CTE

[Amir Neshati]

Jerry and Amir....Sounds like a good show name to me. Though I would want it
called Amir and Jerry!

Amir
  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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