Gear Leg Stiffeners/Dampers?

The Whitman Strut legs are damped laterally due to tyre scrub just like on a Cessna but they can vibrate fore and aft. Do the wood molding and fiberglass stiffener/dampers work - and if so how do they work? Do they add significantly to the fore/aft bending stiffness of the legs and/or do they add some type of "material damping" to the system.

Simple wood moldings and hoop wrapped fiberglass won't add much bending stiffness - especially when compared to the forged steel leg. A significant proportion of axially aligned glass or carbon fiber rovings might add more stiffness, especially if concenrated on the "nose" and "tail" of the leg but the drawings don't mention this.

A damping mechanism might be the wood rubbing on the metal leg as the fiberglass encapsulated assembly shears fore and aft, however the wood is generally adhesively bonded to the leg so the bond would have to break for this to work - unless the internal flexing of the wood acts as a damper - which may be the most likely answer.

Has enyone tried bonding the wood to the leg with a thick layer of RTV before encapsulating it in fiberglass. The "rubber" would then be working hard in shear as the leg tried to vibrate fore and aft and may soak up more energy than if the wood was "hard" bonded.

What about just wrapping the leg with a tube of rubber - say 1/8" thick - and then wrapping that with a layer of say 2/3 axial, 1/3 hoop glass (or carbon) with the axial fibers concentrated on the nose and tail. Leave one end of the rubber exposed, say the lower end, and only bond the composite sleeve at the other end to keep it fixed on the leg. This would form a concentric beam around the steel strut and spaced from it by the layer of rubber. As the two beams bent, one inside the other, the rubber would be forced to shear between them and absorb energy.

It would be easier to build than the wood molding version to boot! Too whacky?

Anyone have any experience of different options?

Tuned mass/spring dampers (like the little dumb bells on power lines) inside the wheel pants?

???

(I need to get out more!)

Jim Sharkey

Yea they work but its a choice

Short answer is yes they help, a little. I know there are several threads on the topic. The common wisdom I'll pass on is fly without and add them later if you feel the need. I flew without and than added them and they made a difference. It gave a more solid feel. The main wheel would shimmy occasional on a fast taxi. I'm glad I did it but would not do it again simply because its weight and more work. My solution could have been just taxi slower.

I used hard wood stiffener (forgot Oak, Walnut or Maple), on the front of the gear, with carbon fiber wraps in 4 locations, it was stout. It felt more solid after I did the mod. Watch a RV taxi and you will see the wheels walk back and forth. I guess a tiny strip of pine and one wrap of glass at the top and one around the bottom its not going to be as effective.

The idea of bonding is fine, some use pro-seal. Chance is the bond will shear sooner or later but it allows more contact area and fit between the dampener wood and metal gear. The wood is not reducing deflection or adding strength in any way. It just is dampening the "spring back". Also consider any WRAP will add thickness to your gear leg. That makes the fairing grow wider (more frontal area) and therefore more draggy.

Douglas Fir

Van's told me to use clear Douglas Fir for the leg stiffners, if needed. We have no shimmy in the first RV-9A but will keep this in mind when I finish the second plane. Douglas fir is supposed to have the highest strength yet light weight, as per Van's.

Pat Garboden
Ozark, MO
RV-9A N942WG slider for sale

RV-9A N942PT (reserved) tip up in wiring stage

Another thing to consider - anything that adds stiffness will focus more stress in the gear leg at its top end. I recall specifically that Van, in the case of the nose gear legs which were redesigned circa '98, recommended against any stiffeners for this purpose. But, the same argument can apply to the mains.

Alex is correct. If you want to see the material stress at various stations along the leg length, input RV leg dimensions into Neil Willford's excellent landing gear spreadsheet. It is (was?) available on the EAA website under Sport Aviation downloads. The taper ratio of a Van's leg is set so max stress is roughly in the middle of the leg, not at the upper socket. Input a straight leg to see the new location for max stress.....and Alex's point. No, adding a wood or composite stiffener isn't quite as radical, but it is illustrative. Education and recreation, right?

A little recognized detail: Some of the fore-and-aft wheel movement may not be longitudinal bending of the gear leg. It can be torsional twisting of the leg, and a typical wood "stiffener" isn't going to make any difference in torsion. To understand the torsional issue, extend the centerline of the gear leg to the ground and compare the point of contact to the center of tire contact. The difference generates a torsional moment. The arm varies with tire diameter, which one reason why inflation pressure can vary shimmy performance. The arm also varies with leg deflection under load. Here's a sketch of an RV-6 leg as an example.



This sketch was borrowed from some exploratory work I did for a biplane. The tire diameters are 16.375 and 14.375. These may not be correct for an RV tire; it is raining in Alabama this AM and I don't feel like getting wet going to the shop for a measurement. With a 16" tire the calculated torsional moment can be considerable.

zip ties

A 9A builder near me attached the wood stiffener to his front fork with a buch of zip ties and it works fine. Its all covered by the glass faring. Easy, quick, and undoable, plus no epoxy mess!

Zap Atraps may be quick and easy but watch out for wear. They have a habit of chewing thru things including engine mounts.

