What aerodynamic principles are used in the design of drones?

UAVs or airplanes will be affected by air resistance during flight. If this effect is not eliminated, it may cause a great loss of power during flight, and even produce unpredictable results on the control of the aircraft. In UAVs, aerodynamics is an aspect that needs to be paid great attention to not only in appearance but also in internal control during the design process. This article introduces the aerodynamic knowledge used in UAV design from a theoretical perspective.

All aerodynamics are based on the laws of motion. In the aerodynamics of aircraft models, the three laws of motion established by Newton are mainly used.

Factors affecting lift and drag
Aerodynamic forces acting on aircraft, including lift and drag, are caused by the mass of air itself. To generate support force, the air mass must be accelerated to generate an upward support force, and to achieve balance, the support force must be equal to gravity. When an aircraft is flying in the air, the wings pass through the airflow, causing disturbances. In addition to the wings, other parts of the aircraft, such as the fuselage, tail, landing gear, etc., will also cause disturbances and energy loss, so that no contribution to lift is obtained. Therefore, the more energy consumed to generate lift, the lower the efficiency of the aircraft. The air quality required for model aircraft flight depends on three factors: 1. The amount of air in a given space, that is, the air density of the model aircraft flight space; 2. The size of the model aircraft; 3. The speed or rate of the model aircraft flight.

1. Air density
Air is composed of a mixture of many other things. Air can be considered to be composed of countless independent molecules, all of which are in a state of violent motion. The temperature of the gas is a measure of the intensity of this motion. When the temperature is low, the molecular motion is slower than when the temperature is high. The moving molecules collide with objects immersed in them, generating gas pressure. Density is a measure of the number of molecules in a given space. In the low-speed aerodynamics of model aircraft, it is not necessary to study the molecular composition of air. The medium of model aircraft flight is fluid, not that air is liquid. Liquid is a fluid that is almost incompressible under certain conditions, while gas is a compressible fluid. The flight speed of model aircraft is far from the level where the compressibility of air needs to be considered. The problem of air compressibility is generally only considered when dealing with jet-powered aircraft, propeller wingtips and helicopter rotors. In high altitude and high temperature environments, the air density is lower than that close to sea level and in low temperature environments. Model aircraft enthusiasts have certain control differences when flying aircraft in plateau areas and in plain areas. The humidity of the air will also affect the density. Dry air is denser than moist air, so humidity affects lift. Glider pilots can use hot air to help their gliders glide. In aerodynamics, the density of air at normal temperature and pressure near sea level is defined as 1.225 kg/m3.

2. Model size
: A larger aircraft will inevitably produce greater turbulence when flying in a standard atmosphere, so it will generate greater air force than a smaller model at the same speed. This force includes lift and drag.

Span loading: The ratio of the model's weight to its span, expressed as the ratio of weight per unit length. Span loading is a very important parameter, a model with a larger wingspan will sweep more air at the same speed than a model with a smaller wingspan. The greater the mass of air swept, the less acceleration is required to obtain the same amount of air.

The model size can be expressed in terms of wing area.

3.
When the wingspan and area of ​​the model are constant, high-speed flight causes greater disturbance to the air mass than low-speed flight.

The ability of the model to obtain lift is almost entirely determined by the angle of attack of the wing and the incoming airflow. The reference for the angle of attack is usually the chord line.

Chord line: A straight line connecting the leading and trailing edges of an airfoil.

Aerodynamic angle of attack: The actual angle between the airflow and the wing. The aerodynamic angle of attack is different from the geometric angle of attack. In traditional models, the size of the wing's angle of attack (geometric angle of attack and aerodynamic angle of attack) mainly depends on the relative changes of the wing and the tail. The main function of the tail is to balance the aircraft to achieve a predetermined angle of attack and maintain this angle of attack.

The angles at which the tail and wings are mounted relative to the fuselage must be distinguished from the angle of attack of the relative airflow. The fuselage may not be aligned with the incoming flow.

