CLARK-Y

• CLARK-Y-PT (Fig. 12.3)

The famed CLARK-Y airfoil needs no introduction; it is perhaps the most popular airfoil ever used on both full-scale and model aircraft. As compared with the AQUILA, the high-speed, low-lift performance is superior. What is surprising is the small price for the improved performance—the maximum lift coefficient (Cimai) is a mere 0.1 less than the AQUILA.

The effects of a laminar separation bubble are apparent only at a Rn of 60k through the mid-lift range, where the drag coefficient reaches a maximum of 0.032 at a Ci of 0.5. However this bubble does not necessarily detract from the performance since the drag rise occurs mostly in the mid-lift range, a range not used by the vast majority of sailplanes at 60k.

At the high-lift, low-Rn regime of the RC sailplane, the drag reduces as the bubble shortens, reaching a minimum of 0.027 at a Ci of 0.9 for 60k. No doubt the CLARK-Y will stay popular for some time to come.

Also see: DF101, S3010, S3021, SD5060 Digitizer plot: Fig. 10.2 Polar plot: Fig. 12.3 Lift plot: Fig. 12.4

Thickness: 11.72% Camber: 3.55%

DAE51

• DAE51-PT (Fig. 12.5)

The DAE51, along with several other DAE airfoils, was designed by Mark Drela using the ISES code developed by him and Giles21’22. As was mentioned in Chapter 1, this same code was used to analyze the new SD-series of airfoils.

The DAE51 was designed for the propeller of the DAEDALUS, human-powered aircraft25. Thus it was not designed for one operating point, but rather for the range of anticipated conditions. The requirements were26:

1. CiM > 1.2

2. Transition ramp optimized for Rn = 125k, 0.5 < Ci < 1.0

3. No bubble “bursting” for Rn greater than 75k.

4. Thickness less than 9%

The achievement of these design goals was demonstrated by no less than a record 74-mile flight from Crete to Santorini across the Mediterranean, a flight that duplicated in reality the mythical flight of Daedalus and his son, Icarus. Although only the third airfoil to be discussed, the variety of performances and the trade-offs made to achieve them are becoming clear. The drag of the DAE51 at Rn of 300k is considerably lower than the AQUILA and CLARK-Y, but the range of lift is significantly less. For application to RC sailplanes a broad lift range can sometimes be recovered by use of a full-span flaps, while maintaining the low-drag characteristics of the unflapped airfoil. With a 20% flap, the low- drag range could probably be extended up to С/ of 1.1, which would make it competitive against the unflapped AQUILA and CLARK-Y.

• DAE51-PT thickened trailing edge (Fig. 12.6)

The DAE51 was tested with a substantially thickened trailing edge (see Fig. 5.1 for contour) to measure the possible loss in performance. Over most of the low-drag range, a 4-5% increase in drag was found. As further examples, the E374 and SD6080 were also tested with thickened trailing edges, and the same trends were observed. Similar results were observed by Althaus at Reynolds numbers 1-3 million27.

Also see: S4061, SD6080, E387, E193 Digitizer plot: Fig. 10.3 Polar plot: Figs. 12.5, 12.6 Lift plot: Fig. 12.7

Thickness: 9.37% Camber: 3.98%

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