a larger total rotor area is being trimmed by the same controller. Their real advantage, though, is redundancy — the presence of spare capacity that lets a system keep working after a component fails.

This is where the choice becomes a judgement about consequences. If one motor stops on a quadcopter, the torque balance and the thrust balance are both destroyed at once, and the machine cannot be recovered; it will tumble. A hexacopter losing one motor can usually keep flying long enough to land, though with reduced control. An octocopter, whose rotors are often mounted as pairs, tolerates a failure most reliably of all.

Table 19.3 – Rotor configurations compared | Feature | Tricopter | Quadcopter | Hexacopter | Octocopter | |—|—|—|—|—| | Number of rotors | 3 | 4 | 6 | 8 | | How yaw is controlled | Tilting tail motor on a servo | Counter-rotating pairs | Counter-rotating pairs | Counter-rotating pairs | | Lifting capacity for its size | Low | Moderate | High | Highest | | Behaviour if one motor fails | Loss of control | Loss of control | Usually recoverable, degraded control | Usually recoverable | | Cost, weight and power drawn | Lowest | Low | High | Highest | | Typical use | Hobby and experimental builds | Photography, training, general purpose | Survey and spraying with heavier payloads | Cinematography, industrial inspection, emergency response |

Nothing here says that more rotors are better. Eight motors mean eight speed controllers, eight sets of wiring, more weight and a larger battery drain, and all of that must be lifted for the whole flight. A survey team choosing a hexacopter over a quadcopter is usually buying two things deliberately: the ability to carry a heavier instrument, and the ability to bring that instrument home if a motor quits over a canal.

19.4 The Subsystems of a Drone

A drone is easiest to understand as four subsystems that must work together: something to produce thrust, something to supply energy, something to decide, and something to carry instructions in and information out. Fig. 19.4-1 shows how they are joined, and it is worth returning to as you read the four subsections that follow.

Diagram

Fig. 19.4-1 – block diagram of a four-rotor drone; blocks: lithium polymer battery, power distribution board, four electronic speed controllers, four brushless DC motors with propellers, battery eliminator circuit, flight controller, sensor block (gyroscope, accelerometer, magnetometer, barometer, GPS receiver), radio receiver, telemetry transmitter, payload; thick arrows labelled “power” from battery to power distribution board, from there to each electronic speed controller and to the battery eliminator circuit, and from the battery eliminator circuit to the flight controller; thin arrows labelled “signal” from flight controller to each electronic speed controller and from each electronic speed controller to its motor; thin arrows labelled “data” from the sensor block to the flight controller and from the flight controller to the telemetry transmitter; an arrow labelled “pilot commands” from the radio receiver into the flight controller; the ground station and handheld controller drawn outside the drone boundary with a dashed radio link crossing it

19.4.1 The Propulsion System

The propulsion system is everything that turns stored electrical energy into thrust: the motor, the propeller it drives, and the electronics that set its speed.

Why brushless motors

You have already met the ordinary DC motor, in which current is fed to the rotating coils through carbon brushes pressing on a commutator. Drones almost never use them. A brushless DC motor (BLDC) reverses the arrangement: the magnets rotate and the coils are fixed to the housing, so there is no sliding contact anywhere in the motor. The switching that a commutator used to do mechanically is done electronically instead.

Brushless DC motor Working principle: Permanent magnets on the rotating part are pulled round by a magnetic field produced in fixed coils. External electronics energise the coils in a repeating sequence, so the field always leads the rotor slightly and keeps it turning. Speed is set by how rapidly that sequence is stepped through. Components: Stator with three sets of coils, rotor carrying permanent magnets, bearings, and an external controller that supplies the switched three-phase current. Advantages: No brushes to wear out or spark, so the motor lasts far longer; high efficiency and high power for its weight, which matters when the motor must lift itself; fine and rapid speed control, which is what makes attitude correction possible. Limitations: Cannot run directly from a battery — it is useless without its controller, which adds cost and weight; most drone controllers have no position sensor and judge the rotor’s position from the motor’s own signal, so starting and very slow running can be rough; and the motor reports no thrust back, so a damaged propeller shows up as unexpected motion rather than as an error. Where it is used: Every rotor of almost every modern drone; also in electric bicycles, cordless power tools and computer cooling fans.

The electronic speed controller

Because the coils must be energised in sequence, a brushless motor needs a driver, and that driver is the electronic speed controller (ESC). It takes direct current from the battery and switches it to the three motor terminals in the correct order and at the correct rate, producing a three-phase output from a two-wire supply.

Each motor has its own ESC. The flight controller does not supply the motor current itself — it could not; it simply sends each ESC a small signal meaning “run at this speed”, and the ESC handles the power. This is the same division of labour you have already seen between a microcontroller and a motor driver module on a ground robot, and for the same reason: a logic pin can supply a few milliamperes, while a drone motor draws tens of amperes.

Propellers

The propeller converts the motor’s rotation into moving air. Two of its dimensions matter: its diameter, which fixes how wide a column of air it works on, and its pitch, which is the distance the propeller would advance in one revolution if it moved through the air like a screw through wood. A higher pitch moves more air per turn and produces more thrust, but demands more torque and more current from the motor.

Propellers are also handed. A given propeller is shaped to push air downward when turning one particular way, and every multirotor therefore needs both clockwise and anticlockwise propellers, matched to the rotation directions established in section 19.2.4.

Fitting a propeller the wrong way round What students do: Fit all four propellers from the same packet, or reverse one after cleaning it, because they look almost identical. What happens: The wrongly fitted rotor pushes air upward instead of downward. On take-off the drone does not rise evenly; one corner is pulled down while the flight controller applies more and more power trying to lift it, and the machine flips over on the ground. The fix: Check the marking on each blade — propellers are labelled or shaped for clockwise or anticlockwise use — and confirm before every flight that each matches the rotation direction of the motor it sits on.

19.4.2 The Power System

Everything a drone does comes out of one battery, and the choice of that battery shapes the whole design.

Drones are powered almost universally by lithium polymer (LiPo) batteries. These are a form of the lithium-ion chemistry you met earlier, packaged in soft foil pouches rather than rigid cells. The reason for the choice is energy density: a LiPo pack stores more energy per gram than any alternative in common use, and on a machine that must lift its own power supply, energy per gram is the figure that decides how long the flight lasts.

A LiPo cell has a nominal voltage of about \(3.7 \text{ V}\), rising to roughly \(4.2 \text{ V}\) when fully charged. Cells are joined in series to reach a useful voltage, and the count is written with an S: a 3S pack is three cells, about \(11.1 \text{ V}\) nominal, and a 6S pack is six cells, about \(22.2 \text{ V}\).

Two more figures are printed on every pack. Its capacity, in milliampere-hours (mAh), states how much charge it holds. Its C rating states how much current it can safely deliver, as a multiple of its capacity — a \(2200 \text{ mAh}\) pack rated 25C can supply \(25 \times 2.2 \text{ A} = 55 \text{ A}\).

Checking that a battery can supply the motors

Given: A four-rotor drone whose motors each draw a maximum of \(12 \text{ A}\) at full throttle. The flight controller, receiver and payload together draw about \(1 \text{ A}\). The battery is a 3S lithium polymer pack of capacity \(2200 \text{ mAh}\) with a C rating of 25.

Find: Whether the pack can supply the worst-case current, and the energy it stores.

