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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

A powered parachute wing is a parachute that has a cambered
upper surface and a flatter under surface. The two surfaces are
separated by ribs that act as cells, which open to the airflow
at the leading edge and have internal ports to allow lateral
airflow The principle at work holds that the cell pressure is
greater than the outside pressure, thereby forming a wing that
maintains its airfoil shape in flight The pilot and passenger
sit in tandem in front of the engine which is located at the
rear of a vehicle. The airframe is attached to the parachute
via two attachment points and lines. Control is accomplished
by both power and the changing of the airfoil via the control
lines. [Figure 4-17]

A powered parachute.
Figure 4-17. A powered parachute.

Moment and Moment Arm
A study of physics shows that a body that is free to rotate
will always turn about its CG. In aerodynamic terms, the
mathematical measure of an aircraft's tendency to rotate
about its CG is called a "moment." A moment is said to be
equal to the product of the force applied and the distance at
which the force is applied. (A moment arm is the distance
from a datum [reference point or line] to the applied force.)
For aircraft weight and balance computations, "moments"
are expressed in terms of the distance of the arm times the
aircraft's weight, or simply, inch-pounds.

Aircraft designers locate the fore and aft position of the
aircraft's CG as nearly as possible to the 20 percent point
of the mean aerodynamic chord (MAC). If the thrust line
is designed to pass horizontally through the CG, it will not
cause the aircraft to pitch when power is changed, and there
will be no difference in moment due to thrust for a power-on
or power-off condition of flight Although designers have
some control over the location of the drag forces, they are not
always able to make the resultant drag forces pass through the
CG of the aircraft. However, the one item over which they
have the greatest control is the size and location of the tail.
The objective is to make the moments (due to thrust, drag, and
lift) as small as possible and, by proper location of the tail,
to provide the means of balancing an aircraft longitudinally
for any condition of flight.

The pilot has no direct control over the location of forces
acting on the aircraft in flight, except for controlling the
center of lift by changing the AOA. Such a change, however,
immediately involves changes in other forces. Therefore,
the pilot cannot independently change the location of one
force without changing the effect of others. For example,
a change in airspeed involves a change in lift, as well as a
change in drag and a change in the up or down force on the
tail. As forces such as turbulence and gusts act to displace
the aircraft, the pilot reacts by providing opposing control
forces to counteract this displacement.

Some aircraft are subject to changes in the location of the
CG with variations of load. Trimming devices are used to
counteract the forces set up by fuel burnoff, and loading or
off-loading of passengers or cargo. Elevator trim tabs and
adjustable horizontal stabilizers comprise the most common
devices provided to the pilot for trimming for load variations.
Over the wide ranges of balance during flight in large aircraft,
the force which the pilot has to exert on the controls would
become excessive and fatiguing if means of trimming were
not provided.

Aircraft Design Characteristics

Each aircraft handles somewhat differently because each
resists or responds to control pressures in its own way. For
example, a training aircraft is quick to respond to control
applications, while a transport aircraft feels heavy on the
controls and responds to control pressures more slowly.
These features can be designed into an aircraft to facilitate
the particular purpose of the aircraft by considering certain
stability and maneuvering requirements. The following
discussion summarizes the more important aspects of an
aircraft's stability, maneuverability and controllability
qualities; how they are analyzed; and their relationship to
various flight conditions.

Stability
Stability is the inherent quality of an aircraft to correct for
conditions that may disturb its equilibrium, and to return to
or to continue on the original flightpath It is primarily an
aircraft design characteristic. The flightpaths and attitudes an
aircraft flies are limited by the aerodynamic characteristics of the aircraft, its propulsion system, and its structural strength.
These limitations indicate the maximum performance and
maneuverability of the aircraft. If the aircraft is to provide
maximum utility, it must be safely controllable to the full
extent of these limits without exceeding the pilot's strength
or requiring exceptional flying ability. If an aircraft is to .y
straight and steady along any arbitrary flightpath, the forces
acting on it must be in static equilibrium. The reaction of
any body when its equilibrium is disturbed is referred to as
stability. The two types of stability are static and dynamic.

 

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