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horizontal tail. The lift caused by fuselage is small and hence is negligible.
The skin-friction drag is the drag caused by shear stresses present at the body
surface where the flow is attached. The region of the body surface where the flow is
attached is called the wetted surface, and the corresponding surface area is called
the wetted area.
The skin-friction coefficient is defined as
Cf = pD_f S
rfhe schematic variation of skin-friction coefficient with Reynolds number for a
flat plate is shown in Fig. 1.12. For approximate calculation of skin friction of
Re
Fig. 1.12 Schematic illustration of slan-friction coefficient of a flat plate with
Reynolds number.
12 PERFORMANCE, STABILITY, DYNAMICS, AND CONTROL
streamlined bodies for attached fiow conditions, the following flat plate results
may be used.
For laminar fiow
1.328
Cf = T~ (1.13)
For turbulent fiow .
0.455
Cf = ( o9~o Re~2 58 (1.14)
where Re is the Reynolds number based on the length of the flat plate.
The pressure drag arises because of flow separation. If the flow does not close
behind the body, forming a rear stagnation point, but separates, positive pressures
acting in front of the body do not balance those acting on the rear of the body,
and pressure drag results. For streamlined bodies, skin friction is the primary
component of the drag, and pressure drag is relatively small. However, for bluff
bodies like circular cylinder, the skin friction is small, and it is the pressure drag
that forms the primary component of the total drag.
Induced drag is a component of the normal force in the freestream direction.
Whereas the skin-friction and pressure drag are caused by fluid viscosity,induced
drag is not directly caused by the effects of viscosity.
1.4 Wing Parameters
The wing section and planform parameters that are useful in esOmating the
aerodynamic characteristics of aircraft are discussed in the following sections.
The cross section AA of an airplane wing, as shown in Fig. 1.13, is known as
an airfoil section. A two-dimensional wing (a wing that extends to infinity in both
directions), having an identical airfoil section at all cross sections, is called an
airfoil. In other words, an airfoil is a two-dimensional wing with a constant airfoil
section.
The objective of analytical, computational, or experimental investigations is to
design an efficient airfo:l section that has a low drag coefficient, a high lift-to-drag
ratio, a high value of maximum lift coefficient, a small value of pitching-moment
coefficient, and a sraooth gradual stall.
Since the early days of aviation, several efforts-ljave been made to design and
develop series of efficient airfoil sections. Notable among these are the works at
Goettingen, Germany; Royal Aircraft Establishment in the United Kingdom; and
National Advisory Committee for Aeronautics (NACA) in the United States. The
NACA airfoils hayve gained wide acceptance throughout the world and have influ-
enced the airfoil design of others. The NACA airfoils have been used on several
commercial and military airplanes that have been built over the years. The follow-
ing is a brief description of NACA airfoil sections. For more information, refer to
Ref. 2.
The typical geometry of an airfoilis shown in Fig. 1.14. The line drawn midway
between the upper and lower surfaces is called the meanline. The leading and
trailing edges are defined as the forward and rearward extremities, respectively,
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