Method: The worst-case current is the sum of all loads. The maximum current a pack can deliver is its capacity in ampere-hours multiplied by its C rating. Stored energy is capacity in ampere-hours multiplied by nominal voltage.

\[I_{demand} = (4 \times 12 \text{ A}) + 1 \text{ A} = 49 \text{ A}\]

\[I_{max} = 25 \times 2.2 \text{ A} = 55 \text{ A}\]

\[V_{nominal} = 3 \times 3.7 \text{ V} = 11.1 \text{ V}\]

\[E = 2.2 \text{ Ah} \times 11.1 \text{ V} = 24.42 \text{ Wh}\]

The pack can supply the demand, since \(49 \text{ A}\) is less than its \(55 \text{ A}\) limit, and it stores about \(24 \text{ Wh}\) of energy.

Check: The margin is only about six amperes, roughly a tenth of the rating, and a pack that is cold or part-worn will do worse. A designer would treat this as too tight and choose a higher C rating, which is exactly the kind of judgement the calculation exists to support.

From the pack, current goes to a power distribution board, a simple plate of heavy copper tracks that splits the single battery lead into one feed for each ESC. The flight controller, receiver and sensors cannot run at \(11.1 \text{ V}\), so a battery eliminator circuit (BEC) — a small voltage regulator — steps the supply down to the \(5 \text{ V}\) they need.

Lithium polymer packs store a great deal of energy in a soft pouch and will burn fiercely if abused. Charge them only on the correct lithium polymer setting of a proper charger, never leave a pack charging unattended, and never charge a pack that is swollen, punctured or has been in a crash. Do not discharge a pack below the minimum voltage marked on it, store packs at part charge in a metal box away from anything that can burn, and hand damaged packs to your teacher for disposal rather than putting them in a bin.

19.4.3 The Flight Controller and Its Sensors

The flight controller, introduced in section 19.1.2, is the board that closes the sense–plan–act loop. It is a small computer built around a microcontroller, and its entire picture of the world comes from the sensors listed in Table 19.4.

Table 19.4 – Sensors carried by a typical drone, what each measures and where each fails | Sensor | What it measures | What the drone uses it for | Where it is weak | |—|—|—|—| | Gyroscope | Rate of rotation about each of the three axes, in °/s | Detecting tilt as it begins; the primary sensor for stabilisation | Its readings drift slowly, so an angle obtained by adding them up wanders over time | | Accelerometer | Linear acceleration along three axes, in m/s² | Finding which way is down, and correcting the gyroscope’s drift | Cannot separate genuine acceleration from tilt; badly affected by propeller vibration | | Magnetometer | Direction of the Earth’s magnetic field | Establishing heading, so the drone knows which way it faces | Disturbed by motor currents, steel structures and power lines | | Barometer | Air pressure | Estimating height above the launch point | Confused by wind gusts and by the drone’s own downwash near the ground | | GPS receiver | Position and altitude from satellite signals | Holding position, following a route, returning home | Needs a clear sky view; unusable indoors, weak under trees and beside tall buildings | | Optical flow or ultrasonic sensor | Motion across the ground, or height above it | Holding position steadily at low height, and landing gently | Optical flow fails over plain or dark surfaces; ultrasonic fails on soft or angled ground |

The gyroscope and accelerometer are usually built into a single package called an inertial measurement unit (IMU), and the pairing is deliberate. A gyroscope responds instantly and accurately to a change in rotation but drifts if you rely on it for long. An accelerometer is noisy from one instant to the next but always has gravity as a fixed long-term reference. Combining them so that each covers the other’s weakness is called sensor fusion, and it is what produces a steady estimate of the drone’s attitude.

The remaining sensors extend that estimate from attitude to position. A magnetometer acts as a compass and fixes the heading, a barometer gives height, and a global positioning system (GPS) receiver gives latitude, longitude and altitude from satellite signals.

19.4.4 The Communication System

A drone carries two radio paths, and it is worth keeping them separate in your mind.

The command link runs from the operator’s handheld transmitter to a receiver on the drone, usually on the 2.4 GHz band. The receiver passes the stick positions to the flight controller, which interprets them as requests for attitude and height rather than as direct motor settings.

The telemetry link runs the other way, from the drone down to a ground station or a phone. It carries telemetry — measurements about the machine itself, such as battery voltage, satellite count, height, speed and position. On camera drones a separate video downlink sends a live picture to a screen or goggles.

Both links weaken with distance and are blocked by buildings, hills and trees. This is not a fault to be designed away but a condition to be planned for, and the plan is the failsafe: a behaviour the flight controller adopts on its own when the command link goes quiet for longer than a set time. A well-configured failsafe holds position for a few seconds, then climbs to a safe height and returns home, and lands where it took off if the link never returns.

Q. If the radio link is lost, why does the drone not simply stop and hover until it comes back?

A. Because hovering costs as much energy as flying, and the battery is finite. A drone that hovers at the far end of its route while the link stays broken will run its pack flat and come down wherever it happens to be — possibly over water, a road or a crowd. Returning home spends the same energy travelling towards the one place where a controlled landing is known to be safe.

Q4. How does a drone find its way to a village it has never visited, and how does it get home if the radio link to its operator is lost?

A. It navigates by coordinates rather than by recognising places. Its GPS receiver reports where it is, the route is stored before take-off as a list of waypoints, and the flight controller repeatedly works out the bearing and distance from its present position to the next waypoint and tilts the drone that way. The launch point is saved at take-off as the home position, so if the command link stays silent longer than a set time, the failsafe takes over: the drone climbs to a safe height, treats home as its next waypoint and flies back, landing where it started.

19.5 Sensor Payloads

19.5.1 What a Payload Is

Everything described in section 19.4 exists to keep the drone flying. None of it is the reason anyone bought the drone.

A drone is a way of putting an instrument in a place a person cannot easily stand, and the instrument is called the payload — the mission equipment a drone carries, which does the useful work but takes no part in flying the machine.

NoteDefinition

A payload is any equipment a drone carries in order to perform its mission, as distinct from the components required to fly the drone itself.

The distinction is worth holding on to, because both a payload and the flight system contain sensors. The drone’s own GPS receiver, barometer and inertial measurement unit are not payload: remove them and the drone cannot fly. A survey camera is payload: remove it and the drone flies perfectly well, having merely lost its purpose.

Payloads divide into two groups by what they do. Most sense — they collect information and send it back. A few act — they release or apply something. This section deals mainly with the first group, since it is the larger and the one the syllabus names, but the second is treated briefly at the end of section 19.5.3.

19.5.2 Imaging Payloads

The commonest payload by far is a camera, and the choice of camera decides what the flight can reveal.

An ordinary visible-light camera, mounted on the gimbal described in section 19.3.4, records what a person standing at that height would see. It answers questions about shape, extent and condition: how far a flood has spread, whether a roof has cracked, where a boundary runs. For photography and video work it is fitted with a high-resolution sensor and a stabilised lens; for survey work the same camera is flown on a grid pattern and its overlapping images are later joined into a single large map.

A thermal camera records infrared radiation instead of visible light. Every object emits infrared in proportion to its temperature, so a thermal camera does not need any illumination at all and produces a usable image in complete darkness. What it shows is not colour but temperature difference, as Fig. 19.5-1 illustrates: a person, an animal, a running motor or a smouldering patch of ground stands out clearly against cooler surroundings.

Photograph

Fig. 19.5-1 – a thermal camera image of an outdoor scene at night in which two or three human figures appear as bright warm shapes against a cool dark background of ground and vegetation, with the temperature scale bar visible at the edge of the frame; sourcing note – the image must be a genuine thermal capture with the warm figures clearly distinguishable; artistic false-colour photographs of ordinary scenes, and images of casualties or accident victims, are unusable

This makes a thermal camera the standard payload for searching. A missing person in scrub at night is nearly invisible to a normal camera and obvious to a thermal one. The same property is used in inspection work, where an overheating joint in an electrical installation or a wet patch of insulation shows as a temperature anomaly long before anything is visible to the eye.

