NACA Report No. 742

NACA Report No. 742 - Wind Tunnel Investigation of NACA 23012 Airfoil with 30%-chord Venetian-blind Flaps was issued by the United States National Advisory Committee for Aeronautics in 1942. It describes the result of wind-tunnel testing using a common airfoil section from the period, equipped with an unusual flap configuration.

Summary
An investigation was made in the NACA 7x10 feet wind tunnel of a NACA 23012 airfoil with 30%-chord venetian-blind flaps having one, two, three and four slats of Clark Y section. The three-slat arrangement was aerodynamically the best of those tested but showed practically no improvement over the comparable arrangement used in the preliminary tests published in NACA Report No. 689. The multiple-slat flaps gave slightly higher lift coefficients than the one-slat (Fowler) flap but gave considerably greater pitching-moment coefficients. An analysis of test data indicates that substitution of a thicker and more cambered section for the Clark Y slats should improve the aerodynamic and the structural characteristics of the venetian-blind flap.

Testing
NACA launched an extensive investigation of various wing-flap combinations for improving safety and performance in flight. One promising combination was a venetian-blind flap, which produced higher lift coefficients, and lower drag coefficients at moderately high lift coefficients, than any flap previously tested. A further development of this arrangement appeared promising, using a 30%-chord flap (or group of flaps), hinged at the trailing edge of the basic airfoil. The basic airfoil was constructed to the NACA 23012 profile, with a chord of 3 feet and a span of 7 feet, from laminated mahogany and tempered wallboard, with a steel trailing-edge plate. The cutout which had been made in the airfoil to accommodate the retracted Fowler flap was retained; thus the lower surface was not smooth leading into the new flap area, but contained a gap or recess. All the slats were made of wood, shaped to the Clark Y profile. They were secured to the wing with four sets of slat-hinge fittings located spanwise to produce minimum slat bending deflection. Each slat required a separate set of hinge fittings. The combinations tested were:
 * the one-slat (Fowler) flap composed of one 30%-chord slat
 * the two-slat combination composed of two 15%-chord slats
 * the three-slat combination composed of three 10%-chord slats
 * the four-slat combination composed of four 7.5%-chord slats.

Conclusions

 * 1) The best arrangements of the venetian-blind flaps tested used differentially deflected slats and 1.5%-chord slot gaps.
 * 2) The results indicate that the best venetian-blind flap arrangement tested was only slightly better aerodynamically than the best comparable arrangement of the preliminary investigation (reported in NACA Report No. 689, 1940). The multiple-slat flaps did not give significantly higher lift coefficients than the single-slat (Fowler) flap and gave considerably greater pitching-moment coefficients. It appears, however, that improvement in high-lift characteristics could be obtained with the use of more highly cambered thick slats than with those of the present investigation.