Acknowledgements: Joseph R. Chambers, former head of the NASA Full-Scale Tunnel and author of NASA SP-2014-614 Cave of the Winds encouraged myself and Evelyn Crellin to reconsider the importance of the DM 1.
This small experimental glider played a vital role in the development of the first jet-propelled delta wing aircraft to fly, the Convair XF-92A. The extensive flight tests that Convair (Consolidated Vultee Aircraft Corporation) and the U. S. Air Force conducted with the XF-92A led the company to develop a series of successful delta wing aircraft. The air force placed the Convair F-102 Delta Dagger into operational service in 1956 and the Convair F-106 Delta Dart three years later. The air force began operating the world’s first supersonic delta wing bomber, the Convair B-58 Hustler, in 1960. Both the air force and the United States Air National Guard operated more than 1,400 of these aircraft from 1956 to 1988.
Before Alexander M. Lippisch designed the DM 1, he had spent much of his career understanding and developing advanced unconventional aircraft. Between 1921 and 1945, he embarked on 113 projects that led to the construction and successful flight of 59 different aircraft including the Messerschmitt Me 163 Komet rocket fighter (a Komet is on display at the Steven F. Udvar-Hazy Center). The Komet became the world’s first operational fighter aircraft powered by a liquid-fueled rocket engine in May 1944.
By the late 1930s, aircraft designers and engineers began to understand that traditional straight wings and relatively thick airfoils were not ideal for flight at the speed of sound and beyond. Sound travels at different speeds primarily due to variations in air temperature. At sea level and under normal atmospheric conditions, it is 1,220 km/h (760 mph). To reduce the drag of the air that quadruples when airspeed doubles, and solve other problems that arise at transonic speeds, Lippisch proposed a delta wing configuration so named because it resembles the fourth letter ‘Delta’ of the ancient Greek alphabet whose uppercase form looks like this: ‘Δ’. This shape has several advantages for high-speed flight. It combines sharply swept leading edges to minimize drag at high speeds with a large surface area needed to make lift at low speeds. The delta had favorable structural characteristics, too, that allowed engineers to build thin wings making less drag without losing the strength needed to withstand the powerful air loads encountered at transonic speeds.
In spring 1943, Lippisch took charge of the Luftfahrt-Forschungsanstalt Wien (Aeronautical Research Establishment Vienna, or LFW). He began working on several projects and among them was a tailless supersonic fighter aircraft called the “P 13 a”. At the time, Allied bombing operations were increasing in scope and intensity so to counter the bombers, Germany needed new fighter aircraft with performance superior to Allied aircraft. The new designs also had to be quick to build using inexpensive materials that were easily obtained. To boost high-speed performance, Lippisch envisioned powering the P 13 with a ram-jet engine consisting of just a few moving parts and operated by burning a mixture of coal dust and heavy oil or gasoline.
The RLM (Reichsluftfahrtministerium, the German Air Ministry) became interested in this project, perhaps because several firms were already developing unusual forms of semi-taillless or all-wing designs for fighter aircraft powered by either jet turbine or rocket engines (in addition to the Me 163 Komet, the jet-propelled, all-wing Horten Ho 229 is also displayed at the Steven F. Udvar-Hazy Center). Lippisch designed the P 13 to be built of aluminum and he hoped it could attain supersonic speeds. The wing would be a cantilever structure (smooth outer surface without external supports such as struts or wires) with a 60° nose angle and a profile thickness of 15% (thickness of the wing as a percentage of the width of the wing from leading to trailing edge).
Beginning in May 1944, experts evaluated the design at Spitzerberg Mountain near Vienna using a smaller model of the P 13, and in August 1944, they studied the aerodynamics of a model in the supersonic wind tunnel at AVA (Aerodynamische Versuchsanstalt, Aerodynamic Research Institute) Göttingen. Following these tests, Lippisch pushed to build a full-size version without an engine. A conventional powered aircraft would tow aloft the experimental glider manned by a test pilot who would study the takeoff, landing, and handling qualities of the design in flight. A young man assisting Lippisch at the LFW was a student from Darmstadt named Wolfgang Heinemann. Heinemann was one of many German students majoring in engineering and aeronautics who joined one of the special groups of students and teachers called Akafliegs (Academic Flying Groups) or Flugtechnische Fachgruppen (Flight Technical Expert Group, FFG). These groups tried to solve various aeronautical problems and were not afraid to tackle challenges at the limits of the aeronautical sciences. Heinemann persuaded Lippisch to have students of the FFG at the Technical University at Darmstadt build the experimental glider. The students began their work in August-Sept 1944. Since FFG Darmstadt numbered their designs sequencially, the new aircraft became the D 33 (Lippisch later claimed the designation should have been ‘P 13 a V1’ (Project 13 a, prototype 1).
