This series of airfoils was designed for very large sailplanes where thin airfoils are not structurally possible. The starting seed was the SD5060; however, the DF-series is thicker and has a different thickness distribution.

The design requirements were:

1. 11% thick

2. flat bottom aft of 30%

3. minimum drag at C; = 0.2.

The DF101 is the initial design; the DF102 and DF103 are variations on the basic airfoil. The DF102 has an increased thickness (about in max., 0.5%) between 2% and 30% on the upper surface. The DF103 has its thickness decreased by approximately the same amount in the same place. See Figs. 11.1 and 11.2.

The purpose of the variations was to test the effects of a controlled contour change in a region previously recognized as important to performance. To make the test more precise, the same physical section was used for all three; it was modified once and became the DF102, and modified a second time to become the DF103. Besides demonstrating the effects of minor, local changes in the airfoil contour, these airfoils as a group provide a measure of the sensitivity of the airfoil performance to surface modifications, whether deliberate or inadvertent.

• DF101-PT (Fig. 12.8)

As the polars show, the initial design is the best, suggesting that deviations either way from a good nominal design are as likely to hurt performance as to help it. Compared to other airfoils, the DFlOl’s performance is quite good; in particular, it has a remarkable lift range for the 11% thickness. At low speed it performs better than the NACA 2.5411 and not quite as well as the CLARK-Y, something that could be expected from the CLARK-Y’s much higher camber (3.55% for the CLARK-Y vs 2.30% for the DF101). At high speeds it is the equal of the NACA 2.5411 and better than the CLARK-Y.

Also see: SD5060, DF102, DF103, CLARK-Y, S3010, S3021

Digitizer plot: Fig. 10.4

Airfoil comparision plot: Figs. 11.1, 11.2

Polar plot: Fig. 12.8

Thickness: 11.00% Camber: 2.30%

• DF102-PT (Fig. 12.9)

Contrary to what might be expected, the addition of upper-surface thickness did not change the low-lift characteristics. Instead, the lift range was extended by 0.1 at the high end. But this is not the aerodynamicist’s “free lunch,” since overall the drag has increased for lift coefficients above 0.5. Furthermore, at Rn’s of 60k and 100k the stall occurs earlier than at the higher Rn’s, a condition that can cause the wing tips to stall before the inboard sections, leading to “tip stall” problems. In conclusion, the DF101 is a better airfoil.

Also see: DF101, DF103, CLARK-Y, S3010 Airfoil comparision plot: Fig. 11.1 Polar plot: Fig. 12.9

Thickness: 11.00% Camber: 2.30%

• DF103-PT (Fig. 12.10)

As compared with the DF102, removing thickness has produced the opposite effect. The maximum lift is reduced, but the stall behavior has improved over the DF101. Still, the DF101 seems to be the best choice of the three.

Also see: DF101, DF102, NACA 2.5411 Airfoil comparision plot: Fig. 11.2 Polar plot: Fig. 12.10

Thickness: 11.00% Camber: 2.30%

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