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Gear Shimmy

calpilot

Well Known Member
My RV-7 has had violent gear shimmy since first flight, occurred around 23 mph. I tried different tire pressure, different profile tires, wood slats, wrapping the main gear with carbon fiber, nothing worked. I took the gear to the manufacture, he said there should be about 1/2 degree to 1 degree of toe in. My gear has about 1/8 degree toe out on the left, and flush on the right. I just spent $600 on a new set of gear from Vans. Both had about 1/2 degree of toe in. The shimmy was GREATLY reduced. I added a sandwich strip of 10 layers of fiberglass sandwiched by carbon fiber, as posted by one of our other members (I apologize for not remembering the members name) attached to the main gear with stainless ADEL clamps. Not only is there no gear shimmy, but the landing loads are absorbed by the flex of the composite "yardstick" clamped to the trailing edge of the gear. After over 30 years, finally an idea that resolved this chronic issue! I have my RV grin back!

Gary Brown
DAR - Oregon
 

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Still trying to understand shimmy

My 9A has a certain shimmy on the mains above 20kts, which continues to just below 15kts as I decelerate during taxi to the run up pad. The taxiway I operate on has some undulations that probably excite the behavior, as I don’t see the shimmy that I recall at other airports. No shimmy has been detected by the butt-o-meter on the runway, takeoff or landing.

I thought I understood the shimmy mitigation, but maybe not. The wood molding I would expect to create damping. Is the FG/CF layup doing the same through the adel clamps, or is it stiff enough to be pushing up the natural frequency of the gear leg to stay out of the zone?

I love the adel clamp idea, I just don’t think it fits under my gear leg fairings.
 
shimmy

it does fit under the fairings! It absorbs lateral landing loads due to its ability to flex laterally, but dampens the fore and aft motion that is the shimmy.

g.
 
Thanks cal pilot, that made my day seeing my idea helping someone with the same problem I had. I am now about 30 hours and at least as many landings and still no shimmy to date since adding the strips. I do also think that rocket bobs idea of an aluminum bar would work equally as well in dampening the fore and aft loads and still let the gear absorb up and down as designed.
 
Thanks cal pilot, that made my day seeing my idea helping someone with the same problem I had. I am now about 30 hours and at least as many landings and still no shimmy to date since adding the strips. I do also think that rocket bobs idea of an aluminum bar would work equally as well in dampening the fore and aft loads and still let the gear absorb up and down as designed.

I think your fix was spot on, the lateral flex of the directional carbon fiber does dampen landing loads, and the stiffness of the fore and aft either prevents the shimmy or dampens it, at any rate, RV GRIN!!! Thank you for posting your fix, it is the bgest!

Gary
 
Aluminum bar

Like many others, I've experienced unpredictable, sometimes violent, shimmy at the end of the roll-out. I can't say that I have been able to correlate it with tire pressure, but I try to maintain about 30psi. I've only got 70 hours on the plane, but the the oversize (380-150) tires are wearing perfectly.

Obviously there can be many interacting modes of shimmy. The videos I've seen suggest the biggest amplitude is fore and aft. Certainly that is consistent with the accumulating wear (damage?) on my gear leg fairings. It is very easy to excite this mode by rolling or hitting the tire with your foot.

This thread appeared just before I started my second annual, so I decided to put in the effort to stiffen the gear leg. The attached photo shows what I came up with. I ordered .125 x 1.5 x 24" 2024 stock from Spruce. Since the greatest flex is at the bottom of the gear leg, I elected to space the attachments more closely at the bottom. Distances from the bottom and Adel clamp sizes:

1" 15
2.5" 15
4.5" 16
7.5" 16
11" 17
16" 17
23" 20

I had to taper the bar at the bottom to help it fit inside the fairing. It would be nice to mount the bar even lower on the gear leg, but that is difficult after the pants/intersection fairings are already painted. I tapered the upper end of the bar just to reduce overall weight.

This is all just redneck/TLAR engineering. So far, I haven't even been scientific about collecting the performance data. I've only done one flight, with the pants/fairing off. I put 39psi in the tires, on the theory that high rolling resistance would only excite the fore-aft shimmy. The first landing I rolled on, and felt what I think was some fore-aft shimmy on touchdown, which was concerning. The next 5 T&G's I dropped in from about 6". The high tire pressure definitely contributed to a very firm gear, but the stiffeners held up just fine. I had no shimmy at the end of the rollout on the final landing.

Next flight I am going to try 30psi in the tires, as my piloting skills need a little more give in the gear!
 

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Thanks cal pilot, that made my day seeing my idea helping someone with the same problem I had. I am now about 30 hours and at least as many landings and still no shimmy to date since adding the strips. I do also think that rocket bobs idea of an aluminum bar would work equally as well in dampening the fore and aft loads and still let the gear absorb up and down as designed.

