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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. |
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