Ground resonance
 

A novel take on ground resonance

Chopping the throttle and applying the rotor brake definitely won't work in that scenario:)

it never is (for ground resonance).

What scenario? There's no link...

I beg to differ if it is on start up or shut down. It saved me in a Wessex that had unequal tyre pressures and started to leap from wheel to wheel on rotor start.

It’s in the twitter link:

https://mobile.twitter.com/bennetdg5454/status/1568792611196108803/video/1

(Unfortunately, PPruNe seems to want to extract the url to a fuller link for some reason. However, this might work now. Click here).

I'll probably catch a lot of flack for this, but I actually feared Ground Resonance in the few fully-articulated choppers I've flown more than Mast Bumping in the 22.

I'm guessing its because I know how to avoid Mast Bumping and exactly what to do if I ever encounter low-g. Where as Ground Resonance seems to come out of nowhere (so I don't know how to avoid it) and I'm not 100% sure of how to get out of it?
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Robbiee - you are far less likely to encounter it in a skidded helicopter unless you have a gross imbalance on the rotor.

Designers avoid matching the frequencies of the rotors with the resonant frequencies of the airframe.

Helicopters with wheels can be more prone to it as there are more components (tyres, oleos etc) that can wear differently and change their resonant frequency.

More blades also introduce more areas where imbalances can occur.

It's less of a problem with teetering heads because they don't have drag dampers - they are actually the most suceptible to GR, but articulated heads have dampers. And contrary to popular belief, you can get it in an AS 350 - a friend of mine got into it on the factory course in the USA of all places. And, as crab says, skids are better than wheels....

Good point re drag dampers Paco - a friend of mine on an early morning walkround didn't spot that the groundcrew had wrapped the red and white flag attached to one of the drag dampers on a Wessex around the drag damper so it was very difficult to spot - it padded a little on rotor start (not unusual) but when he lifted to the hover it was very obvious there was something badly wrong. He ended up having to do a running landing and chop the throttles while rolling since it just bounced way too much on a vertical landing.

One of our QHIs was a 'victim' in a Sycamore on his conversion course at TH. He said the sequence was so fast , that one minute he was sitting, happily watching the rotor revs, and the next reaching for the fire button which wasn't there any more - just him, strapped to the remains !

A Sycamore got in to ground resonance during a mid 1950's SBAC Show at Farnborough.
Bouncing from side to side it then rolled right over on to its back.
Nobody hurt, so Industry giggles all round except in the Bristol Chalet!.

I don't know whose belief that was but a 350/355 can definitely get into ground resonance. There were a number of aircraft years ago that led to a series of close calls and several complete destructions. Can also attest, as mentioned above, that cutting throttles (change RPM) and hitting the rotorbrake will get you out of it but barely. We had no choice as the aircraft was also tied down.

The culprits back then were the forward crosstube shock absorbers and aft skid tube springs being out of spec. Throw in a weak/out of phase frequency adapter and off to the races. Aerospatiale ate a lot of shocks and springs after that episode. Still amazes me how fast it happens and where it can end up. The Lama GR was docile in comparison in my opinion. We had one 355 end up on the pad with the canopy gone, tailboom on the ground, and sheet everywhere in what seemed like mere seconds. Luckily for the pilot the transmission/head assy failed aft and landed in between the engines and he was still in his seat strapped in. Took a few minutes to pry his grip off the cyclic. He wasn't a big fan of the flying tupperware after that.

AS350 ground resonance in action...

I had a touch of likely GR in a well maintained H300. Landed in front of a fuel bowser on a very hot day on a newly laid bitumen pad. As I were running down the skids settled into the soft bitumen and became a fixed point for vibration to work against. Took some getting out.

Wrench1 - that's Airbus's belief! Apparently those springy tabs on the rear of the skids must be very precisely positioned.

