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701 elevator cable.Weird problem

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Geoff Heap

Joined: 12 Jan 2006
Posts: 266
Location: Lindenwold, New Jersey

Posted: Sat May 26, 2007 4:44 am Post subject: 701 elevator cable.Weird problem

Hello all.

The plans call for a bungee from a starboard side fuselage stiffener to the top elevator cable. I just did a hookup of my elevator cables. Alone, they work fine. Picture this if you will.

I set aileron control to central. I tension the elevator cables. I work the elevator. Works fine. Now I move stick right to move ailerons. The elevator cables tighten up. Move the stick left and the elevator cables go slack. I have done trial hookups before and never noticed this. Maybe I didn't move the ailerons at those times. I cannot see anything wrong with the assembly. By design, the Torque tube is on fuselage centerline so that means the elevator bellcrank is off center, to the right. Looking at it that way, it makes sense that this tightening/slacking would happen. However I've never heard discussion of it here. Now, back to the Bungee. The plans make comment that the bungee is necessary to stop the elevator cables touching. Is it a fact that the bungee will also correct this problem of tensioning. In my head I can see how it would work. Am I the last one to know or do I have a weird airplane/assembly.......Geoff

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billmileski

Joined: 07 Aug 2006
Posts: 68
Location: Ledyard, CT

Posted: Sat May 26, 2007 6:32 am Post subject: Re: 701 elevator cable.Weird problem

Geoff,

Don't despair. I did that plenty enough for the both of us. If you search the archives, you'll find plenty of discussion on this.

In short, the cables will tension with aileron deflection.  The trick is to carefully tension the cables so that there is minimum acceptable tension in neutral, and not excessive tension at the extremes. I found that I could minimize the effect, by moving the upper hole on the elevator horn slightly.  I ended up sacrificing one horn in the name of experimentation, and made a second when I felt I had minimized the tension change.  Be very wary of overtensioning. I spent a lot of time with a cable tensiometer, and it's really easy to overdo tension so that you have 30+ pounds per cable at the extreme, and 60 pounds total on the linkages seem excessive.

So basically set the system for maybe 20lbs tension with the aileron in a position to cause max tension, and connect a bungee to the right side, pulling on the cable that makes sense (can't remember upper or lower) with at least a pound or so in all positions. Remember that the full aileron position is rarely used.  Then check the minimum tension, and there should be no real slop, if you have a friend hold the elevator and you move the stick fore and aft.  If you feel increasing resistance when you move the stick to the right, in my opinion you probably have excessive tension. Every time I had a noticeable effort to move to the right, the cable tensions would increase to well over 30lbs.  I had the tools at work to make my own bungees and I made an eye loop with bungee stock, slightly stretched it over a short length of nylon tubing, and slid it over the elevator cable.  

I found that there isn't much room between having no elevator slop, and having reasonably low tension at the extremes. And you might want to play with the location of one of the elevator control horn holes. My first one never would have worked, I moved it by a few mm and found a magic combination.



Good luck,



Bill Mileski

701 912S 90hrs

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Geoff Heap

Joined: 12 Jan 2006
Posts: 266
Location: Lindenwold, New Jersey

Posted: Sat May 26, 2007 6:59 am Post subject: Re: 701 elevator cable.Weird problem

Thanks Bill

Now that I know it's normal I can proceed. Thanks for the input...Geoff

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klaus(at)utdallas.edu
Guest

Posted: Sat May 26, 2007 11:28 am Post subject: 701 elevator cable.Weird problem

Hi,



I am guessing you have the problem addressed on the website



http://www.utdallas.edu/~klaus/Airplane/elevator_cable_guide.html



Happy flying,



Klaus



-- 

Klaus Truemper

Professor Emeritus of Computer Science

University of Texas at Dallas

Erik Jonsson School of Engineering and

  Computer Science EC31

P.O. Box 830688

Richardson, TX 75083-0688

(972) 883-2712

klaus(at)utdallas.edu

www.utdallas.edu/~klaus

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jetboy

Joined: 22 Jul 2006
Posts: 233

Posted: Sat May 26, 2007 3:49 pm Post subject: Re: 701 elevator cable.Weird problem

I think that website fix is for the 601?



The 701 is quite different. Mine has this problem also. It was assembled and rigged by CZAW so it arrived to me with elevator cable tension just applying at stick neural position, and springback effect if the aileron is held over to the right. This is a niusance as its hard to push owtwards with ones arm, flying from the left seat. the bungee installed in the location shown has no influence on this. I was cosidering applying another bungee much closer to the control end of the cables, to add bias tension to the other cable, but i trialled doing this by hand and wasnt convinced it would correct the issue.