I have been sharing your shimmy "fix" with a lot of builders. Seeems after talking with the gear manufacture, there is more variance in the gear leg alignments than one would assume. The hole in the gear legs and the hole in the motor mount weldment should result in a slight bit of toe in, but not in a lot of cases. If a person jacks the aircraft and removes the wheels, then sets the aircraft in the ground attitude, places a 2" X 2" angle iron laterally against both axles, there should be a slight toe in. If they are straight, or have toe out, it may be very hard to correct the problem without welding up the motor mount hole and redrilling.

Thanks again for the suggestion, it surely worked well in my case, (after replacing the $600 gear!!!!!)

Regards,

Gary
DAR - NW
 
I have been sharing your shimmy "fix" with a lot of builders. Seeems after talking with the gear manufacture, there is more variance in the gear leg alignments than one would assume. The hole in the gear legs and the hole in the motor mount weldment should result in a slight bit of toe in, but not in a lot of cases. If a person jacks the aircraft and removes the wheels, then sets the aircraft in the ground attitude, places a 2" X 2" angle iron laterally against both axles, there should be a slight toe in. If they are straight, or have toe out, it may be very hard to correct the problem without welding up the motor mount hole and redrilling.

Thanks again for the suggestion, it surely worked well in my case, (after replacing the $600 gear!!!!!)

Regards,

Gary
DAR - NW
When I got to the stage in the build to lift the fuselage up and install the gear I found that the gear legs had lost the left and right labels. Expecting that there should be some toe in I measured the upper attach hole angle to see if there is a difference between the gear legs ( there is) So if the gear legs are swapped you get toe out. A quick call to Van’s (Eric) confirmed the gear legs are handed and he kindly sent me a picture of the way to confirm left and right gear legs. I did a quick gear leg install the alternate way to confirm that they would fit and to measure the result. Sure enough you get toe out which isn’t all that obvious unless you put a long right angle up against both gear legs and measure about two feet out front and back. So in your case the original gear legs may have just been mislabeled and may be salvagable. Definitely worth checking and confirming the correct gear leg is correctly labeled before the install.

KT
 
Grass Roots...

My RV-7 has had violent gear shimmy since first flight, occurred around 23 mph. I tried different tire pressure, different profile tires, wood slats, wrapping the main gear with carbon fiber, nothing worked. I took the gear to the manufacture, he said there should be about 1/2 degree to 1 degree of toe in. My gear has about 1/8 degree toe out on the left, and flush on the right. I just spent $600 on a new set of gear from Vans. Both had about 1/2 degree of toe in. The shimmy was GREATLY reduced. I added a sandwich strip of 10 layers of fiberglass sandwiched by carbon fiber, as posted by one of our other members (I apologize for not remembering the members name) attached to the main gear with stainless ADEL clamps. Not only is there no gear shimmy, but the landing loads are absorbed by the flex of the composite "yardstick" clamped to the trailing edge of the gear. After over 30 years, finally an idea that resolved this chronic issue! I have my RV grin back!

Gary Brown
DAR - Oregon

Het Gary,
Yep, age old problem and a great solution.
Mine was easier, avoid paved runways....

Smooth every time. :)
V/R
Smokey
 
calpilot and tdk,

A couple of questions.

Looks like you cut away some of the rubber cushion on the outside of the clamp to make it thinner and fit better under the fairing. Is that observation correct?

Where are you sourcing the stainless clamps (or are they aluminum)? The stainless ones on the ACS website have white cushion on them, so look different to what your photos show.

Assuming these are standard adel clamp geometry, any issues with the bolt heads clearing and/or rubbing on the fairing?

Thanks for any info.

Cheers,
Greg
 
We seem to have accidentally split the thread on this. Maybe a moderator can merge them?

The original poster did have to cut the rubber off the adels.
I think he was using stainless clamps because of the carbon fiber? I used regular adels for mine.

I haven't put the fairings back on yet, but it will be tricky. I probably will also have to cut the rubber on the front and back side to keep the thickness down.
 
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I used stainless so as not to have any corrosion issues. And yes I removed the the outer rubber banding top and bottom to clear the fairings. After doing so the fairing fit with no issues. The best part no more shimmy after landing roll out.
 
Pendant comment warning ;)

The operative concept here probably is not damping. A damper removes energy from the system, usually in the form of heat. The only significant mechanism for damping in this application would be surface friction between the leg and the stiffener. Perhaps, but if so, it needs examined on a regular basis for wear and/or corrosion.

More likely, the stiffener does exactly that, stiffens the leg in the fore and aft plane, i.e. increases its natural frequency. If the natural frequency is raised above all the potential forcing frequencies (out of round tires, brake drag at one point in rotation, etc), it will not resonate, what we call shimmy.
 
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