Performance of an unmanned aircraft system in flight

The brains of a drone

All drones have a control system. This system could be called the drone’s brains. The system receives information from the drone’s sensors and systems, processes position, altitude and speed information, and transmits steering information to the motors and/or control surfaces to stabilise or change the flight.

At its simplest, the control system implements the steering commands received from the transmitter directly with the control surfaces, and the system has no functions related to flight stabilisation. The remote pilot monitors the drone’s flight and gives the necessary steering commands via a radio transmitter. Gyroscopic stabilisers on one or more axes may act as auxiliary systems, but they do not offer a full flight stabilisation function. Traditional model aircraft have this type of a control system, and their steering is based on the remote pilot staying in constant visual contact with the model aircraft while controlling it. Typically, a pre-programmed control system has been installed in a free flight fixed-wing model aircraft; its operation cannot be affected during the flight, with the exception of the command that stops the flight irrevocably. In that case, the model aircraft lands in a flight condition in which it can no longer fly normally but still maintains a rate of descent that is safe for the environment.

An important part of the system of a drone equipped with a flight stabilisation system is the Inertial Measurement Unit (IMU). This unit recognises the drone’s position and movement in relation to the ground, as well as changes in the position and movement. The function is necessary for the operation of the automatic stabilisation system.

Another important part of the control system is the Global Navigation Satellite System (GNSS). The most widely known global navigation satellite system is GPS (Global Positioning System). The drone receives accurate position information from the positioning system in addition to the distance to the starting point as well as location information depending on the system. The positioning system also includes a barometer; the information provided by it is used to determine the flight altitude. The information about the flight altitude provided by the barometer is much more accurate than the altitude information from the GNSS.

How do motors work?

The drone’s motors turn either rotors or propellers. As they rotate, rotors generate the lift the drone needs; in a fixed-wing drone, the propellers generate the force propelling the drone in the direction of flight as well as the airspeed required for the drone to fly. In drones that fly like a helicopter, the speed of rotation of the motor must be high enough to generate enough lift to equal the drone’s weight. In a single-rotor drone that resembles an ordinary helicopter, the rotor blade angles are usually adjustable, and the lift can be regulated by changing the blade angle while the rotor’s speed of rotation remains constant. In a multirotor drone, the drone’s steering is based on adjusting the speed of rotation of the motors, which in turn affects the control produced by the torque and asymmetric lift.

The operation of the electric motor and its speed of rotation are controlled by the Electronic Speed Controller (ESC).

The drone can also be equipped with a combustion engine. The engine may be two- or four-stroke, methanol-, petrol- or diesel-powered, or it may be gas turbine-powered. The remote pilot of a drone with a combustion engine must study the engine’s operation and the manufacturer’s instructions carefully. The correct handling and storage of fuel requires special care.

Camera mount with camera stabilisation

The drone’s camera mount is often equipped with a stabilisation function. A camera mount with stabilisation is called a gimbal. The gimbal may have two or three axes. In a two-axle gimbal, the camera is stabilised vertically and horizontally, and in a three-axle gimbal, the motions are smoother as the drone rotates around its own axis.

Each axle of the gimbal is equipped with a motor so that the camera can remain stable in its set position. These motors are very small and break easily, and therefore they must be handled with care. The camera should not be moved and, in particular, the motion of the camera must not be resisted while the gimbal is switched on. The transport cover that locks the gimbal must always be removed before the flight and replaced after the flight.

Drones equipped with rotors

The most common type of drone is a multirotor drone that resembles a helicopter. Such drones are called multirotor drones, because they are equipped with more than one rotor. The most common type of multirotor drone is equipped with at least four rotors or more; however, the number of rotors is always even. These drones are often made out of plastic, but other materials such as carbon fibre or fibreglass can be used for the body. In smaller drones, the rotors are often plastic, and the rotors of heavier drones in particular are made out of carbon fibre.

Thanks to their ability to hover, multirotor drones do not need a runway for take-off or landing.

The drone may also resemble a traditional helicopter, equipped with a main rotor that generates lift and a tail rotor that compensates for the tendency to rotate. In such drones, steering and movement are implemented by adjusting the rotor blade angles, which enables a wide range of movement. A drone that resembles a traditional helicopter is mechanically complex, and steering one is a lot more challenging than a multirotor drone. Flying such a drone requires special training, and the flight area must be large. A helicopter drone is often able to land without motor power using autorotation. This makes a controlled landing possible in case of motor failure.

Fixed-wing drones

Fixed-wing drones resemble traditional aeroplanes, because the lift needed for flying is generated with fixed wings. In drones, however, the shape and placement of wings may vary considerably due to optimisation for the intended purpose. The body and wings of the drone are made out of a variety of light, strong materials that are combined using different methods. For example, the wings may be made out of polystyrene that is cut or pressed into shape and possibly covered with thin wood veneer, such as balsa or obeche, or carbon fibre or fibreglass. Thanks to the lift generated by the wings, the drone is not dependent only on the power from the batteries or other power sources to stay in the air; the energy from the power source is used to ascend or maintain the forward speed as well as horizontal flight. The operation of a fixed-wing drone can be controlled in case of motor failure. The drone can still be steered to a controlled landing without motor power, unlike a multirotor drone that may fall straight down in the worst case.

