Landing Flaps for A-26 Invader

Since reduction in wing areas has been necessary to increase speed, there inevitably results an increase in landing speed so that special attention must be given to maintain safe landing conditions. In designing the Douglas A-26 Invader as one of the fastest medium bombers, it was decided to have a smaller wing than had previously been considered possible for an airplane its size. In order to provide a short landing distance so that the airplane could be used from small fields and advanced bases, a special landing flap was developed.

Wind tunnel tests were conducted in the California Institute of Technology 10-foot wind tunnel on a scale model of the wing. These tests permitted a number of positions of the flap and vane to be investigated until the optimum location was determined. The final position was found to give remarkably high values of lift. From this information, the flap arrangement was designed and finally a second wind tunnel model was built to check and verify the final design. This model was tested at the NACA laboratory at Langley Field where nearly full-scale conditions could be duplicated. The results of this extensive development program is the "Douglas double-slotted" flap or "retractable deflecting slot" flap.

As finally developed and installed on the A-26, the flaps have the following advantages:

  1. —For a given wing area, the landing speed is lower than with other types of flaps.
  2. —Their relatively high drag contributes to braking during the landing roll.
  3. —The glide path is quite steep thus permitting the use of comparatively short landing fields.
  4. —With the flaps fully extended, the airplane flies somewhat nose down rather than nose up as is usually the case. This change in attitude enhances pilot visibility during landings.
  5. —The use of flaps for takeoff gives a large improvement in takeoff distance.
  6. —When not in use, the flaps retract fully into the wing.

A general description of the functioning of the flap may be obtained by referring to the schematic diagram, which shows the flow around a plain flap, a slotted flap and the A-26 flap; here, all three types of flaps are set at the same angle of 52°. Under these conditions the plain flap is almost completely stalled, or in other words, the flow has separated from the upper surface of the flap and is very turbulent. This represents a loss of lift. Adding the single slot to the flap reduces the amount of separated flow. However adding the double slot almost completely eliminates the flow separation. It may thus be seen that the double-slotted flap can be set at higher flap angles than the plain or slotted flap without encountering separation and the lift from the flap is therefore proportionately greater.

The flaps are electrically operated by a motor located in the fuselage, the right and left hand flaps being interconnected in such a way that symmetrical deflection is assured. The flaps themselves are supported on a four-bar linkage which in the retracted position is completely contained within the wing contour. The vane is also supported on this linkage in such a way that it moves with respect to the flap and remains located in its optimum position at all degrees of flap travel.

This article was originally published in the April, 1945, issue of Industrial Aviation magazine, vol 2, no 4, p 88.
The original article includes 2 pphotos and a 3-part diagram.
Photos are not credited.