Item 1246
OTHER:
Flight Dynamics - General - Ground Resonance
It starts when the blades "bunch up" on one side of the rotor disc where they generate an unbalanced centrifugal force that gets in phase with the natural frequency of the helicopter's landing gear. It only occurs on the ground.
"... two NACA flutter specialists, Robert Coleman and Arnold Feingold .... explained in a classic paper that if, for some reason, the blades became displaced unequally about their lag hinges, the rotor center of gravity would be forced away from the center of rotation."
[Source ~ RWP3 p.54]. Two papers by them are available as PDF. files.Ground resonance is only applicable to rotors that have lead-lag hinges. This means that the SynchroLite and the UniCopter are not subject it. The Dragonfly has lead-lag hinges and ground resonance may or may not be a problem because its three blade are tied together in the lead-lag plane. See:
OTHER: Flight Dynamics - Rotor Hub - Offset Teetering Hinge Concept ~ Ground Resonance.
Notes:
Re: Dragonfly:
Another way to prevent ground resonance is to put the lag hinges so far outboard that the in-plane natural frequency is too high to couple with the frequency of he aircraft on its landing gear. This was done on the Brantly. Its lag hinges are about 40% of the way out the blade. [Source ~ RWP3 p.55] Will the 14% Dragonfly be good enough?
Somewhere dampening at the lag bushing and tie-bar interface is mentioned.
See;
DESIGN: Dragonfly ~ Rotor - Disk - Lead-Lag, particularly the lower of the two drawings
Other Sources of Information:
[Source ~ HT, section 12-4 (p.668-693)]
There is also Air Resonance.
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Last Revised: February 4, 2007