Dave,

I answered you about using analog servos. After reflection, it wouldn't  
make any difference which type servo you use because you remove the servo  
amplifier from the system and drive the servo directly. This should remove the  
RF interference issue. Use a DPDT toggle with spring loaded center off. 
Cross  wire outer poles together, power wires to center poles and wire servo 
from one  end  of the switch. Power it at 4.8- 6 vdc. Model servos are 
available in  many sizes. I just looked at one that had 480 oz in of torque. I 
think that  equates to about 30 lbs of thrust on the end of the servo arm.

John Melvin
El Paso, TX


In a message dated 4/8/2010 6:29:47 P.M. Mountain Daylight Time,  
ddunw...@fnwusers.com writes:

Thanks  to all of you so much for your various ideas and solutions on 
electric  trim!  The information has been a Godsend!!!

Looking back through  old newsletters, I came across the idea of an RC 
servo with servo  tester.  I saw this one as problematic from the outset, 
and that is  why I asked for assistance.  RC servos are generally pulse 
width  modulated.   That means that they always have power applied, and  
then you move them by sending various width pulses to them via a 3rd  
line.  A nearby transmitter can raise havoc with such a system,  
especially if the PWM wire isn't shielded.  A typical 5 watt AM  
transmitter might not be a problem, but the typical 200 watt pulse  
transponder box would likely wreak havoc!  After reading all sorts of  
horror stories about runaway servos in certificated aircraft, I decided  
that an RC servo wouldn't be a solution.

As an electrical engineer,  and a 50 year veteran pilot, it seemed to me 
that having constant power  available to ANY type of flight control servo 
was a bad idea.

The  obvious (and probably the safest/cheapest) solution, IMHO, would be  
something like the Allen servo.  The big, bad version is $165.   It 
provides 40 pounds of thrust as opposed to standard RC servos which are  
typically measured in ounces.  I am intrigued by mirror motors,  window 
motors, etc., and I'd love to try that, but we live about 900 miles  from 
nowhere!  Getting a bolt around here is a challenge, let alone  finding 
the right motor from a junk yard!

I have to take another  look at the geometry of what is in my KR2 at the 
moment.  My servo  will have to be in the fuselage as I'm not going to 
turn the airplane  upside down and cut into a finished elevator to 
install a servo.  I  plan on fuselage mounting the servo, and simply 
hooking it to the rod that  goes to the trim tab.

I would like to retain the present trim wheel,  but it is butt-ugly, and 
in a poor location, and, well, it is just  ugly!!!  So there is a good 
chance that I will relocate the wheel and  keep a portion of the existing 
manual system such that anything the servo  does is apparent on the 
wheel, and the wheel is there for  backup.

I'll send information as this new project comes to  fruition!

Thanks again,  everybody!

Dave.



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