Thorp Air Command - T18.net

Hopefully a quick question for you fellows with greater knowledge.

Is it normal for the elevator trim rod to run from the back of the seats to the jack screw near the tail? I've had my aircraft in for maintenance for the past few months and after it being in parts the trim wheel is now quite loose, you'll turn the trim wheel and it will spring back to where it started. The maintenance shop seems to think that it's because the trim rod is too long and not supported.

Thoughts? I've attached a picture for reference.

Cheers all

Me thinks your jack screw is seized or turned to the stop

The mechanic came out to the hanger today and tested it out. All appeared moved as it should through the whole movement range, just seemed to have thisspring back issue.

Mine has some spring to it. It is supported in the tunnel between the seats and at least one location behind the seat.

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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

Cheers for the info Jeff.

Mine always used to have a bit of spring that still allowed it to hold it's position but now it's at the stage where when you turn it about half a turn & let go it just bounces straight back to where it was.

As far as I can tell mine is only supported at the two ends so they might be onto something with a middle support.

I stuck my phone behind the seat and was surprised by what I saw. The first support in the picture would be at the back of the baggage compartment, if I had one. Mine springs back maybe an eighth of a turn. Yours may be binding in the back, possibly working toward seizing up if it has been getting worse.

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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

If you look at the 715 drawing, you will see the plans call for support at the 574 former, and right aft of the baggage compartment. Yes, that a pretty long run for such a thin walled tube. I would make sure that both of those hangers are in place and not binding. Then I would lube the jackscrew and work it stop to stop. It should turn 5.5 rotations. Most likely, the 703 arm is not binding, but I would check there if I still had trouble.
Cubes

Hi Todd,

You said the trim wheel is now quite loose, so I’m assuming you mean it turns very freely now. I’m also guessing that might mean you had more friction in the entire system before, and maybe that masked the symptom that you have now. Like Cubes said, the support at the 754 former is important for this long, 1/4” tube. Excess friction in the jackscrew assembly at the tail is a possible issue, as Bill said. Also, that long tube may have a “permanent” bend in it now, for whatever reason. If it is bent excessively before or during installation, it can cause the “spring back” effect you describe. Ideally, it needs to be nice and straight, and then handled carefully during installation to avoid excessive bending to the point of deformation. Re-straightening it after making sure everything turns freely, and properly supporting it, should cure the issue. Sorry for the long-winded explanation.

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

I was thinking along the lines that John is thinking . In my humble opinion any sort of drag or binding in the system would cause the "tube" to "Bow/flex" and then spring back , especially if it is a tangled web along/thru the supports . Binding at either end would cause the tube to "flex/bow" and "twang the magic twanger"

RB

Thanks everyone for all the helpful information, I appreciate such helpful replies.

I'll try and clear up what I'm talking about. Before I took it into maintenance you could turn the trim wheel and it would stop where you move it to with only the slightest bit of spring back. There was also more friction then there is now. As far as I have been told when they looked at it they went through and lubricated the whole system, the jack screw, joins etc. Now when you turn the trim wheel there is a lot less friction there and, especially when the trim gets towards the nose up end of its travel range, there is just as much spring back as how far you turn the wheel. So if you turn the trim wheel a quarter turn and let go it will bounce back a quarter turn. This can be mitigated by keeping your fingers on the side of the trim wheel after you turn it for a couple of seconds which minimizes this spring back issue.

I think that explanation should be as clear as mud. From what has been said here I'm guessing the problem is in the tube for the trim. I will have to confirm it but as far as I remember the only supports are at the front of the baggage compartment and at the rear just before the jack screw. There isn't one at the back of the baggage compartment as has been mentioned. The tube also flaps around quite a lot and appears to have a curve to it so that would add to the issue. The plans for my aircraft are at the maintenance place atm so when I can get down there I'll have a look over the plans and compare it to my system.

As I'm sure you can all tell I'm still trying to learn all the in and outs of the Thorp and I'm trying to pull it back into line after 40 years of wear and tear so hopefully it won't be too much longer until I can get it all back up and running.

Thanks everyone for all your help,
Todd

I would sit in the drivers seat and move the trim wheel and observe what the tube does ?

RB

I'll bet there are quite a few reading this forum who never heard that phrase before, "Froggy"!

By the way, the original was "plunk your magic twanger", for you youngsters.

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

There is no issue to add another hanger if your bird calls for it. The trim tube needs to rotate and not flop around. I just made a set on the plane I'm rebuilding. Use phenolic. Works great.
Cubes

That sounds like it might be the way to go. Can it cause problems if it is supported too much? I'm assuming as long as it can still rotate freely and doesn't bind on anything it should be alright. I'm thinking while I'm messing around with it I might get a support added at the rear of the baggage compartment and another one before the rear connection. Only problem will be that I will need to redesign the panel at the rear of the baggage compartment so it can somehow still be removed since it needs to be pushed towards the rear of the aircraft atm to be able to remove it.

Always seems to be one thing after another atm.

Cheers,
Todd

I used "floating"nut plates on the supports for my baggage compartment and the floor along with the 3 sides are attached to the nut plates using eye bolts . Serves two purposes . Secures the walls and floor and I can use the eye bolts to to secure my trash with bungee cords . BING , BANG , BOOM !

RB