Its limitation is that it sees only heat. It cannot read a number plate, cannot tell one person from another, and loses much of its advantage in the middle of a hot afternoon when the whole landscape has warmed towards body temperature.

A multispectral camera takes the idea further by measuring reflected light in several narrow bands, including bands just beyond the red end of the visible spectrum. Healthy vegetation reflects near-infrared strongly while absorbing red light for photosynthesis; a stressed or diseased plant reflects less near-infrared. By comparing the bands, the camera produces a map of plant vigour across a field. Section 19.7.2 returns to what a farmer does with that map.

19.5.3 Sensing and Mapping Payloads

Cameras record what a surface looks like. Some missions need to know where surfaces actually are, in three dimensions, and for that the usual payload is a scanner.

Light detection and ranging (LIDAR) measures distance by timing light. The instrument sends out a very short laser pulse, waits for the reflection to return, and computes the distance from the time taken and the known speed of light. Sweeping the beam rapidly across the ground beneath a moving drone, as in Fig. 19.5-2, produces hundreds of thousands of measured points per second. The result is a point cloud — a set of three-dimensional coordinates describing every surface the beam struck.

Diagram

Fig. 19.5-2 – side view of a drone flying left to right above uneven ground carrying a LIDAR unit; a fan of laser pulses drawn from the instrument to the surface below, with two rays highlighted and labelled “outgoing pulse” and “returning reflection”; the ground drawn with a low building on one side and a group of trees on the other; beneath the ground profile, a row of dots labelled “point cloud” following the shape of the terrain, building roof and tree canopy; a label noting that some pulses pass through gaps in the canopy and return from the ground below it

Two properties make LIDAR valuable. It works in darkness, since it supplies its own light. And because a fan of pulses will find gaps between leaves, enough beams reach the ground through a tree canopy to reveal the terrain underneath it — something no camera can do. Surveyors use this to map ground shape under vegetation, to measure the volume of a stockpile, and to model structures such as bridges and transmission towers.

The costs are real. A LIDAR unit is heavy and expensive compared with a camera, its point cloud requires substantial processing before anyone can use it, and reflective or wet surfaces return poor data.

Table 19.5 – Sensing payloads compared | Feature | Visible-light camera | Thermal camera | Multispectral camera | LIDAR | |—|—|—|—|—| | What it detects | Reflected visible light | Emitted infrared radiation | Reflected light in several narrow bands | Time of flight of its own laser pulses | | What it reveals | Appearance, shape, extent | Temperature differences | Plant health and stress | Three-dimensional position of surfaces | | Works in darkness | No | Yes | No | Yes | | Main limitation | Needs light; sees only surfaces | No detail, poor contrast when surroundings are warm | Needs calibration and processing to interpret | Heavy, costly, data-intensive | | Typical mission | Photography, mapping, damage survey | Search and rescue, electrical and building inspection | Crop health monitoring, precision farming | Terrain and structure survey, volume measurement |

Payloads that act rather than sense

Not every payload collects data. An agricultural drone carries a tank, a pump and spray nozzles, and its mission is to put a measured quantity of liquid on a crop. A delivery drone carries a cargo box with a release mechanism, as in the flight described at the start of this chapter. These payloads change the drone’s behaviour in a way a camera does not: a spray tank empties during the flight, so the machine grows steadily lighter and its trim changes as it works.

19.5.4 Payload, Lift Capacity and Endurance

Every payload is bought with flight time. This is the trade-off that governs drone operations, and it follows directly from the physics of section 19.2.

The payload capacity of a drone is the mass it can carry over and above its own airframe, battery and electronics. It is not simply the maximum thrust minus the empty weight, because a machine flying at its thrust limit has nothing left for control. Recall from section 19.2.3 that a useful thrust-to-weight ratio is about 2. A loaded drone whose ratio has fallen towards 1.3 cannot climb briskly, cannot hold position in a gust, and cannot recover from a disturbance.

Endurance — how long the drone can stay airborne — falls as the load rises, and it falls faster than the load rises. The reason is in section 19.2.2: extra weight demands extra thrust, extra thrust demands a higher rotor speed, and the current the motors draw climbs more steeply than the thrust they produce.

How a payload shortens the flight

Given: The four-rotor drone of section 19.2, mass \(1.2 \text{ kg}\), flying on the 3S \(2200 \text{ mAh}\) lithium polymer pack of section 19.4.2. Hovering unloaded, the four motors and the electronics together draw an average of about \(15 \text{ A}\). With a \(250 \text{ g}\) payload fitted, the average draw rises to about \(19 \text{ A}\). To protect the pack, only 80 per cent of its capacity is used.

Find: The approximate flight time in each case, and the percentage change.

Method: Flight time is the usable charge divided by the average current, \(t = C_{usable}/I\). Charge in ampere-hours divided by current in amperes gives a time in hours.

\[C_{usable} = 0.80 \times 2.2 \text{ Ah} = 1.76 \text{ Ah}\]

Unloaded:

\[t = \frac{1.76 \text{ Ah}}{15 \text{ A}} = 0.117 \text{ h} = 7.0 \text{ min}\]

Loaded:

\[t = \frac{1.76 \text{ Ah}}{19 \text{ A}} = 0.093 \text{ h} = 5.6 \text{ min}\]

\[\text{reduction} = \frac{7.0 - 5.6}{7.0} \times 100 = 20\%\]

About \(7.0 \text{ min}\) unloaded and about \(5.6 \text{ min}\) with the payload, a reduction of roughly 20 per cent.

Check: The payload added about 21 per cent to the mass and cost about 20 per cent of the endurance, while the current rose by about 27 per cent — current climbing faster than mass, as expected. Flight times of five to ten minutes are ordinary for a small camera drone on a pack of this size, so the magnitude is sensible.

Q3. Why does hanging a heavier box under a drone shorten the flight as well as slow it down?

A. Because the rotors must lift the extra weight for every second of the flight. Heavier means more thrust, more thrust means faster rotors, and the current drawn from the battery climbs faster than the added weight — in the calculation above, a 21 per cent heavier machine drew 27 per cent more current and lost a fifth of its flight time. The drone also slows because the spare thrust that was available for climbing, accelerating and resisting gusts has been spent on simply staying up.

Suppose the same drone is fitted with a \(4400 \text{ mAh}\) pack, which holds twice the charge but weighs about \(100 \text{ g}\) more, raising the unloaded hover current to about \(17 \text{ A}\). Work out the new flight time, and then say whether doubling the size of the battery has doubled the endurance.

19.6 Flying Within the Law

A drone is not a toy in the eyes of the law, and for a good reason. The airspace above your head is shared. A machine of two kilograms falling from a hundred metres carries enough energy to kill; a machine drifting into the approach path of an airliner endangers everyone on board it; a camera two hundred metres up can look into places its subject reasonably thought were private.

Every country therefore regulates who may fly a drone, where, how high and under what conditions. In India that responsibility rests with the Directorate General of Civil Aviation (DGCA), under the Ministry of Civil Aviation, and the framework in force is the Drone Rules, 2021, with its later amendments. A replacement framework has been proposed and is under consideration, so an operator’s first duty is always to check the current position rather than to rely on what was true last year — or on what is printed in a book.

19.6.1 Weight Categories under the Drone Rules

Almost every obligation in the rules is decided by one number: how much the drone weighs when it leaves the ground.

That number is its all-up weight (AUW) — the weight of the complete machine at take-off, including its battery and its payload. Note that this is the total, not the weight of the bare airframe. A drone of 240 g that is given a 30 g camera has moved into the next category, and every rule that goes with that category now applies to it.

