Thorp Air Command - T18.net

Drawing #500.

I am putting together the push-pull tube for a standard T-18. Instead of swaging the ends of a 1" 6061, I am going with the alternate plans which ask for a 1" SPACER to go into a 1" 2024 TUBE.

I have everything ready to go, including the spacer. However, I am not sure what to use to secure the Heim and spacer to the 1" end. The drawing doesn't mention whether to use a bolt or rivet..... nothing. On the first plan (with the swage) you use a an470 ad-4-12 rivet.

I have a AD4-15 rivet, but even that is too short. I have not used longer rivets than this before...and my supplier doesn't have any longer rivets than AD4-15.

This is probably a dumb question... but, Should I use a longer rivet? I saw that aircraft spruce has them as long as AD4-22. Is this the right approach?

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Dean Cuke
Visalia, California
N772DM
KVIS

Ideally, you would use rivets. Rivet dash lengths are in 1/16" increments. The best thing to do is get the longest rivet you can buy and cut them off to length. You need long rivets for the bellcranks as well. When driving long rivets, you need to allow more length than the standard 1.5 x diameter over-length due to the fact that they squeeze up more before forming the head.

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Bob Highley
N711SH
SN 835
KLAL

Thanks Bob. I'll order in some longer rivets.

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Dean Cuke
Visalia, California
N772DM
KVIS

I am looking at remaking one of the push pull tubes (Drawing 500) for N89ER and I have a couple of questions. First, any hints on the best way to go about the diameter reduction at both ends? Is this something that is readily done in a home shop or should I find a machine shop to do it?

Looking at the alternative geometry that does not have the diameter reduction, but instead has an insert; it looks to me like the larger end diameter will interfere with the bell-crank when the push-pull tube is fully extended. Am I missing something on this?

Thanks for your help.

Regards,

Mark

The original tapered tubes were made in specialty shops in the L. A. area where the proper swedging machines were available. This is not for the home guy due to the posibility of cracking. The threaded rod ends should be long enough to provide clearance at the bell cranks. Look closely at DWG 520 and you will see that the maximum throw on the bellcrank allows plenty of clearance for either type of tube end.

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Bob Highley
N711SH
SN 835
KLAL

Drawing 500 shows a HEIM F-31-14 or equivalent (which is solid instead threaded) on one end of the assembly and a HEIM F-34-14 or equivalent (which is threaded) on the other end. That is how my existing push-pull tubes are made.

If I understand you correctly, I should use a threaded rod end on both ends and build the alternate ends with no swage. That should work, but it was not clear to me that using two threaded ends was allowed.

Thanks,

Mark

The alternate end detail clearly shows using the Heim F-31-14 or equivilent. It just requires longer rivets. If you use two threaded ends, you run the risk of having the assembly unscrew, although I have seen this setup on some RV's. If you properly install the differential belcrank, it will not interfer. If it does, grind some relief on the end of the tube.

_________________
Bob Highley
N711SH
SN 835
KLAL

Bob is of course right. I looked more carefully last night. Sorry for the confusion.

I looked at buying the Heim F-31-14 equivalent from Aircraft Spruce. They list part number REP3H5 as dimensionally equivalent. I was surprised to find that it has a hollow shank. Aside from potentially requiring more length compression before the head forms, does that have any impact on how the rivets set?

Mark

Mark,

To form the head on a rivet through a hollow shank it's tricky. I just did my rod ends and had to install a short aluminum plug in each, just to fill the void after a few failed attempts. I feel a lot better that way too.

Les,
Albuquerque