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  #1  
Old 06-03-2021, 03:06 PM
FinnFlyer FinnFlyer is offline
 
Join Date: Aug 2018
Location: Bell, FL
Posts: 674
Default Non-wood anti-shimmy gear leg fairings?

Is it crazy to consider this idea:

Discard the stock fairings (don't use if not installed yet).
Shape relatively high-density foam in the shape of the gear leg fairings.
Make tunnel for brake line.
Cut out 1 to 2" long spaces for and add front and rear wood blocks front and back in upper, center and bottom positions (shaped as the cut-outs from the foam).
Wrap with fiberglass and resin.
Smooth to final finish.

Too much work?
Will never work?

Obviously proper alignment is critical as no adjustment possible once the foam, wood blocks and fiberglass are bonded with resin. Although one could wax the legs, but then how would the assembly be securely held in place?

I assume that foam would easily be crushed by the back-and-forth shimmy, thus the 6 wood blocks for each leg.

Finn
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  #2  
Old 06-03-2021, 04:36 PM
David Paule David Paule is offline
 
Join Date: Dec 2009
Location: Boulder, CO
Posts: 5,417
Default

It's a whole lot easier and probably more durable and effective to make thick aluminum strips that fit inside the fairings, and attach them to the gear legs with Adel clamps. An RV-6 owner locally did this and it worked perfectly.

I think he used 7 Adel clamps per strip.

And no, I can't say what the optimum size of aluminum is for this. His pieces were fairly thick but I suspect (no data) that thinner would work fine.

I plan to use 1/8" x 1.5" aluminum strips on my RV-3B when it's far enough along.

Dave
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  #3  
Old 06-03-2021, 05:57 PM
PaulvS's Avatar
PaulvS PaulvS is offline
 
Join Date: Aug 2019
Location: Western Australia
Posts: 1,192
Default

I too think that the aluminum strips with Adel clamps is the way to go. This method will allow the gear leg to move up and down as it should, but not fore and aft as it shouldn't. There's also little risk of trapping moisture, which has caused corrosion on some gear legs that have been chemically bonded.
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  #4  
Old 06-03-2021, 07:31 PM
mtnflyr mtnflyr is offline
 
Join Date: Mar 2019
Location: Boulder, Colorado
Posts: 22
Default Aluminum stiffeners

David Paule recommended I use aluminum for my stiffeners after a discussion on how to resolve severe main gear shimmy. Hopefully, you can see these images. The stiffeners are stout and you could probably get away with thinner aluminum, but they solved my problem nicely. Our airfield has some taxiways in pretty tough shape and my main gear are rock solid now. The difference is night and day. Before installing the stiffeners, I thought I was going to break something when I got going faster than a plodding walk on these taxiways. Now, I don't have to watch my speed at all. It's almost fun to taxi.

I used spacers cut on my lathe and alternated the Adel clamps to keep the stiffeners normal to and aligned with the gear leg centerline. I used decreasing-sized Adel clamps as I moved from the top of the gear leg to the bottom and calculated the spacing to place each clamp over the part of the gear leg that corresponded to the design diameter for that Adel clamp.

Getting the stiffener perfectly aligned took some noodling. I ended up chucking a 12-inch section of pipe in my lathe and bored it to a slip fit on the upper bosses of the gear leg. Drilled and tapped for a socket head cap screw in the pipe to lock the rotation when I had the leg where I wanted it. Clamped the pipe in my welding table vise and installed the stiffener with the clamps just snug enough that I could still move it around. Installed the fairings and got the stiffener right where I wanted it. Removed the fairings and tightened the adel clamps. Solid. Be careful drilling the holes in the stiffener. You need to get them the right distance off the adel clamp to keep it vertical without any wobbling. I did some test fitting with aluminum scraps to get the distance right so that I compressed the rubber on the adel clamp. Ended up very solid.

https://www.dropbox.com/s/1dlhrdyhqk...ner1.png?raw=1
https://www.dropbox.com/s/5ovru70pfg...ner2.png?raw=1
https://www.dropbox.com/s/8f75le2wpm...ner3.png?raw=1

Last edited by mtnflyr : 06-03-2021 at 08:25 PM.
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  #5  
Old 06-04-2021, 04:36 AM
fixnflyguy fixnflyguy is offline
 
Join Date: Jan 2006
Location: Winston-Salem, N.C.
Posts: 1,577
Default My thoughts..

