AS350 - Ground Resonance [video]

[Bushrat]

...two ways to stop ground resonance...get off ground...chop throttle...as ground resonance is a function of power... dissipates almost instantly when throttle is cut...no need for weird rotor brake action....have worked on squirrels for over 20 years..lamas as well

[Devil 49]

The theory as I was taught is that articulated systems will diverge at some frequency as the blades 'hunt'. It's usually not noticed because it's transient and low energy anywhere except at operational NR. The airframe is tuned away from most energetic and maximum exposure time, flight NR. That's why the AStar has heel springs, to change the airframe's natural frequency to keep it from being a harmonic to the NR. There are also dampers in the system (forward crosstubes on AS350) to absorb energy and keep it from magnifying the issue. Everything has to be within parameters, from the disk to the ground and the the heel springs should have the actual skid heels clear, a reason to takeoff again and perhaps move and try another SMOOTHER landing so you don't bump a blade out of phase and clear the heel allowing the heel springs to do their thing.

If you stop the process before anything is damaged, a change in the NR arrests the divergence because the excitation is not sympathetic/harmonic. That's a trap to the unwary- you can't accelerate the rotor fast enough from ground to flight idle to keep the airframe intact.

[Savoia]

As I understand it ground resonance is a temporary imbalance in the rotor system caused by external forces.

Posted before in another thread:

Ground resonance is a hazardous condition that can occur any time the rotor of a helicopter or gyroplane is turning while the aircraft is on the ground. Similar in concept to the behaviour of a washing machine when the clothes are concentrated at one point during the spin cycle, ground resonance can occur with a rotorcraft when the spacing between blades become irregular or the damping system, including drag hinge dampers, landing gear oleo struts, or wheel tire pressure, is operating out of limits. Ground resonance occurs at three rotor rpm bands, the first at about one-third of normal rotor speed, the second at a range including normal rotor speed, and the last in a range above normal rotor speed.

Ground resonance is precipitated by a shock to the aircraft arising from excessive motion of a rotor blade in its plane of rotation thereby moving the rotor center of gravity from the axis of rotation. Inadequate damping allows the rotor center of gravity to spiral away from the rotor axis of rotation, causing the rotor to generate unbalanced rotating moments beyond the compensating ability of the damping system. In addition to damping system malfunctions mentioned above, such blade movement can occur as a result of taxying over rough ground with the rotor turning at a speed within a susceptible range, or takeoff/landing in which a shock occurs to one side of the landing gear.

Under extreme conditions, the initial shock can cause violent oscillations that quickly build and result in catastrophic damage of the entire airframe. In some cases, complete destruction occurs, e.g. body panels, fuel tanks, and engines are all ripped about normal rotor speed.

Recovery is possible in some cases. If sufficient rotor RPM exists, immediate takeoff can restore rotor balance. If rotor RPM is low, complete shutdown might be sufficient.

Two bladed semi-rigid see-saw (underslung) type rotor systems are not susceptible to ground resonance because the blades do not lead and lag.

And you may also add Devil49's comments to the above.

But let's say something actually structural did depart the rotable components, a tip fairing or a blade damper or something, then I assume no amount of lifting off recovery is going to save you. Right?

Totally different scenario and almost impossible to predict. Most 'things' that depart dynamic components while in full motion have the potential to cause catastrophe but .. there are always exceptions. I had the 'beanie' hat fly off the head of Hughes 500D in flight and all I got was a mild kick through the cyclic .. but I was lucky!

[AdamFrisch]

Wasn't there a French Squirrel in the Paris-Dakar rally that managed to get down after one MR blade departed, or something? Sounds like a tall tale, but I'm sure I heard it somewhere.

[TRC]

Wasn't there a French Squirrel in the Paris-Dakar rally that managed to get down after one MR blade departed, or something? Sounds like a tall tale, but I'm sure I heard it somewhere.

I very much doubt it...

[birrddog]

There was an EC 130 that had an accident in NY due to partial blade separation. Pilot was Tammy Fein. Landed safely on floats in the Hudson River.

On July 7, 2007, about 1651 EDT, a Eurocopter EC-130-B4 helicopter, N453AE, operated by Liberty Helicopters, Inc., experienced an in-flight separation of a section of one of the main rotor blades during flight and sustained substantial damage during an emergency descent and subsequent autorotation into the Hudson River in New York. The commercial pilot and seven passengers were uninjured. No flight plan was filed with the Federal Aviation Administration (FAA) for the 14 Code of Federal Regulations Part 135 and 136 sightseeing flight, nor was one required. Visual meteorological conditions prevailed at the time of the accident.

[Senior Pilot]

There was a similar incident to an AS350 in Florida, 2006, discussed in Ground Resonance fails AS350 tail boom.

