Jet Model Airplane Aluminum Landing Gear Repair (TIG aluminum)

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11-08-2011, 07:24 AM
jakeru

8 Attachment(s)

Jet Model Airplane Aluminum Landing Gear Repair (TIG aluminum)

I friend and customer of mine who is really into model airplanes (he organizes at least one annual Model Hobby exposition after all) has a jet model airplane with damaged landing gear I repaired for him. And by "jet model airplane", I don't mean a "modeled" jet engine. I mean a real jet engine, i.e., a gas turbine powered model airplane! Sounds pretty crazy.

He said he damaged the landing gear from being crash landed at about 100 miles per hour! :O

Anyway he added the landing gear cost $400 and is no longer available. So the repair route seemed like a natural solution.
Attachment 3960

It has a small compressed air cylinder on board that gets switched to power a ram that lowers the landing gear by the way. (Not shown.) Lot of cute little precision machined CNC aluminum components making up this little assembly.

I told him I gotta see this model jet airplane sometime!

Here is how it looked before welding. I solvent washed it clean (it was quite dirty and also oily from the piston oil lubrication) and wire brushed the end a little before welding.
Attachment 3958

Attachment 3959

Here is how I held is steady for welding to begin with. Welder's finger / third hand comes in handy for small items like this. The other piece was an old piece of practice scrap that was about right for holding it up:
Attachment 3961

I started by running a light arc over the edge to etch it clean. When you find a dirty spot using this method, it makes it worth it because you can clean it before you melt the two pieces together. Here is how it looked after the etching: It wasn't light enough current to not melt the very tip, but that's OK.
Attachment 3962

In this example, it's not pristine clean, but passable. The little black areas are from contamination and better if they are not there. The inside of the tubing was difficult to prepare. I couldn't sand down very much due to the thin tubing. I also couldn't leave a ridge/weld bead sticking inside the tubing, because there is a tight tolerance piece that needed to fit inside the tubing.

Getting the initial first tack weld (or 2 or 3) can be tricky. In this case, getting the gap not too big was critical, as was getting the right orientation and getting the pieces of tubing perfectly straight. Of course getting evertyhing aligned and straight is an iterative process - what matters is that the final, as-welded part is straight. I recall hammering the parts a bit to persuade them.
Attachment 3963

The damage from the crash landing tore the pieces apart and tore the tubing out of round a bit as well.

After that, the game was welding it up without over penetrating. Actually, I couldn't leave a build up area inside the tubing much at all, because tight fitting components had to be in there. I wanted to raise the bead up proud, for strength. Not the prettiest weld bead, but mission accomplished. Inside this tubing there is a precision piston (that doesn't need to slide up here by the weld) and behind it a compression coil spring.

If I had a piece of steel or stainless rod that fit the ID of the tubing precisely, it would have been ideal to stick it in there while welding as a backing apparatus. The tubing however was custom machined to a funky, nonstandard ID. So I didn't have the right size tubing. But this should hold up at least until the next time it is crash landed at 100 miles per hour. :P
Attachment 3965 Attachment 3964
11-08-2011, 07:58 AM
parkour

That looks pretty nice.
Thanks for showing it.
11-08-2011, 03:24 PM
everlastsupport

Nice work Jake. He got lucky it broke where it did and I can see why you were hoping for a pipe or round bar to stick inside as well. But looks like you handled it just fine.

Good work as usual. Did he mention how much other damage there was $$? That is a big boy toy for sure. I've seen them, unreal.
11-08-2011, 03:53 PM
Trip59

Are you guys going to turn it back down (or file it back down?) or is that not critical? Would copper work as a backer as nice as it does on steel?
11-08-2011, 06:15 PM
67cudafb

Looks like tuff piece to repair pretty thin tube you need to post picks of the plane sitting on landing gear and maybe in flight nice job let us know if it all works when it is reassembled
11-08-2011, 06:59 PM
DaveO

I wonder if these landing gear assemblies could be designed with break-away parts, kinda like the shear pin in a snow thrower, that would break on overloading and provide an easier fix-it point. Or maybe those 100 mph landings aren't frequent enough to need them.
11-09-2011, 04:04 AM
Tritium

I bet that is one expensive toy. The turbines for those start around 2 grand and many are around 5 grand. I saw a big jet on YouTube, a B52 that had 8 engines. It crashed in smoke and flames and I bet the owner was in tears over that one.

Here is the video of its maiden flight:

http://www.youtube.com/watch?v=fbITzCI2AU0 :)

Here is it's crash:

http://www.youtube.com/watch?v=YAWwe474YHk :(

Thurmond
11-09-2011, 04:15 AM
DVA

I must first say nice repair. Hope it holds up. I have been flying model airplanes for over 20 years and have learned that sometimes those things like the ground much more than the air.
11-10-2011, 07:30 AM
jakeru

Quote:

Originally Posted by Trip59

Are you guys going to turn it back down (or file it back down?) or is that not critical? Would copper work as a backer as nice as it does on steel?

I could certainly file it down for aesthetics, but it would weaken it for sure if I did that, and since it sounds like strength is more important from what the customer tells me, I will leave it. It is not interfering with the fuselage, (which I have not yet seen) I am told there is a good deal of room. (I'm still not sure exactly how much that means... I had a thought of adding a reinforcing rib but wasn't sure how much room I had exactly.)

Quote:

Originally Posted by everlastsupport

Good work as usual. Did he mention how much other damage there was $$? That is a big boy toy for sure. I've seen them, unreal.

Thanks guys. It sounded like this was the main part damaged, and all that was preventing it from being flown again, although based on the landing gear completely broken apart, I imagine there are at least some other gnarly scrapes and cosmetic damage to the fuselage, wings, etc!

Quote:

Originally Posted by 67cudafb

Looks like tuff piece to repair pretty thin tube you need to post picks of the plane sitting on landing gear and maybe in flight nice job let us know if it all works when it is reassembled

I certainly hope to see some pictures of this thing assembled also, at some point. I will certainly share them with you guys if I do.

Quote:

Originally Posted by DaveO

I wonder if these landing gear assemblies could be designed with break-away parts, kinda like the shear pin in a snow thrower, that would break on overloading and provide an easier fix-it point. Or maybe those 100 mph landings aren't frequent enough to need them.

I can't really speak for how frequent the 100mph crash landings are. Except that if it were me flying the controls, it's probably be way too often. I'd probably press the controls in the opposite direction of how I intended to make the plane go. :P