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Non-wood anti-shimmy gear leg fairings?

FinnFlyer

Well Known Member
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
 
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
 
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.
 
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/1dlhrdyhqkusp0c/stiffener1.png?raw=1
https://www.dropbox.com/s/5ovru70pfggzh2j/stiffener2.png?raw=1
https://www.dropbox.com/s/8f75le2wpmeeg8u/stiffener3.png?raw=1
 
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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.
 
Heavy?

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/1dlhrdyhqkusp0c/stiffener1.png?raw=1
https://www.dropbox.com/s/5ovru70pfggzh2j/stiffener2.png?raw=1
https://www.dropbox.com/s/8f75le2wpmeeg8u/stiffener3.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
 
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
 
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
 
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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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
 
Thanks. That looks like pretty thick (3/16? 1/4"?) alum bars. With the Adel clamps how much weight did it add?

Finn
I didn't weigh the stiffeners. Cut them out of 1/4" scrap I had in my shop. My shimmy problem was severe. You can see it in this video:

https://youtu.be/SfFXGsx36TU

I talked to Dave about how stout to make the stiffeners and we agreed there was no easy way to calculate the forces the gear legs were experiencing - or the effect different stiffeners would have on gear leg harmonics - and to just build a set and see what happens. I went with the overkill approach. It worked so well that I've not tried to put thinner stiffeners on to shave a couple of pounds off. One of the outlying fields here in Colorado has a runway that is in serious need of resurfacing. Rough as ****. Landing there used to give me a shimmy that loosened the fillings in my teeth. Now, I don't even notice the runway condition.

Truth in advertising - I have over 500 carrier landings in F-4's and F-14's, so my notion of how stiff gear legs should be might be a little different than the average RV pilot.

If you are worried about the weight, I'd suggest you make a couple of stiffeners and swap them out to see what different thicknesses give you for behavior. That's the beauty of this design - you can change out the stiffeners quickly with little effort. Dave can tell you that you get much more stiffness in the direction you care about (fore and aft) by making the stiffeners wider than making them thicker. The limit there is that the stiffeners have to fit inside the fairings.

He tried to convince me to go thinner (Dave cares a lot about weight), but I just wanted to solve a very disturbing problem and move on.
 
Yikes!

I thought your upper gear leg fairing was going to fly off!

My issue is I need to stiffen the gear in the up and down plane. I have a long prop on a short gear -4 and would like to lessen the gear splay upon landing.
The problem is I can’t squeeze a conventional stiffener inside the fairing on the top or bottom of the gear leg.
Anyone use a piece of aluminum angle as a stiffener? Would stiffen in both axes.
 
It probably won't be all that effective without making the gear leg deeper (top to bottom). And I'd recommend making the fore-aft direction a different stiffness than the up-down one.

The basic parameters are first, the material. Aluminum has a modulus of elasticity of roughly 10 million psi, and steel is roughly 29 million psi. Modulus of elasticity is the measure of a material's stiffness.

The second parameter is the shape. The section moment of inertia is the incremental area times the square of the area's centroid from the centroid of the whole shape. For stiffer, maximize this. You can find numerous shapes and formulae on line and even calculators for specific shapes. CAD programs can often calculate it, too.

Both work together. For more stiffness, increase (modulus of elasticity) X (section moment of inertia). And yes, it gets a little more complicated when multiple materials are assembled together.

In mtnflyr's case, all we wanted to do was increase the fore-aft version of that ( ) X ( ) product. Since in that direction the aluminum was so far from the original gear leg's centroid, it was very effective even with the lower modulus of the aluminum strip. And it fit within the existing fairing. This was an engineering solution, not a theoretical one, in spite of using some actual engineering knowledge to assist.

And I ought to mention that I read here on VAF some time ago that someone else did this and it worked. That gave me the idea -- thanks to whomever that was. So we didn't even invent this solution.

Dave
 
I thought your upper gear leg fairing was going to fly off!

