Sudden violent stick shake occurrence?

[Learjet]

Whilst flying on a cross-country flight yesterday a friend of mine reported experiencing a sudden and violent stick shake that was so forceful and quick that the stick literally jerked out of his hand in a fore and aft movement! It was so instantaneous that the stick actually smacked his hand again on its return path! He reported that the stick "shook so violently back and forth about ten times - I thought I'd lost a rotor blade!!!" before abating just as quickly (within seconds) and everything went back to normal again - much to his relief! There was no apparent change to the gyros attitude and this entire occurrence lasted only a matter of seconds.

Some background to the incident:

We were 3 Magni M16's flying on a cross country trip, flying straight and level along a wide valley at approx 700-800 ft agl with a light headwind (ASI 95mph / ground speed approx 80mph). Although we were flying in a mountainous area there was no turbulence to speak of and it was, in every sense, a "perfect day for flying". All 3 gyros pretty much followed the same flight path, and although the magni that experienced the shake was behind the others, he was several miles away and highly unlikely (?) to have experienced any wake turbulance from the other 2 gyros ahead. Neither of the preceeding magnis experienced any turbulance, gusts or wind drafts in the flight path.

The pilot (who is very conservative) has several hundred hours on Magni's and his pax (also a gyro pilot) over a thousand hours. The rear stick was removed, so although the pax could not see the stick shake he could feel the severe vibration). The pax did however say that he intermittently felt a "very slight vibration or resonance" (not noticed by the pilot) which apparently seemed to occur at a specific engine or rotor rpm. His speculation is that it may be attributable to the tracking being out?

None of us has ever previously heard of or experienced this type of occurence (and we're talking in the region of more than 2 thousand accumulated hours of Magni flying time here!) and after searching the forum the only similar reference I could find appears to be one in this thread
http://www.rotaryforum.com/forum/showthread.php?t=8459&highlight=violent+stick+shake
in which Doug Riley makes reference to "flutter". What is it and could it have been the cause of the sudden stick shake occurrence?

[Fl90]

I don't think it's flutter. More of an imbalance, maybe ice, something that could detach after a moment of vibration. Also, were there any birds (large, territorial) in the immediate area that may have dived into the rotor? Are there any signs of contact between the hub bar and the teeter towers?

Phil.

[Caribean_gyro]

I once lost a spinner on my rpo and it hit the blade. I shake like crazy and then back to normal. perhaps he hit something.
chuckP

[birdy]

Sounds familiar :)
Wot was the humidity like?

[Rotor-Head]

Is there any way it could have been resonance? Were you in a turn or pulling any additional G's that would have cause a rrps change at that point?

[Learjet]

Thanks for the input guys...

Fl90... definately wasn't ice. It was a warm day (this is Africa!) and the area we were flying over is semi-desert so we can rule out humidity.

An inspection of the gyro (especially the rotors and and rotor head) appeared to be normal with no signs of anything having "detached or bashed!" nor any evidence of a bird strike (my initial thinking as well).

Rotor-head - no additional G's or anything - he was flying along straight and level...

Bizarre!!

[Fl90]

Carefull with your assumption of ice formation. I live in Florida. You stated the flight was at 800' AGL. At 800' here it's also 800' ASL, and ice is possible on a warm day, with the correct conditions of air and airfoil. Remember, your tip speed is a bit higher than a FW that would be flying at that altitude.

Did you check for contact between the sides of the hub bar and the teeter tower?

Phil.

[_MOL_]

I've experienced something similar.
Our conclusion was resonance from the mast/rrpm. I test flew a gyro some years back witch had been modified by the owner, (Engine 582 turned from downside to upside right) and this made the free length of the mast longer. Making it more flexible. We assumes the rrpm and vibration levels fell right into the mast's self resonance, making it bump forward and aft in a violent matter, so hard it felt like someone took the stick and forced it fwd and aft to its stops. Impression was the rotor blades had half a meter split. (About 2 feet). Outright frightening, I tell you.
The design was changed in this case and made the mast too soft.

What I'm saying is that it can be mast resonance, but in the case with the Magni witch hasn’t been changed in its design I assume, so perhaps the mast has got a crack somewhere, or has become fatigued. Perhaps . . Just a thought, and worth checking out.

[GYRO J]

may have been wake turbulace from another gyro

[dragonflyerthom]

Yea I think I would be looking at the mast supports to make sure they haven't loosened or broken or come out. That would be very disconcerning.

[MrGrey]

accidental pre-rotator engagement?