Ha. How typical. However, when the technical guys showed up back then they weren’t as “positive.” As we both know French helicopters never fail, they sure were in a hurry to change the inspection criteria on the springs and shocks from on-condition to a specific interval of checks. As I recall none of our 350/355 parts passed any check at all except for several new E models we had.
FYI: it wasn’t the springs that had a specific position but the shocks on the forward x-tube that did. However, it wasn’t the position check the shocks failed instead it was the extension timing check under load that failed. Should have been 10 seconds or so but most measured at zero seconds. The springs failed by flattening over time instead of having a permanent deflection and were worn thin.

AS 350/355 GR Issues

X-Tube shocks - as Wrench1 says - fails the extension test
Heel springs - they get worn and god help you if you only replace one at a time!
Starflex DU Bushes - At the end of the Star where it goes in the Spherical Bearing - would wear depending on the type of dust you were exposed to and how abrasive it was. The "fix" was the introduction of "carbide" bushes which is the current config although it is possible for the carbide sleeve to move as well. I know of a 355N that had this issue while offshore. Luckily not far from the beach so continued for about 5 minutes and couldn't read the instruments by the time he landed!

https://cimg9.ibsrv.net/gimg/pprune....6a91ffea39.jpg
Sleeve inside item 5


https://cimg6.ibsrv.net/gimg/pprune....a251c44a1f.jpg

I flew E model Serial No.1 once - the original 2 engined "B" model! Fuel or pax your call.

Years ago I got into GR in a B3 after a somewhat rough touchdown from an auto during recurrent training. Luckily they spool back up to 100% so quickly that it was basically a non event to get it off the ground. I would think this would be faster than shutting down and stopping the rotor in almost all cases.

Agreed if you encounter it on landing and if you are quick during the early stages of shutdown but on start-up (where most cases I know about have occurred) chop the throttle and hit the rotor brake.:ok:

ATSB report of a ground resonance accident on an A109E conducting pilot transfer from a ship in Australia in February this year:

https://www.atsb.gov.au/publications...rt/ao-2025-009

Unrelated, but I witnessed ground resonance on an AS350B years back. It developed rapidly after the helicopter touched down. But the pilot reacted swiftly and took off again. He would have been on guard during his next landing attempt which went without incident.

I was only involved with two ground resonance incidents both with skidded aircraft.

The first was a Lama in Scotland which had landed for a hot refuel. As I stepped over the skids with the refueling hose and was removing the fuel cap the pilot took off.
I threw the hose clear and walked out round the back of the skids. I stopped to give the pilot a hard stare. On shut down he explained that ground resonance had set in. Probable cause was the
landing site which was a loose gravel track. That could have been worse if the hose had get caught up.

The second was a skidded 365C. Pilot had three goes before getting it safely on the ground. Much work was done but the aircraft continued to want to go into resonance.
Among the work was new set of undercarriage dampers. Eventually a third set of dampers fixed the problem. Apparently the overhaul shop were setting them up incorrectly.

I instructed on the RAF Puma HC1 OCU for a couple of tours. We used to get a fair number of ex Wessex pilots coming through. The first start up they experienced, the standard “Puma lateral dance” always caused them great concern. The Puma’s forward slanting main rotor mast, and very basic oil filled main rotor dampers allowed the main blades to swivel forwards, so they were no longer at 90° to each other. The mass imbalance caused the whole aircraft to rock left/right until the spin drier effect sorted it all out. Very worrying for pilots with time on the ground resonance prone Wessex.

A certain 72 sqn pilot at Odiham decided to authorised a Wessex sortie with a recently arrived, fresh out of the OCU young pilot. During start up they ended up with the aircraft lying on its side in dispersal after ground resonance set in. During the unit inquiry it was discovered that the flight had been authorised as “Pre Northern Ireland Shake Down” training. A legend was born, plus a new song written for the SH Song Book!

”There’s a Wessex on the ground (on the ground)
With it’s rotors going round (going round)
And it jumps up and down and shakes itself to bits
And gives its passengers the $hits, $hits, $hits!”

:p