Bowden cable trim system questions

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Will Aldridge

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For my elevator trim control I'm going to run a Bowden cable from the left console just behind the throttle all the way back to the tab on the elevator and have a few questions about the limitations of Bowden cables. I have consulted the Tony Bingelis books but they were kind of vague on all subjects I'm asking about below.

1. What's the longest unsupported distance they can span? The longest distance right now would be about 29" (i can add intermediate support if necessary). And is there a longest recommended length? From my trim lever to tab is about 11 ft.

2. How tight a bend can they go around? The only serious changes in direction would be from the bottom of the h-stab to the middle of the elevator spar, that is if i didn't go through the interior of the h-stab and come out through the middle of the spar. There is roughly 3.1" inches between the aft face of the h-stab spar and the forward face of the elevator spar and the spars are both about 4 inches tall so 2 inches from the bottom to the middle where the Bowden cable would go through ( elevator spar is full width).

Or could i just go under the elevator spar and leave enough slack to account for the elevator deflection? Don't like that idea btw.

3. When ordering from ACS how accurate do you have to be? How easy is it to trim it down if it's too long? Fyi i have a Bowden cable stretcher if i order too short (same principle as a lumber stretcher when cutting a piece too short). Do they leave a certain amount of cable extending beyond each end of the sheath?

4. Any other gotchas i need to know about?

Thanks
 

Victor Bravo

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One gotcha, make sure that after everything is done and ready to fly, thereis no play in the elevator tab. ZERO ability to rattle the tab up and down before all the play is taken up.

Since I can't rremember which airplane you are building (how fast or light is it) I don't know whether this installation is a high or low flutter risk, so I am speaking hyper-conservatively here.

But zero play is always better than any play in any airplane.
 

Dan Thomas

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Sep 17, 2008
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Or could i just go under the elevator spar and leave enough slack to account for the elevator deflection? Don't like that idea btw.


4. Any other gotchas i need to know about?
I used this on my Jodel. The cable runs under the stab to a clamp on the elevator, and there's slack to let the elevator move. The cable runs through a slot in the closure panel under the stab. It HAS to be free of any tendency to snag on anything, obviously.

Use BIG wire. I used .090", available from Wicks Aircraft. You don't want ANY chance of it breaking. It will rust some, and rusting causes pitting that weakens it. You can get stainless wire but it's not as strong.

Any clamps used on the wire itself must be designed so that they don't nick the wire but still hold it securely. I drilled a bolt (AN-5, IIRC), and threaded it almost to the head so that the wire is clamped between two nuts and thin washers to avoid any shearing action at the hole in the bolt. Drilling a .090" hole in a too-small bolt would risk bolt breakage.

Cessna allows .025" per inch of tab chord free-play on their tabs on their singles. If the tab has a 5" chord, slop up to .125" at the trailing edge is acceptable. For a fast homebuilt I'd want it considerably tighter than that.

Some Maules used two cable sheaths and ran 1/16" 7X7 control cable in a pull-pull arrangement. Much less likely to break and no slop in it. The cables were tensioned to 15 pounds. If I did the Jodel again that's what I'd do.
 

Will Aldridge

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Joined
Oct 30, 2009
Messages
905
Location
Northern Utah
One gotcha, make sure that after everything is done and ready to fly, thereis no play in the elevator tab. ZERO ability to rattle the tab up and down before all the play is taken up.

Since I can't rremember which airplane you are building (how fast or light is it) I don't know whether this installation is a high or low flutter risk, so I am speaking hyper-conservatively here.

But zero play is always better than any play in any airplane.
Basically a war replica with a foam fiberglass sandwich fuselage, fixed gear and a rotary engine. Ought to push up against 200 kts

I can't answer your questions, but if there are routing issues that make this installation a challenge, I wonder if this is a situation where an electric trim might actually be easier than the manual one?

View attachment 70460

http://www.aircraftspruce.com/pages/el/trim/ractrimservos.php
I've seen those, even had that servo drawn in at one point but want to go mechanical.

I used this on my Jodel. The cable runs under the stab to a clamp on the elevator, and there's slack to let the elevator move. The cable runs through a slot in the closure panel under the stab. It HAS to be free of any tendency to snag on anything, obviously.

Use BIG wire. I used .090", available from Wicks Aircraft. You don't want ANY chance of it breaking. It will rust some, and rusting causes pitting that weakens it. You can get stainless wire but it's not as strong.

Any clamps used on the wire itself must be designed so that they don't nick the wire but still hold it securely. I drilled a bolt (AN-5, IIRC), and threaded it almost to the head so that the wire is clamped between two nuts and thin washers to avoid any shearing action at the hole in the bolt. Drilling a .090" hole in a too-small bolt would risk bolt breakage.

Cessna allows .025" per inch of tab chord free-play on their tabs on their singles. If the tab has a 5" chord, slop up to .125" at the trailing edge is acceptable. For a fast homebuilt I'd want it considerably tighter than that.

Some Maules used two cable sheaths and ran 1/16" 7X7 control cable in a pull-pull arrangement. Much less likely to break and no slop in it. The cables were tensioned to 15 pounds. If I did the Jodel again that's what I'd do.
Thanks for that info, probably ought to look seriously at that setup.
 
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