Biomedical Engineering Reference
In-Depth Information
FIGURE 5.6
Aerodynamic performance of different profiles as a function of angle of attack
Δα
, for a rectangular wing
with aspect ratio
=
6 and Reynolds number
=
20,700
[23]
. (a) Lift coefficient and (b) lift-to-drag ratio. With kind permission
from Springer Science and Business Media, E.V. Laitone, Wind Tunnel Tests of Wings at Reynolds Numbers Below 70,000,
Experiments in Fluids 23 (1997) 405-409.
conventional rounded-nose airfoil at a Reynolds
number of 120,000. From this figure and
Figure
5.6
it is seen that the Reynolds number has a
relatively insignificant effect on thin airfoils
with sharp leading edges, and has a significant
effect on airfoils with a rounded nose. The cir-
cular-arc airfoil has the best performance at the
lower Reynolds number, while the rounded-
nose airfoil has the best performance at the
higher Reynolds number, with substantially
higher maximum lift coefficient.
At low Reynolds numbers (<50,000), the assump-
tions of potential flow break down and the
behavior of the airfoils is quite different than at
the Reynolds numbers typical of full-scale aircraft
(on the order of 10
6
). For example, the Kutta
condition (flow leaves the airfoil trailing edge
smoothly) may not be satisfied at low Reynolds
numbers. Also, the variation of lift with angle of
attack is highly nonlinear at low Reynolds num-
bers, and the lift curve slope may be quite differ-
ent than the potential flow prediction of
2π
per
radian.
Figure 5.8
shows the lift coefficient as a
function of angle of attack for a thin circular arc
profile with a camber of 8% at a Reynolds num-
ber of 3.14
×
10
5
. Although this is significantly
