Thorp Air Command - T18.net

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James Ball

Don't now what happened to the text I put with the photos. Here it is again.
During a routine an annual inspect of a T18 here in AU , we found some cracking in the Main landing gear. I thought I would share so others can look at their T18 in this area. We repaired the weld and added a doubler to both front and back sides.
Cheers

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James Ball

Let us know how this works out as it is a common problem.

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Bob Highley
N711SH
SN 835
KLAL

Was your gear already heat treated? Did you heat treat again?

As my luck would have it, I found the exact same cracks on MLM.

DW

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In theory, theory and practice are the same. In practice, they are not.

OK... question answered. Saw your notes.

Is 550 enough? I worked in a spring shop for a couple of decades, and we tempered 4130 at 825 for an hour or so. (After a full quench of course.)

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In theory, theory and practice are the same. In practice, they are not.

The main landing gear was heat treated on manufacture . We tempered as per notes after repair . Hope that will do the trick . We have repaired other legs and they seem to be ok. I personally don't think the fillet weld was adequate. The weld looked pretty but needs more metal in the weld.

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James Ball

I would need to find and check my old receipt but I thought the tempering temperature was 1100 plus degrees.

Well, I searched the old thorplist and found the results to be quite interesting. There was a lot of advice suggesting to do nothing more than stop-drill the ends of each crack and just monitor. Has that advice truly withstood the test of time?

Personally I'm a little uncomfortable with cracks in major load-carrying parts. I really like jtball's repair shown above. It makes a lot of sense to me to have a tapered tongue to reduce the stress concentration of the gusset.

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In theory, theory and practice are the same. In practice, they are not.

I was one who used the stop drill method and it served me about 1000 hrs. until the crack finally went around the corner. I, too, had pretty but small, inadequate welds. I suspect taxiing on unimproved surfaces was the culprit in stressing the weld.

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Bob Highley
N711SH
SN 835
KLAL

What are peoples experience with distortion after heat treat (or re-heat treat)?

Seems to me as long as the part goes into the quench oil vertically, there shouldn't be much.

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In theory, theory and practice are the same. In practice, they are not.

In my experience, heat treating distorted the gear legs quite a bit. I used the bow and arrow method for straightening the gear.(Hydraulic jack and fairly heavy chain)

Hi James, the welds look good. As Bob has said these gear legs somtimes end up with cracks in this area, I dont believe there is anything weak about this gear leg system, but if it gets banged right then somtimes these cracks show up. This is where the legs show some stress when they are loaded or over loaded. Watch the rest of the structure to make sure the stress point hasnt moved somwhere else after beefing up the one on the front of the legs. The Fillet was a little skimpy, you sure done a fine job......Dan

That T18 will be back soon due to the aged plastic brake lines which I am replacing with braided hose,so I will have better look at the surrounding structure then.

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James Ball

Just curious, where can one get a copy of the drawing showing the gear repair details? - DW

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In theory, theory and practice are the same. In practice, they are not.

I will ask the owner of the drawings, if its OK to put on the forum. The drawings are not approved as such but I use an Approved Person in the Sport aircraft association of Australia to incorporate such a modification. Who gives out certificates of airworthiness certificates for experimental category. I will get back to you soon.

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James Ball