Flight Theory and Aerodynamics. Joseph R. Badick
3.9 is an example of deployed ground spoilers to reduce lift over the wing and increase the effectiveness of the brakes. Some aircraft use the spoilers in flight for roll control at higher airspeeds when the action of the ailerons would be too much of a force. Sometimes spoilers are used as speed brakes to reduce lift on both wings, which allows the aircraft to descend without increasing airspeed. Gliders utilize spoilers to control their descent rate upon landing.
Trim Systems and Tabs
Trim systems are designed to alleviate the pressures on the primary flight controls as experienced by the pilot during aircraft operation. Usually located on the trailing edge of these devices, the pilot (or autopilot) operates the respective trim system in order to position the flight control where minimum pressures are exerted in the system. The two most common trim systems are trim tabs and antiservo tabs.
A trim tab is usually found on the trailing edge of the elevator or rudder, and unlike the antiservo tab, the trim tab moves in the opposite direction to which the primary control moves (Figure 3.10). An antiservo tab is found on the trailing edge of a stabilator, and moves in the same direction as the primary control to which it is attached (Figure 3.11). As previously mentioned, the antiservo tab serves to reduce the sensitivity of the stabilator during pitch to the pilot, as well as reducing control pressure as needed.
Figure 3.10 Trim tabs.
Source: U.S. Department of Transportation Federal Aviation Administration (2008a).
Some smaller aircraft may incorporate a different type of trim tab that does not involve an independent mechanical linkage, this type of trim tab consists only of a metal plate attached to the trailing edge of a primary flight control. When set correctly, this allows the pilot to fly “hands‐off” the flight control while in straight and level flight. Figure 3.12 provides a summary of the many secondary flight controls available, including their location on the aircraft and function.
AIRFOILS
An airfoil or, more properly, an airfoil section, is commonly shown as a vertical slice of a wing (see Figure 3.13). In discussing airfoils in this chapter, the planform (or horizontal plane) of the wing is ignored as it will be discussed in Chapters 4 and 5. Keep in mind during this discussion that an airfoil is also found on the vertical stabilizer, horizontal stabilizer, and rotor blades. Wingtip effects, sweepback, taper, wash/out or wash/in, and other design features are not considered. No single airfoil design is perfect for every flight situation: wind tunnel tests, and computer‐generated designs dictate what airfoil is best for the mission of the aircraft.
Figure 3.11 Antiservo Tab.
Source: U.S. Department of Transportation Federal Aviation Administration (2008a).
Figure 3.12 Secondary control surfaces and their location.
Source: U.S. Department of Transportation Federal Aviation Administration (2018).
Figure 3.13 Airfoil section.
Figure 3.14 Airfoil terminology.
Airfoil Terminology
The terminology used to discuss an airfoil is shown in Figure 3.14:
1 Chord line is a straight line connecting the leading edge and the trailing edge of the airfoil.
2 Chord is the length of the chord line. All airfoil dimensions are measured in terms of the chord.
3 Mean camber line is a line drawn equidistant between the upper surface and the lower surfaces.
4 Maximum camber is the maximum distance between the mean camber line and the chord line. The location of maximum camber is important in determining the aerodynamic characteristics of the airfoil.
5 Maximum thickness is the maximum distance between the upper and lower surfaces, and its location of maximum thickness will also be important when determining aerodynamic characteristics.
6 Leading edge radius is a measure of the sharpness of the leading edge. It may vary from zero for a knife‐edge supersonic airfoil to about 2% (of the chord) for rather blunt leading edge airfoils.
Definitions
Flight path velocity: The speed and direction of a body passing through the air.
Relative wind (RW): The speed and direction of the air impinging on a body passing through it. It is equal and opposite in direction to the flight path velocity.
Angle of attack (AOA or α, pronounced alpha): The acute angle between the relative wind and the chord line of an airfoil.
Aerodynamic force (AF): The net resulting static pressure multiplied by the planform area of an airfoil.
Lift: The component L of the aerodynamic force that is perpendicular to the relative wind.
Drag: The component D of the aerodynamic force that is parallel to the relative wind.
Center of pressure (CP): The point on the chord line where the aerodynamic force acts.
Laminar flow or streamlined flow: Smooth airflow with little transfer of momentum between parallel layers.
Turbulent flow: Airflow where the streamlines break up and there is much mixing of the layers.
Geometry Variables of Airfoils
There are four main variables in the geometry of an airfoil:
1 Shape of the mean camber line
2 Thickness
3 Location of maximum thickness
4 Leading edge radius
In a cambered airfoil (nonsymmetrical), the mean camber line and chord line are different. If the mean camber line coincides with the chord line, the airfoil is said to be symmetrical. Figure 3.15 shows the difference between a cambered and symmetrical airfoil, note