Thorp Air Command - T18.net

I don’t know how long it’s been there. I just happened to notice it today when I pulled the left seat. The repair will be a standard stop drill and scab patch but I am curious if a crack like this is unique to this airplane. Thanks

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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

I haven't heard of this being a common crack, but hopefully you will get input from those with more Thorp knowledge than me.

Looking at drawing 505, it looks to me like I am missing a backplate.

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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

Looks like it may have started at a rivet hole? Might be due to poor fabrication technique... improper or no deburing of the drilled hole, etc. I haven’t heard of any particular problem in that area, but I’ve seen a lot of variance in the way some builders modify it from the plans. FWIW.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

Omitting the backplate would rob that area of a lot of strength. I don’t know about anyone else but I tend to step on the tunnel to get in and out so I don’t mess up the seats. Here is a picture of the back side (ignore the loose wire, it’s not fully installed yet). It looks like the builder tried to compensate for the missing backplate when it broke at the top.

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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

My aircraft does not have a reinforcing plate on the -505 bulkhead, neither does the -505 drawing. Note: my drawings are for a T18CW that came from Lou Sutherland and are modified John Thorp drawings. See attached photo.

My baggage floor is even with the top of the tunnel and has a stringer running full width of the fuselage. My battery box is on the right behind the pass seat.

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

Interesting... The S-18 drawing 580 still shows it. I guess he missed erasing it there. If you look close at the 505 drawing for the S-18 you can still see where it was. One of the curved corners didn't get erased and the dimension from the center to the edge is still on it.

I have to patch it anyway so mine will get a 505-4 back plate large enough to repair the cracks. I am putting in a new floor too. The last time I had the forward tunnel out it took me about 3 hours to put it back so I swore I would fix that the next time I had it out. The current floor is 0.016 thick and won't support anything. And to think all of this work started with a simple electric trim upgrade. I really wanted a luggage compartment but repairs are taking all of my free time so I guess that will wait until next winter.

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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

I looked at a thorp for sale that had a similar crack. It looked as if it was caused by luggage.

-Andy

Could be. Especially if there is a tunnel back there to transfer the load to the frame. The damage on mine actually starts above the elevator push-pull tube. My baggage compartment consists of what can be seen in the pictures so I doubt luggage could have caused the crack without messing up the push-pull tube or possibly even fouling cables. Mine is certainly a lot stronger now than it was. I took the patch out the same distance on both sides so the load is being carried by a large area when I step on the tunnel instead of just the area immediately around the 1/2 flange.

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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

My fix to weak tunnel.

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Bob Highley
N711SH
SN 835
KLAL

I like that. It looks very strong.

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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