[Ling]

Learjet,

After spending some time searching, I found this thread.

I've experienced exactly what one of you guys had on an older Magni, a VPM M16 (with a Rotax 914).

It is an extremely frightening experience, and I never want to experience it again. One feels the whole machine is doing to shake itself to pieces! I have found that since experiencing this, I am forever fearful it will do it again on my current gyro (a MT-03), although touch wood, it hasn't yet.

I spoke to Eric Torr (a well known instructor in South Africa for others on this forum), and he says this can be caused if there is any excessive wear on the teeter bolt or it's bush. He also said tightening up the sideways gimble nuts can also stop it.

On my MT-03 (which has almost 200 hours now), I have noticed, ever so slightly, that the stick shake has been getting ever so slightly more pronounced in the cruise (although she has never done that horrible violent fore and aft movement on me). When I do the 200 hr service, the teeter bolt and bush will be replaced.

I've experienced this horrible violent stick shake about 3 time thus far (all on my old VPM M16). Apparently the VPM was prone to this. Maybe the boffs out there can explain to us exactly what is causing this. To me it almost feels like mast bumping! Very, very unpleasant.

[automan1223]

On a cross country to the outer banks I was flying myself and my wife back from a brunch on Ocracoke. There is always some wind but my direction of flight coupled with the weight and bumpy crosswind made the rotor do things I had never experienced before. I would call it resonance but it was so annoying that I changed my course and took the long way around just to stay over more dry land than usual, changing direction made the gyro fly much quieter and smoother.

For your experience to be sudden unless you caught a rotor or other sheer from altitude unknown or rare, it is possible that it was just that.

Good luck.

[Doug W.]

The same thing happened to me on the way back from Mentone this year. I was on climb out from Decatur, Illinois airport when the nose dropped with a bump and raised with a bump. The stick left my hand and made an 18 inch circle.

What an attention getter! I thought I hit something.

Greg Gremminger, Bob Heimberger and I left Mentone together and stopped for fuel in Decatur. Greg split off to Perryville, MO. I followed Bob to St. Charles, MO and went on home to the Kansas City, MO area.

I called Greg G. as soon as I could to see if he had an idea about what could have happened. I could find nothing wrong after a close inspection.

Greg said he had never experienced it but had heard of it. He called it "Blade Whip". It is when the whole head is trying to teeter on the PIVOT BOLT instead of the TEETER BOLT. The rotor comes around 90 degrees later and the pivot bolt is in it's fixed position. At this point the teeter could be completely opposite of where it needs to be causing it to bump and could raise or lower the nose.

The whole head trying to rotate would explain the large stick circle and shake.

It seems to right itself within a rotation or two.

I am not sure anyone is sure what brings this on. Things must have to be just right. (Or Wrong)

It has never happened to me again. I had not heard reference to this anywhere until this thread.

[fiveboy]

I think a poll would be interesting guys (and gals).

[birdy]

Anyone considered rotor ice???

[ckurz7000]

Anyone considered rotor ice???

Given the geographical location and meteorological conditions (over desert in clear conditions in South Africa, if I remember correctly) I doubt that's the reason. Also, wouldn't rotor ice start building gradually? I expect to see a gradual increase in stick shake accompanied by a loss of performance. I agree with you, however, that once you hvae accumulated some ice, if it breaks away, you'd see a sudden and strong imbalance.

-- Chris.

[helipaddy]

Parkinsons disease can cause it too!

[Mistral]

Naughty! Naughty!
Let's name the phenomenon roto-Parkinson!

[Ling]

The one time I experienced this severe fore and aft violent stick shake was when I hadn't cleaned my rotors for about 6-8 hrs of gyro flying. Quite a few insects had built up on the leading edge, and as soon as I cleaned the rotors, and also regreased the rotor head, it was back to normal. It occurred just as I got to my cruise speed. If I slowed down, the stick shake would return to normal.

I re-iterate what I have said before, for those that haven't experienced it before, it's one of the most frightening things to experience!

[mceagle]

The incident that I have had cause to investigate was also quite violent. It happened after an increase in load after a "normal" manoeuver and was severe enough for the pilot to head for the deck. He just managed to pull out before hitting the ground - with two bent control rods.
I wonder if this phenomen could have been the cause of any "open finding" fatalities?
It would be interesting for pilots to report the brand of rotors and machine to identify any pattern.

[gyrogreg]

Phil, I have experienced this once on an M16. It was at higher airspeed. It came in quickly, and went away quickly when I gripped the stick tighter.

