NASA and Supersonic Cruise

William Flanagan

For an aircraft to attain supersonic cruise, or the capability to fly faster than sound for a significant portion of time, the designer must balance lift, drag, and thrust to achieve the performance requirements, which in turn will affect the weight. Although supersonic flight was achieved over 60 years ago, successful piloted supersonic cruise aircraft have been rare. NASA has been involved in developing the required technol­ogy for those rare designs, despite periodic shifting national priorities.

N THE 1 930S AND EARLY 1 940S, investigation of flight at speeds faster than sound began to assume increasing importance, thanks ini­tially to the "compressibility” problems encountered by rapidly rotat­ing propeller tips but then to the dangerous trim changes and buffeting encountered by diving aircraft. Researchers at the National Advisory Committee for Aeronautics (NACA) began to focus on this new and trou­blesome area. The concept of Mach number (ratio of a body’s speed to the speed of sound in air at the body’s location) swiftly became a famil­iar term to researchers. At first, the subject seemed heavily theoreti­cal. But then, with the increasing prospect of American involvement in the Second World War, NACA research had to shift to shorter-term objectives of improving American warplane performance, notably by reducing drag and refining the Agency’s symmetrical low-drag airfoil sections. But with the development of fighter aircraft with engines exhibiting 1,500 to 2,000 horsepower and capable of diving in excess of Mach 0.75, supersonic flight became an issue of paramount military importance. Fighter aircraft in steep power on-dives from combat alti­tudes over 25,000 feet could reach 450 mph, corresponding to Mach numbers over 0.7. Unusual flight characteristics could then manifest themselves, such as severe buffeting, uncommanded increasing dive angles, and unusually high stick forces.

The sleek, twin-engine, high-altitude Lockheed P-38 showed these characteristics early in the war, and a crash effort by the manufacturer

aided by NACA showed that although the aircraft was not "supersonic,” i. e., flying faster than the speed of sound at its altitude, the airflow at the thickest part of the wing was at that speed, producing shock waves that were unaccounted for in the design of the flight control surfaces. The shock waves were a thin area of high pressure, where the supersonic airflow around the body began to slow toward its customary subsonic speed. This shock region increased drag on the vehicle considerably, as well as altered the lift distribution on the wing and control surfaces. An expedient fix, in the form of a dive flap to be activated by the pilot, was installed on the P-38, but the concept of a "critical Mach number” was introduced to the aviation industry: the aircraft flight speed at which supersonic flow could be present on the wing and fuselage. Newer high­speed, propeller-driven fighters, such as the P-51D with its thin laminar flow wing, had critical Mach numbers of 0.75, which allowed an ade­quate combat envelope, but the looming turbojet revolution removed the self-governing speed limit of reduced thrust because of supersonic propeller tips. Investigation of supersonic aircraft was no longer a the­oretical exercise.[1054]