5. GROUND RESONANCE - MECHANICAL INSTABILITY

H-21 Ground Resonance Accident

One of the first considerations for flight testing a new or modified helicopter is to determine it’s susceptibility to get into a ground resonance situation. This applies primarily to helicopters with a fully articulated rotor system. This was true of all our helicopters.

Without a lot of technical explanation, the condition can only occur when the helicopter is on the ground.  It involves the landing gear oleo, or shock absorbing cylinder, the rotor blade lag dampers, tire pressures and rotor blade weight. The condition may occur at various rotor RPM, collective pitch, and control motion.

When the helicopter gets into this condition, it can be stopped immediately by lifting the helicopter into the air.  The motion in this condition is a divergent lateral oscillation that can destroy the helicopter if permitted to continue. 

If the aircraft is flyable the testing can be performed and lifting off is the way to stop it. If the aircraft is new and has never flown, lifting off is not logical since you may encounter other problems and may not be able to put the aircraft back on the ground.  An alternative for testing a new or modified aircraft that has not flown is to change the frequency of it’s contact with the ground. We have done this a number of times, I will try to explain how we did that.

To change the helicopters relation with the ground, we attached heavy cables from each side of the forward and aft rotor, and from each main landing gear wheel. The cables were all joined  together and attached  to two heavy pipes about 12 foot long. With this arrangement pulling on the pipes would  pull the cables and change the frequency of the helicopters relation with the ground and stop the resonance. 

Two Problems:

1. Who’s going to pull on the pipes, if needed.  Solution: Get twelve laborers from the Employment agency to hold the pipes and pull when the pilot yells “PULL” on the PA system.
2.  If there is an accident these people could get hurt from flying debris. Solution. Build a double 12 foot cyclone fence between them and the helicopter.

This procedure worked well in the past, but not this time. We were testing  new all metal rotor (prior blades were wood). The aircraft went into resonance, the PULL command was given, the men pulled but the cable to the aft rotor failed.  The helicopter was severely damaged as shone in the following pictures.  Testing of the new metal rotor blades was discontinued.

Charles Kessler is a retired flight test engineer for Boeing’s Vertol helicopter division (formerly Piasecki Helicopter Co.). He joined Piesecki in 1947, in the company’s fourth year, and retired from Boeing in 1983. During his 37-year career he took part in the testing of prototypes and alterations of such models as the CH-47 Chinook and Sea Knight, the H-16, HRP-2, and the V-107. He taught the stability augmentation system to the German Luftwaffe. He has written about his experience in a blog called “Early Helicopter Years,” which can be found at http://helicopterstory.blogspot.com/.