CFI Pop Quiz #215 - Resonance

[Chris Burgess]

What action should be taken if ground resonance is encountered during a landing attempt?

A) Attempt to make a takeoff regardless of RPM situation.
B) Close throttle immediately and raise collective pitch to dampen vibrations.
C) Make an immediate takeoff if RPM is in proper range; otherwise, close throttle, and lower collective pitch.

Note: Although this looks helicopter, it can apply to gyroplane and is included in FAA-H-8083-21. It is something the gyroplane student should be familiar with. It is included in the Practical Test Standards for Recreational and Private gyroplane pilots.

[brett s]

C, and if not able to takeoff hope you get lucky - otherwise wait for the machine to stop disintegrating around you :)

[Paul_Zurawski]

(C) is the correct action

[Chris Burgess]

Brett is quite correct. Ground resonance can quickly destroy the aircraft. This condition could be experienced in the Air and Space 18A or any rotorcraft with a fully-articulated rotor system. This is why you must have knowledge of it for a Recreational and Private ratings. Contact with the ground is involved. Proper servicing of the tires, struts, and rotor lead/lag dampers is very important. In a conversation I had with Dr Igor Bensen years ago, he told me some of his injuries were from such and incidence. Reference FAA-H-8083-21.

[Victor Duarte]

I'd say B,
i 've always read that adding collective, if possible, immediately stops resonnance by de-coupling the frame fron the ground. wrong ?

[brett s]

Not if you can't get off the ground in the first place...

[Victor Duarte]

Brett,
sorry, i did the wrong answer because i didn't take the time to read carefully, yes "C" is the good aswer to me, by adding collective, i meant try to take off if rpm allows it, that's what my ex father in law, helo instructor taught me, .. silly me

[Chris Burgess]

If the Air & Space 18A contacts the ground, the flight manual says you must abort the takeoff. There are several reasons for this. Lets say you made a hard, one wheel first landing and the rotors slung to something other than 120 degrees apart (three blades). You are in a lot of trouble. You are at the mercy of "ground resonance" since rotor RPM will not allow a "jump" takeoff, the only way the 18A can takeoff. "C" becomes, close the throttle and as Brent put it "wait for the machine to stop disintegrating around you". Sometimes the 18A would "wallow" around as you did the initial spin-up of the blades. This was mostly due to the blades being out of phase, not 120 degrees apart. As they self center on spin up, this "wallow" goes away and the ship smooths out. Imagine a 5 pound weight strapped to one end of one rotor if you can, that is what ground resonance is like. A severely out of balance rotor that likes to destroy everything attached to it.