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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




The airflow outside of the boundary layer reacts to the
shape of the edge of the boundary layer just as it would
to the physical surface of an object. The boundary layer
gives any object an "effective" shape that is usually slightly
different from the physical shape. The boundary layer may
also separate from the body, thus creating an effective shape
much different from the physical shape of the object. This
change in the physical shape of the boundary layer causes a
dramatic decrease in lift and an increase in drag. When this
happens, the airfoil has stalled.

In order to reduce the effect of skin friction drag, aircraft
designers utilize flush mount rivets and remove any
irregularities which may protrude above the wing surface.
In addition, a smooth and glossy finish aids in transition of
air across the surface of the wing. Since dirt on an aircraft
disrupts the free flow of air and increases drag, keep the
surfaces of an aircraft clean and waxed.

Induced Drag
The second basic type of drag is induced drag. It is an
established physical fact that no system that does work in the
mechanical sense can be 100 percent efficient. This means
that whatever the nature of the system, the required work
is obtained at the expense of certain additional work that is
dissipated or lost in the system. The more efficient the system,
the smaller this loss.

In level flight the aerodynamic properties of a wing or rotor
produce a required lift, but this can be obtained only at the
expense of a certain penalty. The name given to this penalty
is induced drag. Induced drag is inherent whenever an airfoil
is producing lift and, in fact, this type of drag is inseparable
from the production of lift. Consequently, it is always present
if lift is produced.

An airfoil (wing or rotor blade) produces the lift force by
making use of the energy of the free airstream. Whenever
an airfoil is producing lift, the pressure on the lower surface
of it is greater than that on the upper surface (Bernoulli's
Principle). As a result, the air tends to .ow from the high
pressure area below the tip upward to the low pressure area
on the upper surface. In the vicinity of the tips, there is a
tendency for these pressures to equalize, resulting in a lateral
.ow outward from the underside to the upper surface. This
lateral .ow imparts a rotational velocity to the air at the tips,
creating vortices, which trail behind the airfoil.

When the aircraft is viewed from the tail, these vortices
circulate counterclockwise about the right tip and clockwise
about the left tip. [Figure 4-7] Bearing in mind the direction
of rotation of these vortices, it can be seen that they induce
an upward flow of air beyond the tip, and a downwash flow

behind the wing's trailing edge. This induced downwash has
nothing in common with the downwash that is necessary to
produce lift. It is, in fact, the source of induced drag. The
greater the size and strength of the vortices and consequent
downwash component on the net airflow over the airfoil, the
greater the induced drag effect becomes. This downwash over
the top of the airfoil at the tip has the same effect as bending
the lift vector rearward; therefore, the lift is slightly aft of
perpendicular to the relative wind, creating a rearward lift
component. This is induced drag.

Wingtip vortex from a crop duster.
Figure 4-7. Wingtip vortex from a crop duster.

In order to create a greater negative pressure on the top of an
airfoil, the airfoil can be inclined to a higher AOA. If the AOA
of a symmetrical airfoil were zero, there would be no pressure
differential, and consequently, no downwash component and
no induced drag. In any case, as AOA increases, induced
drag increases proportionally. To state this another way—the
lower the airspeed the greater the AOA required to produce
lift equal to the aircraft's weight and, therefore, the greater
induced drag. The amount of induced drag varies inversely
with the square of the airspeed.

Conversely, parasite drag increases as the square of the
airspeed. Thus, as airspeed decreases to near the stalling
speed, the total drag becomes greater, due mainly to the sharp
rise in induced drag. Similarly, as the airspeed reaches the
terminal velocity of the aircraft, the total drag again increases
rapidly, due to the sharp increase of parasite drag. As seen
in Figure 4-8, at some given airspeed, total drag is at its
minimum amount. In figuring the maximum endurance and
range of aircraft, the power required to overcome drag is at
a minimum if drag is at a minimum.