The tail is sometimes designed in a positive V-shaped layout or an inverted V-shaped layout. In this case, the aircraft's balance, pitch and yaw stability are controlled by the two wing surfaces of the V-shaped tail. In addition to the normal wing-tail-vertical stabilizer layout, there are many other layout forms, such as tailless layout, tandem layout, delta wing layout, and canard layout.

Canard layout: refers to a layout in which a small-load wing is located in front of the main wing.

Whether most or all of the load is carried by one or the other wing is a question involving trim and the location of the center of gravity.

Airfoil Section and Lift Coefficient
The efficiency of a wing is greatly affected by the airfoil shape and, to a certain extent, by the camber and thickness of the airfoil.

The fuselage and other parts of the model also generate a certain amount of lift, the size of which depends on their shape and angle of attack. For ordinary models, the contribution of the fuselage to lift is very small. However, the fuselage will produce some forces comparable to lift, which affect the stability of the model aircraft and are always opposite to the balancing force of the stabilizer that keeps the aircraft at a given angle of attack.
Similar lateral instability disturbances are prevented by the vertical stabilizer.

For the convenience of research, aerodynamicists have simplified all the very complex wing shapes and balance factors into a single coefficient, the lift coefficient. This coefficient can describe the lift generated by a model or any of its components. For example, a lift of 1.3 can generate more lift than 1.0.

Factors that affect lift are the size or area of ​​the model, its speed, the density of the air, and the lift coefficient.

The formula is: L = 1/2ρv^2.S.Cl

Flying enthusiasts cannot control air density, but they can obtain a higher lift coefficient by controlling the model's wing angle of attack. They can also increase the wing area, although this will increase the model's weight and lead to an increase in flight speed. With other parameters unchanged, a small increase in speed will lead to a large increase in lift. Under a given area and trim, a heavier aircraft must fly faster than a lighter aircraft, but increasing speed means consuming more energy.

In some cases the model's engines may not provide enough power to ensure flight.

The ratio of wing loading to wing area
Wing lift coefficient and airfoil lift coefficient The lift coefficient of the entire model or wing should not be confused with the lift coefficient of a single airfoil tested in a wind tunnel. The contribution of the tail to the model lift coefficient is a very complex issue. The lift coefficient of an aircraft is usually determined by the wing area.

Bernoulli's Theorem
When air encounters any object, such as a wing, the air will be deflected, some air passes through the upper surface of the wing, and some passes through the lower surface. In this flow process, complex speed and pressure changes will occur. To generate lift, there must be a pressure difference between the upper and lower surfaces.

Bernoulli's principle: When a fluid flows and viscosity losses are ignored, the sum of the pressure potential energy, kinetic energy and potential energy between any two points on the streamline remains unchanged.

Flow past any object, as long as it is streamlined, will produce similar fluid deformations, accompanied by changes in velocity and pressure.

The source of lift
is the point of highest pressure on the wing, the so-called stagnation point, where the air meets the leading edge. The velocity of the air relative to the wing decreases to zero, and Bernoulli's theorem tells us that the pressure is maximum at this point. The air on the upper and lower surfaces must accelerate away from rest at this point. At a given incoming flow velocity, if the angle of attack of a symmetrical airfoil is increased, the pressure difference between the upper and lower surfaces will increase to a certain value. A cambered airfoil may have a geometric zero angle of attack, but the mean pressure and lift are different from those of a symmetrical airfoil, even though the chord line may be at a geometric zero angle of attack. At some negative geometric angle of attack, the pressure on the upper and lower surfaces may be equal, so a highly cambered airfoil has an angle of attack of zero lift, which is the aerodynamic zero point of the airfoil. Although no lift is generated at this angle of attack, the characteristics of the upper and lower surfaces are different due to the camber of the airfoil. The lift coefficient has a definite limit. If the angle of attack is too large or the camber is increased too much, the streamline is destroyed and the flow separates from the wing. Separation changes the pressure difference between the upper and lower surfaces, greatly reducing lift and putting the wing in a stall state. Airflow separation is a common phenomenon in a small range. The airflow may separate on the upper and lower surfaces, or it may separate and then attach. This is called "bubble separation".