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Pilot's Handbook of Aeronautical Knowledge
Aerodynamics of Flight
High Speed Flight

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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




High Speed Flight Controls
On high-speed aircraft, flight controls are divided into primary
flight controls and secondary or auxiliary flight controls. The
primary flight controls maneuver the aircraft about the pitch,
roll, and yaw axes. They include the ailerons, elevator, and
rudder. Secondary or auxiliary flight controls include tabs,
leading edge flaps, trailing edge flaps, spoilers, and slats.

Spoilers are used on the upper surface of the wing to spoil
or reduce lift. High speed aircraft, due to their clean low
drag design use spoilers as speed brakes to slow them
down. Spoilers are extended immediately after touchdown
to dump lift and thus transfer the weight of the aircraft from
the wings onto the wheels for better braking performance.
[Figure 4-63]

Jet transport aircraft have small ailerons. The space for
ailerons is limited because as much of the wing trailing
edge as possible is needed for flaps Also, a conventional
size aileron would cause wing twist at high speed. For that
reason, spoilers are used in unison with ailerons to provide
additional roll control.

Some jet transports have two sets of ailerons, a pair of
outboard low-speed ailerons and a pair of high-speed inboard
ailerons. When the flaps are fully retracted after takeoff, the
outboard ailerons are automatically locked out in the flared

When used for roll control, the spoiler on the side of the
up-going aileron extends and reduces the lift on that side,
causing the wing to drop. If the spoilers are extended as
speed brakes, they can still be used for roll control. If they
are the differential type, they extend further on one side
and retract on the other side. If they are the non-differential
type, they extend further on one side but do not retract on the
other side. When fully extended as speed brakes, the non
differential spoilers remain extended and do not supplement
the ailerons.

To obtain a smooth stall and a higher AOA without airflow
separation, the wing's leading edge should have a well rounded
almost blunt shape that the airflow can adhere to
at the higher AOA. With this shape, the airflow separation
starts at the trailing edge and progresses forward gradually
as AOA is increased.

The pointed leading edge necessary for high-speed flight
results in an abrupt stall and restricts the use of trailing edge
flaps because the airflow cannot follow the sharp curve

around the wing leading edge. The airflow tends to tear loose
rather suddenly from the upper surface at a moderate AOA.
To utilize trailing edge flaps, and thus increase the CL-MAX,
the wing must go to a higher AOA without airflow separation.
Therefore, leading edge slots, slats, and flaps are used to
improve the low-speed characteristics during takeoff, climb,
and landing. Although these devices are not as powerful as
trailing edge flaps, they are effective when used full span in
combination with high-lift trailing edge flaps With the aid
of these sophisticated high-lift devices, airflow separation is
delayed and the CL-MAX is increased considerably. In fact, a
50 knot reduction in stall speed is not uncommon.

The operational requirements of a large jet transport aircraft
necessitate large pitch trim changes. Some requirements are:

• A large CG range
• A large speed range
• The ability to perform large trim changes due to
wing leading edge and trailing edge high-lift devices
without limiting the amount of elevator remaining
• Maintaining trim drag to a minimum

These requirements are met by the use of a variable incidence
horizontal stabilizer. Large trim changes on a .xed-tail
aircraft require large elevator deflections At these large
deflections, little further elevator movement remains in the
same direction. A variable incidence horizontal stabilizer
is designed to take out the trim changes. The stabilizer is
larger than the elevator, and consequently does not need to
be moved through as large an angle. This leaves the elevator
streamlining the tail plane with a full range of movement up
and down. The variable incidence horizontal stabilizer can
be set to handle the bulk of the pitch control demand, with
the elevator handling the rest. On aircraft equipped with a
variable incidence horizontal stabilizer, the elevator is smaller
and less effective in isolation than it is on a .xed-tail aircraft.
In comparison to other flight controls, the variable incidence
horizontal stabilizer is enormously powerful in its effect.

Because of the size and high speeds of jet transport aircraft,
the forces required to move the control surfaces can be
beyond the strength of the pilot. Consequently, the control
surfaces are actuated by hydraulic or electrical power units.
Moving the controls in the flight deck signals the control
angle required, and the power unit positions the actual
control surface. In the event of complete power unit failure,
movement of the control surface can be effected by manually
controlling the control tabs. Moving the control tab upsets
the aerodynamic balance which causes the control surface
to move.