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Pilot's Handbook of Aeronautical Knowledge
Aerodynamics of Flight
Aircraft Design Characteristics

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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




Fuselage and fin for vertical stability.
Figure 4-27. Fuselage and fin for vertical stability.

The aircraft is then momentarily skidding sideways, and
during that moment (since it is assumed that although the
yawing motion has stopped, the excess pressure on the left
side of the fin still persists) there is necessarily a tendency
for the aircraft to be turned partially back to the left. That is,
there is a momentary restoring tendency caused by the fin.

This restoring tendency is relatively slow in developing and
ceases when the aircraft stops skidding. When it ceases, the
aircraft is flying in a direction slightly different from the
original direction. In other words, it will not return of its own
accord to the original heading; the pilot must reestablish the
initial heading.

A minor improvement of directional stability may be obtained
through sweepback. Sweepback is incorporated in the design
of the wing primarily to delay the onset of compressibility
during high-speed flight In lighter and slower aircraft,
sweepback aids in locating the center of pressure in the
correct relationship with the CG. A longitudinally stable
aircraft is built with the center of pressure aft of the CG.

Because of structural reasons, aircraft designers sometimes
cannot attach the wings to the fuselage at the exact desired
point. If they had to mount the wings too far forward, and at
right angles to the fuselage, the center of pressure would not
be far enough to the rear to result in the desired amount of
longitudinal stability. By building sweepback into the wings,
however, the designers can move the center of pressure
toward the rear. The amount of sweepback and the position
of the wings then place the center of pressure in the correct

The contribution of the wing to static directional stability is
usually small. The swept wing provides a stable contribution
depending on the amount of sweepback, but the contribution
is relatively small when compared with other components.

Free Directional Oscillations (Dutch Roll)
Dutch roll is a coupled lateral/directional oscillation that is
usually dynamically stable but is unsafe in an aircraft because
of the oscillatory nature. The damping of the oscillatory mode
may be weak or strong depending on the properties of the
particular aircraft.

If the aircraft has a right wing pushed down, the positive
sideslip angle corrects the wing laterally before the nose is
realigned with the relative wind. As the wing corrects the
position, a lateral directional oscillation can occur resulting
in the nose of the aircraft making a figure eight on the
horizon as a result of two oscillations (roll and yaw), which,
although of about the same magnitude, are out of phase with
each other.

In most modern aircraft, except high-speed swept wing
designs, these free directional oscillations usually die out
automatically in very few cycles unless the air continues to
be gusty or turbulent. Those aircraft with continuing Dutch
roll tendencies are usually equipped with gyro-stabilized yaw
dampers. Manufacturers try to reach a midpoint between too
much and too little directional stability. Because it is more
desirable for the aircraft to have "spiral instability" than
Dutch roll tendencies, most aircraft are designed with that

Spiral Instability
Spiral instability exists when the static directional stability
of the aircraft is very strong as compared to the effect of its
dihedral in maintaining lateral equilibrium. When the lateral
equilibrium of the aircraft is disturbed by a gust of air and a
sideslip is introduced, the strong directional stability tends
to yaw the nose into the resultant relative wind while the
comparatively weak dihedral lags in restoring the lateral
balance. Due to this yaw, the wing on the outside of the
turning moment travels forward faster than the inside wing
and, as a consequence, its lift becomes greater. This produces
an overbanking tendency which, if not corrected by the pilot,
results in the bank angle becoming steeper and steeper. At
the same time, the strong directional stability that yaws the
aircraft into the relative wind is actually forcing the nose
to a lower pitch attitude. A slow downward spiral begins
which, if not counteracted by the pilot, gradually increases
into a steep spiral dive. Usually the rate of divergence in the
spiral motion is so gradual the pilot can control the tendency
without any difficulty.