Thorp Air Command - T18.net

does anybody have a procedure for aligning the mains?

I used a string and a carpenter's square. I started by tying the string across the front of the tires. The string is tied to the wheel nuts. I place the square on the side of the tires horizontally and on the string. For zero toe in the square should contact both the string and tire sides. If it doesn't then shim the axle where it contacts the gear leg. To check the camber just place the square on the floor and the side of the tire where there is no bottom bulge.

Jim

I did similar but I also snapped a chalk line down the center of the airplane to check that the wheels were staying parallel to the fuselage. I don’t know what others are using for shims. My builder used the wheel pants bracket which I removed with the wheel pants he had. I cut my shims on the milling machine out of steel plate.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

For shims, used a piece of aluminum from your after flight cold drink can, it's on the order of 0.004" thick, I needed two layers under just two of the bolts on one axle to set the toe-in. Or Spruce has taper shims https://www.aircraftspruce.com/catalog/lgpages/tapershim.php?clickkey=3020554. Personally I like the price of the home made ones myself.

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Bruce Finney
N18JF T-18C #262
Yelm, WA USA

I needed quite a bit more than what a person could get from a can. I think it would have been on the order of 25 layers at the thickest corner.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

There is a paper I wrote on the subject somewhere on this sight. It involves making aluminum and grease sandwiches so the gear relax. Also, you need to make sure you have the weight that is normally in the plane, in the plane. I recommend using a cheap laser pointer. You can put it on the brake disk and shine it on the floor in front of the plane for a very accurate measuring tool. Also, remember that the motor is NOT on centerline. Do not hang a plumb bob off the spinner.
Cubes

I bought one of the aircraft spruce 500x5 taper shims three years ago and am just now getting around to installing it for this year's conditional. I think it is 1/2 degree but will check the part number when I get back to the airport. Here's a picture of what the Aircraft Spruce shim looks like...

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John Twigg N12055 SN.79 Pittsburgh, PA (KFWQ)

My mechanic and I went round and round hypothesizing how to fix my alignment based on my multiple complaints of the airplane wanting to drift left (a lot!) on landing as well as the highly uneven tire wear on the left tire. After considering the laser idea, snap line, and everything else, Mr. A&P/IA suggested we just use a chunk of aluminum from a soda can as suggested by Bruce Finney previously, and place it so as to counter the obvious caster or camber error (I forget which) based on the observed outside-of-the-tire excess wear. Badda-BING, perfection. The left turning drift disappeared such that I could take a nap after a wheel landing until she slowed down enough for me to put the tailwheel down. Hundreds of hours since then on new tires, both of which wore as evenly as could possibly be deciphered on visual inspection until all treads were too thin, now another set of new tires that are also wearing evenly. KISS, and let us know how you make out with the shim(s).

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Philip Mandel
N118BC
S/N 957
Beaverton OR

I've read the various posts on wheel alignment procedures...

My airplane's wheels have toe out, as evidenced by both tire wear (inside wearing much faster) and a check with straight edges, etc. The camber looks good (tires are vertical.)

I have the wheels on aluminum / grease plates so the gear can relax, and have the plane loaded with full fuel.

After some disassembly and experimentation, it looks like it will need about .030" worth of shims on the aft axle bolts to bring it to zero toe. I can do that, but it will be putting a bend on those bolts, and that doesn't seem copacetic. I could ream the four holes with the shim in place, but not sure that's right, either.

Is bending the gear leg an option? Possibly with a long section of pipe? Or?

Any recommendations?

Thanks a lot,

Peter

I have shimmed something like that and I believe the bolts did bend but I have not seen any negative consequences. The gear leg is extremely strong, I wouldn’t try to bend it, I doubt you can control the amount of twist anyway.

Thanks David, I appreciate your experience and input.

My concern is, after those bolts bend (which requires somewhat considerable torque on the nuts to achieve, I've found,) will they have sufficient strength to keep the axle assembly together? I can envision if one bolts breaks, the other three will quickly follow suit. And then its a bad, very bad day.

Since the axle is easy to replace (in case I screw it all up,) I'm considering oversizing the holes in the shimmed axle base to align with the gear leg mount, and possibly use NAS bolts. Any thoughts on that idea?

I’m not an engineer but I think that would be fine too. No more than .030 would change the direction I don’t think you would need to ream much

And...

That's what I did this morning- took a reamer and went through the four holes. It was much quicker and removed less material than I expected. The bolts then went in snug- but no bending required.

So, I'm happy with this. Looking forward to flying it tomorrow to see if it handles better on landings, etc.

Thanks again for your input, David. It really helps!

Peter