RV-3 landing gear problem

Hey guys
I finally got my RV-3. Flew it for the first time today. The engine mount and gear legs were replaced as part of the purchase. Problem: wobble! Pretty bad during taxi, and roll out on landing.
So...the original gear legs had a wood stiffener fiber-glassed to the back edge of the gear legs, hidden by the leg fairing. It was pretty stiff, and we think the stiffness transferred the landing force up to the mount where it cracked. So at this point, I have the wheel pants and fairings off , and wondering if there is any "new wisdom" on the gear legs. My plane was built in 1984, it had an IO-360 and CS prop on it originally. Now has a new IO-320 and Vans 76 x 79 metal prop.

My gut feeling is that some stiffening is needed, and I am considering strapping the original stiffeners to the legs rather than "bonding" with resin and fiber glass which is a lot stiffer. This should give some dampening and still allow some flex of the gear to avoid transferring so much force to the mount.

I was advised to run the tire pressure around 24# also, which is supposed to help. I only had the chance to make 2 landings today...not pretty, but still need to figure out the sight picture and touch down attitude so my technique will get better, but I do want to get the legs and wheel pants on it once I figure out the stiffener thing.

Any advise would be great.
Thx
Dave

You will find all different tire psi recommendations and they are worth a try. The best thing I did was buy a motorcycle tire balancer and balance the tires. Mine came from Harbor Freight and didn't cost much but works well.

Did the engine mount crack get repaired?

Is the "wobble" a sort of shimmy or is it an actual looseness somewhere?

The landing gear is a simple structure. Any forces and moments in it get transferred to the mount. With the bigger engine and that prop, it might simply have been loaded higher than most.

If you strap the stiffeners to the legs I'd expect the straps to wear. The wood and fiberglass are far softer than the steel and will permit some flexibility even if bonded. It's the additional moment of inertia (combined with the modulus of elasticity) of the wood which make the combination stiffer.

Dave

I would epoxy wood strips in place per the plans. You could make the strips shorter and/or thinner than standard if you want to reduce the stiffness.

I built and flew an RV6 for about 300 hours before I put on gear leg stiffeners, and then about 600 hours more after that. The difference was noticeable and much better than without them; however, I would also say that they weren't really necessary, even though they changed the feel of landing, and taxiing, for the better, in my opinion.
The thought also occurred to me that since Antisplat Aero makes nose job fittings for the A models, a similar piece could be made to be used as gear stiffeners, which could easily be installed or retrofitted, as opposed to making per plans stiffeners.
To me, the top third of the leg is so stiff that the stiffener really only needs to be introduced an a part of the gear leg anyway. A nose job type stiffener could be adjusted for the amount of stiffness you want, by sliding it up and down on the gear leg and it could be rotated to match the trail of the existing gear leg fairing, making installation very easy.
Just a thought. Maybe the guys at Antisplat Aero can chime in here.
Good luck.

Check your wheel alignment. Toe in/out, etc....

After watching enough RV's taxi over rough surfaces, and seeing the spaghetti-like dancing of gear legs, I decided we'd put the stiffeners on the RV-3 legs from the start. My RV-8 has different gear, but Louise's RV-6 has always had them since she's had it, and the gear were well behaved. In the -3, with stiffeners, we still get dancing at some speeds - and the speed is totally dependent on tire pressure.

The best pressure for lowest shimmy seems to be around 21 psi, but I hate to run them that low. Right now, we're at 29, and we get shimmy between about 20 - 30 knots. Speeding up or slowing down makes it go right away. It isn't too bad operationally - the only time it is bothersome is with a long back-taxi at speed.

Just our experience.

Paul

Ironflight said:

Right now, we're at 29, and we get shimmy between about 20 - 30 knots. Speeding up or slowing down makes it go right away. It isn't too bad operationally - the only time it is bothersome is with a long back-taxi at speed.

Just our experience.

Paul

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Sometimes a light tap of the brakes can stop the oscillation. Sometimes not....
I do not have them on my 6. Occasionally I will get some shimmy at very low taxi speeds (less than 10 kts), but a quick tap puts things right. I run my tires between 30-36.....
I had none of this before I put on the Dresser Recaps....(yes, they have been balanced.)

Confirm that you are using the proper tire profile for spring landing gear.

What I did on my -4

I installed wood stiffeners (home made) on the front and aft side of my gear legs as recomended by several. The gear legs were painted with urethane, then the wood stiffeners were sealed with urethane, and bonded to the gear legs with Proseal. After that cured, I wrapped with 2 layers of fiberglass cloth/resin and painted over all of it with Teflon urethane paint. The Teflon paint keeps the fairings from chaffing, and the Proseal keeps any moisture sealed from the steel gear. I have 200 hours on mine now, and been on many different surfaces with no shimmy at all.

Foam

My gear leg fairings are filled with foam which stiffens them a bit. Not as good as wood I'm sure, but good enough for most conditions and lighter/simpler.

mach25 said:

My gear leg fairings are filled with foam which stiffens them a bit. Not as good as wood I'm sure, but good enough for most conditions and lighter/simpler.

Click to expand...

I have seen this done where the foam is simply sprayed into the fairing while on the gear. While I can not comment on how well it works for dampening shimmy, it works really well to hold moisture and cause corrosion on the gear leg. Additionally, it makes it impossible to inspect the gear leg, brake lines etc...
An RV6 on our field did this. After quite a few years, they noticed some rust stains emanating from the bottom of the fairing. They removed the foam to discover pitting and corrosion. They caught it in time and did not loose the gear legs.
Something to consider.

Shimmy

The fact that the gear was replaced raises a huge red flag. I would start by hoisting the gear off the ground, preferably with cargo straps around the engine mount tubing near the firewall. Absolutely NOT by the engine hoist ring.
Loosen the nuts at the upper end of the gear socket and check for play in the bolts. These bolts, in my opinion, should be a light to moderate drive fit. If you can move the bolts with your fingers, the holes are too large. The bolts should be one dash number longer than necessary. This eliminates the possibility of reduced bolt diameter adjacent to the threads carrying a shear load. Torque the nut to the maximum torque, slightly over probably will not hurt.
If the upper end is satisfactory, check for play in the lower end of the socket. While the fit at the lower end is not super critical, any play will contribute to shimmy. While it is hoisted, pull the wheels and check the toe in.
The above is ONLY because the gear has been replaced.
The next step, which would be the first step were it not for the gear leg replacement:
Checking the tire balance is fine, but I much prefer to start with new top of the line Goodyear or Michelin tires and new tubes. The cheaper tires are not only likely to be out of balance but also out of round. I experienced severe nose wheel shimmy on a Beech Baron that I was flying a lot and flying a lot of short legs. The nose gear strut was quite loose. A change to the most expensive Goodyear tire made the shimmy go away completely, even with the, loose oleo strut.
Only after you have done the above would I install the wooden dampner.
If you find that the gear is loose in the socket let me know and I will describe various fix options and procedures.
The Wittman round gear toe in/out changes with aircraft loading. Toe out is the worst scenario as the toe out will tend to increase as ground speed increases. Wittman recommended 1/16' toe in when fitting the gear, most of this goes away as the airplane is loaded, so it is probably close to zero at gross weight, static.
On my first Tailwind, the gear would only shimmy on landing, around 30/40 m/h statute, and would stop immediately with application of light symmetrical braking. Never shimmied on takeoff or taxiing.
Based on 50 plus years experience with the round spring gear, as designed and patented by Steve Wittman.