Thorp Air Command - T18.net

I'm faced with rebuilding my trim mechanism. One of the problems is that my trim mechanism in the tail is not "geometrically compliant." ie I have a solid rod between two u-joints to drive the trim screw.

The drawings (A-701) show a flex shaft going from the bulkhead (STA 191.75) to the end of the trim screw. All of the pictures I've seen here on the forum show solid splined shafts (or square tubing) with U-joints at each and. The splined shafts are slick, but a flex shaft seems quick and easy to make up.

Are people experiencing problems with the flex shaft configuration?

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

Research the recent NL editions. [:D] There are several good write-ups (w/pics) that detail a variety of gizmos that deal with that part of the trim. [;)]

That's why we publish the NL to use as a reference. [:0]

RB

This is a trim Bob and I came up with. Look at the photo album, open up fytrplt album and look at images 007,008,009. We used a universal shaft from a RC model truck, TRAXXAS #5451X and #5458X yoke, a Graingers motor, 2L009, and micro switches from radio shack. The wiring diagram is in my photo album. The flex shaft "IS" a must. As destortion will occur at this point.

The trim yoke pitches fore and aft therefore the shaft/ torque tube must slide/move fore and aft also . If you follow what Bill Williams has posted it's pretty straight foward. I found all the parts locally and used his wiring diagram that was posted with his pictures. I'm currently building the system up right now.

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Stephen R. Peirce
S/N#078 Ph 256-461-0233
Rocket City
Huntsville, Alabama

Not a flex shaft per the drawings, but I know of one builder that used and is currently flying with a graphite "fishing rod!" [8D] Wants to make a low pass over every lake in SOCAL. [:p]

RB

NO the yoke has a different diameter, .312 and is keyed with flat sides. I turned the shaft to .312 and filed flat sides on the shaft.

*** admin posted for Bill Williams ***

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Forum Administrator

I have a jack screw out of an old project and the limit stop nuts have been set up to limit travel to 5 turns. I note that the plans call for 5.4 turns.
Those that have more experience might be able to tell me how much of an issue that missing .4 of a turn is going to be in the air? Is it worth fixing?

If it's out of the airplane why not adjust it to the 5.4 ? I really don't think you would notice the .4 either way ? There probably has to be a +/- factor of .2 ? If it were me I would adjust it to 5.5 to make the math easy ! My math always adds up to Naught from Naught is Naught !

RB

My trim system is 5 turns lock to lock, its been flying like that for 40 years as far as I can tell, no entries in the logs to the contrary. I've not had a problem with running out of trim.

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