Yeah. From the cockpit, it felt like I was going to break the gear leg! Part of the issue was that all three leg retaining bolts had not been fit correctly when the airplane was built in '95 and over the years had ovaled the holes in the mounts so that they all rotated. When hitting the right speed the gear harmonics kicked in and things went south in a big hurry. After several discussions with Van's technical folks, I drilled and reamed the retaining bolt holes to the next larger AN size, bought a bunch of bolts and selected those that gave me a medium press fit into the mount/leg, and assembled the gear into the mounts with Loctite 680. That version of Loctite is designed for this application - retaining cylindrical assemblies. The bolts and Loctite helped, but the severe shimmy was still there, so I asked Dave to help me come up with a stiffener design. That worked better than thought it would.
 
Lightening Holes

I am curious if you could drill lightening holes in the aluminum?

I like this idea the best beacuse it keeps the gear leg exposed to air and not wrap in anything moisture trapping. i think I will do this.
 
I used 1/8" x 1.5" aluminum. "readjusted" the adel clamps so the attachment tangs (to the aluminum bar) are in trail. Seems to have worked, although I still have a bit of shimmy at about 35 mph, but it is easily managed by passing through that speed fairly quickly. And I might add that my tires are significantly larger and heavier than standard, which probably contributes to any harmonic pendulum effect.
 
I am curious if you could drill lightening holes in the aluminum?

I like this idea the best because it keeps the gear leg exposed to air and not wrap in anything moisture trapping. i think I will do this.

Holes may make it more prone to buckling. Think of putting a long 2x4 on its edge and stand on it in the middle. Even worse with a 1x4. Even if held firmly in place at the ends and lower edge, the top edge will tend to move to the sides.

However, perhaps a thick bar with holes may be better than a thin bar without holes.

A triangle-shaped bar with the thick edge against the leg is probably best. Drill holes in that and you have something similar to the webbed triangle arms of those tall cranes.

If only I were a structural engineer ... :)

Finn
 
Holes may make it more prone to buckling. Think of putting a long 2x4 on its edge and stand on it in the middle. Even worse with a 1x4. Even if held firmly in place at the ends and lower edge, the top edge will tend to move to the sides.

However, perhaps a thick bar with holes may be better than a thin bar without holes.

A triangle-shaped bar with the thick edge against the leg is probably best. Drill holes in that and you have something similar to the webbed triangle arms of those tall cranes.

If only I were a structural engineer ... :)

Finn

Seems to me the triangle puts the moment of inertia too close to the gear leg and would be less effective.

I like the idea of an aluminum bar. Maybe like Greg did and re adjusted the clamps. Greg, got any pictures? But I think 1/8 is too thin, maybe 3/16” x 1.5“ ???
 
John has the right idea, I think, about the shape. I prefer a simple rectangle.

If holes were tried, I'd put them adjacent to the gear leg rather than in the middle. Maybe half-round cut-outs. But first try it solid and then start experimenting to find a minimum weight design.

Greg, it might have been you that first mentioned this idea. Thanks very much! It's a good one.

Dave
 
Holes

If the holes are offset towards the gear leg, then I think the gear leg will become one member of the stiffening truss. If the holes were in the middle, then the bar is a contiguous stiffening element and will not rely on the gear leg as a stiffening member ( to complete the “box” structure). I think I would rather keep the “box” structure entirely in the aluminum plate, and not make the leg part of the “box” . This has implications for the forces acting thru the adel clamps. Also I think it is important to not offset the adel clamps from the centerline more than necessary; otherwise I can see the adel clamps trying to impose a torque on the aluminum plate.

But what do I know, All I ever did was stayed at a Holiday Inn Express last night.
 
It's not a truss, it's a composite beam section. Put the holes at the neutral axis of the combined section, accounting for the modulus differences.

Dave
 
John has the right idea, I think, about the shape. I prefer a simple rectangle.
...

