Aeronautical Research in Germany

This volume of the book series “German Aeronautics” is dedicated to German aeronautical research, whose beginnings are here set to coincide with the work and flights of Otto Lilienthal. The deliberations are predominantly related to the airplane. The airship as well as astronautics are only treated marginally, in case of the latter considering aspects of the so called space planes which today, like the American “Space Shuttle”, ascend to earth orbit by rocket propulsion and return gliderlike. They will, in some decades, most likely employ airbreathing engines so that by then today’s sharp differentiation will have been lifted.

Going back to the roots of the German word “Forschung”, i.e. “Research”, one finds in the Old-High-German language “forscon” as “asking for”. Therefore it is justified to date the beginnings of German aeronautical research to Lilienthal. He was the first, who enquired scientifically why a bird flies. This shall not belittle the preceding work, on the contrary. It is, with its early experiments, important. Its failure is due to the fact that systematic “research” was missing and that just “trying” was not sufficient. Some of these early tests shall be described exemplarily to show how far back experimental trials in Germany reached.

The, for Germany, bitter ending of the war was to be anticipated since the summer of 1918 after the successful offensives of the Allies. On September 29, 1918 the supreme command felt compelled to request an immediate cease-fire. Thus, the war was lost. On November 9, the revolution occurred, the Emperor resigned, and a new government, under Friedrich Ebert, was formed. On November 11, the cease-fire agreement was signed. On January 19,1919, a National Assembly was elected, which convened on February 6, 1919 in Weimar and accepted the “Weimar Constitution for the Reich” (“Weimarer Reichsverfassung”) on July 31, 1919, the “Versailles Treaty” was signed; it was ratified on January 10, 1920. This peace treaty, dictated by the Allies, had severe consequences for the aeronautical industry, hence also strongly affected aeronautical research [2].

The “Seizure of Power” on January 30,1933 was immediately felt in the area of aeronautical research as well as in aeronautical industry. Hermann Göring became “State Secretary without Portfolio” (“Reichsminister ohne Geschäftsbereich”) and took over the position of a “State Commissioner for Air Transportation” (“Reichskommissar für den Luftverkehr”). The office “State Commissioner’s Department for Aviation” (“Reichskommissariat für Luftfahrt”) was founded on February 2,1933 and preliminarily attached to the “State Ministry of Transportation, RVM”. Erhard Milch, a board member of German Airlines (Lufthansa), took over as head of the air-transportation department of the RVM, being at the same time Göring’s deputy.

Before describing the environment of German aeronautical research after 1945, technical developments in aircraft design that have occurred during the second half of the twentieth century shall be briefly addressed. These developments are supported by basic research at universities, applied research at research institutions and the industrial experimental development, in total by research.

The age of flflying with objects heavier than air starts in Germany with Otto Lilienthal (1848–1896). After numerous theoretical studies [1] and practical experiments, he was able to glide in the summer of 1891, for the very first time, a distance of 30 m with his flight apparatus at moderate wind speeds. Other nations consider the beginning of aircraft engineering coincident with the first powered flight of the Wright Brothers. There are many “firsts” in flight technology, depending on which “first” we want to honor. Before Lilienthal there were many trials with small unmanned flight models of different kinds and, of course, experiments with large-scale models andpeople as payload or pilots, respectivelyy, but none of them reached the goal they had hoped for. The overall designation was, at that time, “Airshüp Aviation” and was divided into Aeronautics (Balloon technology) and Aviation (Flight technology).

Businessmen and implementers of the aircraft idea like Hugo Junkerrs cannot be envisaged without the application-oriented basic research as background, as performed by Ludwig Prandtl in Göttingen. Ludwig Prandtl is called the “Father of Aerodynamics” [1, 2], which does, however, not allow the conclusion that Germany is the sole “Fatherland of Aerodynamics”. Prandtl (1875–1953) is in line with other, mostly European, researchers and engineers, who considered basic questions of human flight. He has, however, in a unique way, discovered and formed essential elements of aerodynamics as they are known today. His inquisitive personality, which utilized theory and experiment “problem-oriented”, found at the University of Göttingen, which he joined in 1904, an unique combination of science and technology, inspired by the mathematician Felix Klein (1849–1925). This was of decisive importance for the development of aerodynamics.

