FAA Flight Instructor Airplane (FIA)Aerodynamics and Principles of FlightHard
An airplane maintains constant altitude in unaccelerated flight while the pilot gradually reduces power and airspeed decreases from cruise toward the onset of slow flight. To continue producing lift equal to weight at the lower airspeed, the pilot must:
- AIncrease airspeed slightly to compensate for the reduced angle of attack
- BDecrease the angle of attack to reduce induced drag as airspeed decreases
- CMaintain a constant angle of attack regardless of airspeed, since lift is independent of AOA
- DProgressively increase the angle of attack as airspeed decreases, since lift depends on both CL and the square of velocity
Show answer & explanationAnswer & explanation
Correct answer: D. Progressively increase the angle of attack as airspeed decreases, since lift depends on both CL and the square of velocity
Since lift depends on CL × V² (along with density and area), as V² decreases with slower airspeed, CL (and therefore angle of attack) must increase proportionally to keep lift equal to weight. This is why slower flight requires progressively higher pitch attitudes/AOA, up to the critical angle of attack at stall.
Why the other options are wrong
- A. Contradicts the premise that airspeed is decreasing; increasing airspeed would not match the scenario.
- B. Decreasing AOA would reduce CL and fail to maintain the lift needed at lower airspeed.
- C. Lift depends heavily on AOA through the CL term; this option is aerodynamically incorrect.
AOA-Airspeed Tradeoff for Constant Lift
To maintain lift equal to weight at a lower airspeed, angle of attack (and CL) must increase to compensate for the reduced V² term in the lift equation.
- Lift = CL × 0.5ρV²S
- Lower V requires higher CL/AOA for same lift
- This tradeoff continues until critical AOA (stall) is reached
Memory trick: Slow down, pitch up — the wing trades speed for angle to keep the same lift.