Gear stiffener question ??

The mission is to improve the shimmy of round gear. Vans drawing proposes a triangular wooden nose on the front of the rod. My question to experienced RVers is, does the oscillation go up and down or front to rear ? If up and down is not the problem, I don,t want to make the gear stiffer. If stabilization is only needed front to rear, that will define a different approach. Comments appreciated.

Some do the "stiffener" on the front side some on the aft. The stiffener is actually a dampener to help detune the resonant frequency of the gear legs mostly in the fore and aft direction. There are some pre made ones for sale. Others might chime in on how good the pre-made ones are or are not.

Damping

Thanks for the comment Dan. Kevlar and graphite have been suggested for absorption. The graphite people say it is used extensively for vibration attenuation. So I am just deciding how to apply it for maximum benefit without creating a negative characteristic.

Actually both are strong and will increase stiffness, but neither work well for dampening. Wood works really well for this.

WoodmanRog (on this forum) sells pre-made stiffeners.

We also carry them and will have them at Oshkosh. Visit us at booth 4047, in building D.

http://www.flyboyaccessories.com/RV-Gear-Leg-Stiffener-Set-p/2104.htm

With the energy involved, the damping will be negligible - otherwise the fairings would be hot to the touch after landing since damping absorbs energy.

Of the materials mentioned, the wood works because it has a relatively large moment of inertia. Kevlar works because it has more modulus of elasticity than wood, and is placed at the outer fiber, which tends to add to its effectiveness. Carbon works because of its high modulus of elasticity and it's position, also at the outer fiber.

The added stiffness increases the natural frequency mostly in the fore-aft direction but slightly in the transverse direction too. Separating the natural frequencies of the two directions seems to reduce the likelihood of the vibration.

What I plan to do when I get that far is use Woodman Rog's wooden pieces with some uni carbon, along with the +/- 45 degree glass wrapping over all.

Dave
RV-3B under construction

Wooden gear leg stiffeners

I'm not sure how the gear leg oscillates but it does move.
The gear on my aircraft shook so hard that I thought it would tear apart the whole motor mount. Frankly it scared me, this was during the first taxi test. Then I put on wooded gear leg stiffeners per Van's design. The only wooden window trim at our little lumber yard was oak so that is what I used. I never thought about the stiffness, I just wanted the violent shake to go away! I was concerned about the added weight, oak is heavy.. But the end result was awsume. the gear legs still flex and bend but with no shaking.
The wood works great. I can not speak for the other materials because I have no experience with them.

Thanks!

I appreciate you all taking the time to help me ! I will update the thread with pics and results as the project proceeds. Larry

Wood

I had the dubious task of removing some wood gear leg stiffeners from a rv6a that we're rotten in places. I imagine they were on there about 15years. That's a fair amount of time but even so. You may want to soak them in some thin slow-curing epoxy to extend their life and reduce moisture wickin. Just a thought.

Prefab

I installed the premade stiffeners mentioned above. They were easy to install and saved some time because they are preshaped. I wrapped them in fiberglass with epoxy resin. Having just finished the canopy/windscreen installation with some carbon fiber over the canopy interface, carbon fiber is hard to sand. I find the fiberglass easier to work with. Having said that, I sanded and smoothed the fiberglass winding that went around the wood. I found that using 4 inch tape make the whole process easy.
I am not flying yet, so cannot comment on the performance.

Wheel Shimmy

Good discussion by everyone about putting on gear stiffeners. I was referred to the company below when I started getting a shimmy on landing while slowing to just above taxi speed. It was pretty pronounced and ended up being as simple as replacing the tires with Wilkerson retreads which fit perfectly with no mods to the wheel pants. LOTS cheaper than the competition and haven't had any shimmy since.

http://www.wilkersonaircrafttires.com

I never installed the gearleg stiffners. After the initial flights I had shimmy until I lower the tire pressure to approx 27psi, which made me have have second thought about not installing the stiffners. Then I replaced the standard Air Hawk tire that came with the kit with a Desser Elite and Monster retreads and I haven't have the shimmy issue since regardless of the tire pressure I run. I do on occasions have a little shimmy when I hit a larger gap in the taxiway, but it is pretty brief.

Long story short, I personally would not install the stiffeners and bypass the wood and fiberglassing mess.

Just my two cents!

What actually happens without the stiffeners (on some planes) is the round gear legs will want to oscillate in a 360 degree motion. The stiffeners allow the gear legs to only move in a side to side flex. The reason lower tire pressures work is because the flatter tires scrub the pavement and tend to pull the gear legs aft while taxiing preventing them from moving side to side. This works well but you will wear out tires excessively fast. Application of the stiffeners that I make and sell (as well as a couple of other distributers) are according to Van's plans. Installation takes about an hour after you have jacked up the plane and no modification to the standard fairings is necessary if mounted in the aft position as I advise in the instructions. Stiffeners also reduce bounce on landings, a side benefit that no RV pilot needs, right? For video and information contact:
[email protected]

woodmanrog said:

....The reason lower tire pressures work is because the flatter tires scrub the pavement and tend to pull the gear legs aft while taxiing preventing them from moving side to side. This works well but you will wear out tires excessively fast.......[/email]

Click to expand...

I only had to run low tire pressure with the original Air Hawks, I now run 45 psi with the Desser retread tires with good tire wear patterns and very little to no shimmy.