Aerodynamics of Flight

garD method of increasing lift is by increasing velocity if the AOA flow around the body, and a reference area associated with is held constant just short of the “critical,” or stalling, AOA the body. The coefficient of drag is also dimensionless and is (assuming no flaps or other high lift devices). used to quantify the drag of an object in a fluid environment, such as air, and is always associated with a particular surface Lift and drag also vary directly with the density of the air. area. Density is affected by several factors: pressure, temperature, and humidity. At an altitude of 18,000 feet, the density of The L/D ratio is determined by dividing the C by the C , L D the air has one-half the density of air at sea level. In order to which is the same as dividing the lift equation by the drag maintain its lift at a higher altitude, an aircraft must fly at a equation as all of the variables, aside from the coefficients, greater true airspeed for any given AOA. cancel out. The lift and drag equations are as follows (L = Lift in pounds; D = Drag; C = coefficient of lift; ρ = density L Warm air is less dense than cool air, and moist air is less (expressed in slugs per cubic feet); V = velocity (in feet per dense than dry air. Thus, on a hot humid day, an aircraft second); q = dynamic pressure per square foot (q = 1⁄ ρv2); 2 must be flown at a greater true airspeed for any given AOA S = the area of the lifting body (in square feet); and than on a cool, dry day. C = Ratio of drag pressure to dynamic pressure): D If the density factor is decreased and the total lift must equal C . ρ . V2 . S D = D the total weight to remain in flight, it follows that one of the 2 other factors must be increased. The factor usually increased is the airspeed or the AOA because these are controlled Typically at low AOA, the coefficient of drag is low and directly by the pilot. small changes in AOA create only slight changes in the coefficient of drag. At high AOA, small changes in the AOA Lift varies directly with the wing area, provided there is no cause significant changes in drag. The shape of an airfoil, as change in the wing’s planform. If the wings have the same well as changes in the AOA, affects the production of lift. proportion and airfoil sections, a wing with a planform area of 200 square feet lifts twice as much at the same AOA as a Notice in Figure 5-5 that the coefficient of lift curve (red) wing with an area of 100 square feet. reaches its maximum for this particular wing section at 20° AOA and then rapidly decreases. 20° AOA is therefore the Two major aerodynamic factors from the pilot’s viewpoint critical angle of attack. The coefficient of drag curve (orange) are lift and airspeed because they can be controlled readily increases very rapidly from 14° AOA and completely and accurately. Of course, the pilot can also control density by overcomes the lift curve at 21° AOA. The lift/drag ratio adjusting the altitude and can control wing area if the aircraft (green) reaches its maximum at 6° AOA, meaning that at this happens to have flaps of the type that enlarge wing area. angle, the most lift is obtained for the least amount of drag. However, for most situations, the pilot controls lift and airspeed to maneuver an aircraft. For instance, in straight-and-level flight, Note that the maximum lift/drag ratio (L/D ) occurs at MAX cruising along at a constant altitude, altitude is maintained by one specific C and AOA. If the aircraft is operated in steady L adjusting lift to match the aircraft’s velocity or cruise airspeed, flight at L/D , the total drag is at a minimum. Any AOA MAX while maintaining a state of equilibrium in which lift equals lower or higher than that for L/D reduces the L/D and MAX weight. In an approach to landing, when the pilot wishes to consequently increases the total drag for a given aircraft’s 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 g a a wing or airfoil compared to its drag. A ratio of L/D indicates dr e airfoil efficiency. Aircraft with higher L/D ratios are more sit Total drag a efficient than those with lower L/D ratios. In unaccelerated ar p flight with the lift and drag data steady, the proportions of Minimum the coefficient of lift (C L ) and coefficient of drag (C D ) can drag Induced drag be calculated for specific AOA. [Figure 5-5] Airspeed 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.