Double-Slotted Flaps On A-26 Give High-Efficiency Performance

by C R Pettit Administrative Engineer,
Douglas Aircraft Co

Many factors contribute to the superiority of the Douglas A-6 Invader over its predecessor, the A-20, but none is more outstanding than the new double-slotted wing flaps. Without increasing flap weight, these units permit performance factors which, as a matter of fact, are seemingly at variance with aerodynamic data.

To meet the increasing demand for higher speeds, it has been necessary to proportionately reduce wing area thereby increasing wing loadings. The resulting increase in takeoff and landing speeds and longer runway requirements conflict sharply with combat zone realities of small and none-too-smooth fields. To reconcile these performance factors in the design of the A-26 with war-theater conditions, the development of a more effective landing flap was undertaken.

Tests were conducted on a large-scale model of the wing in Caltech's 10-ft wind tunnel, and a number of positions of the flap and vane (affording a double slot) were investigated until an optimum location was determined which gave remarkably high values of lift.

Results of the tests are illustrated in the accompanying sketch showing airflow over various types of flaps set at an angle of 52°. With the plain flap, the airflow lines indicate an almost completely stalled condition. The flow has separated from the upper surface of the flap and is very turbulent, representing almost complete loss of lift. With the conventional slotted flap, the amount of separated flow is reduced. And with the Douglas double-slotted flap, flow separation is practically eliminated. This type can be set at higher flap angles than the other two without encountering separation, and therefore its lift is proportionately greater.

A second wind-tunnel model, built to check and verify the previous findings, was tested at the NACA laboratory at Langley Field, where nearly full-scale conditions could be duplicated. Th result of this development program is the new Douglas double-slotted flap credited with the following advantages:

  1. For a given wing area, lower landing speed is possible, hence landings are correspondingly safer.
  2. Relatively high drag of the flap contributes to the braking during landing roll. The A-20, with a normal weight of 18,600 lb has a landing distance of 2,560 ft, whereas the A-26 with normal weight of 27,600 lb requires but 2,400 ft.
  3. The glide path is rather steep, thus permitting the use of comparatively short landing fields having relatively high boundary obstructions, and with flaps fully extended, the plane comes in somewhat nose down, rather than nose up (as is usually the case) considerably improving pilot's visibility.
  4. The flaps give substantial improvement in takeoff distance requirements. The A—20, when operating in an overload condition of 26,000 lb and with a consequent wing loading of 56 lb, requires 3,100 ft for takeoff, whereas the A-26 with an overload condition of 35,000 lb. and a wing loading of 65 lb takes off in 3,450 ft.

The flaps are supported on four-bar linkage, retractable within the wing contour, thus maintaining aerodynamic cleanliness. The vane is also supported on this linkage and moves with respect to the flap to remain in optimum position for all degrees of flap travel. Electrically operated, the flaps are interconnected in a manner to assure symmetrical deflection.

This article was originally published in the "For Better Design" column of the May, 1945, issue of Aviation magazine, vol 44, no 5, pp 160-161.
The original article includes 3 photos and a diagram.
Photos are not credited.

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