Engineering Applications of Boundary Layer Control

Abstract

TH E rapid evolution of aircraft is now at a stage of development where it is profitable to give more attention to certain aerodynamic refinements, heretofore viewed as being impractical or uneconomical. The present line of progress has very nearly reached its limit in meeting the demands for higher top speeds in transport aviation and reduced minimum speeds in sportscraft. The field of aerodynamics offers some interesting possibilities in increasing the overall efficiency of the airplane by improving the characteristics of the airfoil sections used. With this idea in mind, an evaluation of the state of development in the special branch known as boundary layer control, seems timely. The lift and drag characteristics of an airfoil are very sensitive to flow conditions in the thin layer of air nearest the surface of the wing. Within this thin strata of air, known as the boundary layer, the velocity varies from zero at the surface of the profile to a velocity equal to that of the free air stream. The energy in this layer is very important as it is the inability of the lagging boundary layer to move downstream against the pressure gradient of the wing that results in the flow separation or burble associated with the stall. Various methods of energizing the boundary layer will delay this flow separation to much higher values of lift. The most commonly used method of adding this energy has been the use of a slot on the leading edge of the wing. The properties of such an arrangement, however, are well known and will not be discussed in this paper. Attention will, instead, be directed to the promising results obtained by actual sucking or blowing air from slots in the wings. Owing to the difficulties involved in the obtaining of experimental data in this field, progress has been slow. The work of J. Ackeret, A. Betz and Oskar Schrenk, ' 4 conducted at the Aerodynamic Laboratory in Gottingen, has been outstanding. The experimental work of the group has been largely restricted to the use of suction slots in increasing the lift and reducing the drag of thick wing sections. In this country, M. J. Bamber utilized a relatively thin wing with a back1 J. Ackeret, Removing Boundary Layer by Suction, N.A.C.A. Tech. Memo. No. 392, 1927. 2 Oskar Schrenk, Experiments with a Wing Model from which the Boundary Layer is Removed by Suction, N.A.C.A. Tech. Memo. No. 534, 1929. 3 M. J. Bamber, Wind Tunnel Tests on Airfoil Boundary Layer Control Using a Backward-opening Slot, N.A.C.A. Rept. No. 385, 1931. 4 Oskar Schrenk, Experiments with Suction Type Wings, N.A.C.A. Tech. Memo. No. 773, 1935. ward opening slot. Although both suction and pressure were used for the same slot, the form of the slot was such as to favor distinctly the pressure method of control. Considerable difference of opinion still exists as to the ,type of wing section best suited to application of boundary layer control. For that reason, most experimental work includes tests both of thick sections ( 3 0 % thickness) and thin sections, the latter usually with a flap arrangement. From the standpoint of the lift coefficient, the thick section gives slightly higher values for the same expenditure of power, as compared to the thin section with flap. This advantage is further increased by the small manifold duct losses and the more uniform pressure distribution along the length of the wing. This problem may become quite acute in the case of the thin section, especially in a tapered wing. The thick wing has always appealed to the structural man because of the possibilities offered to reduce weight as a result of increased beam depth. In addition, the added storage space is desirable even to the point of completely housing the power plant inside the wing. All these advantages are to a certain degree offset by the large minimum drag of such a section. This drag can be reduced to a value comparable with that of a normal section, but involves a continuous power output in the form of pressure or suction.