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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




Flight Control Systems

Flight Controls
Aircraft flight control systems consist of primary and
secondary systems. The ailerons, elevator (or stabilator), and
rudder constitute the primary control system and are required to
control an aircraft safely during flight. Wing flaps, leading edge
devices, spoilers, and trim systems constitute the secondary
control system and improve the performance characteristics of
the airplane or relieve the pilot of excessive control forces.

Primary Flight Controls
Aircraft control systems are carefully designed to provide
adequate responsiveness to control inputs while allowing a
natural feel. At low airspeeds, the controls usually feel soft
and sluggish, and the aircraft responds slowly to control
applications. At higher airspeeds, the controls become
increasingly firm and aircraft response is more rapid.

Movement of any of the three primary flight control surfaces
(ailerons, elevator or stabilator, or rudder), changes the airflow
and pressure distribution over and around the airfoil. These
changes affect the lift and drag produced by the airfoil/control
surface combination, and allow a pilot to control the aircraft
about its three axes of rotation.

Design features limit the amount of deflection of flight control
surfaces. For example, control-stop mechanisms may be
incorporated into the flight control linkages, or movement
of the control column and/or rudder pedals may be limited.
The purpose of these design limits is to prevent the pilot from
inadvertently overcontrolling and overstressing the aircraft
during normal maneuvers.

A properly designed airplane is stable and easily controlled
during normal maneuvering. Control surface inputs cause
movement about the three axes of rotation. The types of
stability an airplane exhibits also relate to the three axes of
rotation. [Figure 5-4]

Airplane controls, movement, axes of rotation
Figure 5-4. Airplane controls, movement, axes of rotation, and
type of stability.

Ailerons control roll about the longitudinal axis. The ailerons
are attached to the outboard trailing edge of each wing and
move in the opposite direction from each other. Ailerons are
connected by cables, bellcranks, pulleys and/or push-pull tubes
to a control wheel or control stick.

Moving the control wheel or control stick to the right causes
the right aileron to deflect upward and the left aileron to deflect
downward. The upward deflection of the right aileron decreases
the camber resulting in decreased lift on the right wing. The
corresponding downward deflection of the left aileron increases
the camber resulting in increased lift on the left wing. Thus,
the increased lift on the left wing and the decreased lift on the
right wing causes the airplane to roll to the right.

Adverse Yaw
Since the downward deflected aileron produces more lift as
evidenced by the wing raising, it also produces more drag. This
added drag causes the wing to slow down slightly. This results
in the aircraft yawing toward the wing which had experienced
an increase in lift (and drag). From the pilot's perspective, the
yaw is opposite the direction of the bank. The adverse yaw
is a result of differential drag and the slight difference in the
velocity of the left and right wings. [Figure 5-5]

Adverse yaw is caused by higher drag
Figure 5-5. Adverse yaw is caused by higher drag on the outside
wing, which is producing more lift.

Adverse yaw becomes more pronounced at low airspeeds.
At these slower airspeeds aerodynamic pressure on control
surfaces are low and larger control inputs are required to
effectively maneuver the airplane. As a result, the increase in
aileron deflection causes an increase in adverse yaw. The yaw
is especially evident in aircraft with long wing spans.