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