Weight is used because it is a fair measure of the harm a machine can do. A heavier drone carries more energy in a fall, needs more space to stop, and does more damage to anything it strikes.

Table 19.6 – Drone categories by all-up weight under the Drone Rules, 2021 | Category | All-up weight at take-off | Typical use | |—|—|—| | Nano | Not more than 250 g | Indoor practice, classroom demonstration, light recreational flying | | Micro | More than 250 g and not more than 2 kg | Photography, small surveys, most consumer camera drones | | Small | More than 2 kg and not more than 25 kg | Industrial inspection, mapping, agricultural spraying | | Medium | More than 25 kg and not more than 150 kg | Heavy lift, logistics, large agricultural machines | | Large | More than 150 kg | Specialised commercial and cargo operations |

The rules apply to civil drones up to an all-up weight of 500 kg. As the categories rise, so does everything else: the certification the machine needs, the qualification its operator needs, and the approvals a particular flight needs. Nano drones carry the lightest obligations, which is exactly why they are the class a school uses.

Q. A 249 g drone and a 260 g drone are almost identical machines. Why should the law treat them so differently?

A. Because any boundary drawn on a continuous scale produces pairs like this, and the alternative — judging every machine on its merits — cannot be administered. The boundary is placed where it is because a drone of about a quarter of a kilogram is, in most collisions, unlikely to cause serious injury. Two machines either side of it are indeed nearly the same; two machines at opposite ends of the scale are not, and that is what the line exists to separate.

19.6.2 Airspace Zones and Height Limits

The second question the rules answer is where a drone may fly, and the answer is given as a map rather than as a list of places.

The DGCA publishes an interactive airspace map that divides the country into three kinds of zone. A green zone is airspace where flying is permitted without any flight permission, up to a stated height. A yellow zone is controlled airspace, typically around airports and above certain heights, where permission from the air traffic authority is needed before a flight. A red zone is prohibited: no drone may be flown there without permission from the central government. Areas around airports, defence installations, international borders and certain strategic sites fall into the restricted zones. Fig. 19.6-1 shows how these are arranged.

Diagram

Fig. 19.6-1 – vertical section through the airspace near an airport; the airport at ground level on the right; a red zone shaded above and immediately around it; a yellow zone extending outward and upward from the red zone boundary; a green zone covering the remaining airspace to the left up to a horizontal dashed line labelled 120 m above ground level; a small drone drawn in the green zone below the line; arrows and labels stating that no permission is needed in the green zone below the line, that air traffic permission is needed in the yellow zone, and that the red zone is prohibited without central government permission; a note that the permitted height is reduced closer to the airport

Above the green zone there is a height ceiling of 400 feet, which is about 120 m above ground level. The reason is separation: crewed aircraft do not normally descend below that height except on approach, so the ceiling keeps the two kinds of traffic apart. Closer to an airport the permitted height is lower still, because that is exactly where aircraft are low.

The map is the authority, not memory or habit. A field that was in a green zone last season may not be today, and checking the current map before a flight is part of planning the flight.

19.6.3 Registration, Certification and Operating Conditions

Three separate things must be in order before a flight: the machine, the operator and the flight itself.

The machine

A drone above the smallest class must be registered on the Digital Sky platform, the DGCA’s online single-window system, which issues it a unique identification number (UIN). The number is displayed on the machine, so that a drone found where it should not be can be traced to its owner. Certain categories are exempted, the nano class among them, and a school should confirm the current exemptions rather than assume them.

The model itself must hold a type certificate, showing that the design meets the standards laid down, and third-party insurance is required for drones above the nano class so that anyone harmed by a flight can be compensated.

The operator

Flying a drone above the nano class commercially requires a remote pilot certificate (RPC), obtained by training at an authorised remote pilot training organisation and passing its examination. This is a licence to fly, and it exists for the same reason a driving licence does: a competent operator is the single most effective safety device on the aircraft.

The flight

The operating conditions apply to every flight, whatever the machine and whoever is flying it. A drone is flown in daylight and in good visibility. It is flown within visual line of sight (VLOS) — the operator, or a trained observer beside the operator, must be able to see the actual aircraft, not merely its picture on a screen. One remote pilot flies one drone. Nothing dangerous is carried without specific permission. Flights over crowds are not made, and no one flies a drone who has been drinking.

Two of these carry exceptions that must be applied for in advance rather than assumed: flying at night, and flying beyond visual line of sight. The vaccine delivery described at the start of this chapter was a beyond-line-of-sight flight, and the permission to make it was as much a part of the mission as the drone.

Q5. Who decides how high a drone may fly in India, and which places it may not fly over at all?

A. The Directorate General of Civil Aviation, under the Ministry of Civil Aviation, working through the Drone Rules, 2021 and the airspace map it publishes. The general ceiling in open airspace is 400 feet, roughly 120 m above ground level, which keeps drones below the height at which crewed aircraft normally fly. Where a drone may go is set by the map’s three zones: green, where no flight permission is needed; yellow, mostly around airports and above certain heights, where air traffic permission is required; and red, around defence installations, borders and other sensitive sites, where flying is prohibited without central government permission.

19.6.4 Safety Equipment and Responsible Operation

The rules also reach into the design of the machine, requiring features that make a drone safer to be near and easier to account for.

Geo-fencing is software in the flight controller that compares the drone’s own position against stored zone boundaries and refuses to enter or take off inside a restricted area. It is an unusual kind of safeguard: the machine enforces the regulation on its operator, rather than the other way round.

The return-to-home behaviour of section 19.4.3 belongs in this list too, because a drone that brings itself back on losing its link is a drone that does not wander into someone else’s airspace. Anti-collision lighting makes a machine visible to other aircraft and to people on the ground, and onboard flight logging records what the drone actually did, which is what makes an accident investigable afterwards.

Responsibilities the rules do not spell out

Not everything that matters is written as a requirement. A camera at height sees over walls, into courtyards and onto roofs, and the people below have not agreed to be filmed. Flying a drone over a neighbour’s house, a hospital ward’s window or a private gathering may be legal in a green zone and still be indefensible. Asking permission before filming a place, and switching the camera off when the mission does not need it, cost nothing.

The same applies to livestock and wildlife, which are frightened by a hovering machine long before a person would be, and to the quiet of a residential street on a Sunday morning.

For a school demonstration, fly only in an open area cleared with the school and check the DGCA airspace map for that location beforehand. Keep spectators well back and behind the operator, never fly over anyone’s head, brief one person to watch the drone continuously while the pilot flies, and land at once if anybody walks into the area. Fit propeller guards where the machine allows it, and carry out the pre-flight check — propellers tight and correctly handed, battery charged and secure, control link and failsafe tested — every single time.

19.7 Applications of Drones

Everything in this chapter so far has been about the machine. This section is about the reasons anyone builds one.

There is a pattern worth noticing as you read. In every case below, the drone is not doing something new. People were already photographing landscapes, inspecting crops and searching for survivors long before drones existed. What the drone changes is the cost of getting an instrument to the right place, and lowering that cost far enough turns an occasional survey into a routine one — which is a bigger change than it first appears.

19.7.1 Aerial Photography

The oldest use of an aerial platform is simply to look down from it.

A photograph taken from height shows extent and relationship: how far a flood has spread, where a boundary actually runs, which part of a roof has failed, how a crowd is distributed across a ground. None of this is visible from within the scene. Until drones became cheap, obtaining such a view meant hiring a helicopter or a light aircraft, so it was done rarely and only where the stakes justified the expense.

Two quite different kinds of work follow from this.