Lots of ways to skin the cat here, however I would not consider the bonded one piece fairing damper a great idea. The legs do a whole lot of moving within the hollow glass wrap leg fairing, and virtually every component of the "system" has relative movement. The leg fairings must be wear tolerant in the cuff fairings, the pant intersections and the length of contact with the steel gear. The dampers (wood/metal/fiberglass) simply null the resonance of the steel leg shake frequency and are not intended to make the legs "stiffer". I used the typical wood stiffeners and bonded them to well painted steel legs with Proseal, then wrapped with a couple layers of glass. Be CERTAIN the steel legs are protected from any water entrapment/bare metal areas..these badboys will rust in short order and crack because of it. The fiberglass fairings "float" as designed over the leg/stiffener and I sanded/filled/painted the inner surface with teflon infused urethane, as well as the upper and lower cuff/paint intersection fairings so there is zero chafe spots. the raw finish on the inner surface of the glass fairings will quickly act like a rasp file on your gear legs if not smoothed and sealed. In hind sight, I may have eliminated the stiffeners all together, as many have been fine without..all depends on runway surface, alignment and wheel balance.
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  #6  
Old 06-04-2021, 07:04 AM
FinnFlyer FinnFlyer is offline
 
Join Date: Aug 2018
Location: Bell, FL
Posts: 674
Default Heavy?

Quote:
Originally Posted by mtnflyr View Post
David Paule recommended I use aluminum for my stiffeners after a discussion on how to resolve severe main gear shimmy. Hopefully, you can see these images. The stiffeners are stout and you could probably get away with thinner aluminum, but they solved my problem nicely. Our airfield has some taxiways in pretty tough shape and my main gear are rock solid now. The difference is night and day. Before installing the stiffeners, I thought I was going to break something when I got going faster than a plodding walk on these taxiways. Now, I don't have to watch my speed at all. It's almost fun to taxi.

I used spacers cut on my lathe and alternated the Adel clamps to keep the stiffeners normal to and aligned with the gear leg centerline. I used decreasing-sized Adel clamps as I moved from the top of the gear leg to the bottom and calculated the spacing to place each clamp over the part of the gear leg that corresponded to the design diameter for that Adel clamp.

Getting the stiffener perfectly aligned took some noodling. I ended up chucking a 12-inch section of pipe in my lathe and bored it to a slip fit on the upper bosses of the gear leg. Drilled and tapped for a socket head cap screw in the pipe to lock the rotation when I had the leg where I wanted it. Clamped the pipe in my welding table vise and installed the stiffener with the clamps just snug enough that I could still move it around. Installed the fairings and got the stiffener right where I wanted it. Removed the fairings and tightened the adel clamps. Solid. Be careful drilling the holes in the stiffener. You need to get them the right distance off the adel clamp to keep it vertical without any wobbling. I did some test fitting with aluminum scraps to get the distance right so that I compressed the rubber on the adel clamp. Ended up very solid.

https://www.dropbox.com/s/1dlhrdyhqk...ner1.png?raw=1
https://www.dropbox.com/s/5ovru70pfg...ner2.png?raw=1
https://www.dropbox.com/s/8f75le2wpm...ner3.png?raw=1
Thanks. That looks like pretty thick (3/16? 1/4"?) alum bars. With the Adel clamps how much weight did it add?