You should really contact the dealer in your country - SA has a lot more flying experience with Magni gyros than we have here in the U.S.

I tightened up the pitch and roll pivots - added some friction. Never happened again, but I do keep my hand on the stick at higher airspeeds to damp any tendency that might start this blade "whip". I'm not sure this was technically blade "whip", but it may have been, and I don't know what conditions or configuration might initiate "blade whip". My thoughts were, as Doug W. explained, the rotorhead pivoting in a circular motion. Once the spindle might start moving in a circular motion, all kinds of cyclic and precession things start to happen. I suggest to add some friction in the roll and pitch pivots to not let the spindle start to pivot in a circular motion - if it somehow started to. Another thing might be to hold the stick a bit tighter - at least not let it completely loose or you hand away from it if it started - at higher airspeeds.

There might be a tendency for this to start from something not quite right with the rotor alignment. I would check a few things also:

- Rotor brake not dragging - or possibly dragging. A sudden or unsymetrical torque on the rotor head could cause the spindle to start moving in a circle.

- Same for the prerotator - make sure the belt brake is adjusted to prevent the prerotator from trying to engage. If it did engage it should disengage immediately because the rotor is turning faster than the Bendix. But, make sure the Bendix releases smoothly and easy - doesn't stick up engaged possibly - in flight this could put a sudden torque on the rotor.

- Keep your blades smooth and balanced - this means minimal tracking offset. Dirty blades can start tracking off also.

- Make sure the teeter bearings are smooth. These are not just bushings - they are radial and thrust roller bearings. Do not tighten the teeter bolt at all - it should be tightened just enough to not be able to move the washers with your fingers - then no more than 1/2 flat to align the safety pin hole. (If it takes more than 1/2 flat, add or subtract a thin washer on the bolt.)

- Friction in the pitch and roll pivots. Adjust by tightening or loosening the bolts. Start with the pitch bolt. Turn the rotor sideways to put just the weight of the rotor on the head - not teetering - trim all the way forward so the spring is not pulling on the pitch. Tighten the pitch bolt just enough to hold the stick straight up - friction keeps it from falling back. Then loosen the ptich pivot bolt JUST enough for the stick to fall back slowly. (This may be a lot less than 1 flat on the nut.) Then turn the rotor fore - aft to adjust the roll pivot friction. Then adjust the roll pivot bolt to feel the same friction in roll as you felt in pitch. This may be a bit more friction than the factory puts in, and you don't want enough friction that you can feel it in flight, but I like a bit of friction in the stick to minimize any residual stick shake. I think this also prevents anything from starting the "whip" you may have experienced. If you don't want that much friction, hold the stick tighter!

(Friction in the cyclic is not generally recommended in a lot of gyros becasue that would tend to agravate any instability in that gyro. However, the Magni is super stable and friction in the stick, or holding it tightly, actually can improve stability by forcing the stable direction ptich movements into the rotor to correct for the disturbance. Don't tighten the cyclic on just any gyro!)

Talk to your Magni rep in SA, he may have more to look for or add. Please inform me of anything more you learn.

Thanks, Greg Gremminger

[birdy]

Maybe the shake described in this thread wasnt ice, coz it sounds like it wasnt violent enuff.
Wen i had ice, it wasnt just the stick that shook, the hole machine shook. the shake had that much movement that the shoulder straps of me belt bruised my shoulders, i couldnt 'catch' the stick. That alone says the mast was moveing violently, not just the stick.
All i could do was cut power, coz the only control i could grab was the throttle.

BTW, the second two times it happened was on cold CLEAR mornings.

Anatha BTW, iv never yet cleaned my blades, ever. So grotty blades, in over 4000 hours, has never had a noticable effect on performance or smoothness.


One thing to consider, if your flyn hands off, and theres no/low friction in the control systm, even with silk smooth blades, is that any abrupt disturbance that activates the offset, can start a kind of stick PIO. The stick is moved by the offset, and if theres no dampner to stop it, it will feed back, and each feed back responce will amplify the next.
Will be catastrofic if you dont stop it.

[Xavier AVERSO]

J'ai eu le même problème avec un M16 VPM, le problème venait du jeux latéral sur l'axe de battement, la solution a été réglé par une fine rondelle.
Greg a raison si vous avez ce problème il faut bloquer le manche et réduire les gas.


J' had the same problem with M16 VPM, the problem came from the plays side on l' center beat, the solution was regulated by a fine disc.
Greg rightly if you have this problem it is necessary to block the handle and to reduce the gas.