Dave

I guess the simple rectangle is easier to analyze. But look at it this way:
Have a wide thin bar. It will tend to buckle. To prevent that, add two thin sides to it from near outside of gear leg diameter. Now you have a triangle profile:
/|\
( )

If not opposed to resin and fiber glass (or carbon fiber), perhaps a 2" wide 1/8" alum bar or plate wrapped to the leg with fiber might be a good substitute for the wood stiffeners. There the wrapped fiber will prevent buckling. Maybe the voids need to be filled with foam to prevent buckling and motion of the bar near leg.

The 1/8" bar can definitely be tapered to less that 2" wide at the ends, as already covered in these threads.

If firmly held in place and prevented from buckling, perhaps even a 1/16" thick bar or plate may be enough.

Tempting to just clamp an alum angle to the leg, as some have done, but there the center plate would be missing.

Finn
 
Not my idea, picked it up here on one of the previous-mentioned threads. I don't see any photos on my phone, but am in the midst of an annual so will try to take some pics later in the week (assuming it cools down enough to actually work in the hangar). And I'll check the dimensions, as I'm not certain now whether it was 1/8 or 3/16 I used.
 
Actual numbers

I just ran the calculations as they might apply to my RV-3B. I assumed a point a little below the gear leg's top end and assumed that a 1/8" x 2" strip of aluminum would fit.

A different RV would probably have a thicker gear leg, but I think that the overall effect would be similar.

At this particular point, the aluminum strip represents about 75% of the fore-aft section moment of inertia. The remainder, of course, is due to the steel gear leg. The total fore-aft inertia with the strip is about 3.3 times what it would be without the aluminum strip.

Trying a 1.5" x 1/8" strip, the aluminum provides 57% of the fore-aft inertia. The total fore-aft inertia now is about 2.2 times what it would be without the aluminum strip. But the total fore-aft stiffness of the combined section is only about 66% as stiff as the with 2" wide aluminum strip.

I ran one more calculation using a 3/8" x 1.5" aluminum strip. This is 21% stiffer than with the 1/8"x 1.5" strip - thickness, of course, adds stiffness, but width much more so.

From these calcs, it's apparent to me that the addition of an aluminum strip is a very powerful approach to gear leg shimmy.And more aluminum width is clearly stiffer.

I don't think that buckling would be much of an issue. I expect that the fastenings would fail before the aluminum strip buckles, but I haven't put any numbers on that. Adding thickness would definitely reduce any likelihood of buckling. But in any case there wouldn't be any buckling until the gear leg deflects a considerable amount, and the increased stiffness tends to prevent that.

Dave
 
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Math Rules!

I just ran the calculations as they might apply to my RV-3B. I assumed a point a little below the gear leg's top end and assumed that a 1/8" x 2" strip of aluminum would fit.

A different RV would probably have a thicker gear leg, but I think that the overall effect would be similar.

At this particular point, the aluminum strip represents about 75% of the fore-aft section moment of inertia. The remainder, of course, is due to the steel gear leg. The total fore-aft inertia with the strip is about 3.3 times what it would be without the aluminum strip.

Trying a 1.5" x 1/8" strip, the aluminum provides 57% of the fore-aft inertia. The total fore-aft inertia now is about 2.2 times what it would be without the aluminum strip. But the total fore-aft stiffness of the combined section is only about 66% as stiff as the with 2" wide aluminum strip.

I ran one more calculation using a 3/8" x 1.5" aluminum strip. This is 21% stiffer than with the 1/8"x 1.5" strip - thickness, of course, adds stiffness, but width much more so.

From these calcs, it's apparent to me that the addition of an aluminum strip is a very powerful approach to gear leg shimmy.And more aluminum width is clearly stiffer.

I don't think that buckling would be much of an issue. I expect that the fastenings would fail before the aluminum strip buckles, but I haven't put any numbers on that. Adding thickness would definitely reduce any likelihood of buckling. But in any case there wouldn't be any buckling until the gear leg deflects a considerable amount, and the increased stiffness tends to prevent that.

