Gear leg shimmy

[randylervold]

RV-3ers,

I'm not planning on putting stiffeners on my gear legs in the hope that I won't have to deal with gear leg shimmy. From listening to owners of rod-legged RVs discuss their experience with shimmy for years now I would conclude that less than half of RVs with rod legs shimmy to start with, and even then by managing tire pressure and balancing the wheel pants many shimmies can be fixed. Best guess then is that maybe 25% of the fleet ends up with a shimmy?

The RV-3 has shorter gear legs than all other models other than the early short legged RV-4s. Intuitively I'd think this would reduce the shimmy problem but that could be wrong. So, I'd like to hear from those with flying RV-3s or short-legged RV-4s as to whether they have shimmy or not. Please respond with your experience and let's see if we can get some sort of statistical concensus.

Thanks!

[rv6rick]

I'll add this to the mix....just posted on our Ohio Valley RVators site this morning:
My new long legged RV4 has had a bit of gear leg shimmy that can be
felt inside when taxiing and on take off roll and on landing. Shiner
called yesterday and said that it was REALLY shaking when we did a 3
ship take off last weekend. I'm glad he took the time to mention this
cause I was wondering what it looked like from outside the plane. He
said that the pants looked like they were going to shake off the plane.

The previous owner said that the shaking started when he put
some 'gooey flat fix' in the tires to help hold the air pressure. He
was going to replace the tubes but never got around to it. After
talking to Shiner I decided to change out the tubes NOW rather than
wait till the tires wore out (they're at about 80%). So, I went to the
airport last night and replaced both tubes with the new 'leak stop'
tubes that came with the plane. I even checked the brakes and repacked
the inner and outer bearings on each wheel while it was apart. I then
did taxi tests from 5mph to about 25mph and NO SHAKE. That said.....I
turned on all the lights and did my 3 stop and goes for night flying
currency and, again....NO SHAKE could be felt in the cabin at all. Nice
and smooth the whole time.....hopefully it 'looks' nice and smooth on
the outside as well.

Oh yea.....if you shake the old tubes.....you can feel and hear the
liquid inside of them......kind of like shaking a milk jug with about a
pint of milk in the bottom!!

Be sure to buy the 'leak guard' tubes and don't use flat fix in your tubes!!

Rick in Ohio at the Buffalo Farm - loosing count of RV/Rocket projects
http://rv6rick.tripod.com/ohiovalleyrvators/

[Kahuna]

Randy,
one rv-4 short leg with dampeners, no shimmy.
This is one of those items where the yahoo groups with polls is useful

Good luck with the Formation CLinic This weekend Randy.
Best,

[tin man]

RV3, short legs, glass over balsa wood stiffeners, lamb tires @ 30 lbs, very light to no wheel shimmy, seems to depend on type of landing (real wheelie vs tail low wheelie)
Tom
RV3 978TM
1000+ hours

[AlexPeterson]

Randy, my personal experience (6A) is that there are and will be certain speeds which may excite some vibration. In my case, I avoid 23 knots taxi speed, as I get a bit of a fore-aft vibration on the mains. I have also noticed that as the tires wear (less mass), the speed at which I get a vibration goes up. I did not see this vibration with the first cheapy Van's tires (less mass - see below).

On a related note, I flat sided an almost new pair of Michelin tires last winter, with a prolonged dragging on landing (frozen brakes). I am replacing them with retreads, so it will be interesting to see how their mass compares to the others, and hence how the vibration characteristics change.

For those geeks interested, the natural frequency of a spring mass system is proportional to the square root of the stiffness and to one over the square root of the mass.

[tin man]

My understanding of the wheel shimmy fix is to CHANGE the frequency at which they shimmy by either stiffening gear legs/ balancing wheel pants / changing tire pressure/or basicly what ever it takes.
Tom
RV3

[jhallrv4]

"the natural frequency of a spring mass system is proportional to the square root of the stiffness and to one over the square root of the mass"


Yeah, that's what it felt like to me...

Jeff

[REHughes]

My old RV-3 with no stiffeners and aluminum fairings would shimmy on most landings about half way through the roll-out (perhaps 25 MPH or so) and brake and rudder-pulsing inputs were required to stop the shimmy. The shimmy would occur even with 20-22 psi in the tires, and seemed to be fairly consistent through at least two different tire types.

In a discussion many years ago at Sun 'n Fun, Steve Wittman expressed the opinion that the most likely reason for the -3's shimmy was that the gear rake-back angle in the three-point attitude was about 5 degrees short of what he would generally recommend for a Tailwind-type gear.

The "long" RV-4 gear would reduce this angle slightly, even if the motor mount gear socket angles were not changed, and would thus perhaps make it more susceptible to shimmy than the "short" original gear.

My current -3 has fiberglass-over-foam gear fairings, and I have noticed no shimmy, except perhaps an occasional short highly damped pulse after running over a bump or seam in the runway. My new -3 will have wood stiffeners and aluminum fairings (and balanced wheel pants and tires).

Hawkeye Hughes
Skyote, RV-3

[davehirschman]

I've got a persistent shimmy in the main landing gear of my RV-3 when I accelerate or decelerate through about 10 miles an hour. I've installed wood landing gear stiffeners, balanced the wheel pants and tried different inflation on my 5:00 by 5 tires. Each change has dampened the shimmy -- but it hasn't gone away. I'm out of ideas and looking for suggestions . . .
Thanks,

Dave Hirschman
Atlanta, Ga.
Dave@Hirschman.com


P.S. Just to make things a little more perplexing, the wheels are straight and the tire wear is even.

[Bob Martin]

Dave,
Have you checked your gear mounting bolts?
on my -6....they worked loose and I had to re-torque them.
Worth a check see...
Good Luck