CTE

[george kennie]

Jerry and Amir,
Thanks for straightening me out here. Just another example of my senior short-sightedness.
I attended a contest with a pattern plane with pull-pull cables on the elevators a couple of years
back and the temperature change from Sat. afternoon to Sunday morning didn't seem to be all that
dramatic but there was a significant humidity change and everybody was dialing in significant elevator
trim correction and being totally surprised by the condition. Must have been something like what Jerry
alludes to regarding air density or some other unfathomnable anomoly?? Whatever it was it convinced me
that there was a CTE thing going on and as you can see, I really never thought it through
properly.Thanks for correcting me! I like being wrong as that's when I learn the most valuable
lessons, but then this is the first mistake I've made in 75 years, if you project that figure, I'll
probably never make another one!!!!!  ;-)??
Georgie

Jerry Budd wrote:

> >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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