Finn
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  #7  
Old 06-04-2021, 07:15 AM
FinnFlyer FinnFlyer is offline
 
Join Date: Aug 2018
Location: Bell, FL
Posts: 674
Default

Quote:
Originally Posted by fixnflyguy View Post
Lots of ways to skin the cat here, however I would not consider the bonded one piece fairing damper a great idea. The legs do a whole lot of moving within the hollow glass wrap leg fairing, and virtually every component of the "system" has relative movement. The leg fairings must be wear tolerant in the cuff fairings, the pant intersections and the length of contact with the steel gear. The dampers (wood/metal/fiberglass) simply null the resonance of the steel leg shake frequency and are not intended to make the legs "stiffer". I used the typical wood stiffeners and bonded them to well painted steel legs with Proseal, then wrapped with a couple layers of glass. Be CERTAIN the steel legs are protected from any water entrapment/bare metal areas..these badboys will rust in short order and crack because of it. The fiberglass fairings "float" as designed over the leg/stiffener and I sanded/filled/painted the inner surface with teflon infused urethane, as well as the upper and lower cuff/paint intersection fairings so there is zero chafe spots. the raw finish on the inner surface of the glass fairings will quickly act like a rasp file on your gear legs if not smoothed and sealed. In hind sight, I may have eliminated the stiffeners all together, as many have been fine without..all depends on runway surface, alignment and wheel balance.
Seems there are different opinions here as to what the "stiffeners" do.
One is to make the fore-aft gear leg wider and thus stiffer, like an RV-8 leaf gear. The other is to make wood act as a dampener. However, does seem that wood wrapped with fiberglass also widens and thus stiffens the fore-aft motion.

I suspect that 1/8" thick alum bar will buckle under the fore-aft forces. I do understand the appeal of this method: removable, making it possible to adjust and to inspect for corrosion. Drawback got to be the added weight.

Perhaps the wood/fiberglass method is also somewhat gentler on the engine mount where the gear leg sockets are attached?

Finn
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N46AZ RV-3B Mazda 13B EFI -- Bought -- Flying
N993FL RV-3A Mazda 13B NA 575 hours
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  #8  
Old 06-04-2021, 07:48 AM
David Paule David Paule is offline
 
Join Date: Dec 2009
Location: Boulder, CO
Posts: 5,417
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My working hypothesis was that we needed to separate the fore-aft section moment of inertia from the vertical.

I do not think that dampening is the goal. Dampening requires motion to work since dampening is a function of relative velocity and we didn't want motion in the first place.

Dave
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  #9  
Old 06-04-2021, 07:50 AM
HFS HFS is offline
 
Join Date: Aug 2009
Location: Lemoore, CA
Posts: 500
Default FYI - Van's Original Instructions

Page 10-5 of Van's original builder's manual, (mine from 1985) instructs ... The gear leg/dampening block assembly can be covered by an aluminum fairing as shown in SK-64, or can become part of a foam/fiberglass as shown in SK-66 (See attached photo of SK-66). ...

So you can see, at least in the original design, that the streamlined foam/wood wrapped in fiberglass was an "approved" method for a gear leg fairing. This doesn't mean that this method is without its faults, but merely to illustrate Van's thinking early on.

As an aside, I used this method; and, until a subsequent owner put the airplane up on its nose, wrinkling the firewall and bending the gear legs - hence requiring replacement, this method worked just fine.
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  #10  
Old 06-04-2021, 09:44 AM
FinnFlyer FinnFlyer is offline
 
Join Date: Aug 2018
Location: Bell, FL
Posts: 674
Default

Quote:
Originally Posted by HFS View Post
Page 10-5 of Van's original builder's manual, (mine from 1985) instructs ... The gear leg/dampening block assembly can be covered by an aluminum fairing as shown in SK-64, or can become part of a foam/fiberglass as shown in SK-66 (See attached photo of SK-66). ...

So you can see, at least in the original design, that the streamlined foam/wood wrapped in fiberglass was an "approved" method for a gear leg fairing. This doesn't mean that this method is without its faults, but merely to illustrate Van's thinking early on.

As an aside, I used this method; and, until a subsequent owner put the airplane up on its nose, wrinkling the firewall and bending the gear legs - hence requiring replacement, this method worked just fine.
Thanks David. So that's where I got the idea! Just now I dug out my old RV-3 manual. SK-63 through SK-69 outline the different methods back then. Interesting that the wood strips are held to the gear leg by 2 to 3 layers of 9 oz fiberglass cloth tape before the trailing edge foam and final layers of cloth is added. That would indicate that the foam would be crushed if used alone.

My RV-4 project actually came with the two-piece alum fairings. So I guess I'll use those.

Finn
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