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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