method of increasing lift is by increasing velocity if the AOA  is held constant just short of the “critical,” or stalling, AOA  (assuming no flaps or other high lift devices).  flow around the body, and a reference area associated with  the body. The coefficient of drag is also dimensionless and is  used to quantify the drag of an object in a fluid environment,  such as air, and is always associated with a particular surface  area.  Lift and drag also vary directly with the density of the air.  Density is affected by several factors: pressure, temperature,  and humidity. At an altitude of 18,000 feet, the density of  the air has one-half the density of air at sea level. In order to  maintain its lift at a higher altitude, an aircraft must fly at a  greater true airspeed for any given AOA.  The L/D ratio is determined by dividing the C$_{L}$ by the C$_{D}$,  which is the same as dividing the lift equation by the drag  equation as all of the variables, aside from the coefficients,  cancel out. The lift and drag equations are as follows (L =  Lift in pounds; D = Drag; C$_{L}$ = coefficient of lift;  ρ  = density  (expressed in slugs per cubic feet); V = velocity (in feet per  second); q = dynamic pressure per square foot (q =  1 ⁄ 2  ρ v$^{2}$);  S = the area of the lifting body (in square feet); and  C$_{D }$= Ratio of drag pressure to dynamic pressure):  Warm air is less dense than cool air, and moist air is less  dense than dry air. Thus, on a hot humid day, an aircraft  must be flown at a greater true airspeed for any given AOA  than on a cool, dry day.  If the density factor is decreased and the total lift must equal  the total weight to remain in flight, it follows that one of the  other factors must be increased. The factor usually increased  is the airspeed or the AOA because these are controlled  directly by the pilot.  Lift varies directly with the wing area, provided there is no  change in the wing’s planform. If the wings have the same  proportion and airfoil sections, a wing with a planform area  of 200 square feet lifts twice as much at the same AOA as a  wing with an area of 100 square feet.  Typically at low AOA, the coefficient of drag is low and  small changes in AOA create only slight changes in the  coefficient of drag. At high AOA, small changes in the AOA  cause significant changes in drag. The shape of an airfoil, as  well as changes in the AOA, affects the production of lift.  Two major aerodynamic factors from the pilot’s viewpoint  are lift and airspeed because they can be controlled readily  and accurately. Of course, the pilot can also control density by  adjusting the altitude and can control wing area if the aircraft  happens to have flaps of the type that enlarge wing area.  However, for most situations, the pilot controls lift and airspeed  to maneuver an aircraft. For instance, in straight-and-level flight,  cruising along at a constant altitude, altitude is maintained by  adjusting lift to match the aircraft’s velocity or cruise airspeed,  while maintaining a state of equilibrium in which lift equals  weight. In an approach to landing, when the pilot wishes to  land as slowly as practical, it is necessary to increase AOA near  maximum to maintain lift equal to the weight of the aircraft.  Notice in  Figure 5-5  that the coefficient of lift curve (red)  reaches its maximum for this particular wing section at 20°  AOA and then rapidly decreases. 20° AOA is therefore the  critical angle of attack. The coefficient of drag curve (orange)  increases very rapidly from 14° AOA and completely  overcomes the lift curve at 21° AOA. The lift/drag ratio  (green) reaches its maximum at 6° AOA, meaning that at this  angle, the most lift is obtained for the least amount of drag.  Note that the maximum lift/drag ratio (L/D$_{MAX}$) occurs at  one specific C$_{L}$ and AOA. If the aircraft is operated in steady  flight at L/D$_{MAX}$, the total drag is at a minimum. Any AOA  lower or higher than that for L/D$_{MAX}$ reduces the L/D and  consequently increases the total drag for a given aircraft’s  The lift-to-drag ratio (L/D) is the amount of lift generated by  a wing or airfoil compared to its drag. A ratio of L/D indicates  airfoil efficiency. Aircraft with higher L/D ratios are more  efficient than those with lower L/D ratios. In unaccelerated  flight with the lift and drag data steady, the proportions of  the coefficient of lift (C$_{L}$) and coefficient of drag (C$_{D}$) can  be calculated for specific AOA.  [Figure 5-5]  The coefficient of lift is dimensionless and relates the lift  generated by a lifting body, the dynamic pressure of the fluid  Figure 5-6.  Drag versus speed.  5-5  Lift/Drag Ratio  C$_{D }$ .  ρ  .  V 2  .  S D =  2  Drag  Airspeed  Minimum  drag  Total drag  I n d u c e d d ra g  p a r s i t e d r a g 