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Pilot's Handbook of Aeronautical Knowledge
Aerodynamics of Flight
Forces Acting on the Aircraft

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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




Gravity is the pulling force that tends to draw all bodies to
the center of the earth. The CG may be considered as a point
at which all the weight of the aircraft is concentrated. If the
aircraft were supported at its exact CG, it would balance in any
attitude. It will be noted that CG is of major importance in an
aircraft, for its position has a great bearing upon stability.
The location of the CG is determined by the general design
of each particular aircraft. The designers determine how far
the center of pressure (CP) will travel. They then fix the CG
forward of the center of pressure for the corresponding flight
speed in order to provide an adequate restoring moment to
retain flight equilibrium.

Weight has a definite relationship to lift. This relationship
is simple, but important in understanding the aerodynamics
of flying. Lift is the upward force on the wing acting
perpendicular to the relative wind. Lift is required to
counteract the aircraft's weight (which is caused by the force
of gravity acting on the mass of the aircraft). This weight
(gravity) force acts downward through the airplane's CG.
In stabilized level flight, when the lift force is equal to the
weight force, the aircraft is in a state of equilibrium and
neither gains nor loses altitude. If lift becomes less than
weight, the aircraft loses altitude. When lift is greater than
weight, the aircraft gains altitude.

The pilot can control the lift. Any time the control yoke
or stick is moved fore or aft, the AOA is changed. As the
AOA increases, lift increases (all other factors being equal).
When the aircraft reaches the maximum AOA, lift begins
to diminish rapidly. This is the stalling AOA, known as
CL-MAX critical AOA. Examine Figure 4-9, noting how the
CL increases until the critical AOA is reached, then decreases
rapidly with any further increase in the AOA.

Before proceeding further with the topic of lift and how it
can be controlled, velocity must be interjected. The shape
of the wing or rotor cannot be effective unless it continually
keeps "attacking" new air. If an aircraft is to keep flying, the
lift-producing airfoil must keep moving. In a helicopter or
gyro-plane this is accomplished by the rotation of the rotor
blades. For other types of aircraft such as airplanes, weightshift
control, or gliders, air must be moving across the lifting
surface. This is accomplished by the forward speed of the
aircraft. Lift is proportional to the square of the aircraft's
velocity. For example, an airplane traveling at 200 knots has
four times the lift as the same airplane traveling at 100 knots,
if the AOA and other factors remain constant.

Actually, an aircraft could not continue to travel in level
flight at a constant altitude and maintain the same AOA if
the velocity is increased. The lift would increase and the
aircraft would climb as a result of the increased lift force.
Therefore, to maintain the lift and weight forces in balance,
and to keep the aircraft straight and level (not accelerating
upward) in a state of equilibrium, as velocity is increased,
lift must be decreased. This is normally accomplished by
reducing the AOA by lowering the nose. Conversely, as the
aircraft is slowed, the decreasing velocity requires increasing
the AOA to maintain lift sufficient to maintain flight There
is, of course, a limit to how far the AOA can be increased, if
a stall is to be avoided.

All other factors being constant, for every AOA there is a
corresponding airspeed required to maintain altitude in steady,
unaccelerated flight (true only if maintaining "level flight").
Since an airfoil always stalls at the same AOA, if increasing
weight, lift must also be increased. The only method of
increasing lift is by increasing velocity if the AOA is held
constant just short of the "critical," or stalling, AOA.

Lift and drag also vary directly with the density of the air.
Density is affected by several factors: pressure, temperature,
and humidity. At an altitude of 18,000 feet, the density of
the air has one-half the density of air at sea level. In order to
maintain its lift at a higher altitude, an aircraft must fly at a
greater true airspeed for any given AOA.

Warm air is less dense than cool air, and moist air is less dense
than dry air. Thus, on a hot humid day, an aircraft must be
flown at a greater true airspeed for any given AOA than on
a cool, dry day.

If the density factor is decreased and the total lift must equal
the total weight to remain in flight, it follows that one of the
other factors must be increased. The factor usually increased
is the airspeed or the AOA, because these are controlled
directly by the pilot.

Lift varies directly with the wing area, provided there is no
change in the wing's planform. If the wings have the same
proportion and airfoil sections, a wing with a planform area
of 200 square feet lifts twice as much at the same AOA as a
wing with an area of 100 square feet.