Aerodynamics, Performance, and Weight and Balance
Performance questions for gliders are not about fuel and runway distance -- they test the drag curve, glide ratio, and how weight, bank, and wind change both. Expect a weight-and-balance computation and a qualitative question about speed to fly.
The drag curve sets your speed
- Total drag is induced plus parasite drag. Induced drag decreases with speed; parasite drag increases with the square of speed. Their sum is lowest at the speed for best L/D.
- Best L/D gives the greatest distance in still air. Minimum sink is a slower speed that gives the longest time aloft -- the speed to use in a thermal.
- Any headwind, sink, or downdraft means flying faster than best L/D; strong lift lets you slow toward minimum sink.
Weight, bank, and stall speed
- Stall speed rises with the square root of load factor: 1.41 at 45 degrees of bank, so stall speed rises about 19 percent in a thermalling turn.
- Water ballast increases wing loading. Glide ratio stays roughly the same, but the speed for best L/D rises in proportion to the square root of the weight, and sink rate and stall speed increase.
- Aft CG reduces pitch stability and can make a spin unrecoverable; a forward CG raises stall speed and trim drag.
Worked example
Empty weight 600 lb at 30.0 in; pilot 180 lb at 25.0 in; water ballast 60 lb at 40.0 in. Total = 840 lb; total moment = 18,000 + 4,500 + 2,400 = 24,900 lb-in; CG = 24,900 / 840 = 29.6 in, inside the envelope. Now compare speeds: at 700 lb the best L/D is 55 mph, so at 840 lb it becomes 55 x sqrt(840/700) = 55 x 1.10 = about 60 mph.
Traps that cost marks
Believing ballast improves glide ratio; using minimum sink as the cross-country speed; skipping the CG check when a heavy pilot flies; ignoring the higher stall speed in a steep thermal turn.