The first is photography and video for its own sake — an event, a film, a promotional sequence for a hotel. Here the drone is a flying camera mount, and everything depends on the gimbal of section 19.3.4 holding the picture steady while the machine makes the constant attitude corrections of section 19.2.4.

The second is measurement, and it is the more technically demanding. In photogrammetry, many overlapping photographs of the same ground are taken from slightly different positions, and software compares the shifts of common features between them to reconstruct the shape and scale of the surface. The result is a single accurate map or a three-dimensional model built from ordinary pictures. This is how a drone is used to measure a construction site, a quarry stockpile or the boundary of a piece of land.

Overlap is what makes it work, and overlap is what the flight plan must guarantee. Fig. 19.7-1 shows the pattern that results.

Diagram

Fig. 19.7-1 – plan view of a rectangular field with a drone’s survey path drawn as a lawnmower pattern of parallel straight lines joined by short turns at each end; the camera footprint drawn as a rectangle at three successive positions along one line, with the rectangles clearly overlapping each other along the direction of travel and labelled “forward overlap”; two adjacent flight lines shown with their footprints overlapping sideways and labelled “side overlap”; the flight line spacing marked as the effective swath width; the take-off and landing point marked at one corner

Planning a mapping flight Input: The boundary of the area to be mapped, the required detail on the ground, the camera’s field of view, and the drone’s usable flight time from section 19.5.4. Output: A flight plan — a height, a set of parallel flight lines with a fixed spacing, a ground speed and a photograph interval — that covers the whole area with sufficient overlap. Method:

1. Choose the flight height from the detail required: lower flight gives finer detail but covers less ground per photograph.
2. Compute the ground area covered by one photograph from the flight height and the camera's field of view.
3. Fix the forward and side overlap needed for reconstruction, commonly a large fraction of each photograph.
4. Set the spacing between flight lines from the photograph width less the side overlap; this spacing is the effective swath width.
5. Lay parallel flight lines across the area at that spacing, running along its longer dimension to reduce the number of turns.
6. Set the ground speed and the photograph interval together so that successive photographs achieve the forward overlap.
7. Divide the total path length by the ground speed, and compare with the usable flight time to find how many batteries the survey needs.

Why it works: Photogrammetry recovers shape from the apparent shift of the same feature seen from two positions. Every point on the ground must therefore appear in more than one photograph, and the overlap rules are what guarantee it. Watch out: Wind changes the ground speed along one direction and against it, so the forward overlap achieved is not the overlap planned. A featureless surface — still water, fresh sand, a uniform tin roof — gives the software nothing to match between photographs, and no amount of overlap will fix it.

Ground covered by one survey flight

Given: A survey drone flying at a ground speed of \(5 \text{ m/s}\), with an effective swath width of \(40 \text{ m}\) after side overlap is allowed for. Its usable flight time is \(12 \text{ min}\).

Find: The area it can cover in a single flight.

Method: The path length is speed multiplied by time, with the time converted to seconds. Area covered is the path length multiplied by the effective swath width. The result in square metres is converted to hectares, where \(1 \text{ ha} = 10\,000 \text{ m}^2\).

\[t = 12 \text{ min} = 720 \text{ s}\]

\[\text{path length} = 5 \text{ m/s} \times 720 \text{ s} = 3600 \text{ m}\]

\[\text{area} = 3600 \text{ m} \times 40 \text{ m} = 144\,000 \text{ m}^2\]

\[\text{area} = \frac{144\,000}{10\,000} = 14.4 \text{ ha}\]

About \(14 \text{ ha}\) in one flight.

Check: Fourteen hectares is roughly a square of side \(380 \text{ m}\), which is a believable morning’s work for one battery and matches what small mapping drones achieve in practice. The estimate is slightly optimistic, because the turns at the end of each line cover no new ground.

19.7.2 Agriculture

A farmer’s central difficulty is that a field is not uniform and cannot be watched all at once. One corner drains badly, one strip was sown thin, one patch has taken a pest. Walking the field reveals these things late, and treating the whole field as though it were one thing wastes water, fertiliser and chemical on ground that did not need them.

This is the problem precision farming sets out to solve: measure the field in detail, then apply inputs only where they are needed and in the amount needed. Drones serve both halves of it.

For the measuring half, the multispectral camera of section 19.5.2 is flown over the field on the survey pattern just described. The map it produces shows plant vigour across the whole area, and it shows it before the difference is visible to someone standing among the plants. A farmer can then inspect the weak patches on foot, find out whether the cause is water, nutrient or pest, and treat the cause.

For the applying half, the payload is a tank and a spray boom. Fig. 19.7-2 shows such a machine at work.

Photograph

Fig. 19.7-2 – an agricultural spraying drone in flight a few metres above a crop in an Indian field, with its spray boom and nozzles visible and a fine spray descending onto the crop; the operator visible at a distance in protective clothing; sourcing note – the operator must be shown at a safe distance and wearing protective equipment; images showing people standing in the spray, or children near the machine, are unusable

Agricultural spraying drone Working principle: The drone flies a programmed pattern a few metres above the crop, holding a set height above the ground with the help of a downward-facing sensor. A pump feeds liquid from an onboard tank to nozzles under the arms, and the downwash from the rotors carries the droplets into the canopy rather than letting them drift. The flow rate is matched to the ground speed so that a fixed quantity is applied per unit area, and the flight controller pauses spraying at the turns. Components: Frame and rotors, liquid tank, pump and spray nozzles, flight controller with height-holding sensor, GPS receiver for the spray pattern, lithium polymer battery, ground control application for the field boundary. Advantages: Covers ground far faster than a person with a knapsack sprayer; applies an even dose because speed and flow are controlled together; keeps the operator out of the chemical entirely; and reaches a standing crop or a wet field without trampling it or leaving wheel ruts. Limitations: The tank, not the battery, usually limits the flight — a \(10 \text{ L}\) tank applying \(10 \text{ L}\) per hectare covers only about a hectare before it must land and refill. Spray drift in wind is a real hazard to neighbouring fields, and chemical application from drones is subject to its own approvals and operating procedures over and above the flight rules of section 19.6. The machine also needs cleaning after every use, since the chemicals attack it. Where it is used: Paddy, wheat, cotton and horticultural crops across India, including under the Kisan drone initiatives that promote drone use in farming.

Notice the shift in what limits the mission. For a survey drone, endurance is set by the battery, as section 19.5.4 showed. For a spraying drone the tank empties first, and the design question becomes how large a tank the machine can lift — which returns you directly to the payload and thrust arithmetic of section 19.2.3.

19.7.3 Disaster Response

After a flood, an earthquake or a landslide, the first problem is not rescue. It is information. Which roads are cut, which villages are isolated, where the water has reached, which buildings have collapsed — and the people best placed to answer are the people who cannot safely move.

A drone answers these questions in minutes rather than hours, and it does so without sending anyone into the damaged area to find out. That is its first and largest contribution: a map of the situation, made from above, updated as often as needed.

Its second contribution is search. The thermal camera of section 19.5.2 detects the warmth of a body against cold ground or rubble, in darkness and through light smoke, and a drone flying a systematic pattern covers ground far faster than a search party on foot. Fig. 19.7-3 shows two patterns used for this.

Diagram

Fig. 19.7-3 – two search patterns drawn side by side over an outline of broken terrain; on the left a parallel track pattern of evenly spaced straight lines with the track spacing marked and labelled “used when the search area is known and large”; on the right an expanding square pattern spiralling outward from a marked starting point labelled “last known position”, with the leg spacing marked and labelled “used when a starting point is known”; both patterns marked with an arrow showing the direction of travel

Its third contribution is delivery. Medicines, water purification tablets, a radio set or a rope can be flown across a collapsed bridge in a few minutes, which is the same capability the vaccine flight at the start of this chapter demonstrated in ordinary conditions.

