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I had some requests for more information on the
320 roll trim system.
An aluminum tab with a small hole for spring
attachment is installed under the connecting push
rod bolt head at the bottom of each stick. A
spring connected to the right tab extends to the
left. A cable connected to the left end of the
spring extends to a gear motor mounted on the
floor pan next to the left closeout bulkhead
under the pilots left thigh. The cable makes 2.5
turns about the gearmoter pulley, then crosses
over to a pulley mounted on the right side
closeout bulkhead, under the passengers right
thigh. The cable wraps around that pulley and
goes back to the left where it attaches to a
second spring, which connects to the tab at the
bottom of the left stick.
Components:
Motor--- Pitman GM8712C711, 12 volt, length
5”, diameter 1.4”, weight 10oz., shaft diameter
3/16”, reduction ratio 581.8/1.0, listed in a
servo motor catalogue which is currently packed
for a move. Cost about $30 several years ago.
Springs--- .050”wire, wound to about 3/8”
diameter, 6” long.
Motor Pulley--- Aluminum, flat, diameter 1” width
.75”
Right side pulley---grooved phenolic, ball
bearing, diameter 1”.
Cable--- 0.046”OD stranded cable, plastic
coated.
The pulleys and cable were salvaged from an old
dot matrix printer.
Voltage regulator—adjustable voltage IC (three
pin plastic package with heat sink tab, looks
like a transistor) available at electronic parts
store or Radio Shack, mounted on a 1-inch square
PC board with an adjusting pot and small
capacitor. I RTV’d a one inch square of .020
aluminum to the heat sink tab but it is probably
not required as the duty cycle is very short and
the IC is thermally protected.
Under no load conditions each springs is
stretched to about 11”. When the stick is moved
one spring gets longer while the other shortens.
The system does not add any friction to the
controls and full control deflection is always
available. The cable is not pinned to the motor
pulley so the system cannot overpower the pilot.
The voltage regulator is set to 5.3 volts,
producing a speed of 6 RPM. The briefest blip of
the trim switch at the first hint of a heavy wing
brings it into trim. A higher voltage will
produce some overshoot.
This motor is 5 to 10 times more powerful than
the application requires. The ideal motor would
be smaller, lighter and have a somewhat higher
reduction ratio. If anyone finds a better motor
let us know.
I am not equipped for photos. If anybody wants to
take photos I’m 35 mi. NW of Denver
International, there is a 50/50 chance I’ll be at
Oshkosh.
BILL HANNAHAN
WFHANNAHAN@YAHOO.COM
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LML website: http://www.olsusa.com/Users/Mkaye/maillist.html
LML Builders' Bookstore: http://www.buildersbooks.com/lancair
Please send your photos and drawings to marvkaye@olsusa.com.
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