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  #11  
Old 06-04-2021, 11:23 AM
mtnflyr mtnflyr is offline
 
Join Date: Mar 2019
Location: Boulder, Colorado
Posts: 22
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Quote:
Originally Posted by FinnFlyer View Post
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.
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  #12  
Old 06-04-2021, 06:13 PM
andoman andoman is offline
 
Join Date: Jul 2006
Location: Payson, AZ
Posts: 77
Default 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.
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"The plane won't build itself..." RV-7A N7.62MM sold
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  #13  
Old 06-04-2021, 07:39 PM
David Paule David Paule is offline
 
Join Date: Dec 2009
Location: Boulder, CO
Posts: 5,419
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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
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  #14  
Old 06-04-2021, 09:11 PM
mtnflyr mtnflyr is offline
 
Join Date: Mar 2019
Location: Boulder, Colorado
Posts: 22
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Quote:
Originally Posted by andoman View Post
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.
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  #15  
Old 06-05-2021, 01:35 AM
PilotjohnS PilotjohnS is online now
 
Join Date: Jan 2016
Location: Southwest
Posts: 2,436
Default 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.
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WARNING! Information presented in this post is my opinion. All users of info have sole responsibility for determining accuracy or suitability for their use.

Dues paid 2022, worth every penny

RV9A- Status:
98% done, 2% left to go
Structure done (less gear)
Electrical/Panel done
Firewall Forward 95% done less baffles and airbox
Fiberglass 95%
www.pilotjohnsrv9.blogspot.com
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  #16  
Old 06-05-2021, 07:49 AM
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Greg Arehart Greg Arehart is offline
 
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Location: Sequim, WA/Atlin, BC
Posts: 2,447
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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.
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  #17  
Old 06-05-2021, 08:08 AM
FinnFlyer FinnFlyer is offline
 
Join Date: Aug 2018
Location: Bell, FL
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Quote:
Originally Posted by PilotjohnS View Post
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
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  #18  
Old 06-05-2021, 10:22 AM
PilotjohnS PilotjohnS is online now
 
Join Date: Jan 2016
Location: Southwest
Posts: 2,436
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Quote:
Originally Posted by FinnFlyer View Post
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“ ???
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John S

WARNING! Information presented in this post is my opinion. All users of info have sole responsibility for determining accuracy or suitability for their use.

Dues paid 2022, worth every penny

RV9A- Status:
98% done, 2% left to go
Structure done (less gear)
Electrical/Panel done
Firewall Forward 95% done less baffles and airbox
Fiberglass 95%
www.pilotjohnsrv9.blogspot.com
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  #19  
Old 06-05-2021, 12:51 PM
David Paule David Paule is offline
 
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Location: Boulder, CO
Posts: 5,419
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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
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  #20  
Old 06-05-2021, 05:38 PM
PilotjohnS PilotjohnS is online now
 
Join Date: Jan 2016
Location: Southwest
Posts: 2,436
Default 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.
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John S

WARNING! Information presented in this post is my opinion. All users of info have sole responsibility for determining accuracy or suitability for their use.

Dues paid 2022, worth every penny

RV9A- Status:
98% done, 2% left to go
Structure done (less gear)
Electrical/Panel done
Firewall Forward 95% done less baffles and airbox
Fiberglass 95%
www.pilotjohnsrv9.blogspot.com
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