Metal Gear Leg Stiffeners

[Ironflight]

I want to preserve this information in the RV-3 forum, even though it is partially copied from a thread over on teh RV-4 forum - the RV-3 gear legs are wet noodles enough that some form of stiffener/damper is generally required, and the wooden ones have always been a bit marginal for us.

I just finished doing this modification on our -3, something I have contemplated for quite awhile, but never got around to doing. At ourt annual a couple of weeks ago, I noticed that the wooden stiffeners had de bonded, and weren’t doing much…so it was time. And with only one test flight in the box, I can still give a pretty definitive answer that the metal stiffeners are better! Our wooden stiffeners were on there from day one, and with almost 800 hours, and were doign very little. The truth is, we have always had a shimmy with these spindly RV-3 legs - we just controlled it with lower tire pressure, braking, and avoiding certain taxi speeds.

Installing metal dampers was probably a total of about six hour’s work - I spread it out over a couple of days, but it was straightforward. Based on notes from Dave Paule, I used 1/8” x 1-1/2” aluminum, and seven Adel clamps. I removed the rubber from the Adel clamps, mostly because I was worried about the outside diameter affecting the fit of our gear leg fairings. Largest Adel is -18, smallest is -12, and they are spaced to where they are naturally tight on the gear leg for their diameter - note that this is NOT evenly spaced. There’s a single wrap of Gorilla Tape under each clamp, just to seat them better.

Spacers were required between the clamp and the metal bar stock to put the center of the bar stock through the center of the gear leg. Screws are low-profile #10 Allen heads, 1” long, and fastened with low profile nylon lock nuts. Washers and s machined bushings were used for spacers, and there was approximately a single washer difference between adjacent clamps. The existing gear leg fairing fit right back in place!

Attachment 27479

[Lufthans]

Very useful info Paul, thanks.

Just added to the todo list for the resurrection project of mine....

[rvaitor87]

Hey Paul.
Thats a great idea! the wood on my -3's gear legs has de-bonded about 3 times over the last 20 years

Do you have any information that you can pass along as to exact measurements, length, and anything that you feel vital to know. I had thought about metal along time ago but was told by a non-RVer that it could set up a harmonic vibration, even at higher speeds. So if you would be so kind as to post further information, it really would be appreciated.

That you.

[Ironflight]

Originally Posted by rvaitor87

Hey Paul.
Thats a great idea! the wood on my -3's gear legs has de-bonded about 3 times over the last 20 years

Do you have any information that you can pass along as to exact measurements, length, and anything that you feel vital to know. I had thought about metal along time ago but was told by a non-RVer that it could set up a harmonic vibration, even at higher speeds. So if you would be so kind as to post further information, it really would be appreciated.

That you.

About the only additional information I can give you that wasn’t in the first post was that mine are 25” long. I am pretty sure that if you’re looking to use a two-foot long piece you have handy, 24” would work just as well.

The line of attachment holes is 3/8” in from the edge of the piece. Spacing is where the various clamps are tight on the gear leg. AN3 hardware is used for attachment.

I flew it again yesterday, doing various medium-speed taxied, landings (wheel and three point), and take-offs, and couldn’t induce any shimmy. I’ll want to fly it to different pavements before I declare complete success, but it sure is an improvement over the wood already!

I used to run about 26 psi in the tires to provide some damping - now at 35 for testing, and its fine. I’ll probably take them up to 40 before next flight.

Paul

[David Paule]

Originally Posted by rvaitor87

....I had thought about metal along time ago but was told by a non-RVer that it could set up a harmonic vibration, even at higher speeds.....

A harmonic vibration in the stiffener is unlikely to have any effect, assuming it even happens, which I'd doubt. In any case stiffener vibration would be out of the plane of the stiffener and of low mass, compared to the gear leg, so it certainly would not excite any shimmy. It would also not have a suitable vibration frequency to bother anything.

Dave,
An old stress and dynamics engineer, now firmly retired

[rvaitor87]

Guys.

Thanks for the reply. I plan on doing this in the next 2 weeks, as the weather down in Florida is extremely unpredictable this time of the year, not too much flying is being done. When I do, I will post a report on what I have found.

Thanks again.

[Larry DeCamp]

Great to hear AL works. I expect a modulus vs. weight comparison with steel could be revealing. How would holes in the flat bar enhance the outcome ? Great work David, thanks. Any energy left to optimize or discourage steel ?

[EdH]

Thanks for this… will be a job on mine at sometime.

[KiloWhiskey1]

Planning on doing this next week on our 3.

Has anyone used a taper pin in place of the gear leg/engine mount bolt? If so, could you please post the part number?

[wirejock]

Originally Posted by Ironflight

<snip>...
Installing metal dampers was probably a total of about six hour’s work - I spread it out over a couple of days, but it was straightforward. Based on notes from Dave Paule, I used 1/8” x 1-1/2” aluminum, and seven Adel clamps. I removed the rubber from the Adel clamps, mostly because I was worried about the outside diameter affecting the fit of our gear leg fairings. Largest Adel is -18, smallest is -12, and they are spaced to where they are naturally tight on the gear leg for their diameter - note that this is NOT evenly spaced. There’s a single wrap of Gorilla Tape under each clamp, just to seat them better.

Spacers were required between the clamp and the metal bar stock to put the center of the bar stock through the center of the gear leg. Screws are low-profile #10 Allen heads, 1” long, and fastened with low profile nylon lock nuts. Washers and s machined bushings were used for spacers, and there was approximately a single washer difference between adjacent clamps. The existing gear leg fairing fit right back in place!

Paul
I'm getting ready to order parts for this. You mentioned seven Adel clamps each side, 18 to 12 size Adel clamps. Do you remember how many of each size Adel clamps were used? One each from 18 through 12? However, it may be pairs of certain sizes. I prefer keeping the rubber cushions. That probably changes the sizes as well. Guess I can order extra. Just trying to avoid more shipping charges from ACS.
Thanks.