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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). 

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. 

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. 

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. 

## Lift/Drag Ratio 

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 

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. 

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): 

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. 

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 

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