Avoiding another bent front gear leg - help needed

MrNomad · #1 02-20-2011, 01:56 PM

In an effort to help "A" model owners avoid a bent front gear leg, Chet and I built a test fixture on a boat trailer which we will tow behind my truck to simulate rolling down the runway. Our goal is to observe the load as we tow the fixture thru dips and over bumps, and to measure the gear leg flexing and bending that occurs.

The next phase of this build will be to add 250lbs atop the front wheel to simulate the load supplied by the airplane.

While Chet is a retired EE, we could use some help with those devices we should attach to the fixture to get measurements and decide where to stiffen the front gear & with what materials. If you'd like to help us by loaning us measurement equipment, we promise to handle whatever you loan us for these tests very carefully and to return it to you when the tests are completed.

Chet and I both wrote to Vans and received a polite reply, but no solutions were offered beyond what's already known. If you're tired of reading about bent gear legs and you?d like to help us with test equipment, please contact Chet:

Oldak ?at? Comcast.net or 520-797-7161 8am to 6pm Tucson time.

Thanks.

vic syracuse · #2 02-20-2011, 02:23 PM

Barry, this is a noble undertaking, but I can't help but wonder if you are really duplicating the RV's. Wouldn't it be better to have the leg attached to an engine mount? It seems to me that attatching it the way you have done, with what appears to be a weld at some distance down the leg, you have actually changed the harmonics of the gear leg itself. You might discover a problem, and engineer a fix for it, but it might not necessarily work on the aircraft due to the different configuration you have here.

Vic

MrNomad · #3 02-20-2011, 02:49 PM

It is important that the tests have meaning and produce results that benefit our community so we appreciate any and all comments.

The portion of the gear leg that extends from our fixture is the same as the portion that extends beyond the engine mount on my 9A. We replicated the angle too. Yes, I guess the engine mount might flex too, but (hopefully) not enough to skew the results.

The first test run had me (200lb old man) riding on the trailer fixture but we know that's unsafe so we hope that our fellow builders will loan us devices to video and measure. Tomorrow we will weld some steel onto the fixture and bolt 5 Chev cylinder heads to the fixture to add weight. Chet knows about D/A converters but we don't have any such devices so we're hoping this will become a collaborative effort with results that will benefit all of us.

Basically, we're eager to see where this takes us and how receptive our community is to the idea.

Thanks again.

N941WR · #4 02-20-2011, 03:12 PM

Barry,

When you get it set up, try it with different tire pressures.

It would also be interesting to see if the Matco axle makes any significant improvement over the stock Van's axle.

MrNomad · #5 02-20-2011, 03:36 PM

We will definitely try it with different air pressures. I can readily see where a soft front tire will set up different harmonics than a tire with 35lbs.

We don't have a Matco axle but if someone wants us to mail us one for testing and comparison, we're happy to accommodate. Neither one of us are mechanical engineers so we're looking for ideas and direction from folks more experienced. Basically, we're two old guys who know how to weld.

After I bent my nose gear 200 hours ago, we eliminated the drag the stock seals add to the front wheel by inserting a steel spacer on the axle. Using an air conditioning spacer, we machined the length of the spacer so that the seals still "seal" but no longer add drag.

vgb · #6 02-20-2011, 03:48 PM

I think what you guys are doing is a great idea and have a good Idea for testing. And best of luck.
And don't let anyone discourge you from experimenting to try to solve a problem that DOES exist.

Radomir · #7 02-20-2011, 03:48 PM

That's exactly what that Matco axle that Bill mentioned does.. so if you made your own, you wouldn't need the Matco part..

Quote:

Originally Posted by MrNomad

After I bent my nose gear 200 hours ago, we eliminated the drag the stock seals add to the front wheel by inserting a steel spacer on the axle. Using an air conditioning spacer, we machined the length of the spacer so that the seals still "seal" but no longer add drag.

Stabird · #8 02-20-2011, 03:50 PM

Vic. The gear leg is enclosed in a hardened steel tube similar to that used in Van's engine mounts. The tube is welded at the bottom with welded stiffner supports at the top. In essence the gear support tube is positioned and supported as in the standard engine mount. The rest of the structure is 1 3/4 14 gauge box tubing with added stiffening structure. We feel it will not add any unsual harmonics to the structure and is stiffer then a standard mount.

In our preliminary tests we noted deflections of about 1.5 inches back and forth on a smooth surface and low speeds. Additional testing is planned.

Andrew M · #9 02-20-2011, 04:01 PM

What kind of test equipment are you looking for? How about a high speed video with a black on white grid panel mounted close to the assembly to show scale? Are you planning on testing to failure?
A few points, placing the trailer wheels at the same location as the aircraft mains, and loading the entire trailer as an actual aircrafts may provide the benifit of duplicating a bounce situation, if you want to go that far.
I myself am going with conventional gear, but enjoy the process of experimenting, looks like fun!

rvbuilder2002 · #10 02-20-2011, 04:04 PM

Quote:

Originally Posted by MrNomad

It is important that the tests have meaning and produce results that benefit our community so we appreciate any and all comments.

And it is in that context that this is offered.

Your effort is noble, but I don't think it is likely to provide any useful information. In fact it could actually cause a greater misunderstanding.

The reason I believe this, is the design of your test rig.

A tri-gear RV can bob up and down in pitch as the gear legs flex. If you tow the trailer by hitching it to a vehicle, you have removed any allowable pitch movement from the equation. The test rig would need to be designed so that you are simulating all three gear with the springiness (highly technical term) of the three gear legs (logical way would be use three actual gear legs and wheels/tires).

Then you would need to have the equivalent weight effected on the three gear legs at a typical vertical and longitudinal C.G. position so that a similar moment of inertia is simulated when the test rig bobs about the pitch attitude. And, it needs to be towed in a manor that would not induce any resistance to the pitching motion induced by the legs flexing.