Aero-Tips!
A good pilot is always learning -- how many times have you
heard this old standard throughout your flying career? There is no
truer statement in all of flying (well, with the possible exception
of "there are no old, bold pilots.")
Aero-News has called upon the expertise of Thomas P. Turner,
master CFI and all-around-good-guy, to bring our readers -- and us
-- daily tips to improve our skills as aviators. Some of them, you
may have heard before... but for each of us, there will also be
something we might never have considered before, or something that
didn't "stick" the way it should have the first time we memorized
it for the practical test.
Aero-Tips 06.20.06
One of the most important things to learn in the very first
lesson or two flying an airplane is to make proper use of the
trim. This lesson will serve you whether you pursue a
recreational or sport certificate, take the traditional Private
Pilot route, or work your way up through the Airline Transport
Pilot certificate. Trim will be one of your best friends in cruise
flight, and is essential to successfully flying on instruments.
In a rather involved and somewhat math-heavy discussion, the National Aeronautics
and Space Administration (NASA) tells us that "when there is no
rotation around the [airplane's center of gravity] the aircraft is
said to be trimmed." From the pilot's seat this means that flight
controls are displaced to that the airplane is not pitching (nose
up/down movement), rolling (wings changing their angle of bank
relative to the ground), or yawing (swinging side-to-side relative
to the airplane's vertical axis). Note that the airplane can be at
any angle of pitch, bank and/or yaw relative to the horizon; it's
the lack of change in the current angles that implies the airplane
is "in trim."
Put it on my tab
You might be pushing or pulling hard on the yoke and/or rudder
pedals to keep movement around the c.g. nil, and the airplane in
trim. Most airplanes, though, have devices to reduce the amount of
work you need to do to keep the airplane trimmed. Trim
tabs are fixed or moveable parts of control surfaces that
help relieve the pressure needed to displace the controls.
- Fixed tabs are attached to the trailing edge
of controls, and are set during airplane production and adjusted as
necessary afterward to apply control pressures normally seen in
cruising flight. These are sometimes called
ground-adjustable tabs. You'll usually see these
thin metal fixed trim tabs on airplane rudders and sometimes
ailerons.
- Moveable or in-flight adjustable trim tabs are
hinged so you can move them using controls in the cockpit. These
are most helpful when the trim requirement changes with airspeed,
power changes and/or angle of attack. Almost all airplanes have a
moveable trim tab on the elevator for pitch control.
Airplanes use different techniques for adjusting trim. Bungee
chords might be used to put a load on control cables, deflecting
the control surface without a trim tab. Most Beech Bonanzas use
this for aileron trim. Some aircraft, for example the Piper Cub,
have jackscrews that change the angle of incidence of the
horizontal stabilizer to change pitch trim. In a few cases,
including Mooneys through some large transport airplanes, the
entire tail section of the airplane may pivot to accomplish pitch
trim, a surprisingly reliable and effective system. Regardless of
the design involved, trim systems do the hard work of maintaining
trim so you don't have to.
Aero-tip of the day: Learning to trim the
airplane is the first step in reducing pilot workload, and
achieving consistent, predictable performance.