There are honest limits, and a responder needs to know them. Endurance is short, so coverage comes in fifteen-minute pieces with battery changes between. Wind and heavy rain ground the machine exactly when conditions are worst. A thermal camera shows heat, not identity, and a warm engine block looks much like a person. And after a disaster the airspace is usually busy with helicopters flying at low level, so drone flights must be coordinated with the agencies running the response rather than launched independently by anyone who owns a drone.

Table 19.7 – What each application demands of the drone | Requirement | Aerial photography and survey | Agriculture | Disaster response | |—|—|—|—| | Main payload | Stabilised or survey camera | Multispectral camera, or tank and nozzles | Thermal camera, sometimes a delivery box | | What limits the mission | Battery endurance | Tank capacity when spraying, battery when scouting | Battery endurance and weather | | Autonomy needed | Programmed grid, closely monitored | Programmed pattern with height held above the crop | Programmed search pattern, with the operator reacting to what appears | | Precision that matters most | Position accuracy, for a map that measures correctly | Height above crop and flow rate, for an even dose | Speed of coverage, over fine detail | | Chief regulatory issue | Privacy, and permission where the area is not a green zone | Approvals and procedures for chemical application | Coordination with crewed aircraft working the same area |

Read across that table and a general lesson appears. The drone is nearly the same machine in all three columns; what changes is the payload it carries and the constraint that binds it. Choosing a drone for a job is therefore mostly a matter of identifying which constraint will bite first — energy, capacity, weather or the law — and designing the mission around that one.

19.8 Summary

What a drone is

  1. A drone, or unmanned aerial vehicle (UAV), is an aircraft carrying no pilot on board, flown either by remote control from the ground or autonomously by onboard computers acting on sensor data. The aircraft together with its ground control station, radio link and supporting equipment is an unmanned aircraft system (UAS).

  2. Most drones are multirotor aircraft: their lift comes from several rotating propellers rather than from fixed wings. This allows vertical take-off and precise hovering, at the cost of short endurance, because energy is spent continuously merely to stay up.

  3. A multirotor is inherently unstable and stays airborne only because it is corrected continuously. The flight controller runs the sense–plan–act loop hundreds of times a second: sensors measure the attitude, the controller compares it with the attitude commanded, and the motor speeds are trimmed.

  4. Autonomy is the extent to which a machine decides its own actions from its own sensor data. Drones are grouped as manual, semi-autonomous and autonomous. Autonomous is not the same as automatic: an automatic machine repeats a fixed sequence, while an autonomous one chooses its action from what it senses at the time.

The physics of flight

  1. Four quantities govern flight: weight \(W = mg\) acting downward, lift opposing it, thrust along each rotor axis, and drag opposing motion. In a multirotor there is no wing, so the rotor thrust is the lift.

  2. A propeller produces thrust by accelerating air downward; by Newton’s third law the air pushes the rotor upward. Thrust rises approximately with the square of rotor speed, and falls when the air is less dense.

  3. A drone hovers when total thrust equals weight, so the net vertical force is zero. Greater thrust makes it climb, less makes it descend. A thrust-to-weight ratio of about 2 is a common design target, leaving spare force for manoeuvre and gusts.

  4. Everything a multirotor does is produced by differential thrust, since no rotor can be tilted. Speeding up the rear pair and slowing the front pair tilts the body, so that \(T\cos\theta\) supports the weight and \(T\sin\theta\) drives the drone forward; total thrust must be raised as the machine leans, or it loses height.

  5. Yaw is produced by torque, not by thrust. Rotors are arranged in counter-rotating pairs whose reaction torques normally cancel; speeding one pair while slowing the other leaves a net torque that turns the drone about its vertical axis at constant height.

Structure

  1. The frame holds every part in a fixed geometry, which is what makes the flight controller’s assumptions true. It must be stiff and light at once. Common materials are carbon fibre, aluminum and moulded plastic composite.

  2. Arm length sets the leverage of each rotor: longer arms allow larger, more efficient propellers and a steadier machine, shorter arms give agility. Motor mounts must hold the shaft perpendicular to the frame and damp vibration, since propeller vibration corrupts accelerometer readings.

  3. Landing gear protects the battery, payload and sensors on touchdown and provides ground clearance. Payloads are mounted near the centre of the frame, isolated from vibration, and a camera is carried on a gimbal that keeps it pointing steadily while the airframe tilts.

  4. Rotor configurations are the tricopter, quadcopter, hexacopter and octocopter. A tricopter needs a tilting tail motor because an odd number of rotors cannot cancel torque in pairs. A quadcopter is the simplest self-balancing arrangement. Six and eight rotors add lift and redundancy: a quadcopter cannot survive a motor failure, while a hexacopter or octocopter usually can.

Subsystems

  1. Propulsion uses brushless DC motors, which have no brushes to wear, high efficiency and fine speed control, but cannot run without an electronic speed controller. Each ESC switches battery current to the three motor terminals in sequence; the flight controller only sends it a small speed signal. Propeller pitch is the distance it would advance in one revolution, and propellers are handed, so each must match its motor’s direction of rotation.

  2. Power comes from lithium polymer packs, chosen for energy per gram. A cell is about \(3.7 \text{ V}\) nominal, and cells are stacked in series (3S, 6S). Capacity is stated in mAh and maximum safe current as a C rating, so a \(2200 \text{ mAh}\) 25C pack can deliver about \(55 \text{ A}\). A power distribution board feeds the ESCs and a battery eliminator circuit supplies \(5 \text{ V}\) to the electronics.

  3. The flight controller’s picture of the world comes entirely from its sensors: the gyroscope measures rotation rate but drifts, the accelerometer is noisy but has gravity as a reference, and combining them — sensor fusion — gives a steady attitude estimate. The magnetometer fixes heading, the barometer gives height, the GPS receiver gives position, and optical flow or ultrasonic sensors help near the ground.

  4. Navigation is arithmetic on coordinates: a route is a list of waypoints, and the controller repeatedly computes the bearing and distance from its present position to the next one. The launch point is stored as home, which is what makes return-to-home possible.

  5. Two radio paths exist: a command link from the operator’s transmitter to the drone, and a telemetry link carrying battery voltage, position and height back down. Both weaken with distance, so a failsafe defines what the drone does on its own when the command link goes silent.

Payloads

  1. A payload is the mission equipment a drone carries, as distinct from what it needs in order to fly. Remove the payload and the drone still flies; remove the flight sensors and it does not.

  2. A visible-light camera shows appearance and extent. A thermal camera detects emitted infrared and works in darkness, showing temperature differences rather than detail. A multispectral camera compares reflectance in narrow bands to map plant vigour. LIDAR times its own laser pulses to build a three-dimensional point cloud, works in darkness, and can find the ground through gaps in a tree canopy.

  3. Some payloads act rather than sense — a spray tank, a cargo box — and these change the drone’s weight and trim during the flight.

  4. Payload capacity is what can be carried above the airframe, battery and electronics while leaving enough spare thrust for control. Endurance is estimated as \(t = C_{usable}/I\), and it falls faster than the load rises, because current climbs more steeply than the thrust demanded.

Regulation in India

  1. Drone flying is governed by the Drone Rules, 2021, made by the Ministry of Civil Aviation and administered by the Directorate General of Civil Aviation through the Digital Sky portal. Obligations scale with all-up weight — the total weight at take-off, including battery and payload.

