Ground resonance in a 206 ?

[The Nr Fairy]

I understood the factors in developing ground resonance to be missing in a 206, because it's only got 2 blades and no oleos. Am I wrong ? Can't see anything to contradict me in Wagtendonk or Cyclic and Collective says "ground resonance is a phenomenon of multi-bladed helicopters".

Prompted by a footnote in this accident report (http://www.aaiu.ie/AAIUviewitem.asp?id=8520&lang=ENG&loc=1652).

[Gordy]

Found it---I think they were just saying "if" not that it could get ground resonance--it is badly written.

Go ahead and bash me for pointing this out, but oh well---I did see this on page 4 section 1.4:

"The pilot jumped out and moved/ran some 20 yards away, as the wife unbuckled her seat belt with difficulty. She then alighted from the helicopter and assisted her husband by opening the rear doors from the outside, as he had experienced difficulty in locating the opening handle on the inside."

This is the kind of pilot I want to fly with---NOT.

[thecontroller]

i saw that. made me chuckle.

i also like the bit about them getting offered a replacement helicopter ride and them refusing!

[Revolutionary]

I wonder if the pilot was yelling and flinging his arms as he was running away from the helicopter.

[Gordy]

I wonder if the pilot was yelling and flinging his arms as he was running away from the helicopter.

Kinda reminds me of Monty Python and the Holy Grail---"run away, run away" with Brave sir Robin and his coconut clapping minstrel......:D

[NickLappos]

Your source materials are clearly poor in describing ground resonance, even the vaunted Wagtendonk. The number of blades has NOTHING to do with ground resonance, ground resonance is only possible if the blades have lead-lag motion. Most 2 bladed helos do not have any lead-lag, nor does the 3-bladed V22. They cannot experience ground resonance as a result.

If a helo has oleos, they are there to PREVENT ground resonance.

Ground resonance is a condition where the rotor blades hunt back and forth as the aircraft rocks on the ground. The blades and the aircraft motion build up a resonance that makes extremely high vibratory energy that can take the helicopter apart. During rotor runup, the rotor passes through two regions where the rotor and the fuselage can tune to resonance, one at about 30% and the other at about 60% Nr. The rotor blade dampers and the oleos are designed to damp these vibrations and make ground resonance entirely disappear.

If you experience ground resonance, cut the throttles immediately, this makes the blades all go to the lead position, and helps quell the resonance immediately. If you then apply the rotor brake, the blades are also tossed forward and pinned there by the rotor deceleration, a very quick cure. The pilot in this case made a very poor desicion to not cut the throttles, and his helo paid the price. Even if he had to switch hands, or stand on his head, cutting the throttle was #1 on his list. Cutting fuel is NOT a good idea, it takes too much time to starve the engine.

[thecontroller]

the 206 has no lead-lag hinges. so how can this case be ground resonance?

[Gordy]

The pilot in this case made a very poor desicion to not cut the throttles, and his helo paid the price. Even if he had to switch hands, or stand on his head, cutting the throttle was #1 on his list. Cutting fuel is NOT a good idea, it takes too much time to starve the engine.
Dont think he needed to cut the throttle----the engine had quit already and he was too busy running away and saving his own ass.........

[spinwing]

Mmmmmm .......


I always taught/thought that "Ground Resonance" developed because the centre of mass of a rotating body was displaced from its centre of rotation which then caused the rotating out of balance.

This can obviously happen with an "articulated" head (Allouette, Hughes 300 series etc) when a dodgy M/R blade or U/C damper allows the blades to phase shift.

I guess a Jetranger (206) could only do it if it were started after re-fitting a rotor head with incorrectly swept blades after maintenance .... can't at the moment think how else ???? .... Don't think you would get it up to full rpm though :E :=


Also remember during basic training on the dreaded Hu 269 that if it was vibrating on lift off a hovering Auto would fix it .... ie allowing the blades to re-phase themselves! ... this was before the advent of the elastomeric dampers of course.

Cheers

[NickLappos]

Spin,
Your explanation is also correct, it just depends on if you are looking at the blade or the whole helo. In effect, when the blades lead/lag in unison they move their effective CG away from the mast, they are going as you describe. But this blade motion is because they are each individually not properly damped as they lead/lag, so the real source is that blade motion that resonates with the fuselage rocking motion (usually in roll, because the roll inertia is much, much lower than the pitch inertia.) In fact, most causes of ground resonance are due to one bad damper and one bad blade.

[havoc]

If you experience ground resonance, cut the throttles immediately, this makes the blades all go to the lead position, and helps quell the resonance immediately. If you then apply the rotor brake, the blades are also tossed forward and pinned there by the rotor deceleration, a very quick cure.

Nick, can you clarify something, during ASTAR B2 training a couple of years ago, upon landing we got into the start of ground resonance and the IP immediately picked the aircraft off the ground. Would chopping the throttle be the recommended technique?

Side note, the mech insisted that the oleos were fine, after several more incidents the aircraft ended up at a different base (scheduled to be relocated prior to the occurences). Their maintenance staff replaced the oleos and the problem was fixed.

[Head Turner]

I have experienced ground resonance in AS350's and in every case the cause was identified as being faulty oleos. The choice of stopping the engine or lifting off into a hover is a pilots decision..the choice will depend upon the erpm/rrpm at the onset of GR. But, remember that GR builds up very quickly and the decision must be immediate

[Gomer Pylot]

My experience in the AS350 is that it normally happens with full rpm, and picking it up to a hover solves the problem. Once, however, I got it after I reduced the throttle (engine lever) to idle, and it got very bad very quickly. The first thing I did was to pull the throttle completely off, and then pull the rotor brake. It finally stopped shaking after turning about 45 degrees on the pad, and observers said the heels and toes of the skids were coming a foot off the ground. I've had them try to start during runup, and immediately retarding the throttle solved it. In the case above, all maintenance could find was some wear in the spherical bearings.