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Pilot's Handbook of Aeronautical Knowledge
Aircraft Structure

Lift and Basic Aerodynamics

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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




Lift and Basic Aerodynamics

In order to understand the operation of the major components
and subcomponents of an aircraft, it is important to understand
basic aerodynamic concepts. This chapter briefly introduces
aerodynamics; a more detailed explanation can be found in
Chapter 4, Aerodynamics of Flight.

Four forces act upon an aircraft in relation to straight-and level,
unaccelerated flight These forces are thrust, lift,
weight, and drag. [Figure 2-1]

The four forces.
Figure 2-1. The four forces.

Thrust is the forward force produced by the powerplant/
propeller. It opposes or overcomes the force of drag. As a
general rule, it is said to act parallel to the longitudinal axis.
This is not always the case as explained later.

Drag is a rearward, retarding force, and is caused by
disruption of airflow by the wing, fuselage, and other
protruding objects. Drag opposes thrust, and acts rearward
parallel to the relative wind.

Weight is the combined load of the airplane itself, the crew,
the fuel, and the cargo or baggage. Weight pulls the airplane
downward because of the force of gravity. It opposes lift,
and acts vertically downward through the airplane's center
of gravity (CG).

Lift opposes the downward force of weight, is produced by
the dynamic effect of the air acting on the wing, and acts
perpendicular to the flightpath through the wing's center
of lift.

An aircraft moves in three dimensions and is controlled by
moving it about one or more of its axes. The longitudinal or
roll axis extends through the aircraft from nose to tail, with
the line passing through the CG. The lateral or pitch axis
extends across the aircraft on a line through the wing tips,
again passing through the CG. The vertical, or yaw, axis
passes through the aircraft vertically, intersecting the CG.
All control movements cause the aircraft to move around
one or more of these axes, and allows for the control of the
airplane in flight [Figure 2-2]

One of the most significant components of aircraft design is
CG. It is the specific point where the mass or weight of an
aircraft may be said to center; that is, a point around which, if
the aircraft could be suspended or balanced, the aircraft would
remain relatively level. The position of the CG of an aircraft
determines the stability of the aircraft in flight As the CG
moves rearward (towards the tail) the aircraft becomes more
and more dynamically unstable. In aircraft with fuel tanks
situated in front of the CG, it is important that the CG is set
with the fuel tank empty. Otherwise, as the fuel is used, the
aircraft becomes unstable. [Figure 2-3] The CG is computed
during initial design and construction, and is further affected
by the installation of onboard equipment, aircraft loading,
and other factors.

pitch, roll, and yaw
Figure 2-2. Illustrates the pitch, roll, and yaw motion of the aircraft along the lateral, longitudinal, and vertical axes, respectively.