| Home | Privacy | Contact |

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

| First | Previous | Next | Last |

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




Drag is the force that resists movement of an aircraft through
the air. There are two basic types: parasite drag and induced
drag. The first is called parasite because it in no way functions
to aid flight, while the second, induced drag, is a result of an
airfoil developing lift.

Parasite Drag
Parasite drag is comprised of all the forces that work to slow
an aircraft's movement. As the term parasite implies, it is
the drag that is not associated with the production of lift.
This includes the displacement of the air by the aircraft,
turbulence generated in the airstream, or a hindrance of air
moving over the surface of the aircraft and airfoil. There are
three types of parasite drag: form drag, interference drag,
and skin friction.

Form Drag
Form drag is the portion of parasite drag generated by the
aircraft due to its shape and airflow around it. Examples
include the engine cowlings, antennas, and the aerodynamic
shape of other components. When the air has to separate
to move around a moving aircraft and its components, it
eventually rejoins after passing the body. How quickly
and smoothly it rejoins is representative of the resistance
that it creates which requires additional force to overcome.
[Figure 4-5]

Form drag.
Figure 4-5. Form drag.

Notice how the flat plate in Figure 4-5 causes the air to swirl
around the edges until it eventually rejoins downstream. Form
drag is the easiest to reduce when designing an aircraft. The
solution is to streamline as many of the parts as possible.

Interference Drag
Interference drag comes from the intersection of airstreams
that creates eddy currents, turbulence, or restricts smooth
airflow For example, the intersection of the wing and the
fuselage at the wing root has significant interference drag.
Air .owing around the fuselage collides with air .owing
over the wing, merging into a current of air different from
the two original currents. The most interference drag is
observed when two surfaces meet at perpendicular angles.
Fairings are used to reduce this tendency. If a jet fighter
carries two identical wing tanks, the overall drag is greater
than the sum of the individual tanks because both of these
create and generate interference drag. Fairings and distance
between lifting surfaces and external components (such as
radar antennas hung from wings) reduce interference drag.
[Figure 4-6]

A wing root can cause interference drag.
Figure 4-6. A wing root can cause interference drag.

Skin Friction Drag
Skin friction drag is the aerodynamic resistance due to the
contact of moving air with the surface of an aircraft. Every
surface, no matter how apparently smooth, has a rough,
ragged surface when viewed under a microscope. The air
molecules, which come in direct contact with the surface of
the wing, are virtually motionless. Each layer of molecules
above the surface moves slightly faster until the molecules
are moving at the velocity of the air moving around the
aircraft. This speed is called the free-stream velocity. The
area between the wing and the free-stream velocity level is
about as wide as a playing card and is called the boundary
layer. At the top of the boundary layer, the molecules increase
velocity and move at the same speed as the molecules
outside the boundary layer. The actual speed at which the
molecules move depends upon the shape of the wing, the
viscosity (stickiness) of the air through which the wing or
airfoil is moving, and its compressibility (how much it can
be compacted).