Introduction
This chapter focuses on the flight control systems a pilot uses
to control the forces of flight and the aircraft’s direction and
attitude. It should be noted that flight control systems and
characteristics can vary greatly depending on the type of
aircraft flown. The most basic flight control system designs
are mechanical and date back to early aircraft. They operate
with a collection of mechanical parts, such as rods, cables,
pulleys, and sometimes chains to transmit the forces of the
flight deck controls to the control surfaces. Mechanical flight
control systems are still used today in small general and
sport category aircraft where the aerodynamic forces are not
excessive. [Figure 6-1]
Flight Controls
Chapter 6
Anti-torque pedals
Cyclic stick
Collective lever
Cyclic
CyclicCyclic
Cyclic
YawYaw
YawYaw
CollectiveCollective
CollectiveCollective
Figure 6-3. Helicopter flight control system.
Hydraulic pressure
Hydraulic return
Pivot point
LEGEND
Elevator (UP)
Control stick (AFT—nose up)
Control cables
Power cylinder
Neutral
Neutral
Control valves
NeutralPower disconnect linkage
Figure 6-2. Hydromechanical flight control system.
Elevator
Control stick
Cable
Pulleys
Push rod
Figure 6-1. Mechanical flight control system.
As aviation matured and aircraft designers learned more about
aerodynamics, the industry produced larger and faster aircraft.
Therefore, the aerodynamic forces acting upon the control
surfaces increased exponentially. To make the control force
required by pilots manageable, aircraft engineers designed
more complex systems. At first, hydromechanical designs,
consisting of a mechanical circuit and a hydraulic circuit,
were used to reduce the complexity, weight, and limitations
of mechanical flight controls systems. [Figure 6-2]
As aircraft became more sophisticated, the control surfaces
were actuated by electric motors, digital computers, or fiber
optic cables. Called “fly-by-wire,” this flight control system
replaces the physical connection between pilot controls and
the flight control surfaces with an electrical interface. In
addition, in some large and fast aircraft, controls are boosted
by hydraulically or electrically actuated systems. In both
the fly-by-wire and boosted controls, the feel of the control
reaction is fed back to the pilot by simulated means.
Current research at the National Aeronautics and Space
Administration (NASA) Dryden Flight Research Center
involves Intelligent Flight Control Systems (IFCS). The goal
of this project is to develop an adaptive neural network-based
flight control system. Applied directly to flight control system
feedback errors, IFCS provides adjustments to improve
aircraft performance in normal flight, as well as with system
failures. With IFCS, a pilot is able to maintain control and
safely land an aircraft that has suffered a failure to a control
surface or damage to the airframe. It also improves mission
capability, increases the reliability and safety of flight, and
eases the pilot workload.
Today’s aircraft employ a variety of flight control systems.
For example, some aircraft in the sport pilot category rely on
weight-shift control to fly while balloons use a standard burn
technique. Helicopters utilize a cyclic to tilt the rotor in the
desired direction along with a collective to manipulate rotor
pitch and anti-torque pedals to control yaw. [Figure 6-3]
For additional information on flight control systems, refer
to the appropriate handbook for information related to the
flight control systems and characteristics of specific types
of aircraft.
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
Lateral axis
(longitudinal
stability)
Aileron—Roll
Rudder—YawElevator—Pitch
Longitudinal
axis (lateral
stability)
Vertical axis
(directional
stability)
Aileron Roll Longitudinal Lateral
Rudder Yaw Vertical Directional
Elevator/
Stabilator Pitch Lateral Longitudinal
Primary
Control
Surface
Airplane
Movement
Axes of
Rotation
Type of
Stability
Figure 6-4. Airplane controls, movement, axes of rotation, and
type of stability.
Figure 6-5. Adverse yaw is caused by higher drag on the outside
wing that is producing more lift.
Lift
Drag
Lift
DragAdvers e yaw
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 aircraft is stable and easily controlled
during normal maneuvering. Control surface inputs cause
movement about the three axes of rotation. The types of
stability an aircraft exhibits also relate to the three axes of
rotation. [Figure 6-4]
Ailerons
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 aircraft
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 6-5]
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