PHAK 5-4

Figure 5-4. Some aircraft have the ability to change the direction of thrust. Stall CL CLMAX CD L/D L/DMAX 18 16 14 12 10 8 6 4 2 0 .2000 .1800 .1600 .1400 .1200 .1000 .0800 .0600 .0400 .0200 0 Coefficient of drag (CD) Lift/drag Angle of attack 0° 2° 4° 6° 8° 10° 12° 14° 16° 18° 20° 22° CL 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 Figure 5-5. Coefficients of lift and drag at various angles of attack. shift control, or gliders, air must be moving across the lifting surface. This is accomplished by the forward speed of the aircraft. Lift is proportional to the square of the aircraft’s velocity. For example, an airplane traveling at 200 knots has four times the lift as the same airplane traveling at 100 knots, if the AOA and other factors remain constant. L = CL . ρ . V2 . S 2 The above lift equation exemplifies this mathematically and supports that doubling of the airspeed will result in four times the lift. As a result, one can see that velocity is an important component to the production of lift, which itself can be affected through varying AOA. When examining the equation, lift (L) is determined through the relationship of the air density (ρ), the airfoil velocity (V), the surface area of the wing (S) and the coefficient of lift (CL) for a given airfoil. Taking the equation further, one can see an aircraft could not continue to travel in level flight at a constant altitude and maintain the same AOA if the velocity is increased. The lift would increase and the aircraft would climb as a result of the increased lift force or speed up. Therefore, to keep the aircraft straight and level (not accelerating upward) and in a state of equilibrium, as velocity is increased, lift must be kept constant. This is normally accomplished by reducing the AOA by lowering the nose. Conversely, as the aircraft is slowed, the decreasing velocity requires increasing the AOA to maintain lift sufficient to maintain flight. There is, of course, a limit to how far the AOA can be increased, if a stall is to be avoided. All other factors being constant, for every AOA there is a corresponding airspeed required to maintain altitude in steady, unaccelerated flight (true only if maintaining level flight). Since an airfoil always stalls at the same AOA, if increasing weight, lift must also be increased. The only