Thorp Air Command - T18.net

I noticed an anti-servo tab on my aircraft had some slop in it and traced it to loose skin at the inboard end along with loose rivets in the hinge. Basically, the inboard rib is starting to "float" inside the tab. I ordered some parts from Cubes and started looking at what it is going to take to fix it but I have to admit, with 30 years of maintenance experience behind me, I am at a loss of how to do this properly. The inboard rib is a solid wedge which means the the rivets must be deliberately installed at an angle to the surface. I have rarely seen this done in a successful manner on assemblies even slightly miss-drilled from perpendicular to the surface. To get a good set on a squeeze rivet, the rivet must be hit square on one end and bucked square on the other. Even with back riveting the rivet needs to be hit square. This angle is large with no way to hit the rivet square without getting into the surface so I thought I would ask for some advice before I destroy some perfectly good parts.

Thanks,
Jeff

_________________
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 tapped the holes and used screws, #4, I think.

_________________
Bob Highley
N711SH
SN 835
KLAL

Might have been #6s. Over thirty years ago. Also, insure the stainless strap service mod is complied with.

_________________
Bob Highley
N711SH
SN 835
KLAL

The strap is there. The two rivets holding it in place through the hinge are loose and contributing to the slop as well but that is an easy fix.

_________________
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

Jeff, when I did mine, I cut the rivets to a minimum length and back riveted with a small set. It worked fine. I didn't hit the skin at all.
Cubes

It's been a while since I did mine , but I am pretty sure I did it like Cubes did ? Came out Muey Bueno ! Although I do like Bob's way with the threaded screws !

RB

I made a small rib out of 1/2 aluminum stock and used number eight screws in place of the rivets, also used a screw on the hinge. I do not have the Stainless strip.

I am not an engineer but it seems to me attaching the rib to the front would firm up the assembly enough to negate the need for the strap.

Initially I was thinking about making a rib like the production aircraft use. Pertty much the same as the outboard end only heavier aluminum or steel to take the load.
Thanks for the input.

_________________
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

The stainless steel "strap" is on the SDR list .

RB

Yep, and near as I can tell, it was added to reinforce that area because only the top and bottom skin (.016 thick each on the T-18) was holding the rib the control arm is attached to. Running a screw through the front adds the front skin and the hinge as support for the rib. I am NOT recommending anyone ignore an SDR, just suggesting there may have been a simpler fix. Like I said, I am not an engineer. I am just a lowly mechanic and Bill's fix, assuming I correctly interpreted what he has done, more closely resembles how production aircraft are built and having the strap didn't prevent me from needing to repair that area.

_________________
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

Easier to add it now should in the future you decide to sell and the buyer wants to see if all the SDR's have been complied with .

RB

_________________
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

DOH !

RB

My set up is 100 times stronger than the .020 stainless strap, I have scratched my head for 20 years trying to figure how the SS strap makes this joint stronger.

I am not familiar enough with Thorps to know if others are having problems with the inboard rib. I think the strap could have a twisting movement however, if you compare mine to the drawing, you will see the rib wasn't installed IAW the drawing. The strap was added after the aircraft was built and was installed using cheap pop rivets instead of the called out AD rivets. Cherry Max rivets would be acceptable too. Add all this to the fact that the front screw was loose, it isn't surprising I have work to do. The other side is assembled the same way but I think it hasn't degraded as far because both screws are tight.

I should add that, assuming the SDR was done when it came out or soon after, this assembly likely worked fine for many years and 800+ hours.

_________________
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