On September 11-12, 1944, Allied bombers struck Darmstadt and hit the building that housed the FFG experimental D 33 glider project. Everything the students could salvage from the rubble was moved to Prien Airport at Chiemsee in Bavaria where since 1924, the students of FFG Munich had operated a large workshop. Prien had been the starting point of many famous gliding events in the years 1918-1939 such as attempts to cross the Alps in gliders and to set altitude records. Now Prien airport and the FFG Munich workshop became the new home of the D 33, where both FFG groups - Darmstadt and Munich - combined their efforts to continue building the aircraft. This collaborative effort led to a new designation for the glider: ‘D’ for Darmstadt, ‘M’ for Munich, and ‘1’ to signify the first design to result from this collaboration.
Students glued, screwed, bolted, nailed, and welded together the cantilever fuselage and various components made of wood, plywood, and steel tubes. They covered the entire glider with 1/16-inch 3-ply birch plywood. To cover the very thick leading edges of the wings and vertical stabilizer, the students had to first heat the plywood with steam. These very thick sections were unsuitable for high-speed flight and suggested that Lippisch had designed the D 33 for experiments at low flying speeds. The students provided the pilot a window on the cockpit floor. This allowed some visibility ahead of and beneath the glider’s nose when the aircraft was pitched up at high angles of attack. Lippisch and the students must have anticipated this condition during landing. To evaluate how the glider handled in flight with the center of gravity at various locations, the pilot could hand-pump 35 liters (9 gallons) of water between two tanks inside the nose and tail of the aircraft. Armament was not planned for this experimental glider.
The students fashioned the wheeled, three-strut, tricycle undercarriage from steel. Contrary to a recent published account stating that the gear was fitted with shock absorbers that had 60 cm (2 ft) of travel, direct observation of the DM 1 aircraft in the Mary Baker Engen Restoration Hangar at the Udvar-Hazy Center confirms that the struts are solid steel with no capacity to absorb shocks. To reduce the stress of landing at the high angles of attack required for delta wing aircraft, the struts are set so close together that the glider appeared ready to tip over. Lippisch may have imagined the test pilot would land on a wooden skid or even the smooth belly of the aircraft since touching down on the gear legs without shock absorbers would probably have damaged the delicate internal wooden structure, which NASM treatment specialist Matt Nazarro likened to the fragile insides of a wooden guitar. The design called for ground technicians to retract the undercarriage after they had mounted the glider piggyback onto a larger powered aircraft, so the gear may only have provided a convenient way to move the aircraft around on the ground.
Authorities had planned to carry the experimental glider into the air piggyback atop a twin-engine and propeller-driven Siebel 204 A aircraft. The DM 1 pilot would have released from the carrier aircraft at altitude and descended with additional thrust from two solid-fuel rockets at an estimated speed of around 800 kph (500 mph). A former coworker of Alexander Lippisch, test pilot Hans Zacher from the DFS (Deutsches Forschunginstitut für Segelflug, German Research Institute for Gliding), was designated to perform the DM 1 test flights. However, Zacher joined the project at a late stage, and the war ended before the students could finish construction.
On May 3, 1945, American troops occupied Prien Airport and found the incomplete glider. German historian and author Hans-Peter Dabrowski wrote in “Flying Triangle,” (Klassiker der Luftfahrt, July 2014, p.61) that when U. S. Army General George S. Patton and other high-ranking officers visited Prien on May 9, 1945, the advanced design features of the aircraft impressed them and Patton ordered the students to resume construction and complete the aircraft. Dabrowski also wrote that Dr. Theodore von Kármén argued vehemently to finish the DM 1, and that Major A. C. Hazen of the Air Technical Intelligence Section, U. S. Army Air Forces in Europe, became the project manager.
Hazen worked closely with Hans Zacher who remained involved in the DM 1 work. One day another group of American visitors came to study the DM 1. They were unknown to Zacher until one day when he casually mentioned that he had studied the work of Walter Stuart Diehl, the famous American pioneer of aerodynamics and author of the authoritative Engineering Aerodynamics (1928), who actively participated in and strongly influenced continuing advances in aerodynamics and hydrodynamics. To Zacher's big surprise, one of his counterparts identified himself as Walter Diehl, and from this encounter, a lifelong friendship arose between Zacher and Diehl.
