CTE

[Jerry Budd]

>Sorry to disagree here, but it would appear that the conclusion 
>being drawn by the respondants is that the pushrod expansion and 
>contraction is the major cause of the problem. Not so, in my 
>experience! Going to pull-pull cables, whether they be steel or 
>kevlar thread will not get rid of the elevator trim change under 
>consideration here.The coefficient of thermal expansion between the 
>cables, which is very small, and the balsa in the fuse, which is 
>significantly greater(glass too) will generate a differential of 
>sufficient magnitude that the "devil" will still attack you when you 
>least expect it.

Maybe I'm missing something here?  If you use pull-pull cables the 
only effect you will see as the fuselage expands/contracts thermally 
is a slight change in cable tension.  Since the upper and lower 
cables would change tension by the same amount (I'm assuming the 
cables are ~ the same length and are made of the same material) there 
would be no trim change (unless the cables are "slack" in which case 
a further reduction in tension would effectively add hysteresis to 
the system).

>The best solution that I have observed is to put the servos in the 
>stabs. Short of this, I have been using servos mounted just at the 
>L.E. of the stab and using a very short pushrod(approx. 6"). My 
>current thinking is that I will still mount the servo in the same 
>area and go with cables.Very light and direct. As regards Kevlar, I 
>currently have a pattern practise ship that I do almost all of my 
>flying with that has kevlar on the rudder and it has two years of 
>mucho sequences to it's credit with no apparent wear or abrasions to 
>the thread. At this point in time I have still not acquired the 
>necessary intestinal fortitude to use kevlar on the elevator.
>Georgie

The problem with having separate elevator servos is that maintenence 
of the servo pots becomes a critical item (as the servo pots wear the 
servo's output changes giving asymmetric elevator deflections) which 
has an adverse effect on pitch/loop tracking.  Experience with this 
type of setup shows the pot wear can become an issue in as little as 
35-50 flights.  You can stay on top of the problem by frequently 
checking your throws using [shameless plug to follow] one of my Laser 
Measurement Systems to reset the throws accurately after changing the 
servo pots:

http://www.buddengineering.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=LMS&Category_Code=P

but it becomes a real pain to be changing the servo pots out every 
35-50 flights or so!

Using a single elevator servo effectively eliminates this as an issue 
but you do give up some redunancy.

Thx, Jerry
-- 
___________
Jerry Budd
mailto:jbudd at qnet.com
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