The Humpback L-1

The History of Boundary Layer Control in the United States of America by Joseph Flatt. A paper on pp.122-143 of Boundary Layer and Flow Control — Its Principles and Application, Vol.1, Part 1, History of Boundary Layer and Flow Control Research in Various Countries, ed: Gustav Victor Lachmann (Pergamon Press, NY 1961)

An excerpt from pages 123-125 transcribed by Jack Erickson on 1/9/05:

Perhaps the most outstanding and active American proponent of boundary-layer control prior to 1946 was Professor E A Stalker. He conducted numerous wind tunnel and aeroplane design studies in the late 1930s and in 1942 was given a $50,000 contract by the USAAF to modify Stinson-Vultee L-1 [40-255] with a suction flap arrangement. The contract was actually to the Dow Chemical Co of Bay City MI, where Stalker was employed.

    Modifications consisted essentially of a new plywood wing that contained full-span, double-segment flaps together with full-span suction slots and ducts, plus addition of a suction blower in the fuselage driven by an auxiliary 80hp Franklin O-180-1.

    The two-section flap covered 40 percent of the chord—the front section deflecting to 35 degrees and the rear to 82 degrees in the fully down position. The outboard section of the aft flap had an additional independent travel of 22 degrees and 33 degrees in the flap's down and up positions, respectively, which provided lateral control. A large hump in the fuselage housed the blower, and louvres on the rear of the hump controlled the pump's exit flow.

Its first flight was made on 6 Mar 1944 at Tri-City Airport (p: Maj R E Horner), followed by 19 flights by Dow test pilot R B Gorrill. After a limited amount of testing by Stalker's group, the L-1 was transferred to Wright Field for additional tests. During early stall tests it entered an uncontrollable spin and crashed, killing pilot Lt P A Hobe.

    Quoting from a USAF historical summary: "This first effort was something of a state-of-the-art experiment. The mechanism for boundary layer control was bulky and complicated and, although it worked satisfactorily (a maximum lift coefficient of 3.6 was obtained), the benefit to aircraft performance was difficult to measure—indeed, the gain was practically cancelled by the increased weight of the special wing, engine, and ducting.

    "Besides, the L-1 was perhaps an unfortunate choice for this experiment since the standard model itself had slotted leading edges, flaps, and an unusually large wing area, all of which made it a high-lift aircraft in its original configuration. When Stalker on 20 Jan 1945 reviewed the L-1 programme, he noted that 'mechanical, structural, and weight difficulties have thus far prevented successful application to military or commercial airplanes,' despite the general acceptance of laboratory and theoretical demonstrations of boundary layer control. He felt that the major drawbacks made evident by the L-1 modifications were very poor lateral control and restrictive weights of wing and blower. He also forecast future problems from lack of internal wing space for fuel, guns, or wheels, as well as structural difficulties in adapting boundary layer control to thin wings."

    In retrospect, Professor Stalker's identification of the primary difficulties—mechanical, structural, and weight—proved to be very correct even for modern installations. After this programme, no progress of note was made until after World War II.

Ref: J C Solvason, "Final Report on the L-1 Boundary Layer Control Aeroplane," AAF # 40-255, USAF MRTSEAL-2 4586-5-1 (June 1945).