FAA Private Pilot Helicopter (PRH)Performance and LimitationsMedium
A helicopter's flight manual shows that the never-exceed speed (VNE) placard decreases as pressure altitude increases. What is the primary aerodynamic reason for this reduction?
- AReduced air density requires a higher retreating blade angle of attack, bringing it closer to stall at lower forward airspeeds
- BEngine power available decreases, making higher airspeeds structurally unnecessary
- CTail rotor authority decreases significantly at higher altitudes
- DThe advancing blade experiences increased dissymmetry of lift at altitude
Show answer & explanationAnswer & explanation
Correct answer: A. Reduced air density requires a higher retreating blade angle of attack, bringing it closer to stall at lower forward airspeeds
As density altitude increases, the air is less dense, so the retreating blade must operate at a higher angle of attack to produce the same lift. This brings the retreating blade closer to stall, so the maximum safe forward airspeed (VNE) must be reduced.
Why the other options are wrong
- B. Power available affects climb performance, not the structural/aerodynamic VNE limit.
- C. Tail rotor authority is a separate concern and is not the reason VNE is reduced with altitude.
- D. Dissymmetry of lift exists at all altitudes and is not the specific altitude-driven cause of VNE reduction.
VNE and Altitude
VNE decreases with altitude because reduced air density requires a higher retreating blade angle of attack, increasing the risk of retreating blade stall at lower forward speeds.
- VNE placards often show reduced speed with altitude
- Retreating blade AOA increases with lower air density
- Higher AOA at altitude brings stall closer at a given airspeed
Memory trick: Thin air, thick trouble — retreating blade stalls sooner.