Page 65 - Aerodynamics for Engineering Students

48 Aerodynamics for Engineering Students

slightly less than the cruising lift coefficient. In such a case the aeroplane will have the
benefit of a low value of the drag coefficient for the wing throughout most of the
flight, with obvious benefits in performance and economy. Unfortunately it is not
possible to have large areas of laminar flow on swept wings at high Reynolds numbers.
To maintain natural laminar flow, sweep-back angles are limited to about 15".
The effect of a finite aspect ratio is to give rise to induced drag and this drag
coefficient is proportional to Ci, and must be added to the curves of Fig. 1.26.

Drag coefficient: (lift coefficient) *

Since

it follows that a curve of C, against Ci will be a straight line of slope (1 + s)/7rA. If
the curve CO, against Ci from Fig. 1.26 is added to the induced drag coefficient, that
is to the straight line, the result is the total drag coefficient variation with G, as
shown in Fig. 1.27 for the two types of section considered in Fig. 1.26. Taking an

Fig. 1.27 Variation of total wing drag coefficient with (lift coefficient)'

A=w

0 9
Fig. 1.28 Idealized variation of total wing drag coefficient with (lift coefficient)' for a family of three-
dimensional wings of various aspect ratios