曝光台 注意防骗
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Supcrcritical
Fig. 1.10 Schematic pressure distribution o'ver circular cylinder.
Fig. 1.11 Schematic illustration of swing of cricket ball
REVIEW OF BASIC AERODYNAMIC PRINCIPLES
11
on the top surface, the fiow separation is turbulent because of the transition caused
by the "seam," which trips the local laminar boundary layer, causing a transition to
turbulent fiow whereas, on the bottom surface, the flow separation is laminar. As a
result, the pressure distribution is asymmetric, and the cricket ball develops a net
upward force as shown. If the ball is so oriented that the whole pict,ure is rotatecl
by 90 deg, the ball develops a cross force, which will make the ball swing inward
or outward with respect to the batsman, depending on the orientation of the seam.
1.3 Drag of Bodies
The total drag of bodies exposed to airstream at low speeds consists of 1) skin
friction, 2) pressure drag, and 3) induced drag. Induced drag is also called drag
due to lift. Whereas the contributions to skin friction and pressure drag come from
all parts exposed to the airstream, contributions to induced drag come only from
lifting surfaces. At low speeds, contr:ibutions to lift come mainly from wing and
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
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