VAF Forums

VAF Forums (https://vansairforce.net/community/index.php)
-   RV-3 (https://vansairforce.net/community/forumdisplay.php?f=10)
-   -   Metal Gear Leg Stiffeners (https://vansairforce.net/community/showthread.php?t=207548)

Ironflight 06-26-2022 12:57 PM

1 Attachment(s)
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 06-30-2022 06:15 AM

Very useful info Paul, thanks.

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

rvaitor87 06-30-2022 02:52 PM

RV-3 gear leg
 
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:mad:

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 06-30-2022 03:29 PM

Quote:

Originally Posted by rvaitor87 (Post 1617135)
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:mad:

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 06-30-2022 06:01 PM

Quote:

Originally Posted by rvaitor87 (Post 1617135)
....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 06-30-2022 06:36 PM

RV-3 gear legs
 
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 06-30-2022 07:27 PM

Why not steel ?
 
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 ?

PilotjohnS 06-30-2022 08:38 PM

leery
 
I would be leery of steel. I think the thickness of the aluminum helps to keep the stiffener from buckling. Cant see how the steel would accomplish this without also hindering the flex in the up and down direction. But what do i know, my only engineering degree is TLAR (that looks about right)

David Paule 06-30-2022 09:02 PM

Quote:

Originally Posted by Larry DeCamp (Post 1617181)
....How would holes in the flat bar enhance the outcome ?....

They would save weight and reduce the effectiveness of the stiffener, but probably not by a huge bunch. They'd be best located adjacent to the gear leg since the stiffener's effectiveness is based upon its width from the gear leg.

Dave

David Paule 06-30-2022 09:08 PM

Quote:

Originally Posted by PilotjohnS (Post 1617194)
I would be leery of steel. I think the thickness of the aluminum helps to keep the stiffener from buckling. Cant see how the steel would accomplish this without also hindering the flex in the up and down direction. But what do i know, my only engineering degree is TLAR (that looks about right)

A small factoid might be of interest. Aluminum, steel and magnesium all have about the same stiffness to density (i.e., weight) ratio. But the buckling resistance is based upon both the stiffener's thickness and the stiffener's modulus of elasticity (more of both is better here), so the most weight efficient material for buckling resistance is the lowest density of these.

That said, we don't know what the stress is in the stiffener and so we can't estimate how close it is to buckling.

I doubt that a steel stiffener would significantly adversely affect up and down bending, especially if it were made thinner so as not to be three times heavier than an aluminum one would be.

Dave


All times are GMT -6. The time now is 03:46 AM.