Apart from the fundamental research activities in aerodynamics, engine development was a decisive area of work. The science of heat engines was at the beginning of the aircraft engineering no longer young, but the requirements regarding a very much improved weight/ power ratio or the improvement of the altitude capability of fast rotating Otto and Diesel engines represented a considerable challenge. As regards the test facilities, new ground had to be broken. * see Volume 2 “Flugmotoren und Strahltriebwerke” (Die deutsche Luftfahrt)

At the beginning of the development of flight devices heavier than air, the question of how to create lift to overcome gravity naturally stood in the foreground. Another important role in the 19th century played the problem of gaining inherent stability, mainly longitudinal stability [1]. It was no coincidence that the first thorough theoretical investigations of the characteristics of aircraft during flight dealt indirectly (F.W. Lanchester [2]) or directly (Bryan and Williams [3]) with longitudinal stability.

All flight pioneers at the end of the 19th century had the common desire to imitate the gliding of the large longdistance flyers as storks or swans. So what was initially closer than to look for constructions that would spread-out the wing area and not consider wing profiles? Lilienthal’s wing construction of the fiber material wood with a fabric covering and wire bracing was the attempt to realize in light-weight construction the necessary wing area and to give the wing the shape of a cambered plate with a rounded nose.

The science of aeroelasticity encompasses those physical processes and problems that result from the interaction between elasto-mechanical systems and the surrounding airflow. Aeroelastic processes, like the swaying movement of trees, plants and oat fields in the wind, daily occur in nature. But also in many technical fields significant aeroelastic phenomena happen in numerous forms such as the wind-induced flutter of towers, masts and bridges, the flutter of tube clusters in heat exchangers, blade flutter in turbo machinery or the oscillation of ocean platforms as well as shields and dams in rivers. Generally, one speaks here of fluidelastic processes.

Until 1936 when the first helicopter Fw 61 of Focke Wulf came to fly, many inventers and designers tried to develop a practicable helicopter. But the designers failed due to the lack of scientific and technological knowledge so that their devices showed a very unstable flight behaviour and could hardly leave the vicinity of the airport. Henrich Focke and his colleagues constructed with the Fw 61 the first helicopter worldwide that was really operational. Further designers were in Germany Anton Flettner and Friedrich von Doblhoff, who developed pioneering propulsion and design principles.

Parachutes serve to slow down objects (for instance human beings, cargo, aircraft and capsules) in the air, whereby equilibrium is established between the load attached to the parachute and the drag of the parachute. Depending on the design of the parachute, a steady sinking or flight-path speed, respectively, with an unsteady landing/roll air bubble (canopy) results, which guarantees a safe landing of man and material. According to the application, one distinguishes between aircraft brake parachutes, stabilising parachutes, cargo parachutes, and man-rescue parachutes.

Within the subsequent chapter, many statements of contemporary witnesses are being used. Thus, it represents a summary of the situation during the years after World War Two, when German aeronautical research was suspended, and many scientists and engineers went abroad. Many of them returned after 1955 to the German Federal Republic or the German Democratic Republic, respectively, and were able to put, in continuation of their activities, their accumulated knowledge and experience to use. Thus, a renewed transfer of knowledge occurred.

During the ten years of forced abstention in Germany after the war, numerous important developments occurred without the participation of German aeronautical industry and research. Although the fundamentals of this development had, to a large degree, been established before 1945 by German research institutions and industry, they had no more been applied in Germany. These fundamentals included: the aerodynamically clean design of high-speed aircraft,the swept-forward and swept-back wing for the high subsonic and supersonic speed regimes,the supercritical wing profile,the Delta-wing with control flaps at the trailing edge,the aircraft with variable-sweep wingsthe jet enginethe arrangement of jet engines in pods below the wing,the controlled helicopter flight.

The year 1945 represented also a caesura for aerodynamics. Teaching and research, as far as not prohibited by the occupation forces, commenced slowly again at the end of the forties/beginning of the fifties. Many of the leading scientists and engineers were no longer working in Germany or had not yet returned, test facilities were dismantled or brutally destroyed like the wind tunnels in Brunswick in 1948 and institutes had been dissolved.

Already in the early phase of the first axial flow compressors for jet engines some aspects of this turbomachinery have been of importance in having exerted a substantial influence on the whole development of jet engines. To these aspects belong the off-design conditions and the aerodynamics of the blading of the axial flow compressor as well as the necessary performance improvement of the compressors in order to produce larger propulsive forces (thrust). These influences will be dealt with here, where also the findings of the early phase will be addressed because these topics refer to typical contributions made in Germany. From that it follows that the subject is not new.

