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Old 10-07-2021, 03:12 AM
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Piper J3 Piper J3 is offline
Join Date: Nov 2014
Location: Hinckley, Ohio
Posts: 2,567
Default Expanded Scale for Trim Indicator

I have set up my trim indicator as an expanded scale. Instead of showing the entire range of trim servo, I have set the end points to show only the portion of the trim corresponding to the flight envelope. End points on the scale are slightly greater than is experienced in flight. This allows a more meaningful and intuitive view of the usable window of trim movement.

Why show movement of trim that is not ever going to be used in flight? This is similar reasoning as used on an analog Volt Meter where the user selects the appropriate scale so readings are more accurate and easier to interpret. Why measure 5 volts on a 500 volt scale?

I recently had my pitot tube break in flight causing very erratic airspeed readings that were meaningless. When I slowed for landing, I set the trim to 1/8 from top of scale and I knew airspeed would be ~63 mph (55 knots) hands-off at idle power setting.

The photo below shows the Takeoff, Cruise, and Landing positions for trim. Because the scale is expanded, the motion of the indicator is more sensitive and allows more accurate positioning especially for takeoff.
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Jim Stricker - EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2007
80 hrs Flying Aeronca Chief 11AC N86203
1130 hrs Flying 46 Piper J-3 Cub N6841H
Bought Flying RV-12 #120058 Oct 2015 with 48TT - Hobbs now 750

LSRM-A Certificate 2016 for RV-12 N633CM
Special Thanks... EJ Trucks - USN Crew Chief A-4 Skyhawk
MJ Stricker (Father - CFI) - USAAF 1st Lt. Captain B-17H
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