FAA Commercial Pilot Airplane (CAX)Aerodynamics and PerformanceEasy
An airplane is in stabilized, level flight at 100 knots. The pilot smoothly accelerates to 150 knots while maintaining level flight and constant weight. What must happen to the wing's angle of attack?
- AIt must increase to generate the additional lift needed at higher speed
- BIt must decrease, since less angle of attack is needed to produce the same lift at higher speed
- CIt increases briefly then returns to its original value
- DIt remains unchanged because lift depends only on airspeed, not angle of attack
Show answer & explanationAnswer & explanation
Correct answer: B. It must decrease, since less angle of attack is needed to produce the same lift at higher speed
Lift = CL x ½ρV² x S. Since weight (and therefore required lift) is constant, as V² increases the required CL (a function of angle of attack) must decrease. The pilot reduces back pressure, lowering the angle of attack as speed increases.
Why the other options are wrong
- A. Backwards — higher speed requires LESS angle of attack for the same lift, not more.
- C. There is no oscillation; the change is a smooth, monotonic decrease.
- D. Angle of attack directly controls CL, which must change to keep lift constant as speed changes.
Lift Equation and Angle of Attack
For constant lift (weight) in level flight, angle of attack and airspeed are inversely related: as speed increases, less angle of attack is needed to generate the same lift.
- Lift = CL x ½ρV² x S
- In level flight, lift must equal weight
- Higher speed → lower required CL → lower angle of attack
Memory trick: Faster wing, lazier angle — speed and AOA trade places to hold lift steady.