Dave

Thanks for running the math. This really seems to be the way to go. In my mind, the aluminum makes the gear leg perform more like the RV8 flat plate-style gear leg. Or by legs that have a drag link.
 
The cause of the shimmy is miss alignment, to munch toe in, to much toe out effects directional control. Obviously if get gear leg is stiff enough the shimmy goes away. We’ve the best success in correcting the aiignment.
 
The cause of the shimmy is miss alignment, to munch toe in, to much toe out effects directional control. Obviously if get gear leg is stiff enough the shimmy goes away. We’ve the best success in correcting the aiignment.

That's not been the experience of most of the folks who have battled shimmy and wrote about it on this forum. You'll find a bunch who discovered out of round and out-of-balance tire/wheel assemblies. (not the case with my airplane). Some had too much pressure in their tires exacerbating the natural harmonics of the gear and reducing the pressure solved the shimmy. (again, not my issue). Very rarely did I read about someone who discovered that their gear had too much toe in or toe out. We were worried about that as we tried to diagnose our shimmy problem and measured the alignment with weight off and weight on wheels, even going so far as to measure alignment using grease plates (thanks for that pointer, Dave Paule). Side note - the right way to do that measurement is with two plates with grease between them, as I found out after cleaning some greasy tires. This experimental stuff is a learning journey. We shared our measurements with Van's every step of the way and they told us that our alignment was right where it needed to be (after fixing the loose retaining bolt issues). Shimmy still there. Better fitting retaining bolts and gear leg stiffeners fixed it - for my airplane. So, it seems, this isn't a one-solution problem. Could be out of balance tires exciting the natural harmonics in the gear legs, could be low breakout force on the nose gear, could be binding nose gear wheel bearings (had some evidence of that with my airplane), could be loose gear legs (part of my problem), or it could be alignment.
 
Question for David Paule

Maximizing section modulus is clearly a high yield design parameter. Material selection might be as well . So far there is little mention of the different performance potential of wood, glass, aluminum and carbon applied to this “stiffener” design.

Since David has the skills , how would you suggest we optimize the 2” depth we have to work within, regarding material selection ?
 
Guess

Maximizing section modulus is clearly a high yield design parameter. Material selection might be as well . So far there is little mention of the different performance potential of wood, glass, aluminum and carbon applied to this “stiffener” design.

Since David has the skills , how would you suggest we optimize the 2” depth we have to work within, regarding material selection ?

OK my guess:
carbon
aluminum
glass
wood

I am not sure why Van's originally used wood, but probably because it was the lightest weight material at the time. For what I know, the stiffer the material, the higher the shimmy frequency, and lower the displacement. So carbon fiber clearly is the preferred option. There is a forum contributor who wrapped the gear legs in fiberglass/ carbon fiber to eliminate the shimmy, but that stiffens the leg up in all directions. This fix just stiffens the leg in the offending direction with only a slight increase in the preferred "soft" direction.

The best fix for this, in my opinion, is a drag link or trailing link; but that is "ugly" with a capital U.
 
RV-4 leg size

If David will run the numbers for different materials, the RV-4 long legs are 1.425" at top and 0.875 " at the bottom, according to drawing.

Finn
 
Optimization?

Personally, I think that aluminum is just fine. What we don't know is what is the desired ratio of fore-aft stiffness to vertical stiffness. So this is sort of a trial and error proposition.

Would you want it optimized for weight? Cost? Ease of construction?

FinnFlyer, I'll run that number later today.

Dave
 
I looked at a diameter of 1.4" near the top of the RV-4 long legs and tried both 1.5" x 1/8" and 2" x 1/8" aluminum.

The 1.5" aluminum combined with the steel leg gives about 1.7 times the vertical stiffness.

The 2" aluminum combined with the steel leg gives about 2.3 times the vertical stiffness.

