Goodbye wooden gearleg stiffeners

rocketbob · #1 10-14-2019, 08:13 AM

About 5 years ago a friend of mine installed 380 tires on his RV7. I fly this airplane regularly. With the 380 tires a shake would develop right around 19 KTS GS which required firm braking to slow thru that speed fairly rapidly to get the shake to stop. Lived with it but it was pretty annoying. Things tried to fix the shake: carefully balancing the tires, new tires instead of recaps, new wooden gear leg stiffeners, various tire pressures, tried one gearleg with a stiffener without the other having one thinking a resonance was developing between the two. Nothing really worked but there were varying degrees of effect on dampening the oscillation. That is, until the new idea we came up with and implemented a few weeks ago.

Wrap the gear legs with 3K 5.8oz carbon fiber. No stiffeners.

Using a 48" roll of carbon fiber, squeegee wetted a 4ft or so square, 8" wide strips were cut using a rotary knife which were then rolled up and wrapped around clean gearlegs bottom to top in one pass overlapping 50% which amounts to two layers of carbon.

No peel ply or vacuum bagging was used as this was considered an experiment.

The layup was done with mains on the floor.

After about a dozen or so hours still no shimmy or shake at all and it seems to be holding up fine.

The gear feels absolutely perfect now with no tendency to rebound or skip. Tires are 35psi. In no way does the gear feel stiffer, but feels well dampened. For this reason I don't think any additional stresses are placed on the engine mount.

David Paule · #2 10-14-2019, 08:51 AM

What was the fiber orientation?

Since carbon can contribute to dissimilar metal corrosion, how did you protect the bare steel? Or did you use some sort of primer between them, and if so, what kind?

Given that carbon has a Young's modulus that's generally considerably lower than steel, can you give us a comparison of the EI with and without the carbon?

Thanks very much!
Dave

rocketbob · #3 10-14-2019, 09:28 AM

We didn't cut in a bias direction off the roll as it wasted too much much material. Discussed it, didn't do it. So whatever orientation it is cutting across the 48" width of the roll.

The gearleg was cleaned bare. I have never had any problems with carbon fiber corroding steel or aluminum when bonding directly to each other. I believe this concern is overblown.

As far as any engineering analysis goes, none was done, nor is anything going to be done. Consider this an experiment that is working.

scsmith · #4 10-14-2019, 11:04 AM

carbon bonded directly to aluminum will corrode the aluminum rapidly if exposed to an electrolyte. Don't dunk it in sea water. Exposed to atmospheric moisture, the aluminum will corrode slowly and degrade the bond over time (years) because moisture (as vapor) can diffuse through the epoxy.

carbon and steel are not as galvanically dissimilar as carbon and aluminum. a bond to bare steel and not exposed to electrolyte will last a long time. But not indefinitely.

An easy way to prevent problems is to first cover the bare metal with a very thin ply of fiberglass, then put on the carbon. The fiberglass ply will provide an electrical insulator/isolator. But I still would not expose a bond like this to electrolyte.

The gold standard would be to apply AC-130 SolGel, followed by BR-6747 primer to the bare metal.

Bavafa · #5 10-14-2019, 11:28 AM

Quote:

Originally Posted by scsmith

carbon bonded directly to aluminum will corrode the aluminum rapidly if exposed to an electrolyte. Don't dunk it in sea water. Exposed to atmospheric moisture, the aluminum will corrode slowly and degrade the bond over time (years) because moisture (as vapor) can diffuse through the epoxy.

carbon and steel are not as galvanically dissimilar as carbon and aluminum. a bond to bare steel and not exposed to electrolyte will last a long time. But not indefinitely.

An easy way to prevent problems is to first cover the bare metal with a very thin ply of fiberglass, then put on the carbon. The fiberglass ply will provide an electrical insulator/isolator. But I still would not expose a bond like this to electrolyte.

The gold standard would be to apply AC-130 SolGel, followed by BR-6747 primer to the bare metal.

Plus one
Obviously Steve has the credentials, my info is just based on readings.

okdonn · #6 10-14-2019, 12:26 PM

Bob, just to be clear, did you spiral wind the 8" x4' length of carbon on the legs, overlapping 50%?
Thanks for sharing this discovery/experiment!

rocketbob · #7 10-14-2019, 12:29 PM

Quote:

Originally Posted by okdonn

Bob, just to be clear, did you spiral wind the 8" x4' length of carbon on the legs, overlapping 50%?
Thanks for sharing this discovery/experiment!

We had 3 or 4 layups, can't remember. You can use whatever width you want as long as you wrap 50% overlap. Might be easier to order carbon fiber tape to do this as the edge unravelling can be a pain.

Gusmax · #8 10-14-2019, 12:40 PM

I would have a difficult time doing this to my beautiful titanium F1 gear legs because they look so nice!

rocketbob · #9 10-14-2019, 12:56 PM

Quote:

Originally Posted by Gusmax

I would have a difficult time doing this to my beautiful titanium F1 gear legs because they look so nice!

I'm in the same boat.

RV7Guy · #10 10-14-2019, 08:46 PM

If you are wrapping it over the powder coated gear leg there shouldn't be a problem with the carbon over the steel leg.

I think an ideal situation for this is carbon fiber sleeve available from CST Sales. Of course this would have to be done before you mount the brakes. With this material you pull it over the gear and tape it to the gear just below the mount. Run it down as far as you need and pull it tight. Tape the bottom. Wet it out with epoxy squeezing the epoxy into the carbon with gloved hands. Wipe off excess with a paper towel.

Interesting concept using carbon as a dampener.