Flight Training - August 2021 - 15

HOW IT WORKS
BY THOMAS A. HORNE
TWEAKING PITCH
TAKING THE LOAD OFF
SOME STUDENTS CAN BE CONFUSED ABOUT THE PITCH TRIM
wheel: that small, round, thin disc used to relieve control pressures.
Sure, the function of the control yoke/stick and rudders
are easy to comprehend. But the pitch trim wheel is a big part
of the control system mix. It comes into play every time an
airplane changes configuration-such as extending or retracting
flaps, turning, climbing, descending, or holding a selected
airspeed. And every time power is increased or decreased.
Whenever those changes occur, the airflow-and air
load-over the horizontal stabilizer and elevator also changes.
Increase power, and the nose will rise. Decrease power, and the
nose drops. If you're trying to hold altitude, you could find yourself
pushing or pulling on the control yoke with extra effort. You
need a little help to eliminate these extra forces and reduce your
workload. That's where the pitch trim wheel comes in.
The trim wheel moves the trim tab at the aft edge of the
elevator, which in turn adjusts the position of the entire elevator.
Move the trim wheel just the right amount, and the trim tab
moves the elevator to compensate for the effects of any change
in lift produced by the tail surfaces. In flight, you'll find the trim
wheel very handy in positioning the tab so that you can fly handsoff-whether
you're climbing, descending, or straight and level.
How does the pitch trim system work? The trim wheel in the
cockpit is attached to a sprocket, and a chain rides in the sprocket's
teeth. The chain is swaged (crimped tightly) to a long, woven
stainless steel cable that runs through pulleys and guides under
the panel and the cabin floor. This cable run goes all the way back
to the tailcone, where it makes a couple right-angle turns through
two more pulleys. Finally, it's swaged to another chain. This chain
also rides in a sprocket. When the sprocket turns, it moves the
trim tab actuator, which moves the trim tab up or down. You can
see the actuator's connection to the trim tab-and should inspect
it for integrity during every preflight inspection.
Having done its job moving the trim tab actuator, where does
the chain go from there? It's attached to another cable, which
runs through pulleys and guides back to the trim wheel in the
cockpit. The system is a closed loop with chains at either end.
Electrically powered trim control systems are more complicated
and use switches and servos to move the cable and
sprockets. Instead of manually moving a trim wheel, you'll press
a sliding switch on the control yoke to trim the nose up or down.
A big problem can occur with electrically controlled trim
systems: The trim motors may " run away, " sending the airplane
into uncommanded pitch movements. To stop that, pull the trim
motor's circuit breaker. Now you're back to manually trimming
with the trim wheel, just like you learned when you started flying.
tom.horne@aopa.org
Trim tab
Trim wheel
Sprocket
AOPA.ORG/FT
15
STEVE KARP
http://www.AOPA.ORG/FT

Flight Training - August 2021

Table of Contents for the Digital Edition of Flight Training - August 2021

Contents
Flight Training - August 2021 - Intro
Flight Training - August 2021 - Cover1
Flight Training - August 2021 - Cover2
Flight Training - August 2021 - Contents
Flight Training - August 2021 - 2
Flight Training - August 2021 - 3
Flight Training - August 2021 - 4
Flight Training - August 2021 - 5
Flight Training - August 2021 - 6
Flight Training - August 2021 - 7
Flight Training - August 2021 - 8
Flight Training - August 2021 - 9
Flight Training - August 2021 - 10
Flight Training - August 2021 - 11
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Flight Training - August 2021 - 15
Flight Training - August 2021 - 16
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Flight Training - August 2021 - 18
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Flight Training - August 2021 - Cover3
Flight Training - August 2021 - Cover4
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