An airplane's power-off stall speed is 55 KCAS. During a full-power stall in the same configuration, the measured stall speed is only 48 KCAS. What primarily accounts for this reduction in stall speed with power applied?
- AReduced induced drag at high power settings lowers the required lift
- BIncreased parasite drag at high power counteracts the tendency to stall
- CGround effect reduces the induced drag and stall speed during the power-on test
- DPropeller slipstream and thrust vector effects increase airflow energy and effective lift over portions of the wing, delaying the stall to a lower airspeed
Show answer & explanationAnswer & explanation
Correct answer: D. Propeller slipstream and thrust vector effects increase airflow energy and effective lift over portions of the wing, delaying the stall to a lower airspeed
With power applied, propeller slipstream increases airflow velocity over the wing root and portions of the wing/fuselage, and the vertical component of thrust helps support the airplane, both of which allow the wing to generate the needed lift at a lower indicated airspeed before reaching critical angle of attack, thus lowering the measured stall speed.
Why the other options are wrong
- A. Induced drag is not the primary factor determining stall onset in this scenario.
- B. Increased parasite drag would not lower stall speed; drag and stall speed are not directly linked this way.
- C. Ground effect is unrelated to power-on versus power-off stall speed differences, which occur regardless of altitude above the ground.
Power-On vs Power-Off Stall Speed
Power-on stall speed is typically lower than power-off stall speed because propeller slipstream increases airflow energy over the wing and a vertical thrust component helps support the airplane, delaying the stall.
- Slipstream increases effective airflow speed over the wing root area.
- A component of thrust acts to help support weight at high pitch attitudes.
- This is why power-on stalls occur at a lower indicated airspeed than power-off stalls.
Memory trick: Power blows extra air over the wing, delaying the stall to a slower speed.