fluid_mechanics

516 Chapter 9 ■ Flow over Immersed Bodies
120
100
NACA 64(1) – 412 airfoil
Re = 7 × 10 5 Stall 1.5
80
60
1.0
α = 6°
α = 8°
C __ L
C D
40
α = 4°
C L
0.5
α = 2°
20
α = 0°
V9.15 Stalled
airfoil
0
–20
0
α = –2°
α = –4°
–40
–8 –4 0 4 8
α = –6°
–0.5 0 0.005 0.010 0.015 0.02
α , degrees
(a)
(b)
F I G U R E 9.34 Two representations of the same lift and drag data for a typical airfoil:
(a) lift-to-drag ratio as a function of angle of attack, with the onset of boundary layer separation on the
upper surface indicated by the occurrence of stall, (b) the lift and drag polar diagram with the angle of
attack indicated (Ref. 27).
C D
V9.16 Bat flying
In many lift-generating devices the important quantity is the ratio of the lift to drag developed,
ld C LC D . Such information is often presented in terms of C LC D versus a, as is shown
in Fig. 9.34a, or in a lift-drag polar of C L versus C D with a as a parameter, as is shown in Fig.
9.34b. The most efficient angle of attack 1i.e., largest C LC D 2 can be found by drawing a line tangent
to the C L C D curve from the origin, as is shown in Fig. 9.34b. High-performance airfoils
generate lift that is perhaps 100 or more times greater than their drag. This translates into the fact
that in still air they can glide a horizontal distance of 100 m for each 1 m drop in altitude.
F l u i d s i n t h e N e w s
Bats feel turbulence Researchers have discovered that at certain
locations on the wings of bats, there are special touch-sensing
cells with a tiny hair poking out of the center of the cell. These cells,
which are very sensitive to air flowing across the wing surface, can
apparently detect turbulence in the flow over the wing. If these hairs
are removed the bats fly well in a straight line, but when maneuvering
to avoid obstacles, their elevation control is erratic. When the
hairs grow back, the bats regain their complete flying skills. It is proposed
that these touch-sensing cells are used to detect turbulence on
the wing surface and thereby tell bats when to adjust the angle of attack
and curvature of their wings in order to avoid stalling out in
midair.
V9.17 Trailing edge
flap
As is indicated above, the lift and drag on an airfoil can be altered by changing the angle
of attack. This actually represents a change in the shape of the object. Other shape changes can
be used to alter the lift and drag when desirable. In modern airplanes it is common to utilize leading
edge and trailing edge flaps as is shown in Fig. 9.35. To generate the necessary lift during
the relatively low-speed landing and takeoff procedures, the airfoil shape is altered by extending
special flaps on the front andor rear portions of the wing. Use of the flaps considerably enhances
the lift, although it is at the expense of an increase in the drag 1the airfoil is in a “dirty” configuration2.
This increase in drag is not of much concern during landing and takeoff operations—
the decrease in landing or takeoff speed is more important than is a temporary increase in drag.
During normal flight with the flaps retracted 1the “clean” configuration2, the drag is relatively
small, and the needed lift force is achieved with the smaller lift coefficient and the larger dynamic
pressure 1higher speed2.