  2. The weight categories are Nano (up to 250 g), Micro (above 250 g up to 2 kg), Small (above 2 kg up to 25 kg), Medium (above 25 kg up to 150 kg) and Large (above 150 kg).

  3. The airspace map divides the country into green, yellow and red zones. In a green zone no flight permission is needed up to 400 ft, about 120 m, falling to 200 ft in the band 8 to 12 km from an airport perimeter. Yellow zones require air traffic control permission and red zones are closed without Central Government permission.

  4. A drone must be registered and carry a unique identification number, and the model normally needs a type certificate. The pilot needs a remote pilot certificate, except for nano drones and micro drones flown non-commercially. Third-party insurance is required for drones other than nano machines.

  5. Flights are normally within visual line of sight; beyond visual line of sight needs specific approval. Releasing goods or agricultural chemicals is separately regulated. Mandated safety features include geo-fencing, a real-time tracking beacon, return-to-home and anti-collision lights, from which nano drones are excluded.

Applications

  1. In aerial photography, the drone makes a costly viewpoint cheap and, because it repeats a route accurately, turns photography into measurement. Photogrammetry flies an overlapping grid and joins the images into a single corrected map of a site.

  2. In agriculture, a multispectral survey produces a vigour map that reveals stress before it is visible on foot, and a spraying drone treats the crop from a few metres up, using rotor downwash to drive droplets into the canopy while keeping the operator out of the mist. Together these support precision farming.

  3. In disaster response, drones supply current information quickly from ground that has just changed: survey flights to map usable routes, thermal flights to search in darkness, close inspection of damaged structures, and delivery of small urgent loads.

  4. Across all three, the aircraft is the same general-purpose carrier. What changes is the payload and the flight pattern, and the engineering judgement lies in matching payload, endurance and flight plan to the problem.

19.9 Key Terms

  1. Accelerometer — a sensor that measures linear acceleration along three axes, giving the drone a long-term reference for which way is down.
  2. All-up weight — the total weight of a drone at the moment of take-off, including the airframe, battery and any payload.
  3. Arms — the structural members extending from the frame that carry the motors and propellers away from the centre.
  4. Attitude — the orientation of a drone in the air, described by its pitch, roll and yaw.
  5. Autonomous drone — a drone that carries out a mission planned before take-off without operator input during the flight.
  6. Autonomy — the extent to which a machine decides its own actions from its own sensor data, without an instruction supplied by a human at the time of acting.
  7. Barometer — a sensor that measures air pressure, used by the flight controller to estimate height above the launch point.
  8. Battery eliminator circuit (BEC) — a voltage regulator that steps the battery supply down to the 5 V needed by the flight controller and receiver.
  9. BEC — battery eliminator circuit.
  10. Beyond visual line of sight (BVLOS) — operation in which the drone is not kept in the operator’s direct sight, requiring specific approval.
  11. BLDC — brushless DC motor.
  12. Brushless DC motor (BLDC) — a motor with rotating magnets and fixed coils, switched electronically instead of by brushes.
  13. BVLOS — beyond visual line of sight.
  14. C rating — the multiple of its capacity at which a battery can safely deliver current.
  15. Command link — the radio path from the operator’s handheld transmitter to the receiver on the drone.
  16. Counter-rotating — an arrangement in which rotors spin in opposite directions so that their reaction torques cancel.
  17. DGCA — Directorate General of Civil Aviation.
  18. Differential thrust — producing motion or rotation by running some rotors faster than others.
  19. Directorate General of Civil Aviation (DGCA) — the authority that administers drone regulation in India.
  20. Drag — the resistance of the air to a drone’s motion, acting opposite to the direction of travel.
  21. Drone — an aircraft that carries no pilot on board and is flown either by remote control or autonomously by onboard computers.
  22. Electronic speed controller (ESC) — the driver that switches battery current to the three terminals of a brushless motor in sequence and at a commanded rate.
  23. Endurance — how long a drone can remain airborne on one battery charge.
  24. ESC — electronic speed controller.
  25. Failsafe — the behaviour a flight controller adopts on its own when the command link has been silent for longer than a set time.
  26. Flight controller — the board that reads the motion sensors and sets the speed of every motor, closing the stabilisation loop.
  27. Frame — the central structure that holds every part of the drone in a fixed geometry.
  28. Geo-fencing — a feature by which the flight controller compares its position with stored zone boundaries and refuses to cross them.
  29. Gimbal — a motorised pivoting cradle that keeps a camera pointing steadily while the airframe tilts and turns.
  30. Global positioning system (GPS) — the satellite system from which a receiver obtains position and altitude.
  31. GPS — global positioning system.
  32. Green zone — airspace in which no flight permission is needed for a drone up to 400 ft, about 120 m, above ground level.
  33. Gyroscope — a sensor that measures the rate of rotation about each axis; accurate over short intervals but subject to drift.
  34. Hexacopter — a multirotor with six rotors.
  35. Hover — the flight condition in which total thrust equals weight, so the net vertical force is zero and height is held.
  36. IMU — inertial measurement unit.
  37. Inertial measurement unit (IMU) — a package combining a gyroscope and an accelerometer in one device.
  38. Landing gear — the structure beneath the frame that takes the shock of touchdown and holds the drone clear of the ground.
  39. Lift — the upward force opposing weight; in a multirotor it is supplied entirely by rotor thrust.
  40. LIDAR — light detection and ranging.
  41. Light detection and ranging (LIDAR) — an instrument that measures distance by timing the reflection of its own laser pulses.
  42. LiPo — lithium polymer.
  43. Lithium polymer (LiPo) battery — a pouch-cell lithium battery chosen for the energy it stores per gram.
  44. Magnetometer — a sensor that detects the Earth’s magnetic field and is used to establish heading.
  45. Manual drone — a drone whose path is chosen continuously by the operator through a handheld controller.
  46. Motor mount — the bracket at the end of an arm that holds the motor perpendicular to the frame and damps vibration.
  47. Multirotor aircraft — an aircraft whose lift comes from several rotating propellers rather than from fixed wings.
  48. Multispectral camera — a camera that measures reflected light in several narrow bands, used to map plant vigour.
  49. Octocopter — a multirotor with eight rotors.
  50. Payload — the mission equipment a drone carries, as distinct from the components it needs in order to fly.
  51. Payload capacity — the mass a drone can carry above its own airframe, battery and electronics while retaining enough spare thrust for control.
  52. Photogrammetry — building a measurable map of a site from overlapping photographs taken from different positions.
  53. Pitch (of a propeller) — the distance a propeller would advance in one revolution if it moved through the air like a screw.
  54. Pitch (of the aircraft) — rotation of the nose upward or downward.
  55. Point cloud — a set of three-dimensional coordinates describing every surface a scanner’s beam has struck.
  56. Power distribution board — a plate of heavy conductors that splits the single battery feed into one supply for each electronic speed controller.
  57. Precision farming — applying water, fertiliser or pesticide according to what each part of a field actually needs.
  58. Quadcopter — a multirotor with four rotors arranged as two counter-rotating pairs.
  59. Red zone — airspace in which drones may not be flown without permission from the Central Government.
  60. Redundancy — spare capacity that allows a system to keep working after a component has failed.
  61. Remote pilot certificate (RPC) — the certificate required to fly a drone, obtained by training at an authorised organisation and valid for ten years.
  62. Return-to-home (RTH) — the function by which a drone flies back to the launch position stored at take-off.
  63. Roll — rotation in which one side of the drone dips relative to the other.
  64. Rotor — a propeller together with the motor that spins it.
  65. RPC — remote pilot certificate.
  66. RTH — return-to-home.
  67. Semi-autonomous drone — a drone that holds its altitude and position by itself while the operator still chooses the route.
  68. Sensor fusion — combining sensors so that each covers the other’s weakness, producing a steadier estimate than either could give alone.
  69. Telemetry — measurements about the drone itself, such as battery voltage, height and position, sent down to the ground.
  70. Telemetry link — the radio path that carries telemetry from the drone to a ground station or phone.
  71. Thermal camera — a camera that forms an image from emitted infrared radiation, showing temperature differences rather than detail.
  72. Thrust — the force produced along a rotor’s axis by accelerating air through it.
  73. Thrust-to-weight ratio — the maximum total thrust a drone can produce divided by its weight.
  74. Tricopter — a multirotor with three rotors, using a tilting tail motor to control yaw.
  75. UAS — unmanned aircraft system.
  76. UAV — unmanned aerial vehicle.
  77. UIN — unique identification number.
  78. Unique identification number (UIN) — the registration number issued for a drone on the Digital Sky platform, tied to its serial numbers.
  79. Unmanned aerial vehicle (UAV) — another name for a drone.
  80. Unmanned aircraft system (UAS) — the drone together with its ground control station, radio link and supporting equipment.
  81. Visible-light camera — a camera that records a scene as the eye would see it.
  82. Visual line of sight (VLOS) — operation in which the operator or a trained observer keeps the drone in direct sight throughout the flight.
  83. VLOS — visual line of sight.
  84. Weight — the downward force of gravity on the drone’s mass, \(W = mg\).
  85. Yaw — rotation of the whole body about the vertical axis to face a new direction.
  86. Yellow zone — controlled airspace in which flying a drone requires permission from air traffic control.

