FAA Private Pilot Helicopter (PRH)Helicopter Aerodynamics and SystemsHard
A helicopter with a fully articulated rotor system is parked with rotors turning at operating RPM in gusty wind conditions on a soft, uneven surface. The pilot notices the aircraft beginning to vibrate with an increasing lateral oscillation that is not correlated with rotor RPM. What condition is developing?
- ADynamic rollover
- BGround resonance
- CMast bumping
- DRetreating blade stall
Show answer & explanationAnswer & explanation
Correct answer: B. Ground resonance
Ground resonance is a self-excited, potentially destructive vibration that develops when main rotor blades on a fully articulated (lead-lag equipped) rotor become unequally spaced (bunched) due to a shock such as a hard landing or ground contact on soft/uneven terrain, causing the rotor's center of mass to shift and match the natural frequency of the landing gear/airframe.
Why the other options are wrong
- A. Dynamic rollover involves a pivoting skid/wheel and excessive roll rate, not lateral vibration matching a frequency.
- C. Mast bumping involves the rotor hub contacting the mast on semirigid systems, unrelated to lead-lag bunching.
- D. Retreating blade stall occurs only in forward flight at high airspeed, not while on the ground.
Ground Resonance
A violent, self-reinforcing vibration on fully articulated rotor helicopters caused by uneven blade spacing (from lead-lag hinges) coinciding with the natural frequency of the landing gear on the ground.
- Requires a fully articulated (lead-lag) rotor system
- Triggered by a hard landing, taxiing on rough ground, or one-wheel/skid touchdown
- Can rapidly destroy the helicopter if not corrected by immediate takeoff or shutdown
Memory trick: Ground resonance = 'bunched blades bounce the whole bird apart.'