|
Fig.J-31 |
Note in Fig.J-30 that the lift coefficient increases in proportion to the attack angle
- at least in the region of small attack angles, before the onset of flow separation. Unfortunately, drag generally also increases with attack angle. As a result, finding the optimum angle at which takes on the maximum possible value
is no straightforward task. It is best solved by using the polar plot, Fig.J-31, invented by
|
Otto Lilienthal |
Otto Lillienthal.
This is a dependence of the lift coefficient
on the coefficient of drag
. Fig.J-32 shows its use in finding the optimum gliding angle
. This diagram may also be
used for interesting solutions of other problems, such as finding the attitude at which the overall aerodynamic force is smallest - or largest.
The most often soufgt point on the polar diagram curve is the point
... the one through which passes the straight tangent line leading from the origin.
 |
Fig.J-32 |
Lilienthal himself has shown the importance of this point for determining the conditions in the gliding flight. Note the difference between the polar curve of the wing and of the complete aircraft (most other components of the aircraft just add to the drag, the resultant ircraft polar curve therefore usually doe not much differ from the wing polar shifter to the right ). The applications need not be only in the area of flight mechanics: determining the aerodynamic fineness
Fig.J-33 |
 |
/
and its maximum is equally important for adjustment of blower blades and many similar devices. The polar diagram is investigated in designing bridges (problem of lifting the bridge structure by wind) and similar problems - with horizontal side force replacing the vertical lif) are encountered
in car bodies, ship hulls and sails, buildings, ... .
Conditions corresponding to the point
(Fig.J-33) are sought whenever it is desired to find mimimum stressing of a structure by the aerodynamic or hydrodynamic forces or if it is desired to attain maximum velocity. This point is, of course, found on the horizontal co-ordinate, at
= 0 , for symmetric bodies.
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This is page Nr. J12 from textbook
Vaclav TESAR : "BASIC FLUID MECHANICS"
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