A pilot is flying a helicopter at high forward airspeed and low density altitude. On the advancing side of the rotor disk, the combination of rotational velocity and aircraft forward speed causes the local airflow over the blade tip to approach the speed of sound. What aerodynamic phenomenon results, and how does it differ from retreating blade stall?
- AAdvancing blade compressibility, which causes shock wave formation and increased drag on the advancing side, unlike the flow separation stall on the retreating side
- BConing, which increases as airspeed increases on the advancing side
- CRetreating blade stall, which occurs simultaneously on both sides of the disk
- DDissymmetry of lift, which is corrected automatically by blade flapping on the advancing side
Show answer & explanationAnswer & explanation
Correct answer: A. Advancing blade compressibility, which causes shock wave formation and increased drag on the advancing side, unlike the flow separation stall on the retreating side
As airspeed and tip speed increase, the advancing blade tip can approach the speed of sound, causing shock wave formation, a sharp increase in drag, and possible control vibration—known as advancing blade compressibility. This is a high-speed effect on the advancing side, distinct from the low-speed flow separation (stall) that occurs on the retreating side due to dissymmetry of lift.
Why the other options are wrong
- B. Coning is caused by lift and centrifugal force balance, not by transonic airflow effects.
- C. Retreating blade stall occurs on the retreating side due to low relative airflow, not simultaneously with compressibility.
- D. Dissymmetry of lift is the underlying cause of both effects but is not itself the compressibility phenomenon described.
Advancing Blade Compressibility
A high-speed aerodynamic phenomenon where airflow over the advancing blade tip approaches or exceeds the speed of sound, creating shock waves, increased drag, and vibration.
- Occurs on the advancing (high relative wind speed) side of the disk
- Distinct from retreating blade stall which occurs on the low-speed side
- Aggravated by high forward airspeed and high rotor RPM
Memory trick: Advancing blade races toward the sound barrier and hits a wall of shock