What aerodynamic principles are used in the design of drones?

When the airflow
passes through the airfoil at a certain angle, it will deflect, resulting in upwash in front of the airfoil and downwash behind it. The appearance of this deflection breaks the balance of the airflow. The movement of the streamline is like a rotating column of air, that is, a vortex. Such a vortex will cause deflection, upwash and downwash of the flow. The magnitude of the vortex rotation speed will determine how much lift is generated. In fact, the airflow flowing through the upper and lower surfaces of the airfoil does not circle. Many experiments show that this rotating vortex can indeed generate lift. The main value of this attached vortex is that it allows the flow flowing through the airfoil to be calculated by the strength of the ideal vortex circulation. This method is particularly useful when calculating the lift distribution along the span of the real wing. At the end of the wing, the attached vortex exists, but it becomes a pair of wingtip vortices that drag together. This pair of vortices does rotate and can be observed.

All parts of the drag and lift-to-drag ratio
model, including the wings, tail, fuselage and every part exposed to the air, will generate drag. Even the parts inside the aircraft engine cover and wheel fairing will generate drag as long as air flows through them.

With lift comes drag. The factors that affect drag are the speed of the flight, the density of the air, the shape of the model and its size. The drag coefficient, like the lift coefficient, combines all the characteristics of the model.

D = 1/2Ρv^2SCd；

Lift-to-drag ratio = L/D

For horizontal flight, the lift-to-drag ratio is a constant (ignoring fuel consumption). The thrust can be adjusted by changing the throttle, which in turn can change the drag. At low speed, in horizontal flight, the drag is reduced to a certain value, and the lift is still equal to the gravity, so the lift-to-drag ratio increases. This trend of drag reduction will not continue to the lowest speed. The total drag coefficient will increase sharply when the speed is reduced to a certain value, which is enough to offset the decrease in speed. Therefore, at this speed, the model reaches the maximum lift-to-drag ratio.

Vortex drag
is the combination of vortices that are dragged from the wing tip or any surface, which produce lift. The presence of vortices is directly related to lift: the higher the lift coefficient for a given wing, the greater the effect of the vortices. The lift-to-drag ratio decreases at low speeds, with the increase in vortex drag being a major factor. The vortex drag of the model increases greatly with decreasing speed.

Airfoil
drag is the resistance caused by the different pressure distribution around the object when the air flows through it. Skin friction drag or viscous drag is caused by the contact between the air and the model surface. Skin friction drag is largely determined by the speed of the airflow, while the speed of the fluid flowing to the rear is determined by the shape of the object. When considering the airfoil, form drag and friction drag are usually considered together, so we often call it airfoil drag.

Boundary layer

The biggest difference in aerodynamics between model and full-scale aircraft is the boundary layer, which is a thin layer of air near the wing or any surface over which air flows. Two properties of air, mass and viscosity, determine the behavior of the boundary layer. Viscosity can be roughly described as the stickiness of any fluid. Viscosity, like the density of air, cannot be controlled, and like air, varies with temperature and air pressure. Inertia resists changes in position or velocity. Viscosity resists shear flow and keeps the fluid in contact with the surface of the object. When the fluid in the boundary layer covering the surface is accelerated or decelerated, the forces caused by mass and viscosity interact, sometimes reinforcing each other and sometimes canceling each other out. At high speeds for a full-scale wing, where the velocity of the fluid is high and the radius of curvature of the surface is relatively large, mass inertia is dominant, and the effect of viscosity, while not negligible, is small. At low speeds for a model wing, viscous forces are relatively more important.

Reynolds number
There are two different types of flow: laminar and turbulent (discovered experimentally by Osborne Reynolds). They can transform into each other under certain conditions. The type of flow at any point in the boundary layer depends on the surface waviness, roughness, the mainstream velocity at a certain distance from the surface, the distance the fluid flows over the surface and the ratio of the density to the viscosity of the fluid. Any change in these factors will bring about changes in the boundary layer. Reynolds combines all quantities except the surface conditions into a single quantity, the Reynolds number.

Reynolds number = density/viscosity* velocity*length, expressed in symbols as: Re = ρVL/μ

Reynolds number effect: The ratio of the inertial force and the viscous force produced by the mass in the boundary layer relative to the velocity of the fluid at each point is important. This ratio will vary slightly with seasonal conditions and altitude.

The Reynolds number for the boundary layer
is different from the Reynolds number for the chord length of the wing and the Reynolds number for the boundary layer itself. When the airflow reaches the leading edge of the wing, it begins to separate into two streams at the stagnation point, one flowing over and one flowing under. The Reynolds number at this point in the boundary layer is zero because the distance from the surface is zero. The boundary layer flow starts at the stagnation point and moves along the wing surface, and the Reynolds number at each point depends on the distance measured along the wing profile from the stagnation point at that point. Therefore, the Reynolds number within the boundary layer also increases with the distance from the stagnation point.