Another famous visitor to Prien Airfield was Charles A. Lindbergh. According to 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 summer 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 weight measured 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, and they 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 USA 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 Wind 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 the firms tested small models in wind tunnels to determine the high-lift characteristics of these designs. When intial 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 company’s test models. During this process, the NACA researchers altered the blunt leading edges of the DM 1 wings by adding sharp leading edges 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 revealed strong swirling vortex airflows over the top surface of the wings at low speeds and high angles of attack.
This video showes the DM 1 inside the FST at Langley during a test on 1 August 1946. Smoke makes the airflow visible: https://www.youtube.com/watch?v=_zsZxaVP4v0. 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 aircraft for landing. The vortices also helped the pilot maintain directional control about the yaw axis using the rudder. The test revealed the importance of wing sweep angle to generating the vortex, and the requirement to position the vertical fin and rudder to take advantage of the vortex flow.
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 fitted with a thin wing required for transonic flight speeds 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 wing 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 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).
We have found no written confirmation that Convair incorporated the NACA data from the DM 1 directly into their groundbreaking work to design and build the first jet-propelled delta wing aircraft. Yet it must be so. When on 18 September 1948, the Convair XF-92A became the world’s first delta wing aircraft to fly, it marked the triumphant culmination to a project that Convair had started in September 1946. The goal then was to develop and evaluate the aerodynamic characteristics of the delta wing configuration by flight-testing an experimental prototype aircraft called the Model 7002. The air force renamed this aircraft the XF-92A in May 1949. Recall that NACA tested the DM 1 in the wind tunnel in April, June, August, and November 1946. Interest in the DM 1 probably led Convair Chief of Aerodynamics Ralph H. Shick to visit DM 1 designer Alexander Lippisch in July 1946. In October, Lippisch and two other German scientists moved for a few weeks to Convair’s home airfield in San Diego. Convair had plenty of time to study the DM 1 wind tunnel data, consult with Lippisch and his colleagues, and then incorporate the results during design and construction of the Model 7002.
The NACA stored the DM 1 in a shed at Langley Field in January 1948 and then offered it to the Smithsonian in November 1949. Unaware of the significance of the DM 1, Paul Garber declined the offer so the NACA transferred the glider to the Air Force Museum at Wright-Patterson Air Force Base, OH. As the decade passed, the U. S. Air Force placed into service hundreds of delta-wing fighters and flight tested the supersonic delta-wing Convair B-58 Hustler. These developments helped to convince the Smithsonian to acquire the DM 1 when the Air Force Museum offered it in 1959. The glider was too large to ship in one piece so craftspersons removed the wing tips and vertical fin before the aircraft arrived at the Smithsonian. Collections staff moved the DM 1 from the Paul E. Garber Facility to the Mary Baker Engen Restoration Hangar at the Steven F. Udvar-Hazy Center in 2011.
Joe Chambers summed up the DM 1 legacy this way: “The DM 1 activity was one of the first at Langley to exploit control of vortex flows for high-angle-of-attack performance, and led to an ongoing program of increasing interest with industry and [the Department of Defense] over several decades that ultimately led to the use of structural airframe devices called wing leading-edge extensions.” In 2016, leading edge extensions are fitted to many domestic and foreign military aircraft and they function just as the Lippisch DM 1 did in the Full-Scale Tunnel at Langley after NACA engineers added the thin metal leading edges in 1946. Note, too, that many high-performance jet aircraft are equipped with twin vertical tails that designers can position more directly in the path of the vortex flow than a single vertical tail. Examples of jets with leading edge extensions and twin vertical tails are the McDonnell Douglas F/A-18 Hornet, McDonnell Douglas F-15 Eagle, Lockheed Martin F-22 Raptor, Lockheed Martin F-35 Lightning II, Mikoyan-Gurevich MiG-25 Foxbat, Mikoyan MiG-29 Fulcrum, and the Sukhoi Su-27 Flanker.
Wingspan: 5.9 m (19 ft 5 in)
Length: 6.6 m (21 ft 7 in)
Height: 3.2 m (10 ft 5 in)
Weights: Empty, 297 kg (655 lb)
Gross, 460 kg (1,017 lb)
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, 3/29/15, 4/20/15.
Dabrowski, Hans-Peter. “Flying Triangle,” Klassiker der Luftfahrt, July 2014.
Lindbergh, Charles A. The Wartime Journals of Charles A. Lindbergh, (New York, 1970).
Lippisch, Alexander M. The Delta Wing-History and Development, Gertrude Lippisch translator, (Ames, Iowa, 1981).
Lippisch DM 1 Curatorial files, National Air and Space Museum.
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.
Russ Lee, Evelyn Crellin, rev 11/26/16