The development of the modern flying quality requirements started rather corresponding in Germany, Great Britain and the USA in the middle of the 1930’s. Almost at the same time comprehensive reports were published in the USA and Germany in 1942 and in 1943 [1,2] resuming the state of art. This simultaneity reveals objective challenges, well-founded in the technical development and the growing importance of aircraft in the civil as well as in the military field. Efforts of developing flying quality requirements were directed towards improving the design of aircraft. Thus the research activities on the flying quality requirements are leading towards a new approach insofar as the handling qualities are respected in an earlier state of the design process of an aircraft. This basic approach went distinctly beyond the former activities of air-traffic-security in the I92o’s following the international convention from 1919, which should assure a minimal security standard of flying [3]. Regulations for security standards represent only one aspect of flying quality requirements. The other one is within the context of standardising and optimising the handling qualities.

All metallic materials have the characteristic ofisotropy, which means their physical and mechanical properties are independent of direction. Within light-weightdesign of structures, discrete reinforcements in the direction of the main loads, certain directional stiffness characteristics, were already very early realized to reduce the weight of structural elements. The introduction of monocoque constructions into aircraft engineering — and hence the consequent load-conformal design of load-bearing structures by means of beam, rod and plate elements — means a large step towards ultra light-weight structural design. The discrete stiffening of the skin by strips and beams (integral construction) with pronounced orthotrop stiffness directions, i. e., stiffness directions normal to each other, became a design principle of modern light-weight structural design.

The interruption of activities in the area of aeronautical engineering after World War Two, issued by the Allies, also had the consequence that almost all Germans concerned with aero elastics emigrated to the West, like USA, England, and France. Russia recruited by force the rest of the engineering personnel, or turned it to other occupations within Germany. The DVL in Berlin-Adlershof, located in the Eastern Occupation Zone, disappeared completely, and activities at the AVA in Göttingen ceased on August 21,1945. Hans Georg Küssner and a small team still worked there until the end of 1946 for the British “Ministry of Supply” on the “Göttingen Monographs” [1], edited by Albert Betz, where the results of the aero elastic research between 1939 and 1945 were summarized. The impressive research work “1939–1946” related to unsteady aerodynamics presented there was again as summary published by Hans Georg Küssner and Hans Billing [2] in 1953. Starting in 1947 and lasting until 1953, all scientific work in the field of aeroelasticity in Germany ceased.

Since the beginning of the helicopter development (see “First Helicopters and Rotor Systems”) there was a need to place hinges and bearings as close to the rotor center as possible to guarantee low rotor stiffness and hence good control characteristics at low cabin vibrations. This design concept is characterised by the distance of the flapping hinge (delta hinge) from the rotor center and demonstrated in the subsequent figure, which covers the time period from the beginnings until today.

During the first half of the 20th century, German aeronautics succeeded in establishing concepts and setting up trend-setting large-scale test facilities. Especially in the years immediately prior to and during World War Two, politics and government support led to a facility technology that represents peak performances. Examples are, for instances, the former engine high-altitude test facility of the “Bavarian Motor Company” (“Bayerische Motoren Werke, BMW”) in Munich-Milbertshofen, which is still in operation at the American Arnold Engineering Development Center (AEDC), located near the city of Tullahoma in the State of Tennessee [1], and the large wind tunnel erected in the alpine Ötztal for energy-supply reasons in the immediate vicinity of a power station; Americans and the French fought bitterly after the war over this wind tunnel, which is still today in Modane (France), as wind tunnel S1, one of the wind tunnels used by the Frenxh “Office Nationale des Etudes et Recherches Aéronautiques, ONERA” within the Airbus program [2]. Both facilities were, like many other German test facilities, after the war diassembled and as reparations installed and operated by the Victorious Powers.

The following considerations are fundamentally looking upon aeronautical research as a subdivision of the greater spectrum of research in the natural and engineering sciences. Aeronautical research, however, has the peculiar characteristic that it involves a multitude of disciplines — as already exposed in Part 1. Thus, the content of aeronautical research — involving aerodynamics as well as materials and structures, and propulsion as well as avionics — encompasses an extraordinarily wide scope.

Throughout the history of every field of research, thus also research in aeronautics, major accomplishments have been achieved by extraordinary individuals. However, when discoverers or inventors create such achievements, their environment such as society, universities and state are virtually absent and may even try to show, that this new idea or concept is unrealistic and not usable. This was also the case in the initial phase of flying with apparatuses heavier than air.