Laminar Boundary
Layer Laminar flow causes much less surface friction than turbulent flow. In a laminar boundary layer, the air flows in a very smooth manner. It is as if each microlayer of the fluid is a separate thin layer or sheet, and as they slide over the other layers, there are only slight viscous or viscous stresses between the two layers. There are no air particles moving up and down between the layers. The lowest layer sticks to the surface, and the layer above it flows smoothly over this thin layer, and the next layer and so on. Until the outermost layer of the boundary layer moves at almost the same speed as the mainstream.

Transition
Small surface defects, such as roughness, paint spots, flying particles, or imperfections in the model skin and turbulence caused by protrusions in the spar, tend to disturb the laminar boundary layer. But at low boundary layer Reynolds numbers, viscosity tends to damp these disturbances, allowing the laminar flow to successfully pass them. At a certain point, the laminar flow will reach a critical point, at which point the small pulsations caused by surface irregularities will continue to persist without being damped out, and a short distance after this point, any small disturbance will overcome the damping effect. A significantly corrugated or rough surface will quickly cause this phenomenon, that is, at low Reynolds numbers, the laminar flow is suddenly disrupted and transitions to a turbulent flow.

Turbulent boundary layer
In a turbulent boundary layer, there is no system of tiny slip layers, but instead there are air particles that have great freedom of movement, moving up and down outside the usual mainstream direction. Although any individual particle moves at an unsteady speed, the average speed in the lowest part of the turbulent boundary layer near the surface is much greater than before the transition. This results in increased surface friction, but because the particles move faster, they have more momentum and are less likely to stop. The thickness of the turbulent boundary layer continues to increase as the Reynolds number increases. A smooth surface without contamination, ripples, or other imperfections can delay the transition. On such a surface, the transition occurs further back in time, and the critical Reynolds number in the boundary layer is higher. Rough surfaces or surfaces with relatively large ripples or bumps will shift the transition forward, reducing the critical Reynolds number.

Laminar Separation
On the upper and lower surfaces of the front part of the wing, the pressure decreases as the airflow accelerates from the stagnation point. The outer laminar flow is dragged by viscosity, and the accelerated airflow transfers the acceleration momentum layer by layer, so the entire boundary layer gains momentum, so the increased speed helps to maintain laminar flow, so that even large bulges or defects on the wing can be overcome without transition. When the airflow reaches the minimum pressure point, the mainstream speed begins to slow down, and the force pulling the outermost laminar flow decreases. This will inhibit the outer boundary layer and cause it to slow down as well. The effect of this slowing down is transmitted from the outside of the boundary layer to the inside, just like the phenomenon mentioned above that the mainstream accelerates the drag layer flow. The laminar flow closest to the surface never moves very fast. It only needs a small deceleration to stop. Therefore, a small distance behind the minimum pressure point, the laminar flow of the slowest outer boundary layer is interrupted. The airflow is stagnant at this point and prevents the inflow of laminar air above. The longer the deceleration continues, the more the boundary layer slows down. As the range of stagnation resistance increases, it forces the rest of the boundary layer to leave the wing surface together. This is laminar separation.

Bubble separation
occurs under favorable conditions, such as slow airflow deceleration after the minimum pressure point, and turbulent reattachment occurs behind laminar separation. The resistance of the stagnant air disturbance to the boundary layer is equivalent to a small bump or bulge on the wing. If the Reynolds number is large enough at this point, the airflow can turn to turbulence. The increase in the thickness of the turbulent boundary layer brings the airflow back to the wing surface, leaving the stagnant area (also called the separation bubble) below. After that, the turbulent boundary layer continues to overcome the pressure gradient and may reach the trailing edge of the wing without separating. The lowest air particles in the boundary layer continue to move backward because they have greater momentum to overcome the pressure that attempts to stop them. There is a localized isolated annular flow in the separation bubble, which is formed by the forward flow of the air layer closest to the surface. It is manifested as the formation of a very flat vortex that expands in the span direction. There is also a lateral vortex behind the bubble, which is more or less arranged in the chord direction. Laminar separation bubbles almost always occur on model aircraft wings, and equipment such as turbulence generators are usually used to prevent their occurrence. At high angles of attack, the point of minimum pressure on many airfoils moves forward, with the separation bubble following closely behind, sometimes very briefly. The turbulent boundary layer behind the bubble may not have enough energy to fully attach the airflow to the airfoil again, and separation may occur at a point before the trailing edge of the wing is reached. As the angle of attack continues to increase, the separation point moves almost to the leading edge of the wing, eventually causing the vortex to break up. This is why most model wings stall. A direct result of low Reynolds numbers is premature stall. On large wings at high speeds, laminar flow does not remain far behind the leading edge of the wing due to the high Reynolds number. And imperfections that are still small will cause the transition to occur too early without the formation of a separation bubble. Therefore, full-size powered aircraft do not have laminar separation problems. When separation does occur, it usually begins at the trailing edge of the wing.