At the beginning of the module “UAS General Knowledge”, we will outline the principles of flight and take a look at the reason why drones can take off at all.
At first glance, air appears almost like “nothing” – if something has disappeared, it is said to have “dissolved in the air”. In fact, air is much more than nothing, and it is only the characteristics of air flowing around a body that makes it possible for an aircraft to fly. Actually, it is only the aerodynamic force that lifts the drone and let it hover in the air.
In addition to these forces, we will take a look at the directions of movement of the drone and how the forces are used for control.
First of all, it is important to know that a wing and a rotor blade have a similar cross-section – in principle, a rotor blade is a small wing. The cross section of a wing profile usually has a round front edge and tapers to the rear.
On this airfoil – or rotor blade – the airflow creates four forces that act in four different directions:
- The lift pulls the wing up
- The weight counteracts this downwards
- The thrust is necessary in order to generate an airflow at all
- Drag opposes the forward movement
In a steady and straight flight, all forces acting on the wing are in equilibrium, so that no resulting force remains. No resulting force means that the aircraft or rotor blade does not become faster or slower, nor does it climb or descent. If this balance is intentionally or unintentionally disturbed, the drone climbs or descents, becomes faster or slower.
The aim of the typical wing shape is mainly to maintain an optimal balance between lift and drag and to enable lift even at different speeds. This can be achieved by “tilting” a surface slightly towards the airflow, which creates a pressure difference between the top and bottom of the airfoil. This angle between the wing chord and the airflow direction is called angle of attack.
This effect can also be experienced when driving a car by first holding the flat hand out of the window so that the thumb is pointing in the direction of travel, which means that the airflow hits the edge of the hand directly. The airflow on the upper and lower side of the hand is similar, no lift is generated. However, if the hand is tilted in a way that the thumb is pointing upwards, you can immediately feel how the hand is pulled up. You increased the angle of attack, which leads to different airflows around the upper and lower side of the hand. Now, lift is created.
By increasing the angle of attack of a wing, the air above the wing is accelerated, causing the pressure to drop. Below the wing, the air “presses” a little against the surface, which creates a higher pressure here. The result is a force on the wing, which “pulls” it upwards – this is lift.
In addition to the angle of attack, the speed of the airflow also plays a role in the strength of the lift: the faster the wing moves through the air, the higher the lift. In the case of a drone with propellers, the lift increases accordingly with a higher rotation speed of the propellers.
A multirotor drone or multicopter has three directions of movement:
- up and down,
- rotating around its own axis
- sideways, forward and backward.
Note that a drone’s propellers do not all rotate in the same direction. As a rule, the opposite propellers turn in the same direction, the directly neighbouring propellers in the opposite direction. This prevents unwanted rotation around its own axis.
Climb and Descent
To take-off with a drone, the total upward force generated by all propellers must be higher than gravity. This is achieved by rotating the propellers faster.
Once the desired flight altitude has been reached, the speed of the propellers is reduced until there is a balance between lift and weight.
In order to descent with the drone again, the lift generated by the propellers must be slightly less than gravity.
Rotation around the Axis
To turn around its axis, the total force of one pair of opposing propellers must be greater than that of the other pair.
An example: if, as illustrated on the right, propellers 1 and 4 turn faster than 2 and 3, the drone turns clockwise around its own axis. This can be achieved by increasing the force on propellers 1 and 4 or reducing it to propellers 2 and 3. However, if only the force on propellers 1 and 4 is increased, the drone would also climb due to the increased lift. To prevent this, the speed of propellers 2 and 3 must be reduced at the same time.
Whether a drone moves forward, backward or sideways is often a matter of opinion when flying a drone, since each side can be in front or in the back. As a rule, the remote pilot sets the perspective.
For example, if the force on propellers 1 and 3 is increased in the illustration on the right and the force on propellers 2 and 4 is reduced, the drone moves to the right.