• SD7090-PT (Fig. 12.148)
At first glance the SD7090 appears to be a lower lift version of the S3Q21. A comparison plot of the profiles (see Fig. 11.9) shows that the SD7090 has less camber than the S3021, is somewhat thicker, and the thickness distribution is significantly different along the forward, upper surface. For the SD7090 this part of the airfoil plays an important role in the stall characteristics.
As Fig. 12.151 shows, the SD7090 has a fairly sharp stall, unlike the S3021. This is caused by a sudden “bursting” of the leading edge separation bubble at high angle of attack.
• SD7090-PT loose/tight covering, Rn = 300,000 (Fig. 12.149)
As received, the covering on the surface was typical of plastic film over balsa— somewhat wavy, with a wavelength longer than that of the raw balsa grain. After the model was tested in this condition the Monokote was shrunk tightly over the surface sheeting and made to follow the balsa grain by firmly pressing a cloth over the hot Monokote as it cooled. The resulting surface showed a marbled appearance.
As Fig. 12.149 shows for a Rn of 300k, this difference between the surface finishes did not affect the performance. (The shift in the angle of attack was caused by an inadvertent sharp jolt to the angle of attack sensor prior to the start of the run.)
• SD7090-PT trips, Rn = 300,000 (Fig. 12.150)
At 300k various size trips were tested at 30% chord. The lowest trip was 0.04% (0.005 in) and had only a small effect. But as the trip height increased so did the drag, especially at low angles of attack where the flow would otherwise have been laminar to almost 60% chord. These tests illustrate how even small roughness elements effect performance. This should mainly be considered for F3B and cross-country flying where Rn’s of 300k and higher are common.
Also see: E374, SD6060, NACA 2.5411, CLARK-Y
Digitizer plot: Fig. 10.58
Airfoil comparision plot: Fig. 11.9
Polar plot: Figs. 12.148-12.150 Lift plot: Fig. 12.151
Thickness: 9.99% Camber: 1.87%