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Gear shimmy video

13brv3

Well Known Member
Greetings,

I purchased an RV-3B a few months ago, and I'm finally getting around to looking at the gear leg shimmy. I mounted a GoPro on one of the tie down attachments just to see what this actually looks like, then intentionally provoked it. Pretty interesting if you've never seen it.

https://youtu.be/GaKSJmu1zgw

BTW, this is a good angle to critique landings, so no videos of that :)

FWIW, I'm familiar with the shimmy causes and fixes, so this is mainly being posted for your entertainment. Don't say I never gave you anything :)

Cheers,
Rusty
 
Fun times. Good example. Now, let's see those landings ;)
(You can always just share your best ones)
 
Haw many takes?

Must have been more than one?

And if anyone can point me to the gear fix thread I would appreciate it. I too bought a RV3 with enough shimmy for the rocketts!
 
And if anyone can point me to the gear fix thread I would appreciate it. I too bought a RV3 with enough shimmy for the rocketts!

There's no single thread, but a search will find a bunch of different threads.

Bottom line is that the shimmy happens at the resonant frequency for your gear legs. The primary factors for the frequency are the weight of the wheel/brake/pants, and the stiffness of the gear leg. For the most part, the weight of the wheel assembly can't be changed too much, so the main thing we can change is the stiffness of the gear legs. If you do some searches for gear leg stiffeners, or dampers, you'll find more info, but most people end up with a wood strip wrapped in epoxy to stiffen the gear leg. The idea is to raise the resonant frequency to a point higher than normal taxi speeds.

Another "fix" you see a lot is the recommendation to lower the tire pressure. I'm thinking that also changes the frequency by effectively making the tire smaller, but it may also add some dampening by raising the rolling resistance.

The other recommendations are usually along the lines of making sure your tires are as close to perfectly round as possible, and balanced. These go along the lines of trying to prevent the vibration that will get amplified by the resonance. Tuning forks are quiet if you don't hit them :)

This plane has some form of gear leg stiffeners, but I haven't taken the aluminum gear leg fairings off to see how they're constructed. Adding the stiffeners totally resolved the shimmy on my previous RV-3B, so I'm hoping I can rework them and have the same success. I'll also be getting rid of those aluminum fairings and replacing them with fiberglass. They guy who built this plane had a thing for polished aluminum.

Cheers,
Rusty
 
Beautiful landing Rusty, thanks for posting those videos. Let me know if you plan to sell those aluminum fairings, I am actually looking for some.

Cheers, Gary
 
Beautiful landing Rusty, thanks for posting those videos. Let me know if you plan to sell those aluminum fairings, I am actually looking for some.

Cheers, Gary

Thanks Gary. I'll let you know about the fairings. I guess they would probably work on a 4 if it's the short gear version.

Rusty
 
Greetings,

I purchased an RV-3B a few months ago, and I'm finally getting around to looking at the gear leg shimmy. I mounted a GoPro on one of the tie down attachments just to see what this actually looks like, then intentionally provoked it. Pretty interesting if you've never seen it.

https://youtu.be/GaKSJmu1zgw

BTW, this is a good angle to critique landings, so no videos of that :)

FWIW, I'm familiar with the shimmy causes and fixes, so this is mainly being posted for your entertainment. Don't say I never gave you anything :)

Cheers,
Rusty
Rusty,

What causes the shimmy and what is the fix?

Thanks
Angelo
 
Shimmy is caused by a combination of things being present at the same time. Excitation force repeating at the same frequency as the resonant frequency of the overall structure (spring gear with a weight (tire) on the end of it. Minimal dampening (absorption) of the vibrational energy. Removing/change anyone of them will "fix" (reduce the tendency of) the shimmy. On our airplanes, the easiest thing to change is the tire pressure. Maintaining pressure between 25-30psi did the trick for me. Lower pressure would work too but that may cause other problems.

My tire gauge was way out of calibration resulting in tire pressures way higher than I was reading. Higher pressure caused (amount other things) less drag, less contact with the runway surface. Drag aids to dampen shimmy.

