Pushrod to cable conversion?

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Eugene

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Elevator control on my airplane is accomplished by pushrod system. They used aluminum tube inside of tail boom that rides on the rollers. And it's working pretty good, but new tail will be a little bit longer and a little bit higher. I don't see any another way to extend this, but only convert to cable system. Maybe somebody have done this before and can help me on this one.

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wsimpso1

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Cables? Ugh. Increased control friction will muddy up the control feel, you will need two control horns and cable runs to both, any asymmetry in the runs will give loose cables somewhere in the control throw... Just don't.

A simple bellcrank will serve to change direction and then either a couple rollers along the way (if the tube is to stay small), or bump the tube size to carry 1.5 times max compression without buckling over the length. Another way to support a long push-pull tube is to stick in an idler (or ball roller guide) at one or two places along the length, which greatly reduces the tube diameter needed.

What size is the current tube?

You calculate max control moment (chapter 8 of TOWS) on the elevator system at Vne, divide the moment by the control horn arm length and multiply by 1.5 to get max pushrod forces. Euler and Johnson calculations from Shigley to check sizing and lengths between supports on the pushrods.

Billski
 

Pops

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First, not addressing going to a cable system.
On the top drawing, the pushrod is at 90 degs to the elevator hinge line so you have equal amount of elevator up and down travel.

On the lower drawing you would have more elevator up travel and less down travel.

For a cable system you will have to have 2 cables, one attached to the bottom elevator horn and another attached to an added top horn.
Or, put a bellcrank at the end of the tube to attach the cables and a pushrod to the elevator horn.
 

Geraldc

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Edit Billski wrote about this as I was changing drawing.
For a change of direction you can use an intermediate crank with
pushrods .
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Eugene

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OK, no more cables! I will get that out of my head and start thinking about bellkranks.
Thank you! I definitely learned something today and I can sleep better now!

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Eugene

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On the lower drawing you would have more elevator up travel and less down travel.
Yes I recognized that, but was too late already. I was more obsessed with finding a way to do it. I will think about it and come up with a better picture. Thank you!
 

BBerson

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What is that on the bottom of the big tail boom tube in the drawing (post 7). Is that a structural stiffener?
 

TFF

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Most planes don’t fly upside down so less down than up is not an issue. Some planes are even and some are not. The question comes of geometry. Is your plane designed that way because of control geometry or just a limit because it can’t be used for flying? Combination of both?
 

wsimpso1

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Elevator deflection on my airplane was designed to be 30° up and 15° down. Wondering if this is more or less standard on all airplanes?
Some go even, some have less down travel than up. You need to be able to stall the airplane in positive g just to land it. Since landing also requires rotation in pitch and doing all this in ground effect, many airplanes need quite a bit of elevator to do all that. We do not need to ever fly at stall speed while inverted and in ground effect, and many airplanes have smaller negative g limits than positive, so less down travel fits there.
 
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