Jet Model Airplane Aluminum Landing Gear Repair (TIG aluminum)

[jakeru]

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!

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


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.




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:


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.


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.


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.