NACA-ARR-L5C24

Abstract

Tests have been made to study the effectiveness of small pressure differences, such as exist between the inside of airplane ducts and the external airplane surfaces suitable for duct exits, in removing the duct boundary layer through slots in the duct wall end thereby reducing separation losses. Two-dimensional diffusers of 150 and 300 included angles and some 90&#176; bends were tested. The 30&#176; diffuser was tested with end without a resistance at the large end. Several different types of boundary layer were set up at the diffuser inlets. <br>All the diffuser tests indicated that the expansion losses could be reduced at least half by the removal of approximately&#160;4 to 10 percent of the total air flow; and the pressure required to blow out the boundary layer was small relative to the pressures normally available in airplane ducts. The slots in the diffuser arrangements were generally formed merely by cutting narrow strips from the two diverging walls of the diffuser. Not more then one slot was used on each surface, and none were usually required on the two parallel walls of the diffuser. Effective boundary-layer control for the inner corners of the bends required a slot with a lip that projected into the duct in order to help "peel off" the boundary layer and also required somewhat higher internal pressures than were used with the diffusers.