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Stabilator Cables Strain F-1203A Bulkhead?

G

GDS

Well Known Member
The right Control Column Mount Assembly is riveted to the bulkhead, F-1203A. (page 21-11)

Following instructions on page 32-14, when I tightened the stabilator control cables, the tension on the Control Column Mount Assembly pulled the bulkhead aft, creating a bow of about 3/8 inch in the bulkhead where the assembly was attached. I had to loosen the cables all the way to get everything back in shape. My plan is to install the floor plates to give it more rigidity and then go back to tension the cables.

Anyone else have this problem?
 
How did you tension the cables ?

Something does not sound right. Would seem you have WAAAAAY too much tension on the cables.

John Bender
 
I agree with John, something is not right. There should be no deflection when you are tighten up the cables, 3/8" is huge. Time to double check everything.
 
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set mine using a cable tension gage to 40-45Lbs. No distortion experienced. Do have a question tho. I could get the stab to hit its stops before the sticks hit theirs, but the gap bet. sticks and stops are only about .060". Is this typical, and is it enough?
Dick Seiders
 
Hi Dick,

You should have no problem. The stick fore and aft location is controlled by the length of each elevator cable, you can ease one and tighten the other to move the stick a little forward or aft then tighten both the same amount to the required tension if you wish a different stick location. If you have installed the safety clips you will need new ones or you may wish to keep what you have until you have flown it and if it needs to be moved for your comfort it can be.

Best regards,
Vern
 
Tension was "mechanic tight"

I am borrowing a tension meter today to check the cables. I set tension "mechanic tight" using a 1.5 inch-long allen wrench in the turnbuckle. Don't see how I could have over-tightened with such a small mechanical advantage. Everything moves correctly and is very smooth through the whole range of motion. It could be that I set tension in warm weather and they contracted significantly with cold weather? We range from 70 daytime to 20 at night.

Regarding the gap between stick and bulkhead, I have about 1/8" gap when the stick is pulled aft, which seems adequate to me. After getting that gap set I'm not sure I will have the <3 thread showing on the cable ends as they go into the turnbuckle. This is one of those 1 hour jobs that may take a couple of days...
 
Borrow the tension meter

Don't know how you held each cable from turning, but with a clean smooth thread, you can develop a lot of tension on the cables. Sounds like you will find the problem there.

Are you doing the construction in sequence ? ( did not jump ahead at any point in this section )

John Bender
 
I followed the instructions in sequence. After the problem, I worked ahead to "Miscellanea" and installed the floor plate, part F1227. This part gives additional integrity to the bulkhead and the entire control column mount assembly.

Today I re-tensioned the cables and all is well. I used a tensiometer and took them to 40. I definitely had the cables too tight before, probably as high as 80?

I would suggest to other builders that they delay tensioning the stabilator cables until after completing the following "Miscellanea" section, where you install the floor part F1227.

When they were too tight the control-stick had a little resistance and the stabilator would stop at any position. At the correct tension, the control movement was very smooth and the stabilator would not stick in mid-position.

As John Bender suggests, it did not take much muscle to over-tension the cables!

(Side note, Our consulting engineer tells me that it was not due to temperature changes. The contraction/expansion of the aluminum airframe would be greater than that of the steel cables, so intense cold would make the cables loose, not tight)
 
Hey Vern, thanks for the feedback. I am still puzzled tho. I set the sticks to equal gap as you outlined, but the gap could not be made greater than the 1/32-1/16 gap. The cables are of equal length right? So setting them to equal gap is the best you can do assuming correct tension at 40-45lbs. I am curious to know how others are getting a larger gap. I/8 for example as indicated by GDS in original post.
Dick Seiders
 
dick seiders said:
Hey Vern, thanks for the feedback. I am still puzzled tho. I set the sticks to equal gap as you outlined, but the gap could not be made greater than the 1/32-1/16 gap. The cables are of equal length right? So setting them to equal gap is the best you can do assuming correct tension at 40-45lbs. I am curious to know how others are getting a larger gap. I/8 for example as indicated by GDS in original post.
Dick Seiders
Click to expand...

Hi Dick,

Yes the cables are meant to be the same length but that is hard to do so it would not be unusual to find one a small amount longer, which is why we put turnbuckles in both cables. If the cables were very precisely made only one would need to have the turnbuckle. There are other areas where small tolerances can cause issues like yours in this system as well. In the end as long as the rear stops are the ones hitting first you are good to go.

Best regards,
Vern
 
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