19.10 Objective Questions

  1. A drone weighs 240 g by itself. A camera of 40 g is bolted to it before the flight. Under the Drone Rules, 2021, the loaded machine falls in the category:
  1. Nano
  2. Micro
  3. Small
  4. Medium
  1. A quadcopter turns to face a new direction while holding its height. This is achieved by:
  1. tilting the two front rotors on their mounts
  2. increasing the speed of all four rotors together
  3. speeding up one counter-rotating pair and slowing the other by a matching amount
  4. deflecting the airflow with a small rudder below the frame
  1. Assertion (A): A quadcopter cannot be recovered if one of its motors stops in flight. Reason (R): With one rotor stopped, the thrust balance and the torque balance are both destroyed at the same time.
  1. Both A and R are true, and R is the correct explanation of A.
  2. Both A and R are true, but R is not the correct explanation of A.
  3. A is true but R is false.
  4. A is false but R is true.
  1. Match each sensor in Column A with what it measures in Column B.
Column A Column B
a. Gyroscope i. Distance, from the time taken by a reflected laser pulse
b. Barometer ii. Rate of rotation about each of the three axes
c. Magnetometer iii. Direction of the Earth’s magnetic field
d. LIDAR iv. Air pressure
  1. Fill in the blanks.
  1. In a green zone, a drone may be flown up to ________ ft above ground level without flight permission.
  2. A brushless DC motor cannot be connected directly to a battery; it must be driven through an ________.
  3. The total weight of a drone at the moment of take-off, including its battery and payload, is called its ________.

  1. Write the full form of each of the following as used in this chapter: UAS, ESC, IMU, BVLOS, RPC, LIDAR.

  2. The magnetometer of a drone is mounted right beside the heavy power leads running from the battery to the speed controllers. Predict, in one line, what the operator will notice during flight.

19.11 Check Your Concepts

  1. Define a payload. Distinguish it from the equipment a drone needs in order to fly, giving one example of each from the same machine.

  2. Differentiate between an automatic machine and an autonomous one, with one example of each. State why the distinction matters when a drone is sent to fly a route over a village.

  3. Explain the working principle of a brushless DC motor, and give reasons why it cannot simply be wired across a battery in the way an ordinary DC motor can.

  4. A drone tilts nose-down by an angle \(\theta\) in order to fly forward. Explain, using the components of the tilted thrust, why the total thrust must be increased at the same time if the drone is to hold its height.

  5. A four-rotor drone has a total mass of \(1.5 \text{ kg}\) including its battery and camera. Each of its four motor-and-propeller units can produce a maximum thrust of \(700 \text{ g}\). Taking \(g = 9.8 \text{ m/s}^2\):

  1. Find the thrust each rotor must produce for the drone to hover.
  2. Find the thrust-to-weight ratio of the machine.
  3. Comment on whether this drone has enough spare thrust to work in gusty conditions.
  1. A survey team must map a canal embankment while carrying a heavy and expensive LIDAR unit, flying partly over water. They can buy either a quadcopter or a hexacopter of the same overall size. Compare the two on lifting capacity, behaviour after a motor failure, and running cost, and recommend one for this work with reasons.

19.12 Diagram, Circuit & Output Reading

  1. A student describes the power arrangement of a drone as follows:

“The 3S lithium polymer battery feeds a power distribution board. From the board, one lead goes to each of the four electronic speed controllers, and one lead goes directly to the flight controller.”

Identify the fault in this arrangement, state what would happen to the flight controller, and name the component that is missing.

  1. A quadcopter has been assembled and all four propellers were fitted from the same packet, every one of them marked for clockwise rotation. The motors themselves are wired correctly, with one diagonal pair turning clockwise and the other anticlockwise.
  1. State what the two anticlockwise rotors will do to the air.
  2. Describe what the operator will observe when the throttle is raised for take-off.
  1. The four motors of a drone each draw a maximum of \(11 \text{ A}\), and its electronics draw a further \(1 \text{ A}\). Three battery packs are available.
Pack Capacity C rating Mass
P 2200 mAh 20C 180 g
Q 2200 mAh 35C 200 g
R 4400 mAh 20C 340 g
  1. Work out the maximum current each pack can safely deliver, and state which packs can meet the worst-case demand.
  2. Of the packs that qualify, say which would give the longer flight and what it costs the drone in return.
  1. A school stands 10 km from the perimeter of an airport, in an area shaded green on the airspace map published on the Digital Sky portal. A teacher plans a short demonstration flight with a 900 g drone from the school field.
  1. State the height limit that applies at this location.
  2. Name two requirements, other than the height limit, that must be satisfied before this flight is made.

19.13 Case-Based Questions

  1. A farmers’ co-operative is considering a spraying drone for its members’ plots. A demonstration is arranged on a rectangular field measuring \(300 \text{ m}\) by \(80 \text{ m}\). The machine offered has the following working figures:
Quantity Value
Spray swath treated on each pass 5 m
Ground speed while spraying 4 m/s
Time taken to turn at the end of a pass 12 s
Area covered by one full tank 2,400 m²
Flight time on one battery, with a full tank 8 min
  1. Find the number of passes needed to cover the field, and the total flying time including turns.
  2. Find how many tank refills the field will need, and hence say whether the tank or the battery is the constraint that interrupts the work more often.
  3. The co-operative expects the drone to finish the field in the flying time you calculated. Explain why the real time on site will be considerably longer, and state one regulatory requirement that applies to spraying agrochemicals from a drone.
  1. A landslide has cut a hill road at about nine in the night. Two houses above the slip are unaccounted for, and a district team reaches the site with one drone carrying both a visible-light camera and a thermal camera. The team must decide what to fly, and quickly, while a rescue helicopter is expected in the area before dawn.
  1. Choose the payload the team should use for the first flight, and justify the choice from the conditions at the site.
  2. State one limitation of that payload that the team must keep in mind while reading its images, and explain what could go wrong if they forget it.
  3. List two checks the team must make before taking off — one technical and one concerning the airspace — and give a reason for each.