Making a pitch trim indicator for a Glasair

I have a Glasair 1 FT with the newer style trim system that uses a trim
wheel connected to a gear train that winds and unwinds stainless steel
control cable and adjusts the relative tension of springs bias the
elevator up and down. This system works well in flight. The existing
indicator was a rotary wheel attached to the top of the gear train with
arbitrary markings. It spun as you cranked on the trim wheel. It is
in a hard to see location and I cut it off since we never used it. I
would like to create an electronic trim indicator based on a Wheatstone
bridge. This bridge resistance divider would drive a linear miniature
panel zero center (milli)ammeter of the "mag negative", "zero", "max
positive" type. I don't know what the full scale + and - should be of
the meter. Should it be +/- 1 ma, +/- 10ma, etc? I would couple the
moving trim cable to a linear taper slide potentiometer that would be
nominally centered in its motion range when the trim was set to what
has been determined to be the takeoff trim position. As the trim wheel
is rotated, the moving control cable moves the slide pot and unbalances
the circuit, moving the needle on the meter which indicates relative
trim up or trim down. The circuit would include one adjustable pot to
intially balance the meter at 0 for nominal takeoff trim.
I would like to power this off of unregulated aircraft bus voltage.
Would the current be insensitive to actual voltage? I would image it
would be insensitive at the zero position, since if the four
resistances are equal, the voltage on each side of the ammeter is the
same and hence no current regardless of voltage. I guess that the
sensitivity would vary with voltage nominal changes when off zero.
Is there someone who could suggest nominal values for the four
resistors and the range of the (milli)ammeter?
It this idea even practical?
Nominal aircraft bus voltage is about 14.4 volts.

The current through your meter can easily be calculated
by applying ohm's law.
i.e if using a 10K pot and 2 resistors of 4K7,
max. current would be 14.4V divided by 10K or
about 1,5 mA.
I am afraid you will not easily find a potmeter that
can stand the vibration, dust and oil present in any
aircraft over a reasonable amount of time.
Also the coupling to the trim cable might not be an easy task.

My approach to this problem would be a series of opto-detectors
and a vane on the trim cable. Each of the opto-detectors
could drive one LED on the panel. This setup has no
moving parts except the vane, does not suffer from
variations in the supply voltage, and offers digital readout
which not only looks cooler but is actually clearer.
Only drawback is the multiwire cable to the panel.
This could be avoided by using more electronics.

"karel" wrote

I am afraid you will not easily find a potmeter that
can stand the vibration, dust and oil present in any
aircraft over a reasonable amount of time.
Also the coupling to the trim cable might not be an easy task.

Fiddlesticks! You just are not going to find it at Rat Shack.
--
Jim in NC

"Morgans" wrote in message
...

"karel" wrote

I am afraid you will not easily find a potmeter that
can stand the vibration, dust and oil present in any
aircraft over a reasonable amount of time.
Also the coupling to the trim cable might not be an easy task.

Fiddlesticks! You just are not going to find it at Rat Shack.







And your positive addition of information is



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karel wrote:
"Morgans" wrote in message
...

"karel" wrote

I am afraid you will not easily find a potmeter that
can stand the vibration, dust and oil present in any
aircraft over a reasonable amount of time.
Also the coupling to the trim cable might not be an easy task.

Fiddlesticks! You just are not going to find it at Rat Shack.


I think what you are looking for is available from Ray Allen Company, formerly MAC trim systems.




And your positive addition of information is



( .................. fill out here .............)

On Tue, 12 Sep 2006 20:28:56 -0000, "karel"
wrote:


"Morgans" wrote in message
...

"karel" wrote

I am afraid you will not easily find a potmeter that
can stand the vibration, dust and oil present in any
aircraft over a reasonable amount of time.
Also the coupling to the trim cable might not be an easy task.

Fiddlesticks! You just are not going to find it at Rat Shack.







And your positive addition of information is

Try Mouser Electronics.
The have a rather large catalog.




Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com