At least this is how I understand it.

Bevan.
 
Shimmy

I currently have a -3 that I built. I also had one that I bought. Both had major shimmy. Both were "cured" by installing Desser Monster Retreads maintaining tire pressures of 20-25 psi.

Bill Newkirk
 
I currently have a -3 that I built. I also had one that I bought. Both had major shimmy. Both were "cured" by installing Desser Monster Retreads maintaining tire pressures of 20-25 psi.

Bill Newkirk

It would be interesting to know how the weight of those tires compared to what you were using. Changing the weight should change the frequency. Also I'd wonder if retreads are different enough in diameter to make a difference? The tires have plenty of wear on them, and I'd be up for changing them if that's all it takes.

Rusty
 
Shimmy

The Monster retreads are a little heavier but their main feature is a really stiff sidewall. My current -3 shook so violently on landing that the engine would stop because of fuel starvation, even after adding gear stiffeners.

Bill
 
Wood strip/Fiberglass leading edge

Rusty,
By your video it looks like you have the stock aluminum fairing. Do you have the wood leading edge wrapped in fiberglass installed?
I have an RV-4 and opted to not use the lighter AL gear leg fairings supplied with the kit but went with a wood/fiberglass leading edge (as recommended in the plans, wrapped with 2" fiberglass cloth) and then foam behind the gear leg and then all wrapped with 3 layers of fiberglass ( I think 6oz....its been a while) cloth, so ending up with 6 layers of fiberglass on the leading edge.
I have never had any issues with shimmy or cracks in the legs and I have had many rough strip and/or rough landings.

Marc
 
Marc,

As I mentioned earlier in the thread, the gear legs have some sort of wood stiffeners, but until I take the fairings off, I won't be able to see how they were constructed. Clearly they're not working, so I'll probably be reworking them, since the stiffeners fixed the problem with my previous RV-3B.

Cheers,
Rusty
 
Anyone with an RV3,4,6,7,9 tail dragger who has experienced wheel shimmy more than a couple of times need to carefully check the lower engine mount to airframe attach area. I guarantee you that these 3/8 bolts will be loose. I have seen them up to two full turns loose. The working back and forth at this attach area caused by the shimmy puts tremendous strain on these corner fittings and the bolt. Good luck.
 
Greetings,

For further entertainment, I decided that I'll try to keep this thread updated as my testing progresses. It may take a while since the hanger is a looooooong way away, and it's winter.

The last time I was at the airport, I removed the fairings to see what sort of stiffeners exist. They're not exactly as specified, but at least they were in the ballpark. The most obvious problem is that one of them had come loose from the gear leg, and since the leg is tapered, it slides down, and becomes even more loose. I'll give you one guess which one that was :p

I cut the loose stiffener off, and besides coming loose, it was also coming apart, and the ends were somewhat water rotted. The other one appears to be pretty solid, though there is some hollow sound as I tap on the upper end. Since I could never stand to have the two not match, and since I figure that one has some water rot as well, I'm going to remove it on my next trip, and plan to replace them both.

I also brought the fairly well worn Condor tires home, and have a motorcycle wheel balancer on order. If the tires appear round, I will balance them as best I can, and then reinstall them. If they are not round, I'll order new tires. The Desser monster retreads seem to be well recommended.

The first step in my experiment will be to install known round and balanced wheels, but no stiffeners, fairings, or wheel pants for the next flight. I'll try to determine if there is shimmy, and if it's at a higher or lower speed. Sadly, I didn't note the GPS ground speed when it happened before, but I have a pretty good feel for it.

Following that test, I'll install the wheel pants for another test, then stiffeners, and finally fairings. I really don't think the gear leg fairings will have much effect, so I'll probably try to get it shimmy free before installing those.

Cheers,
Rusty
 
"The wood is no longer needed"

The Glasair folks ran into the same problem, but their thinking is not along the same lines as ours. They actually came up with a fix - using composites of course.

I have flown versions of the F1 and HR2 before and after this setup. HUGE difference.:eek:

If someone would show me how, I can add the PDF into this thread - or send it along for another smart guy to add it? Either way you all get the benefit.
 