[500e]

Tear along the dotted line comes to mind,

[ReverseFlight]

Speaking from personal experience. I once landed AS350 carefully on the flat strip at the rear of the starboard skids, then applied a little too much downward pressure on the collective and ground resonance kicked in immediately - it happens too easily. I lifted off at once and repeated the landing, this time with micro-millimeters of pressure easing on the collective for an uneventful setdown. Never to be repeated again.

[nigelh]

devil49. I think you are wrong . I don't believe you have ever seen anyone try to "land through " this and I don't believe it has ever occurred in flight .
I have been around 350,s and flown them for circa 30 yrs and have never come across this happening in flight . I believe it can ONLY occur when in contact with the ground ...sorry !!!

[fly911]

What would you expect to find in the report that would exonerate the pilot? Lack of training I suppose.

That incident went south pretty quickly. My observation only: Right after the start of the video, the pilot had some pitch in, as the squirrel was rotating (hopping) to the left. That would not have happened in flat pitch IMO. The pilot may have been too afraid to bring that heaving mass into the air and figured "almost into the air would do almost as good". Another possibility is that the pilot was too afraid to take his hand off of the collective long enough to reach for the throttle quadrant. Very scary for the pilot. Not a time for indecision.

[ShyTorque]

The sound track on the video gave me the impression that the Nr was increasing but not up to flying speed as the resonance began. It continued after it began decreasing again after the throttle was retarded. I don't therefore think he had a chance to lift off.

(One thing that I'm absolutely sure about is that ground resonance can't occur in the air, btw. The answer's in the name).

[Savoia]

As one suspected (#7) - induced by the landing.

[SASless]

What caught my eye is the way the cabin roof separated....then the tail boom. The 350/355 roof has always been a weak link....and if the tail boom will fail as it did.....well....not very impressed! They are plastic helicopters.

[RVDT]

They are plastic helicopters

Correct - just a plastic Alouette and approaching 40 years old! It meets the standards in force in 1974 so.......................

To get resonance in a 350 something is busted or worn out. They do not just decide to do it just for something to do.............

The more we learn, the more we forget..............

[Gomer Pylot]

Aerospatiale (now Eurocopter) claims (or did) that the AS350 cannot experience ground resonance. It can, however, experience dynamic divergence. A fart by any other name....

I've had it start many times, and lifting to a hover stops it immediately, assuming the throttle is full open and Nr is sufficient. I once had one go into dynamic divergence after landing. Everything was fine until I reduced the throttle to idle, and then it went bonkers. The bystanders (pilots and mechanics) who didn't run and hide said the skids were coming almost a foot off the ground, and the aircraft turned about 45 degrees on the pad. When it started, I had the throttle at idle, and I wasn't about to try to bring the rpm back up, so I put the throttle in cutoff and pulled the rotor brake full on. It only took a few seconds to stop the movement, and there was no further damage. The inspection revealed only a slight amount of wear in the spherical bearings, nothing else. The springs on the heels of the AS350 can be a real problem, especially on wavy concrete. I don't know if that was the problem, but I've had dynamic divergence start many times on concrete pads in the marsh. I always waited a few seconds after landing before retarding the throttle, and that almost always was sufficient to prevent bouncing, but not that time.

[paco]

It would appear that those springy leaf things need to be very precisely fitted for them to work properly.

Phil

[Savoia]

Gomer, thank you for this input verifying the correct action when not using the lift-off option.

I always waited a few seconds after landing before retarding the throttle, ..

This is exactly the correct procedure in the Ecureuil in order to be prepared for the potential onset of GR - especially on level and hard surfaces where forces are communicated more effectively from the ground to the disk.

It would appear that those springy leaf things need to be very precisely fitted for them to work properly.

Ciao Phil! Definately and a lot of the time they become 'flat' and need to be re-aligned in order to restore their 'springiness'!

ps: I was going to drop you a PM after Christmas - I was exposed to non-stop Abba and Boney-M during the festive season the former bringing back memories of John Broome. Boy he was keen on that group. Did you also have to suffer the tape deck in the Towers LongRanger playing 'Dancing Queen' hour after hour, lol! Them' were the days!

[RVDT]

those springy leaf things

become worn in thickness as well and need to be replaced regularly after landings on abrasive surfaces.

Watched someone fit "one" new item opposite a worn one due to supply issues from EC on a 355. Bad move!

The other issue that people miss is a worn/loose DU sleeve on the end of the star although there shouldn't be many of those left by now as the new star has "carbide" bushes which are an "improvement" or "not" depending on your point of view.

[unstable load]

I know a guy that had a very similar incident in a Twin Squirrel, on landing the machine went berserk and started thrashing around. Fortunately it was on a large gravel parking lot and it slid around the show while he was chasing the throttles to close them off.
By the time he got them closed the machine had moved about 15 meters on the pad in eccentric circles.

It turned out the starflex had delaminated and threw the blades out of sync once he landed.
A LOT of bits had to be replaced on that ship.