As mentioned in the discussion of the AQUILA airfoil, its high-speed per­formance is severely compromised by its high-camber. The S2091 was designed primarily to be an improvement over the AQUILA by extending the polar to lower lift, while maintaining the AQUILA’s low-speed, high-lift characteristics. Details of how this was achieved through airfoil design may be found in Refer­ence 23.

• S2091A-PT (Fig. 12.79)

• S2091B-PT (Fig. 12.80)

Data on two models are shown. The first model, version A, was found to be too thin as verified by hand-held templates. In addition, the leading edge was rough in certain areas from the fiberglass beneath the paint. Undoubtedly this influenced transition in a manner similar to that of a trip strip. The model was later re-contoured and given the В designation. Only the В version was digitized for coordinates.

The effect of the roughness of version A as compared with the smooth version В is to decrease the drag at 60k. Smaller improvements are found for 100k, while no improvement exists at 200k.

Referring to the В version data (Fig. 12.80), the goal of extending the low – lift end of the polar beyond that of the AQUILA airfoil has been achieved. Futhermore, it comes as some surprise that at Rn’s above 60k the maximum lift coefficient is increased by 0.1 over that of the AQUILA. In summary, the S2091 is an advance in performance over the AQUILA, but this comes mostly through the relaxation of the flat-bottom requirement. Of course a flat lower surface does not necessarily mean the airfoil is deficient, as illustrated by the performance of the DF101 and the nearly flat-bottom S3021.

• S2091B-PT Gurney Flap type A (Fig. 12.81)

• S2091B-PT Gurney Flap type В (Fig. 12.82)

• S2091B-PT Gurney Flap type C (Fig. 12.83)

The S2091B-PT was tested with a so-called Gurney flap, often used on facing car wings which develop a download to increase traction. As can be seen in Fig. 5.5, a Gurney flap is a simple, thin tab on the order of 1% chord, which is perpendicular to the lower side of the airfoil at the trailing edge. For these tests the tab was 0.017% chord thick (0.002 in) brass shim stock with a length of 0.6% chord for type A, 1.2% for B, and 2.6% for C. •

For the type A Gurney flap, the whole polar is shifted upwards in lift coeffi­cient by 0.1—an impressive result for such a simple modification. Similar results are found for type B, but diminishing returns begin to appear for type C. It can be reasonably expected that the efficiency of the small 0.6% and 1.2% Gurney flaps in increasing lift is not unique to the S2091. (Note that there is a small drag penalty associated with the Gurney flap.)

Also see: AQUILA, E214, SD7037, SD7032 Digitizer plot: Fig. 10.29 Polar plot: Figs. 12.79-12.83 Lift plot: Fig. 12.84

Thickness: 10.10% Camber: 3.91%