And if the 2" aluminum was 1/4" thick, the combined fore-aft stiffness would be about 2.5 times the vertical.

Worth noting is that I haven't looked at the Adel clamps at all. They'd introduce some flexibility to the system. How much? I don't know. My recollection, which I shared with mtnflyr, is that 7 clamps was sufficient.

Dave
 
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Thanks Dave.

I guess I really, really need to fly over to that paved runway, land and taxi to see if I have shimmy. But at least I now have a wealth of solutions!

Finn
 
We haven’t worked on any A model’s,All 3s 4s and 6s. I’ve been working on 3s 4s sense we had to drill our own gear legs, we didn’t have a lot success with good alignment so the shimmy seemed to be a way life. My comments are basted on my experience, the more the toe in the more severe the shimmy, when aligned to neutral the shimmy goes away. So fare 100% successful.
 
Realize

I realized the problem is being describe based on a RV4 experience. But I intend to put at least some form of dampener on my RV9A. If I am going to do something, this seems easier than shaping Home Depot wood and stuff.( after all this is more expensive than a Telsa Model X, and some of those used home depot wood during manufacturing) I might as well do what you all figure out worked the best; then I never have to worry about it. Is there any harm in going over kill?
 
PilotJohnS material priorities

I am one person that has done the 100% carbon fiber stiffener. A CF plate was used like David’s aluminum “ beam web” and then carbon was wrapped around the gear leg and the back of the carbon plate. It is somewhat effective but not 100%. Some low speed conditions can still generate shimmy.

Comments on carbon fiber:

Impressive Carbon Fiber stiffness specs are for filaments. An epoxy bound CF matrix has a Young’s modulus similar to aluminum 😨 So why do race cars use carbon ? My guess is shape options, vacuum bag and autoclave make a difference.

UP / Down gear flex concerns. Because the section modulus is narrow , the carbon skin has little leverage to apply much affect. However, the top skin is in compression when the ship is on the gear. Slight waves form in the carbon wrap but there is no hint of delamination at these strain levels.

I hope these observations add value to the conversation 😊
 
I am one person that has done the 100% carbon fiber stiffener. A CF plate was used like David’s aluminum “ beam web” and then carbon was wrapped around the gear leg and the back of the carbon plate. It is somewhat effective but not 100%. Some low speed conditions can still generate shimmy.

Comments on carbon fiber:

Impressive Carbon Fiber stiffness specs are for filaments. An epoxy bound CF matrix has a Young’s modulus similar to aluminum 😨 So why do race cars use carbon ? My guess is shape options, vacuum bag and autoclave make a difference.

UP / Down gear flex concerns. Because the section modulus is narrow , the carbon skin has little leverage to apply much affect. However, the top skin is in compression when the ship is on the gear. Slight waves form in the carbon wrap but there is no hint of delamination at these strain levels.

I hope these observations add value to the conversation 😊

Where did you get the CF plate?
How does its weight compare to alum?
Cost?

Finn
 
CF plate

I just laminated roll stock under polyethylene with compression. Thickness is about .060" and weight is 1.5 gram per square inch ( fully saturated with resin ) ie shiny both sides.
 
aluminum bar on the 6150 steel?

With the aluminum bar on the 6150 steel, do we need to worry about corrosion between 2 dissimilar metals?

Thanks

Tim
 
oh, so the clamps are around the leg and then the aluminum bar is bolted to the clamp where the bolt runs through the clamp?

Thanks

Tim
 
There are a couple of old threads on this...but I can't find them now.
Here's a pic of what I did. It is a tight fit, and you want to get them as far down as possible as that is where the gear leg bends. It was a significant improvement on my plane, which has oversize tires, but I still get some shimmy on rollout on rare occasions.
 

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Scrub and release

Thanks Tom,

I got off my phone and now using the computer and just found the other fotos as well to see how its done.

Mine as well have the over sized 380*150 tires that were ok at 28 pounds or less for 99% of my landings with no fairings.

