Aerodynamics and Aircraft Performance
Forces and the lift equation
Lift = one-half rho V-squared S C-L, so lift varies with air density, airspeed squared, wing area, and angle of attack. The wing always stalls at the same critical angle of attack regardless of weight, airspeed, or attitude; weight changes the speed at which that angle is reached. Induced drag dominates at low speed and decreases as speed increases, while parasite drag rises with the square of speed. Their sum is total drag, and maximum lift-to-drag ratio defines best glide and maximum range.
Load factor and V speeds
Load factor is the ratio of lift to weight. In a level turn, load factor equals 1 divided by the cosine of the bank angle. Stall speed increases with the square root of load factor. Maneuvering speed is published at maximum gross weight and decreases as weight decreases, so at light weights you must slow down sooner in turbulence.
Density altitude and ground effect
High altitude, high temperature, and high humidity reduce density and raise true airspeed, takeoff roll, and climb distance, but they do not change indicated stall speed. Ground effect reduces induced drag and causes floating and premature liftoff.
Traps
- The wing does not stall because airspeed is low; it stalls because the critical angle of attack is exceeded.
- Wake turbulence sinks; stay above and upwind of the preceding aircraft's flight path.
Worked example
At a 60 degree bank, load factor = 1 / cos 60 = 2, so stall speed rises by the square root of 2, about 1.41. A 50-kt airplane stalls near 71 kt. At 75 degrees, load factor is 3.86 and the multiplier is about 1.96.