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Pilot's Handbook of Aeronautical Knowledge
Flight Controls

Flight Control Systems

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Found on many gliders and some aircraft, high drag devices
called spoilers are deployed from the wings to spoil the smooth
airflow, reducing lift and increasing drag. On gliders, spoilers
are most often used to control rate of descent for accurate
landings. On other aircraft, spoilers are often used for roll
control, an advantage of which is the elimination of adverse
yaw. To turn right, for example, the spoiler on the right wing
is raised, destroying some of the lift and creating more drag on
the right. The right wing drops, and the aircraft banks and yaws
to the right. Deploying spoilers on both wings at the same time
allows the aircraft to descend without gaining speed. Spoilers
are also deployed to help reduce ground roll after landing. By
destroying lift, they transfer weight to the wheels, improving
braking effectiveness. [Figure 5-19]

Spoilers reduce lift
Figure 5-19. Spoilers reduce lift and increase drag during descent
and landing.

Trim Systems
Although an aircraft can be operated throughout a wide range
of attitudes, airspeeds, and power settings, it can be designed
to fly hands-off within only a very limited combination of
these variables. Trim systems are used to relieve the pilot of
the need to maintain constant pressure on the flight controls,
and usually consist of flight deck controls and small hinged
devices attached to the trailing edge of one or more of the
primary flight control surfaces. Designed to help minimize
a pilot's workload, trim systems aerodynamically assist
movement and position of the flight control surface to which
they are attached. Common types of trim systems include trim
tabs, balance tabs, antiservo tabs, ground adjustable tabs, and
an adjustable stabilizer.

Trim Tabs
The most common installation on small aircraft is a single
trim tab attached to the trailing edge of the elevator. Most trim
tabs are manually operated by a small, vertically mounted
control wheel. However, a trim crank may be found in some
aircraft. The flight deck control includes a trim tab position
indicator. Placing the trim control in the full nose-down
position moves the trim tab to its full up position. With
the trim tab up and into the airstream, the airflow over the
horizontal tail surface tends to force the trailing edge of the
elevator down. This causes the tail of the airplane to move
up, and the nose to move down. [Figure 5-20]

movement of the elevator
Figure 5-20. The movement of the elevator is opposite to the
direction of movement of the elevator trim tab.

If the trim tab is set to the full nose-up position, the tab moves
to its full down position. In this case, the air .owing under
the horizontal tail surface hits the tab and forces the trailing
edge of the elevator up, reducing the elevator's AOA. This
causes the tail of the airplane to move down, and the nose
to move up.

In spite of the opposing directional movement of the trim
tab and the elevator, control of trim is natural to a pilot. If
the pilot needs to exert constant back pressure on a control
column, the need for nose-up trim is indicated. The normal
trim procedure is to continue trimming until the aircraft is
balanced and the nose-heavy condition is no longer apparent.
Pilots normally establish the desired power, pitch attitude,
and configuration first, and then trim the aircraft to relieve
control pressures that may exist for that flight condition.
Any time power, pitch attitude, or configuration is changed,
expect that retrimming will be necessary to relieve the control
pressures for the new flight condition.