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Ground resonance AS-350


RagMan
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A little bondo will fix that right up!

 

That's going in a playlist I've made specifically for helicopter vids to support my ground lessons.

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If you have flight power get it off the ground fast, (recon he had 6\8 seconds then it was self destrcting) then put it down real soft preferably on a flat surface, if not kill the motor & hang on.

The 350 cab seems to be a weak point tear on dotted line.

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According to the manufacturer, it's not possible to get into ground resonance in an AS350. It is possible, however, to encounter 'dynamic divergence'. I've had a couple diverge very dynamically. If it starts to shake while you still have full rpm, just pick it up to a hover. If you've already gone to idle, however, about all you can do is ride it out. I had that happen once, and I just cut the throttle completely off, pulled the rotor brake, and hoped. It turned about 45 degrees on the pad, and spectators who had the courage to keep looking instead of jumping behind something solid said the skids were coming a foot off the ground. No damage though, and the inspection found only slight wear in the spherical bearings. The springs on the heels are a real problem, especially if the pad is just a little wavy instead of perfectly level. I always waited until it was smooth after landing, but that one time there was no indication of anything until I retarded the throttle, and there was no way on god's green earth I was going to take it back up.

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It looked to me like the resonance began upon shutdown. In that instance by the time you tried to restart, get to flight idle, and lift it would probably be far too late. It looks to me like the only option they had was to ride it out.

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As a semi-ridged guy with no real training in ground resonance, I must ask; What could he have done to stop it (since he was already on the ground)?

:o

 

You reverse whatever it is you're doing that starts the "dynamic divergent excitation" (ground resonance). Pardon my french, but the difference between the two isn't significant to a mere pilot.

 

If you're accelerating the rotor, you take it back to idle. If you're landing or just landed and still have flying RPM, you take off again, and you try to do either as soon as possible. If every powered landing starts the process again, you do a hovering auto, landing with less than cruise NR means it's not harmonic anymore.

The school answer is that you have 3 turns of the rotor after full development (at 390 RPM that's about a half second after the first "bump") before it's irreversible, potentially destroying the airframe. But that's at full chat, with all the rotor inertia and engine power trying to break stuff...

When it's developing, it might rock/pitch but it's soon violent enough that you know something bad is happening, you better be correcting it now.

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The springs on the heels are a real problem, especially if the pad is just a little wavy instead of perfectly level.

 

I was always under the impression that the springs were to counteract the problem.

 

It's happened me a few times, and I've seen it happen to coworkers during landing that those skids start doing their little tap dance, but it was always pretty easy to just raise collective and bring it back into the hover. I've never had it happen while on the ground in the flight gate, where taking off would be the correct action. However, if you listen closely to the video it sounded like he was just bringing it up into the flight gate during the onset, and as he started to retard the fuel control(which would be the correct action in that situation) the resonance got worse. Maybe if he had pulled the roter brake sooner and forced all 3 blades into their lead position it might have ended a little better. But as the motion got more violent, it was probably quite difficult to reach the brake in time.

 

On another note, I've heard many times how weak that cantilevered cockpit structure is, but it didn't sink in until I saw this video!

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I have encountered the start of ground resonance approximately 5 times while landing the Astar and resolved it every time by promptly putting the collective full down. On one occasion it occurred while running on to a washboard runway while doing a hydraulics off landing. That one got exceptionally bad yet it immediately stopped once the collective was full down. I know this goes against the grain of what we have all been taught but it works. If it ever fails me I would immediately pick the aircraft up.

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Yes, the springs were added in an attempt to tame the dynamic divergence. But like a number of things on the AS350, the result was less than perfect. When everything worked as it should, I enjoyed flying the AStar. It was smooth, comfortable, had a heater (!), and you had lots of fuel and range. I've made vertical takeoffs at max gross weight (at least that much, if the roughnecks didn't lie at all about their weights) and then increased collective to get to cruise power. But when something went wrong, you had your hands full. It autorotates like a rock, and it's so cheaply built that things do go wrong. Everyone knew they wanted to start shaking on touchdown, and acted accordingly. There was a small platform on the airport at New Iberia which everyone used for practice offshore landings during training and checkrides. If you were flying an AStar, you never touched down on it, you just came to a hover and then departed. Every AStar tried to go into resonance on it, every time. I don't recall many tears from anyone when they were phased out of the oil patch.

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There was a small platform on the airport at New Iberia which everyone used for practice offshore landings during training and checkrides. If you were flying an AStar, you never touched down on it, you just came to a hover and then departed. Every AStar tried to go into resonance on it, every time. I don't recall many tears from anyone when they were phased out of the oil patch.

 

I'll be landing on that very platform in a few weeks at recurrent, in the 206 & 407, not the Astar though.

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Worn out, damaged or unloaded steel springs can lead to divergence. Unloaded means to set the machine down where the tabs are not in contact with the surface or on a curved surface (as possibly in this case). The “E” dimension is >85mm or 3.35in and as a preflight item is to see light under the rear portion of the skid tube.

 

 

Edited by Spike
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More prone to skid equipped helos.

 

Ahh yea, had it backwards. Thanks!

 

 

 

You were right the first time, per the book. It’s more prone to wheeled helicopters.

 

Your PTS reference on Ground Resonance is the Rotorcraft Flying Handbook (FAA-H-8083-21; page 11-7).

 

The swirling (unbalanced) forces are being applied to the top of the mast. The center of mass of the hull is well below the rotor head, and the result is a combination of pitching and lateral rocking (some call it "padding").

 

The inertia of a helicopter about the pitching axis is higher due to the amount of mass lengthwise; however, there’s less inertia about the roll axis, so lateral movement will dominate. The hull not being infinitely stiff and supported on a sprung undercarriage (Wheels and struts), will have a natural resonant frequency of rocking. Skids add stiffness and increased surface resistance across a wider area.

 

This was an issue on older wheeled helicopters (S-55 etc.). This wheels more prone to ground resonance issue for the most part, has been eliminated with current wheel and strut designs.

 

The first mathematical analysis of ground resonance was due to Robert Coleman and Arnold Feingold who were working at NACA (the forerunner of NASA). It was concluded early that ground resonance is purely a mechanical phenomenon and that aerodynamic forces are not significant.

 

G. TASK: GROUND RESONANCE

REFERENCE (S): FAA-H-8083-21; POH/RFM.

 

Objective. To determine that the applicant:

 

1. Exhibits knowledge of the elements related to a fully articulated rotor system and the aerodynamics of ground resonance.

 

2. Understands the conditions that contribute to ground resonance.

 

3. Explains preventive flight technique during takeoffs and landings.

 

Pagesfromfaa-h-8083-21.jpg

Edited by iChris
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