FAA Flight Instructor Airplane (FIA)Aerodynamics and Principles of FlightHard
Airplane A has a weight of 3,000 pounds, a wing area of 200 square feet, and stalls at 50 knots in level flight. Airplane B has the same weight of 3,000 pounds but a wing area of only 150 square feet. Using the fact that stall speed varies with the square root of wing loading, what is Airplane B's approximate stall speed?
- A50 knots
- B58 knots
- C53 knots
- D65 knots
Show answer & explanationAnswer & explanation
Correct answer: B. 58 knots
Wing loading A = 3,000/200 = 15 lb/ft²; Wing loading B = 3,000/150 = 20 lb/ft². Since stall speed varies with the square root of wing loading, Vs(B) = 50 × √(20/15) = 50 × √1.333 = 50 × 1.155 ≈ 57.7 knots, which rounds to approximately 58 knots.
Why the other options are wrong
- A. 50 knots ignores the higher wing loading of Airplane B and would only be correct if wing loading were unchanged.
- C. 53 knots underestimates the effect of the wing loading ratio on stall speed.
- D. 65 knots overestimates the stall speed increase for this wing loading ratio.
Wing Loading and Stall Speed
Wing loading (weight ÷ wing area) directly affects stall speed; stall speed varies with the square root of the ratio of wing loadings when weight or wing area changes.
- Formula: Vs2 = Vs1 × √(WL2/WL1)
- Higher wing loading = higher stall speed
- Smaller wing area with same weight increases wing loading and stall speed
Memory trick: Smaller wings, same weight, means a sneakier (higher) stall speed