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Introduction to UAV Systems. Mohammad H. Sadraey
Читать онлайн.Название Introduction to UAV Systems
Год выпуска 0
isbn 9781119802624
Автор произведения Mohammad H. Sadraey
Издательство John Wiley & Sons Limited
3 Aerodynamics
3.1 Overview
Aerodynamics is the science that involves the study of the behavior (i.e., dynamics) of air when confronting a moving object (e.g., air vehicle). The UAV has a number of components that are characterized by their aerodynamic outputs (e.g., lift), two of which are wing and tail. A wing/tail is considered as a lifting surface in which the lift is produced due to the pressure difference between lower and upper surfaces. In contrast, surfaces such as aileron, rudder, and elevator are referred to as the control surfaces. Lifting surfaces are generally fixed while control surfaces are always moving up/down or left/right. Both lifting and control surfaces are functions of their aerodynamic features.
The primary forces that act on an air vehicle are thrust, lift, drag, and gravity (or weight). They are shown in Figure 3.1. The UAV components that have a direct contact with moving air contribute to aerodynamic features. A number of elements have considerable contributions on an air vehicle’s aerodynamic features. They are mainly: (1) wing, (2) tail, (3) fuselage, (4) engine cowling, (5) landing gear, and (6) payload. Three primary aerodynamic components of an air vehicle are: (1) wing, (2) tail, and (3) fuselage.
The primary aerodynamic function of the wing and tail is to generate sufficient lift force or simply lift (L). However, they have two other undesirable products, namely drag (D) and nose‐down pitching moment (M). The fuselage is not fundamentally considered as an aerodynamic component based on its function. However, fuselage has a considerable role in creating drag, while it generates a little lift.
In this chapter, aerodynamic forces (mainly lift and drag), airfoil, pressure distribution, friction drag, induced drag, drag polar, air vehicle drag, boundary layer, and aerodynamic efficiency will be briefly covered.
3.2 Aerodynamic Forces
The primary function of the UAV aerodynamic components (e.g., wing and tail) is to generate sufficient lift force or simply lift (L). However, they have two other aerodynamic products, namely drag force or drag (D) and nose‐down (Figure 3.2) pitching moment (M). While a UAV designer is looking to maximize the lift, the other two (drag and pitching moment) must be minimized. In this chapter, lift and drag are presented, while in Chapter 5, the pitching moment will be introduced.
Figure 3.1 Forces on an air vehicle during a level flight
Figure 3.2 Aerodynamic lift, drag, and pitching moment
The aerodynamic forces of lift and drag [8] are functions of the following factors: (1) aircraft configuration, aircraft/wing angle of attack (α), (3) aircraft geometry, (4) airspeed (V), (5) air density (ρ), (6) Reynolds number of the flow, and (7) air viscosity:
(3.1)
(3.2)
where ρ is air density, V is velocity, S is the wing planform area, and CL and CD are the lift and drag coefficients respectively. The calculations of lift and drag coefficients will be presented in the coming sections.
The lift force or simply lift is always defined as the component of the aerodynamic force perpendicular to the relative wind. The drag is always defined as the component of the aerodynamic force parallel to the relative wind (V∞). In other words, lift is always perpendicular and drag is always parallel to the relative wind. Figure 3.2 shows an airfoil section and the directions of lift and drag.
In reality, the aerodynamic force is located at the center of pressure (cp), which is moving with the variations of angle of attack (α). However, the aerodynamic center (ac) which is frequently selected to be the center of lift, is located nearly at the quarter chord (i.e., 1/4 of C). The pitching moment is the bi‐product of moving the location of aerodynamic force from cp to ac. The moment can be taken with respect to any point, but traditionally is taken about a point 25% rearward of the wing leading edge, known as the quarter chord. The aerodynamic center has a desired property – the variation of moment coefficient with respect to the angle of attack is zero (i.e., moment coefficient remains constant).
3.3 Mach Number
It is customary to compare the speed of an air vehicle (V) with the speed of sound (a). The Mach number is defined as the ratio of airspeed over the speed of sound:
(3.3)
The speed of sound at the sea level standard condition is 340 m/s. M is used to define four different flight regimes for airspeed: (1) subsonic, (2) transonic, (3) supersonic, and (4) hypersonic.
When the flight speed is less than the speed of sound – where M < 1 – it is defined as subsonic speed. When 0.8 ≤ M ≤ 1.2, the flight regime is loosely defined as transonic. If the flight speed is less than the speed of sound, but M is sufficiently near 1, the airflow expansion over the top surfaces of the wing/tail/fuselage may result in locally supersonic regions. A flowfield where M > 1 everywhere is defined as supersonic. At supersonic speeds, a shock wave (e.g., normal, oblique, and bow) is created by nature to adjust the flow properties (e.g., air pressure and temperature).
The flow regime for M > 5 is given a special label, hypersonic flow. For values of M > 5, the shock wave is very close to the surface, and the flowfield between the shock and the body (the shock layer) becomes very hot indeed, hot enough to dissociate or even ionize the gas. The aerodynamic characteristics of an air vehicle is strongly a function of Mach number, some of which will be discussed in this chapter.
3.4 Airfoil
The most important element of a lifting surface (e.g., wing/tail) for aerodynamic purposes is its cross‐section or airfoil. Any section of the wing cut by a plane parallel to the aircraft xz plane is called an airfoil. It usually looks like a positive cambered section where the thicker part is in front of the airfoil. A typical airfoil section is shown in Figure 3.3, where several geometric parameters are illustrated. The leading edge is often curved (for a subsonic air vehicle) and the trailing ledge is sharp.
If