CTE

[Wayne Galligan]

I am running an MK bellcrank and c.f.  pushrod in my carbon fiber/coremat fuse Aries.  Titanium ends and ball links hooking it all up except for the control horn I used Du-bro nylon clevis.   I don't remember but maybe a flight or two that I had to change the elevator trim even a click.  Not only is the c.f fuse dimensionally stable but I have no formers in it outside of the two rails for the rudder and elevator servo.  I have replaced the nylon control horns once due to wear (MK ballbearing clevis would eliminate this problem) but other then that it has worked fine and with hardly any trim problems

Wayne G.
  ----- Original Message ----- 
  From: Amir Neshati 
  To: discussion at nsrca.org 
  Sent: Tuesday, November 25, 2003 2:18 PM
  Subject: Re: CTE




  Both sides of a the cable will expand or contract the same, so the trim will still not change due to different CTEs
  They may get tighter or looser, but still no trim change.....I flew a few hundred flights on a glass fuse. with a carbon
  pushrod, had the trim sensitivity set to 20% and would take as much as 6 of  these little clicks to trim each flight.
  As it got warmer, the problem I encountered was that I would take off, on the trim pass the plane would head for the ground,
  so I would start adding the up trims....about a minute in the flight the fuselage would cool and now the fuselage would
  contract a bit, so comes in a click or two of down trim now, but  noticed the rudder pull-pull would never change trim with
  any changes in temps ..so, I gave pull-pull a try on the elev. and after 400 flights in temps. ranging from 50 to 100 degrees
  it still worked great. I tightened the cables only one time after the first day or two of flying and didn't touch them again.

  The only thing I can see here is that due changes in air density (the CTE for those tiny molecules of air)
  a tiny bit of elevator change may be experienced, and one may change the trim by a tiny click and be done.
  Where as with a pushrod, I had to change elev. trim by 4~6 clicks at first, then after a minute or so into the flight
  as the fuselage temps. stabilized due to the cooling air flowing  over it, would have to re-trim again...

  I weighed a carbon pushrod once that was ready to install and weighed around 1.5 ounces and I believe the 
  40 lbs. coated fishing line stuff weighed only a few grams...Kevlar would probably weigh only a couple of grams.

  Have fun,
  Amir






    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. 
    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 
    Amir Neshati wrote: 

      Pull-pull will eliminate the trim change... Amir 
        ----- Original Message -----
        From: Nat Penton
        To: discussion at nsrca.org
        Sent: Monday, November 24, 2003 7:24 PM
        Subject: CTE
         Gray what is the CTI for balsa ? I have carbon push rods / balsa fuse and my elev trim moves around considerably. I suppose it is because of humidity variation between the shop and outdoors ?? The pushrods are 50" long.      NatPenton   
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