I enquired to ZAC but got no response, I am following this up again now.



I feel that if the elevator control horn that the cables connect to was folded to the left so that the cable holes were in line with the aileron tube centreline, the tension would only increase, both directions of stick travel. This mod might require two bends about 5 degress, each about 1" either side out of the centreline of the horn?



My concern is that doing this might render the horn more prone to collapse under overload, because it would have less strength from being bent by the slight sideways loads that would now occur, however these loads probably already exist while the aileron is over to the left or right anyway?



We just need the OK from the factory, as to whether there is a reason for it to be this way. Its not much fun taxying out with left down aileron and wind behind the tail, causes the stick to shake around as the elevator wobbles (read flutter) against the loose cables.



Ralph

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Ralph - CH701 / 2200a

Geoff Heap

Joined: 12 Jan 2006
Posts: 266
Location: Lindenwold, New Jersey

Posted: Sat May 26, 2007 6:52 pm Post subject: Re: 701 elevator cable.Weird problem

Jetboy wrote "the bungee installed in the location shown has no influence on this. I was considering applying another bungee much closer to the control end of the cables, to add bias tension to the other cable,"

......Ralph....At this time I have installed a bungee about 18" to 24" back from the elevator bellcrank/horn. It's attached to a right side vertical stiffener. It had mostly the effect that I expected. I set neutral tension with right aileron. Now when I move to central and onwards to left aileron the bungee takes up the slack and maintains approx neutral tension. About 90 % of the problem went away. Next step is tweaking the position. I 'm sure I didn't get THAT lucky on my first attempt. I can probably get it a little better. Dissapointing though to find this problem now. I never heard of it before. However. what I don't like is that a bungee will be holding my elevator steady(??) while I am flying. It's probably all Zed's fault .........Geoff

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billmileski

Joined: 07 Aug 2006
Posts: 68
Location: Ledyard, CT

Posted: Thu May 31, 2007 5:41 pm Post subject: Re: 701 elevator cable.Weird problem

Ralph,

I have seen CZAW 701s in which they moved the lower hole of the elevator control horn (inside fuse) up a fraction of a cm to avoid the lower cable rubbing along the fuse channel just aft of the control horn.  Unless the lower hole position on the elevator itself is relocated similarly, this will make the tension increase more dramatic with aileron deflection.  And you might want to consider measuring your tensions because significant springback in my case meant like an increase to 40-50 pounds per cable.



Bill



   	  jetboy wrote: 	
		  I think that website fix is for the 601?



The 701 is quite different. Mine has this problem also. It was assembled and rigged by CZAW so it arrived to me with elevator cable tension just applying at stick neural position, and springback effect if the aileron is held over to the right. This is a niusance as its hard to push owtwards with ones arm, flying from the left seat. the bungee installed in the location shown has no influence on this. I was cosidering applying another bungee much closer to the control end of the cables, to add bias tension to the other cable, but i trialled doing this by hand and wasnt convinced it would correct the issue.



I enquired to ZAC but got no response, I am following this up again now.



I feel that if the elevator control horn that the cables connect to was folded to the left so that the cable holes were in line with the aileron tube centreline, the tension would only increase, both directions of stick travel. This mod might require two bends about 5 degress, each about 1" either side out of the centreline of the horn?



My concern is that doing this might render the horn more prone to collapse under overload, because it would have less strength from being bent by the slight sideways loads that would now occur, however these loads probably already exist while the aileron is over to the left or right anyway?



We just need the OK from the factory, as to whether there is a reason for it to be this way. Its not much fun taxying out with left down aileron and wind behind the tail, causes the stick to shake around as the elevator wobbles (read flutter) against the loose cables.



Ralph

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jetboy

Joined: 22 Jul 2006
Posts: 233

Posted: Fri Jun 01, 2007 12:57 am Post subject: Re: 701 elevator cable.Weird problem

Thanks Bill (and others)  I will check those things at the next interval 10 hrs time. Have had no reply yet from ZAC. The drawing for the bellcrank has the top hole at 120mm and lower one at 118.5, this is the standard Zenair dwng, there is no CZAW alteration on this part.  I will measure mine when I check it. 



I have been unable to find rigging tension data for the 701 in the build manuals or plans. My guess is there is none. How do you set the rudder when the cable tension depends on the set  of the nosewheel? (answer= with the nose off the ground)



 Looking at the plans, it is probably not possible to get the bellcrank holes ovet to the actual centre of axis of rotation. So I will be trying a 2nd Bungee to get the bias I'd prefer. I can live with it as is. But it would be nice to have confirmation that it is as intended.



Ralph

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