FAA Private Pilot Helicopter (PRH)Helicopter Aerodynamics and SystemsHard
A pilot is starting the rotor system of a two-bladed helicopter parked with the nose into a strong, gusty crosswind. During the low-RPM startup, the blades begin to flap excessively, and one blade dips dangerously close to the tail boom. What phenomenon is occurring?
- ARetreating blade stall
- BBlade sailing
- CGround resonance
- DDynamic rollover
Show answer & explanationAnswer & explanation
Correct answer: B. Blade sailing
Blade sailing occurs during rotor start-up or shutdown at low RPM when strong or gusty winds cause excessive semirigid rotor blade flapping, risking a tail boom or ground strike; centrifugal force has not yet built up enough to rigidify the rotor disk.
Why the other options are wrong
- A. Retreating blade stall occurs in forward flight at high airspeed, not during startup.
- C. Ground resonance is a self-excited vibration from an unbalanced fully articulated rotor, not wind-induced flapping.
- D. Dynamic rollover involves a pivoting skid during hover, not blade flapping.
Blade Sailing
Excessive up-and-down flapping of rotor blades during startup or shutdown at low RPM, most often in strong or gusty wind conditions, that can cause a blade to strike the tail boom.
- Occurs at low rotor RPM before centrifugal stiffening takes effect
- Most common with semirigid (teetering) two-blade rotors
- Mitigated by facing the helicopter into the wind and using proper cyclic technique during start/stop
Memory trick: Sailing blades flap like sails in the wind before they stiffen