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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

Preface

Acknowledgements

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

Appendix

Glossary

Index

Thrust line affects longitudinal stability.
Figure 4-23. Thrust line affects longitudinal stability.

 

Power changes affect longitudinal stability.
Figure 4-24. Power changes affect longitudinal stability.

Conclusion: with CG forward of the CL and with an
aerodynamic tail-down force, the aircraft usually tries to
return to a safe flying attitude.

The following is a simple demonstration of longitudinal
stability. Trim the aircraft for "hands off" control in level
flight. Then, momentarily give the controls a slight push to
nose the aircraft down. If, within a brief period, the nose rises
to the original position and then stops, the aircraft is statically
stable. Ordinarily, the nose passes the original position (that
of level flight) and a series of slow pitching oscillations
follows. If the oscillations gradually cease, the aircraft has
positive stability; if they continue unevenly, the aircraft has
neutral stability; if they increase, the aircraft is unstable.

Lateral Stability (Rolling)
Stability about the aircraft's longitudinal axis, which extends
from the nose of the aircraft to its tail, is called lateral
stability. This helps to stabilize the lateral or "rolling effect"
when one wing gets lower than the wing on the opposite side
of the aircraft. There are four main design factors that make
an aircraft laterally stable: dihedral, sweepback, keel effect,
and weight distribution.

The most common procedure for producing lateral stability
is to build the wings with an angle of one to three degrees
above perpendicular to the longitudinal axis. The wings on
either side of the aircraft join the fuselage to form a slight V or
angle called "dihedral." The amount of dihedral is measured
by the angle made by each wing above a line parallel to the
lateral axis.

Dihedral involves a balance of lift created by the wings' AOA
on each side of the aircraft's longitudinal axis. If a momentary
gust of wind forces one wing to rise and the other to lower, the
aircraft banks. When the aircraft is banked without turning,
the tendency to sideslip or slide downward toward the lowered
wing occurs. [Figure 4-25] Since the wings have dihedral,
the air strikes the lower wing at a much greater AOA than the
higher wing. The increased AOA on the lower wing creates
more lift than the higher wing. Increased lift causes the lower
wing to begin to rise upward. As the wings approach the
level position, the AOA on both wings once again are equal,
causing the rolling tendency to subside. The effect of dihedral
is to produce a rolling tendency to return the aircraft to a
laterally balanced flight condition when a sideslip occurs.

The restoring force may move the low wing up too far, so
that the opposite wing now goes down. If so, the process
is repeated, decreasing with each lateral oscillation until a
balance for wings-level flight is finally reached.

 

4-16