I then spent all last winter doing the pressure recovery wheel pants and was happy with those until I had to land.......................

I've messed with tire pressure from 21 psi the best but still unacceptable to 48 psi and scary. I've check all the bolts, balanced the tires (another story on junk from Amaz) and ready to give up and go back to the 500*5 tires with the new wheel pants to see if that would resolve the problem. Otherwise I fly with no wheel pants to stay with the big tires

I'm a short gear RV 4 with the wood stiffeners but they aren't helping in this case.

I should have some time this winter to try this and see if it works for me. Otherwise back to the smaller tires.

My theory looking at this is that the wheels are out of alignment slightly starting the scrub and release leading to our shimmy. I'll confirm what I have as well when I'm doing this. Unfortunately I thought I could install shims to correct for alignment like I did in my C-177 A but that's not possible with this type of gear.

What alloy of Aluminum did you use?

Thanks

Tim
 
Gear

The Wittman round gear is only perfectly aligned at one weight and one angle between the fuselage and the ground. As the airplane is loaded the axle will move toward toe out and much of the camber will be removed at high gross weight.
The flat gears do not have this issue and with the flat gear it is relatively easy to experiment with toe in/out with shims between the axle and gear.
 
Tom, I think there would be an improvement is your stiffener were longer, and extended farther up and farther down.

Tim, I plan to use 2024-T3 or T4, because I have some on hand. I think that 6061-T6 would be fine. They have nearly the same Young's modulus, which is what we want here. It's within 3%. While the 2024 is stronger, that probably won't matter here. I say "probably" because I don't know for sure. I haven't ran any calcs on that. But I'd think that buckling would be the failure mode and in that case the 2024 would be that same 3% better. You'd have a greater improvement by making the strip thicker.

Dave
 
Jrs14855 comment

The comment appears to be logically accurate. I recently measured tow in on my 4 with Vans mount and predrilled gear legs. They were 1 degree tow in. Thus fuel and people should move tem closer to zero and hopefully not tow out .
 
wood to the metal stiffeners?

Just wondering if anyone has gone from the wood to the metal stiffeners and what the improvement was?

If the wood is installed correctly it would appear visually to offer substantial rigidity but until I get my fairings off to see how mine were installed I'm only assuming at this point.

Then again when dealing with harmonics maybe the rubber in the Adel fasteners are the big help here?

Thanks

Tm
 
Shimmy

The shimmy is only in the left gear on my Wittman Tailwind. Lowering tire pressure helped some. Shimmy was mostly on landing at around 25 statute on the GPS.
I just installed the metal stiffner on the left gear only. 2024, 1/4" x 1 1/2". I wanted 3/16' but it wasn't readily available. I removed the rubber from the clamps and modified the ends so the bar is clamped between the clamp ends at all locations. Seven clamps, the bar is as long as space permits.
Two landings on very rough pavement, noticeable improvement but still some shaking.
My gear is 13/16" diameter at the "ankle" I know the RV6 is 7/8" and I regret not using 7/8". What is the diameter of the RV4 gear?
I am going to do some more testing including on relatively smooth runways.
 
Not sure what the diameter is on the 4. Will check in a couple of weeks.

I can live with the odd shimmy every few dozen landings, but not every landing.

I'll pull the gear leg faring's off to see what my wooden stiffener's look like. If everything looks good I may forgo the science experiment with the aluminum bar unless some one else has experience otherwise and go back to the smaller tires and original wheel pants if needed.

Tim
 
With the wooden stiffeners, you might be able to improve things by gluing on some unidirectional carbon to the long edge that's farthest away from the gear leg. Maybe. Hold the carbon taut while gluing to keep it really straight. You can use 5 minute epoxy, a dab at each end, for that, holding it tight while it cures. Come in with the main epoxy after the 5 minute stuff is well-cured.

Don't know how this compares at all but it's a smaller change, if you already have the wooden pieces attached.

Dave
 
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