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Mechanics of Fluids

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where Ɣ0 represents the circulation in the centre <strong>of</strong> the aer<strong>of</strong>oil <strong>of</strong> span b.<br />

At a distance x from the centre the circulation Ɣ = Ɣ0{1 − (x/ 1<br />

2 b)2 } 1/2 .<br />

∴ Le =<br />

� b/2<br />

−b/2<br />

−ϱUeƔdx =−ϱUeƔ0<br />

� b/2<br />

−b/2<br />

An introduction to elementary aer<strong>of</strong>oil theory 409<br />

� �<br />

1 − x/ 1<br />

2b � �<br />

2 1/2<br />

dx<br />

=−ϱUeƔ0bπ/4 (9.44)<br />

Combining eqns 9.42, 9.43 and 9.44 we obtain<br />

Di = viLe<br />

Ue<br />

= −Ɣ0Le<br />

2bUe<br />

= 4Le<br />

ϱUebπ<br />

Le<br />

2bUe<br />

= 2<br />

ϱπb 2<br />

� �2 Le<br />

Ue<br />

= 2<br />

ϱπb 2<br />

� �2 L<br />

U<br />

Division by 1<br />

2ϱU2 S gives the result in terms <strong>of</strong> the dimensionless coefficients:<br />

� � �<br />

CDi = C 2 L<br />

πb 2<br />

S<br />

= C 2 L /(πAR) (9.45)<br />

The vortex drag is shown to decrease as the aspect ration AR <strong>of</strong> the wing<br />

increases. It may also be shown that the elliptical distribution <strong>of</strong> lift here<br />

assumed is the condition for the vortex drag to be a minimum for a given<br />

value <strong>of</strong> the lift.<br />

Equation 9.45 enables the effect <strong>of</strong> vortex drag to be separated from other<br />

components <strong>of</strong> drag, and thus data obtained at one aspect ratio can be<br />

converted for wings with the same aer<strong>of</strong>oil section but a different aspect<br />

ratio. The change in the effective angle <strong>of</strong> attack is<br />

� �<br />

CL<br />

arctan(vi/U) = arctan(Di/L) = arctan(CDi/CL) = arctan<br />

πAR<br />

Phenomena associated with the viscous flow round wings are considered<br />

in Section 8.8.6 and the effect <strong>of</strong> compressibility is briefly treated in<br />

Section 11.11.<br />

Example 9.7 A wing <strong>of</strong> span 10 m and mean chord 2 m has a lift<br />

coefficient <strong>of</strong> 0.914 and a drag coefficient <strong>of</strong> 0.0588 for an angle <strong>of</strong><br />

attack <strong>of</strong> 6.5 ◦ . If the distribution <strong>of</strong> lift over the span is elliptical, what<br />

are the corresponding lift and drag coefficients for an aer<strong>of</strong>oil <strong>of</strong> the<br />

same pr<strong>of</strong>ile and effective angle <strong>of</strong> attack, but aspect ratio 8.0?<br />

Solution<br />

Aspect ratio = 10 m/2 m= 5<br />

CDi = C2 L<br />

πAR = 0.9142 /5π = 0.0532

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