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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

Damped versus undamped stability.
Figure 4-19. Damped versus undamped stability.

Longitudinal Stability (Pitching)
In designing an aircraft, a great deal of effort is spent in
developing the desired degree of stability around all three
axes. But longitudinal stability about the lateral axis is
considered to be the most affected by certain variables in
various flight conditions.

Longitudinal stability is the quality that makes an aircraft
stable about its lateral axis. It involves the pitching motion
as the aircraft's nose moves up and down in flight. A
longitudinally unstable aircraft has a tendency to dive or
climb progressively into a very steep dive or climb, or even
a stall. Thus, an aircraft with longitudinal instability becomes
difficult and sometimes dangerous to fly.

Static longitudinal stability or instability in an aircraft, is
dependent upon three factors:
1. Location of the wing with respect to the CG
2. Location of the horizontal tail surfaces with respect
to the CG
3. Area or size of the tail surfaces

In analyzing stability, it should be recalled that a body free
to rotate always turns about its CG.

To obtain static longitudinal stability, the relation of the
wing and tail moments must be such that, if the moments
are initially balanced and the aircraft is suddenly nose up,
the wing moments and tail moments change so that the sum
of their forces provides an unbalanced but restoring moment
which, in turn, brings the nose down again. Similarly, if the
aircraft is nose down, the resulting change in moments brings
the nose back up.

The CL in most asymmetrical airfoils has a tendency to
change its fore and aft positions with a change in the AOA.
The CL tends to move forward with an increase in AOA and
to move aft with a decrease in AOA. This means that when the
AOA of an airfoil is increased, the CL, by moving forward,

tends to lift the leading edge of the wing still more. This
tendency gives the wing an inherent quality of instability.
(NOTE: CL is also known as the center of pressure (CP).)

Figure 4-20 shows an aircraft in straight-and-level flight. The
line CG-CL-T represents the aircraft's longitudinal axis from
the CG to a point T on the horizontal stabilizer.

Longitudinal stability.
Figure 4-20. Longitudinal stability.

Most aircraft are designed so that the wing's CL is to the rear
of the CG. This makes the aircraft "nose heavy" and requires
that there be a slight downward force on the horizontal
stabilizer in order to balance the aircraft and keep the nose
from continually pitching downward. Compensation for this
nose heaviness is provided by setting the horizontal stabilizer
at a slight negative AOA. The downward force thus produced
holds the tail down, counterbalancing the "heavy" nose. It is as if the line CG-CL-T were a lever with an upward force at CL and two downward forces balancing each other, one a strong force at the CG point and the other, a much lesser force, at point T (downward air pressure on the stabilizer). To better visualize this physics principle: If an iron bar were suspended at point CL, with a heavy weight hanging on it at the CG, it would take downward pressure at point T to keep the "lever" in balance.

 

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