Thanks Mark

Mark,
Would really appreciate seeing the .pdf.
I've thought about other options besides pine / oak trim molding. One is milling alum blocks the shape of the airfoil and bolting on the gear somehow. Another idea is milling some polyurethane or some other dense non-biodegradeable material that would offer some form of damping properties.
Craig


The Glasair folks ran into the same problem, but their thinking is not along the same lines as ours. They actually came up with a fix - using composites of course.

I have flown versions of the F1 and HR2 before and after this setup. HUGE difference.:eek:

If someone would show me how, I can add the PDF into this thread - or send it along for another smart guy to add it? Either way you all get the benefit.
 
My plane (RV7) had no gear leg stiffeners. The gear would only occasionally shimmy.
The main reason for installing "Woodman's" stiffeners was to eliminate the little skipping and bouncing on landing;perfect landings were elusive. It is an entirely better landing plane with the stiffeners! PS : it never shimmy's now.
 
The Glasair folks ran into the same problem, but their thinking is not along the same lines as ours. They actually came up with a fix - using composites of course.

I have flown versions of the F1 and HR2 before and after this setup. HUGE difference.:eek:

If someone would show me how, I can add the PDF into this thread - or send it along for another smart guy to add it? Either way you all get the benefit.

I'm not sure I can upload it, but I'll try. Either way, I'd love to have a copy if you don't mind emailing it to me.

BTW, one of my tires was out of round, and about 2.5 oz out of balance. It wasn't the one that was doing the worst shimmy though. The other was better, but Desser Monster retreads inbound.

Thanks,
Rusty
[email protected]
 
Provided by F1Boss

Gear%20Leg%20Stiffiner-X2.jpg
 
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Thanks for posting that. I wonder how many people have used that method? The shimmy seems somewhat different for everyone, and you can find people who say the wood dampers worked perfectly, and others who think they didn't do much good. I guess I'd want to see a fairly high rate of success before I wrap that much fiberglass on anything.

Rusty (I hate fiberglass)
 
Too Much

I like the fiberglass solution and not sure why I had never considered it.
But, the 6 layers on a round rod is 12 layers of cloth. Pretty sure 12 layers will not fit between the spring steel rod and the VANS gear leg.
Now, from what I understand from the internet, the Rocket leg fairings are larger inside so it may accomidate 12 layers.

FYI......I reinforced a set of Vans leg fairings with 2 layers of glass tape internally on each side (4 total) and installed the pine wood stiffeners per the instructions. The fairing would not fit over the legs. I had to remove the stifferners, because removing the 4 plys inside the fairings was impossible. That is why I am so interested in this thread and really liked the idea of glassing the leg.
 
I read somewhere on VAF that adding balance weight to the wheel fairing nose helped a lot. Don't remember where I saw that, though.

Dave
RV-3B, skinning fuselage
 
For anyone playing along, I ended up replacing the tires and tubes. I got a cheap balancer, and found that the left tire was significantly out of round, and would have required 2.5 oz of weight to balance. The right tire was fairly round, but still would have needed about 1.25 oz of weight.

The new 6 ply rated Desser Monster retreads with Airstop tubes are nice and round, and spot on balance. I literally couldn't identify a heavy spot on either tire/wheel combo. I had to open up the wheel pants a bit more to fit the monster tread, but it's back together now.

I decided to remove the left stiffener too, even though it wasn't loose like the right one was. I figure I'll have them both be the same when I install new stiffeners. For today's test, I had the wheel pants on, but no gear leg fairings or stiffeners. I'm going to leave the tires at 30 psi since the goal is to fix the problem with the stiffeners.

I didn't note the groundspeed for the original test in the video, but it was way faster than I hit shimmy today. Today, I could feel them starting around 7-8 kts, and by 10-12kts they were both going wild. The good news is that this is exactly what I would have expected, so it seems predictable. The tires are heavier, so I effectively added weight to the end of the "spring", and also made the spring less stiff. Both of those changes would reduce the resonant frequency, and that's just what happened.

