Thorp Air Command - T18.net

I am performing tail surgery on 633PM. I stripped a rear nylock nut plate while switching tailwheel setups, and after removing the feathers, discovered some ishoos that required a more serious resolution. The flange coming bottom forward off the rear bulkhead was in bad shape - see photos below. I will post pics of the fix tomorrow. I know the plane was ground looped some years ago and I suspect that was the culprit in creating the fatigue cracks, tho perhaps not the sole reason. The fix involves a new Chromoly angle riveted to the bulkhead with the bottom spanning and rivited to the longerons, with a chromoly spacer between the angle and the skin (the thickness of the longerons, which I seem to forget at the moment)The bottom flange of the angle will pick up two rivets each side on the longerons and will also transfer load up through the vert stab spar, per the original design.

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Kinda hard to see in the bottom one, but there are cracks behind each bolt hole.

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Fraser MacPhee
N926WM
Serial #279-1
Angel Fire, NM (KAXX)

Not shown in the photos above is the angle bracket/plate that was removed from the rear bulkhead, so some of you thing that was the only reinforcement - the angle bracket flange had cracked as well - once I spotted that, I knew I had ishoos, and upon further probing, that's when I found the bulkhead flange had turned turtle and surgery was schedooled.

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Fraser MacPhee
N926WM
Serial #279-1
Angel Fire, NM (KAXX)

Lovely! You are on the right track to fix the problem. My main concern is with the amount of protective gear the guy with the sander is wearing. The cracking is due to the side loads imposed by the old spring and Maule tailwheel. The newer Trusty setup gives less moment arm to impose load on this fitting. However, inspection of this area is a must on every condition inspection no matter what tail spring/tailwheel setup you are using.

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Bob Highley
N711SH
SN 835
KLAL

Good time to install that electric trim you may have wanted.

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Dave M
Sacramento
ex-T-18 owner

A few more pics of PDT's (Plane Dokter Todd, the hangar neighbor) werk - I was his gorgeous assistant. Fotos below

This is what was left of the original

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Bolted in position ready for riveting to the bulkhead and drilling out the longeron holes

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From the back - note the tabs under the bulkhead on the longerons - at first this concerned me, as the the bulkhead was flanged/tab'd to the longerons, but as I drew out the load paths, and with the thickness of the new piece, it made sense. Also note the spacer between the bracket and the bottom skin.

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The bracket with the longeron holes drilled out and just prior to riveting in place - we mounted the tailwheel first and got everything lined up before drilling them. Some of you may think I'm nutz, but I elected to put 4 3/8s bolts, two per side through the longerons/plate connection. I thought it was a bit much to ask rivets to carry that thickness. I may be wrong, and it leaves four hex heads hanging in the wind, but that's my story and I'm sticking to it.

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Fraser MacPhee
N926WM
Serial #279-1
Angel Fire, NM (KAXX)

Just did this same fix on our 1978 Thorp about two months ago. The cracks to the back bulkhead and the steal plate are almost exactly the same as I saw on N6CM. I wonder how many other Thorps are flying around with this condition?

Brent Schultz
Corona, CA

Brent's correct, this is a common problem with the Thorp tailwheel and as Bob suggested should be part of the condition inspection.

Inspected mine since it's apart and sure enough it was also cracked and repaired somewhere in it's history. Here's a photo of the past repair.

Chuck

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Chuck Kincer
Richmond, Me
575CK
KIWI