Thorp Air Command - T18.net

When I separated my old worn Jack Screw from the old worn bearing, they were together with a nice press fit. Now my new Jack Screw, which was made according to drawing 704, turns out to have a loose fit with the new (and old) bearing, much to my surprise.

Digging deeper, I compared the drawing and the bearing dimensions, and I see the Jack Screw is supposed to be 0.3745/0.3735, and the bearing inside bore is specified at 0.375 which indeed guarantees a loose fit. Am I missing something? This doesn't seem right to me.

I couldn't find anything in Thorplist related to this specific problem. Any insights?

More importantly... how to fix? Either make a new screw with a press fit dimension or Loctite the screw to the bearing.

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

Dick, I think you repeated yourself on the clearances. I'm with you, it should be a snugs fit. Not necessarily a press fit but a snug fit. I would use LOCTITE 680. The area you should be concerned is the the screw and the aluminum block. Did you know builders put a plastic shim between the bulkhead and bearing to take out the end play.

When I had my jack screw assembly made (should I decide to use it), I supplied the bearing so the jack screw could be machined to fit the bore of the bearing. A nice snug fit. [8D] The threads on the "block" would be a greater concern. If the play in the bearing bore is not that much, then a little dap will do ya of Loctite as suggested will work (in my opinion). [^]

RB

Guys,

Looking at it, you know how obvious it is that the jackscrew is used to control axial position of the threaded block, and ultimately the trim tab position. So what's important is to be certain there's no axial play. Use the nylon button mentioned earlier for a shim as necessary behind the head of the screw. Whether the shaft rotates in the bearing ball or the ball rotates, it shouldn't be a great concern, as long as it's well confined and gets lubrication. It's a spherical bearing ball to accomodate the changing angle, it's not a typical ball-bearing. It's not high speed and there's no radial load.

The screw on mine fits nice and snug, but the ball is really loose, fore & aft, which is worse. I'll replace it for that reason. Just be sure the head is captured well so the screw has no axial play, and there's no slop at the trim tab.

Happy trails...
Les Krumel, Albuquerque

I've been waiting for Rich Brazell to write up for the Tiger Tales (or forum) about the use of the RAC servo on the stabilator pitch trim (same as aileron roll trim). He used that and I converted my Dayton gearmotor screw drive to Rich's installation. Bill Beswick had one also but mounted differently (vertical instead of horizontal). Tom Worth

Tom: Thanks for the kind words on my trim. It has worked 4.0 with almost 80 hours on it. I hesitated posting anymore comments on it from when it was installed as the initial writeup in a prior NL caused such a hugh pile of piss ants ! The only negative report is that it is tad bit fast. If I ever get around to it I can install a RAC speed reducer to fix that small baileywick. I'll see how it works (speed wise) with the new Infinity stick grips. They have the "coolie hat" which should help with the speed control ? I currently have the RAC G207 and G205 stick grips installed. Pulled the aft panels (during the first annual) below the fin and the RAC servo unit and linkage looks just fine. I also installed (2) CB's in the panel (one for elevator and one for aileron) that can be pulled in less than 2 seconds should I have runaway trim.

The aileron trim has also been trouble free as is the RAC rudder trim. The rudder trim is only used with the AP on in cruise. I will probably sell the two RAC stick grips for 1/2 price. They look new and are already wired.

RB

Rich,
I am also using a RAC servo on the pitch trim servo tab. The biggest potential issue with this set up is that if the mechanism or control rod breaks, there is nothing to keep the tab from possibly fluttering. So, I added a backup safety--an additional arm on the control link, with a cable to the cockpit that can be pulled. It will provide up trim when pulled, but should stop any tab flutter issues. You are correct in that the RAC servo is a bit fast (mine is the one with the 1.2" throw and I extended the arms on the control link to match the throw of the servo, as well as reduce the force on or needed by the servo to actuate). So it takes a very momentary blip of the switch to adjust trim. Interestingly, when I fly by myself with no baggage to speak of, I need to use the trim. However, when we flew to Oshkosh, with two persons in the cockpit and a nearly full baggage compartment, I hardly needed to adjust trim between climb, cruise or descent.

By the way, the RV-12 uses the RAC servo in the same way to control the servo tab on its stabilator. Though the speeds don't get up to those of a Thorp.

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Dave M
Sacramento
ex-T-18 owner

A trim tab is different than the Thorp servo tab. I would guess if the RAC TU'ed you would not have enough time to pull the trim cable before the tail came off. Flutter happens at lighting speed. I have taken a RAC apart and the gears are not that strong, I'll use the screw jack specified in the plans.