Unlike helicopter drones, fixed-wing drones require enough speed for take-off and landing. A drone may take off from a runway, it can be thrown into the air, or a catapult launching device may be used. The drone requires a sufficiently large runway for landing, or the drone can land with a parachute.

Hybrid drones equipped with wings and rotors

Fixed-wing drones come both with and without propellers. Multimotor fixed-wing drones can also be used like multirotor drones, and they are able to hover. The ability to hover has been implemented in different ways with different power unit solutions in hybrid drones, either with rotating motors or with separate hover motors that enable hovering as well as vertical take-off and landing. This combines the benefits of helicopters in take-off and landing as well as the greater speed of fixed-wing drones and their low power requirements in horizontal flight.

Everything that was not originally a part of the drone is considered payload. The carrying capacity of a drone depends on what type of drone is used. For some drones, flying with a payload is not recommended at all. For example, fixed-wing drones usually cannot carry as large loads as multirotor ones. The placement of the payload is also more critical for fixed-wing drones; outside the body, the load may affect the operation of a fixed-wing drone significantly.

The remote pilot must study the manufacturer’s instructions and the drone’s carrying capacity stated by the manufacturer. The maximum allowed take-off weight, the total weight (drone + payload) stated by the drone manufacturer, cannot be exceeded. The drone classification and operating category must also be taken into account in the take-off weight.

Common equipment is also included in the payload

You might think that payload consists primarily only of objects or things transported from point A to point B, but common equipment attached to the drone, such as cameras, gimbals and propeller covers, are also included in the payload. Using such equipment during flight may cause the performance to deteriorate, because air resistance and total mass increase with the equipment. This must always be taken into account when flying a drone that carries a payload.

Taking the drone’s centre of gravity into account

The drone manufacturers have specified the centre of gravity (CG) of each drone; its location cannot usually be changed or recalibrated manually. The location of the centre of gravity must always be checked when flying a drone with a payload attached. If necessary, the placement of the payload must be changed in order to move the centre of gravity to the permitted zone. The payload must be attached

• as close to the centre of gravity as possible
• as firmly as possible
• as close to the body of the drone as possible, because a poorly attached moving load may affect the centre of gravity during flight and cause loss of control.

The location of the centre of gravity varies depending on the type of drone, and in some models it can be recalibrated. Sometimes recalibration is even necessary due to the placement of the payload. When considering payload placement, it must also be ensured that the load does not cover the drone’s batteries and cause a risk of the batteries overheating.

Drone batteries, the most common power source

Drone batteries require careful service and maintenance. The batteries must be removed and charged as soon as possible after each landing. The batteries must be kept or stored protected from light in a cool place with fireproof surfaces. If the batteries are kept or stored with too low a charge (less than 10%) instead of charging them after the flight, they may become permanently damaged and pose a hazard during the next flight. In case of fire, many batteries require special extinguishers in order to extinguish the fire.

Concerning batteries, note that

• a battery is a consumable product, and it cannot be recharged indefinitely
• the battery capacity decreases after each charge

You can check the maximum recommended number of charges in the manufacturer’s user manual.

How many cells do your batteries have?

Batteries consist of several cells. The weight of the drone determines how many cells its battery needs to have. Even though a battery works, that does not always mean that all of its cells work correctly. The condition of cells cannot be checked visually, but most drone systems will warn you of potential faults or damage.

As a rule, you can check the condition of the battery cells from the display of the remote control system. The electric potential of cells is measured in volts (V), and they should be at the same level in all of the battery’s cells. If the power in one of the cells differs from the others, you should charge the battery full (100%) and check if all of the cells reach the full level. If this does not happen, the battery in question should not be used for flying a drone.

The most common type of battery for flying drones

LiPo (lithium-polymer) batteries are the type that is most commonly used in drones. Even though other types of batteries also exist, a LiPo battery tolerates the high discharge rate the best. The ability of the LiPo battery to store electricity in relation to its weight is also good.

The way to calculate the effect has been described below.

How is the energy calculated?

The amount of electrical energy in a battery can be calculated by multiplying voltage with capacity. The unit of electrical energy is watt-hour (Wh). Example: if the cell’s voltage is 3.5 V and its capacity is 3 Ah, the amount of electrical energy is 10.5 Wh.

V x Ah = Wh

How is power calculated?

The power output of a battery cell is measured in watts (W), and it is calculated by multiplying the voltage with electric current, whose unit is ampere (A). The power is dependent on time, meaning that if the cell is discharged by using a high electric current it has a lot of power, but its capacity is consumed and the battery discharges faster:

• If the cell is discharged with a current of 3 A, the power is 10.5 W and the discharge lasts for one hour.
• If the same cell is discharged with a current of 10 A, the power is 35 W and the discharge only lasts for approximately 15 minutes.

V x A = W