Next step will be to install stiffeners as close to Van's drawings as possible. They allow for mounting these in front or rear of the gear leg, and I'm going to mount them on the front side since that's less in the way of my brake lines. With cold weather and a distant hangar, it might be a while before that happens.

Rusty
 
When it's time to mount the stiffeners, be sure to TAPE them in place and make sure the fairings fit before permanent attachment.
 
Has anyone had success attaching a stiffener without the fiberglass? I've seen pictures of people using Ty-wraps, though there wasn't a report of how well it worked. I also know there's some really heavy duty reinforced packing tape that would probably work a lot better. You would probably have to replace it periodically, but it might be at least a temporary option.

The reason for asking is the sudden realization that it's too cold to apply the fiberglass. It's not impossible, but it would take a lot of extra effort to try to heat the epoxy and work surface. It would also be really interesting to be able to easily test different types of stiffeners, though I realize the fiberglass cloth does become part of the stiffener.

I need to make a 3 hour round trip to the cold (by my standards) hangar today, and since I need to remove the wheel pants again to increase the opening a bit more, I might do a taxi test without them. I'd expect that to increase the taxi speed where the shimmy occurs, and if it helps, I can easily see putting this whole stiffener project off until Spring when hopefully I'll have a hangar closer to home.

Cheers,
Rusty (I hate cold, and fiberglass)
 
Stiffener report

My 7 had, regardless what I did a shake right at taxi speed so I finally decided to install stiffeners. I read where tape worked well so I tried it and it did improve the shaking but not to my satisfaction. I just this week removed the tape and did the fiberglass attachment, It worked very very well, NO shaking and what a pleasure to taxi now. I wish I had done it a long time ago.
As someone earlier said tape stiffeners on first to get proper alignment with current fairings. Lots of work to glass them on but IMO well worth the effort.
 
Thanks for the comments Larry. When you added the stiffeners with tape, did the speed of the vibration increase? If so, I'm assuming the speed increased even more with the fiberglass, probably to a point faster than you normally maintain a steady ground speed.

Rusty (snowing...)
 
Alternative

For many years the standard attach method for Wittman Tailwind was the steel strapping bands used in packaging. Maybe four straps in the length of the gear leg. You would need to find someone to loan you a tool as I believe they are quite expensive.
Another separate issue I have not seen addressed is the fit of the gear in the lower end of the socket. It has become common on the Tailwinds to weld a bushing across the socket and cut a slot thru the center of the bushing and the end of the socket. AN5 bolt in the bushing, tighten to remove any play between the gear and socket.
 
Success ridding RV-3B of shimmy

I just had great success eliminating my main gear shimmy in my RV-3B. My shimmy issue was primarily on taxi and exacerbated by concrete taxiways with expansion joints.

So after reading through this thread and others on the subject I ordered two Goodyear Flight Custom III 6-ply 5.00-5 tires and Michelin AirStop tubes. I went for the 6-ply for the stiffer sidewall to minimize flexing of the tire (i.e. shifting the natural frequency and reducing elastic perturbation). Load capacity rating was of no consideration. I went with the Michelin tubes after multiple failures of "leak guard" brand in various aircraft.

I filled the new tires to 45psi. Now things are smooth and nice on taxi like they should have always been. I am very satisfied.

My gear legs don't have the stiffeners (although I have a set of wooden ones in a box I had procured). Also, my wheel pants now ride higher allowing more clearance over hangar door angles. The ground stance looks better too.

So now the one and only thing that bothered me about my RV-3B has gone away. For now anyway. I fly it much more often which is super great.

Jim
 
That's great new Jim. My plane might as well be in storage since the combination of work and weather have kept me away from the airport. I did manage one flight with a new set of dampers clamped to the gear legs, and there was no shimmy at all. I've since rotated the stiffeners to the front of the leg instead of the back, and await confirmation that it still works. Way too cold for epoxy, so it will stay temporarily clamped until later.

BTW, I never tried higher than 30 psi. Maybe I'll try that without the stiffeners before committing to epoxy.

Rusty
 
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