Lippisch DM 1 Reconsidered - Part 3

In this four-part series, curators Russ Lee and Evelyn Crellin take an in-depth look at the Lippisch DM 1, an experimental German glider. At the conclusion of Part 2, U.S. Army General George S. Patton ordered the students to resume construction of the glider at the Prien Airport. A number of American visitors arrived to witness the construction of the DM 1, including the famous American pioneer of aerodynamics Walter Stuart Diehl.

Another famous visitor to Prien airfield was Charles A. Lindbergh. According to German historian and author Hans-Peter Dabrowski, Lindbergh inspected the DM 1. We know that Lindbergh crisscrossed Germany with the U. S. Naval Technical Mission investigating the newest developments in aircraft and missiles made by German scientists and engineers. In June 1945, he arrived at Prien airfield and talked at some length to Dr. Felix Kracht about a supersonic, swept-wing rocket glider and a ramjet engine that used coal for fuel, but in his book, The Wartime Journals of Charles A. Lindbergh, Lindbergh does not say he personally inspected the DM 1.

What is certain is that construction work on the glider resumed during the summer of 1945 and ended a few months later. The finished aircraft spanned 6 m (19 ft 8 in), the tip of the vertical tail reached  3.2 m (10 ft. 7 in.), and empty it weighed 374 kg (825 lb). Joe Chambers wrote in his book, Cave of Winds: The Remarkable History of the Langley Full-Scale Wind Tunnel, that in August, American officials considered testing the DM 1 in Germany by launching it from atop a twin-engine Douglas C-47 transport, but they also may have considered towing it aloft on a cable behind the C-47. Whatever their initial intent, the Americans soon abandoned the idea of flying the glider and set about moving it to the U.S. for further evaluation. American personnel placed the aircraft into a large wooden crate designed and built specifically to protect it in one piece. Men in a truck hauled the crate away on November 9 and dropped it off in Mannheim, Germany, where workers loaded it aboard a ship that sailed to Rotterdam. The DM 1 moved from Rotterdam to Boston and arrived there on January 19, 1946. Two days later, the Army Air Forces Material Command asked the National Advisory Committee for Aeronautics (NACA) to evaluate the DM 1 using the Full-Scale Tunnel (FST) at the Langley Memorial Aeronautical Laboratory at Langley Field, Virginia.

Another ship carried the glider down the East Coast to Norfolk where a truck moved the aircraft to Langley Field. Joe Chambers noted that aerodynamicists tested the DM 1 in three phases in April, June, and November 1946. American companies such as Convair had developed an independent interest in delta-wing aircraft and they tested small models in wind tunnels to determine the high-lift characteristics of these designs. When initial NACA tests of the DM 1 failed to produce the amount of lift at angles of attack that U.S. companies had expected, the work turned to modifying the German glider until its performance matched that revealed by the small models. During this process, the NACA researchers began to appreciate the importance of the rather blunt leading edges of the DM 1 wings. Technicians added sharp leading edges similar to classical stall strips to the wings, they reshaped the vertical fin and removed it for some tests, and they modified the control surfaces. Aerodynamicists and engineers conducted extensive flow visualization tests using small strands of wool attached to the upper surfaces of the wings. Wind tunnel tests continued and the modified wings exhibited strong swirling vortex airflows over the top surface of the wings at low speeds and high angles of attack.

This video shows the DM 1 inside the FST at Langley during a test on August 1, 1946. Smoke makes the airflow visible. At video time 1:22, a metal strip attached to the right wing leading edge can be seen causing a powerful vortex to stream over the wing. This vortex was critical to preventing the wing from stalling when flown at the high angles of attack required to slow down the delta for landing. The vortices also helped the pilot maintain directional control about the yaw axis using the rudder.

These findings were important. They gave the designers of delta-wing aircraft confidence to proceed with building and flight testing an experimental piloted delta-wing aircraft equipped with a thin wing required for transonic flight because they knew the delta would be stable and controllable at the low speeds needed for takeoff and landing, thanks to the strong vortex flow generated by the sharp leading edge at high angles of attack. Designers had known for years that flight at transonic speeds required a thin and low-aspect ratio form to minimize drag. What no one understood before NACA’s work with the DM 1 was how to stabilize and control these configurations at low airspeeds so that pilots could land using a conventional aircraft landing gear. After all, there was no point in taking off and flying fast enough to break the sound barrier if landing was impossible. The Langley Laboratory team that studied and modified the DM 1 deserves mention: Sam Katzoff, J. Calvin Lovell, and Herbert A. Wilson, Jr. (Chambers, Cave of Winds, 190-226). NACA’s work was critical to transforming the delta wing concept into a practical application, but the basic idea about sharp leading edges that generate vortex flow dates to the inter-war period. In a paper describing the DM 1 tests at Langley, NACA aerodynamicists Herbert Wilson and J. Lovell cited the work of German aerodynamist H. Winter who observed votices form over rectangular plates that were thin and flat. Winter published his observations in 1936 (see below, Sources).

Bradley, Robert E. “The Birth of the Delta Wing,” American Aviation Historical Society, Winter 2003. Chambers, Joseph R. Cave of Winds: The Remarkable History of the Langley Full-Scale Wind Tunnel, (NASA SP-2014-614), 2014. Chambers to Lee email, 4/20/15. Wilson, Herbert A., and Lovell, J. Calvin. “Full-Scale Investigation of the Maximum Lift and Flow Characteristics of an Airplane Having Approximately Triangular Plan Form,” NACA Research Memorandum RM No. L6K20, 12 February 1947, Langley Memorial Aeronautical Laboratory, Langley Field, VA. Winter, H. “Strömungsvorgange an Platten und Profilierten Körpern bei kleinen Spannweiten [Flow Phenomena on Plates and Airfoils of Short Span],” VDI-Special Issue (Aviation), 1936, translated by S. Reiss and published in NACA Technical Memorandum No. 798, July 1936, Washington, D. C.