E374

• E374A-PT (Fig. 12.24)

• E374B-PT (Fig. 12.25)

The E374 is often selected because its low camber (2.2%) makes it more of a high-speed type than a floater, and because its thickness (10.9%) allows it to be used for the large spans found in cross-country sailplanes. Recently, for example, Joe Wurts flew the E374 to a new cross country distance record of 141 miles. As with the E205, E193, and E387, the E374 shows the effects of a laminar separation bubble through the mid-lift range at the lower Jin’s.

Two models of the E374 were produced for these tests. Overall, both models were very accurate; the A version has slightly more aft camber and is slightly thinner rearward of 50% chord than the B. For reasons that cannot be satis­factorily explained, the A version is marginally the better of the two in terms of performance, even at the lower C’s where it would be thought that the additional camber would be detrimental.

• E374B-PT u. s. bumps xjc = 50%, type A (Fig. 12.26)

• E374B-PT u. s.t. xjc = 20%, А/с = .17%, w/с = 1.0% (Fig. 12.27)

On the more accurate E374B model, two kinds of trips were tested: upper – surface, three-dimensional bumps (type A) and the more commonly tested two­dimensional trip strip. It is a widely held view that three – dimensional boundary layer disturbances produced by bumps or zig-zag tape are more unstable than the initially two-dimensional disturbance produced by a continuous trip strip28. Because the trips were placed at different locations, comparisons of their relative effectiveness cannot be made. What can clearly be seen from these results is that a trip strip at 20% leads to improved performance for Rn less than 200k. For the bumps, the 150k case is most interesting. Below Ci of 0.5 the bubble is effectively tripped; however, above 0.5, there is no benefit. Most likely the leading edge of

the bubble is upstream of the trip, which makes it ineffective because it is in the recirculating region of the bubble.

• E374B-PT u. s. wavy clay, x/c = 0% to 15%, h/c = .20% (Fig. 12.28)

To investigate the sensitivity in performance to gross, upper-surface waviness near the leading edge, a generous amount of artist’s clay was smeared over the forward 15%. The wavelength, which was random, was on the order of 7% with a maximum height of 0.2%. All edges were carefully smoothed. Although the waviness was greater than anything expected in typical construction, the results do provide clues to potential performance losses due to, say, hasty repairs near the leading edge. Interestly, as Fig. 12.28 shows, there are no large adverse effects.

• E374B-PT thickened trailing edge (Fig. 12.29)

The effect of a thickened trailing edge was also measured. The shape is given in Fig. 5.2. As with the DAE51, there is a small drag increment everywhere, indicating once again that a thin trailing edge is better.

Also see: SD6060, SD7090, E205, NACA 2.5411, CLARK-Y Digitizer plot: Figs. 10.11, 10.12 Polar plot: Figs. 12.24-12.29 Lift plot: Fig. 12.30

Thickness: 10.91% Camber: 2.24%

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