
- is generated in asymmetric flow, Fig.J-27. In a similar manner to the expression for the aerodynamic drag, the lift force
[
] is expressed by means of the lift coefficient
 |
|
Fig.J-27 |
as
Analogous expression us used also for the resultant aerodynamic force
[
]
Mechanism of lift generation is outside the scope of the present one-dimensional approach. The most important case is the circulation lift, used on aeroplane wings. It depends on phenomena caused by sharp trailing edge, a general feature of aeroplane wing sections such as those shown in the example Fig.J-29
 |
|
Fig.J-29 Examples of lift coefficient dependence on attack angle for a family of classical wing sections show the typicallinear relationship. |
The basic fact is that the coefficient of circulation lift increases with increasing attack angle. This is valid even for the simplest case of a flat plate - where, however, already at relatively small attack angles the aerodynamic performance deteriorates considerably due to separation of flow at the sharp leading edge. To suppress this effect, common low-speed wing section have rounded leading edges (Fig.J-29). Properties may be also improved by camber of the mean line "s" as shown also in Fig.J-29.
 |
Fig.J-30
This example
of another
classical
aeroplane wing
section
dependence
of drag
(and lift) on
attack angle
shows that what
may seem to be
a good idea
according to
Fig.J-29 -
to use the wing
at as large
attack
angles as
possible -
would bring
unacceptably
large drag.
|  |
In the examples Fig.J-29 and J-30 the camber is so large that the bottom surface of the wing is flat. Camber, of course, leads to asymmetry -lift generated already at zero attack angle. This may be important: drag coefficient, as shown in Fig.J-30, tends to increase with increasing attack angle and shapes permitting generation of lift at small attack angles may be an advantage.
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Vaclav TESAR : "BASIC FLUID MECHANICS"
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