DanH said:

Alex is correct. If you want to see the material stress at various stations along the leg length, input RV leg dimensions into Neil Willford's excellent landing gear spreadsheet. It is (was?) available on the EAA website under Sport Aviation downloads. The taper ratio of a Van's leg is set so max stress is roughly in the middle of the leg, not at the upper socket. Input a straight leg to see the new location for max stress.....and Alex's point. No, adding a wood or composite stiffener isn't quite as radical, but it is illustrative. Education and recreation, right?

A little recognized detail: Some of the fore-and-aft wheel movement may not be longitudinal bending of the gear leg. It can be torsional twisting of the leg, and a typical wood "stiffener" isn't going to make any difference in torsion. To understand the torsional issue, extend the centerline of the gear leg to the ground and compare the point of contact to the center of tire contact. The difference generates a torsional moment. The arm varies with tire diameter, which one reason why inflation pressure can vary shimmy performance. The arm also varies with leg deflection under load. Here's a sketch of an RV-6 leg as an example.



This sketch was borrowed from some exploratory work I did for a biplane. The tire diameters are 16.375 and 14.375. These may not be correct for an RV tire; it is raining in Alabama this AM and I don't feel like getting wet going to the shop for a measurement. With a 16" tire the calculated torsional moment can be considerable.

Click to expand...

Thanks Dan - I'll follow up on that.
Jim Sharkey

Yesterday I said the torsional moment varies with leg deflection. That moment can get very large as the arm increases significantly at full load. Here's another drawing borrowed from last year's biplane work. Not exactly an RV install, but if I recall correctly the leg seen here is from an RV-6. The tire and leg deflection at 3.7G is calculated, not a guess, another detail from the Willford gear design spreadsheet.



The torsional arm at full deflection (blue line) is 9.9 inches in this example. Rolling resistance and any other force pushing the tire rearward is seen as a twist of the leg in it's fuselage socket (force x arm) and can be many hundreds of foot lbs. Viewed from above, the tire motion due to leg torsion would describe an arc, a change in toe so to speak, which has all kinds of interesting effects. The actual motion is a combination of leg bending and torsion. A conventional wood stiffener may have an effect on bending (depending on attachment), but do little in torsion.

<<This would form a concentric beam around the steel strut and spaced from it by the layer of rubber. As the two beams bent, one inside the other, the rubber would be forced to shear between them and absorb energy.>>

Ain't forgot about you Jim. Interesting concept. Have a friend who kicked around something similar for a different application. The outer shell needs to be engineered for a specific stiffness if I recall that project correctly. The problem in this application is the concentration of stress at the top of the leg. If the outer shell is stiff enough to do any good if becomes, in effect, the same as a thickened leg section. To avoid that, the shell would need to extend up through the socket.

DanH said:

Yesterday I said the torsional moment varies with leg deflection. That moment can get very large as the arm increases significantly at full load. Here's another drawing borrowed from last year's biplane work. Not exactly an RV install, but if I recall correctly the leg seen here is from an RV-6. The tire and leg deflection at 3.7G is calculated, not a guess, another detail from the Willford gear design spreadsheet.



The torsional arm at full deflection (blue line) is 9.9 inches in this example. Rolling resistance and any other force pushing the tire rearward is seen as a twist of the leg in it's fuselage socket (force x arm) and can be many hundreds of foot lbs. Viewed from above, the tire motion due to leg torsion would describe an arc, a change in toe so to speak, which has all kinds of interesting effects. The actual motion is a combination of leg bending and torsion. A conventional wood stiffener may have an effect on bending (depending on attachment), but do little in torsion.

<<This would form a concentric beam around the steel strut and spaced from it by the layer of rubber. As the two beams bent, one inside the other, the rubber would be forced to shear between them and absorb energy.>>

Ain't forgot about you Jim. Interesting concept. Have a friend who kicked around something similar for a different application. The outer shell needs to be engineered for a specific stiffness if I recall that project correctly. The problem in this application is the concentration of stress at the top of the leg. If the outer shell is stiff enough to do any good if becomes, in effect, the same as a thickened leg section. To avoid that, the shell would need to extend up through the socket.

Click to expand...

The idea is not to add significantly to the bending stiffness of the leg but introduce a mechanism to absorb energy as it flexes. That's why I suggested only attaching the "concentric tube" at one end - a bit like holding a sheaf of paper at one end and letting it flex by having the sheets slide over each other.

Jim

PS - any better direction to the program download site?
Thanks

Jim,
No problem...the landing gear tool is at:

http://www.eaa.org/sportaviation/2004/S409-LandingGearDesign.xls

The companion article is in the Sept 04 issue.

There are a whole bunch of useful spreadsheets and design tools at:

http://www.eaa.org/sportaviation/moreinfo.asp

<<only attaching the "concentric tube" at one end>>

Ahhhhhh.....

I think you have an interesting idea. Keep in mind rubber doesn't have much damping (ability to remove energy from the system, usually as heat). Check published "damping factor" figures for something like a Centaflex rubber torsional coupler for an example. If it could do much actual damping due to internal friction it would melt. It is really a spring, ie an isolator.

That's not to say this idea won't work; internal damping is low, but there is some. And there's nothing to say you couldn't put sliding friction to work in a shell and core system. The really neat thing about your shell idea is that it should provide some damping in all axis. Go for it!

And after 25 hours of flying....

....and 90 landings it doesn't need anything

Jim Sharkey
RV6 - Phase 1