- 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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This is page Nr. J11 from textbook Vaclav TESAR : "BASIC FLUID MECHANICS"
Any comments and suggestions concerning this text may be mailed to the author to his address tesar@fsid.cvut.cz

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