Helicopter Breaks Apart After Landing

[Garry Russell]

I can see the second one now

Is is similar to sound feedback... where the forces rock and and forth increasing unless you give distance to break the effect?

[Chris Trott]

Not really because the problem is two separate systems working against each other. The rotors creating asymmetric lift, and the landing skids bouncing on the ground and rebounding the helicopter. With landing gear, they were able to somewhat dampen out this effect, but as the Chinook test shows, it's not always 100% effective. As a result of the Chinook test, Boeing/Vertol redesigned the CH-47 landing gear substantially to try and fix the problem. It was mostly successful, however the CH-47's have a very large warning in the manual about certain rotor RPMs that are not to be held under any circumstance.

BTW, I had to pull out my Mi-24D manual to confirm, but the Russian helicopters also have a similar warning about not holding certain RPM ranges due to harmonics and resonance when operating the Rotor in manual mode. The Mi-2 has a different range, but it's the same. I've always believed this to be partially due to the Russian design relying on dynamic balance for the rotors and not static balance as most Western helicopters do. This is the reason for the well-known "Hoplite Hop" and "Hind Sway" that you see when those helicopters start up.

[SkippyBing]

It's not strictly to do with asymmetric lift although one of the mechanisms that mitigates it comes in to play.

In any rotor system with more than two blades there's a lag hinge which allows the individual blades to move in the plane of rotation independent of the other ones. This allows for the change in drag as the lift changes through a complete rotation, essentially centrifugal force* holds the blades perpendicular to the rotor hub and then the forces of lift and drag move it from this position by varying amounts as it moves round the disc. The forces balance out around the whole disc so the movements of the individual blades cancel out any in-balance and the whole thing only vibrates a bit, because it's a helicopter.

With GR (ground resonance) at least one of the blades (but not all) will be out of position, i.e. lagged, and remains so due to feedback from the airframe preventing it moving back into position. This causes an in-balance in the mass distribution of the rotor disc which as seen in the video will handily be at a frequency that starts to shake the entire airframe with fairly terminal consequences. Think of it like a car wheel with too many balance weights on one side. In most cases the feedback from the airframe is due to a fault in the undercarriage, i.e. a spring/damper that's no longer working properly, this makes it more rigid than usual when it comes into contact with the ground. Alternatively it may be due to a faulty damper on the lag hinge on the rotor, in that case you're more likely to experience GR when starting up, in which case you should shut down ASAP.

In the case of the Squirrel as in the original video there is a steel tongue that extends from the aft end of each skid that acts as a damper, just holding the ends of the skids off the ground. There have been cases where people have landed on an uneven surface such as rippled concrete and these have been taken out of the equation leading to GR, the solution being to lift off as soon as it's detected and reposition before touching down again. Obviously I don't know if that's what happened to the Brazilian example but it's one possibility.

For two bladed helicopters there's no lag hinge as the blades balance out, one leading while the other lags and vice versa, hence you won't get GR as you shouldn't get an in-balance in the rotor disc unless one blade experiences damage/something falls off.

*which we'll take as being a real thing for the purposes of this explanation.

[petermcleland]

In my time on Sycamores the risks of Ground Resonance was reduced by using a low inflation pressure in the tyres...I remember once crashing in a field as a passenger with the inertia dampers locking and unlocking...The two pilots made a good job of plonking into a field and with no damage except to our nerves. We abandoned it there and proceeded to our respective destinations by train.

The one mistake the ground recovery team made was to fail to inflate the tyres to a higher pressure, suitable for towing on the road for an extended period...It was being towed along the A1 when the soft tyres overheated and burst...The A1 was blocked for some time and I think it made the National papers!

[Garry Russell]

Thanks guys

Is this GR always there, but only rarely rears up and gets out of control??

[DaveB]

Reading what the troops have written, it seems the potential for GR is always present when the aircraft is on the ground with rotors turning and burning. I guess a certain amount is present by the nature of the beast but as long as it's not upset too much, helicopters remain safe
The main thing with the report thus far is that vibration started before the aircraft landed so the effect of GR came to the fore because something was already not right. It does seem like double jeapordy doesn't it

ATB
DaveB

[Chris Trott]

GR is certainly always a possibility, especially with the lag joints, but as long as you've got a decent dynamic balance on the rotors, then the risk is fairly minimal.

[Garry Russell]

Sounds a bit like riding a bike

The potential for falling off is constant...but usually you don't but occasionally nearly do

Of course flying is not natural and helicopters are an extreme form of natural physics busting

[DaveB]

You still have the ability to make me laugh Mr Russell.. I'm glad to say

ATB
DaveB