Collection Item Summary:
During World War II, Bruno Nagler designed this sole prototype as a backpack portable helicopter. Although intended for sport or commercial courier duties, the German Air Ministry funded the project for military purposes with specifications for one-hour endurance and a 50-kilometer (31-mile) operational radius. The blade-mounted engines proved unreliable because the tremendous centrifugal forces of rotation interfered with carburation. The NR 54 V2 underwent ground testing at the end of the war, but it never flew.
The blade-mounted engines eliminated the need for a heavy and complex transmission or anti-torque tail rotor. The pilot only had two controls. One overhanging stick tilted the rotor in the desired direction, while another controlled engine RPM. The conical structure above the rotor is the fuel tank. The "strap-on" helicopter idea did not die with the NR 54 V2, and the U.S. military experimented with similar concepts in the 1950s.
Collection Item Long Description:
Nagler-Rolz NR 54 V2 Tornistergeraet (Portable Equipment)
At the onset of World War Two, German rotary-wing pioneers, Heinrich Focke and Anton Flettner, gave Germany an early lead in military helicopter development. The success of Focke and Flettner designs encouraged Nazi Germany to fund further rotary-wing research in anticipation of producing helicopters that could support its military objectives. One of the most unusual aircraft to emerge from this effort was a small, portable helicopter - the NR 54 V2.
The designer was an Austrian, Bruno Nagler. In 1929, he assisted fellow Austrian, Raoul Hafner, on his RI Revoplane helicopter. The RI and its successor, the RII, managed to become briefly airborne during testing in Britain, but they lacked the performance or control required of a practical helicopter. Hafner later developed the Rotachute and Rotabuggy towed gyrogliders for Great Britain in World War Two.
In 1934, Nagler had constructed the Helicogyro, which could operate as either a helicopter or autogyro. In helicopter mode, the 90-horsepower Pobjoy engine powered a two-bladed rotor, with torque countered by a vertical control surface placed in the rotor downwash. In autogyro mode, a pusher propeller moved the aircraft in forward flight with the rotor freewheeling to provide lift. In 1937, the Helicogyro flew in Britain after considerable fine-tuning. However, Nagler realized that the increased mechanical complexity of helicopters put them at a severe disadvantage in terms of production cost.
Nagler then shifted his focus to the creation of the lightest and most mechanically simple helicopter possible for use as an individual sport aircraft or means of delivering parcels to outlying areas with limited road access. However, the impending war provided a new impetus for his designs.
German military interest in helicopters for observation and liaison duties aboard U-boats and other naval vessels provided Nagler with an opportunity to concentrate his research on single seat, lightweight helicopters without concern for funding. In 1938, Nagler received a research contract to construct a new helicopter. By this time, he had joined with Franz Rolz to create the Nagler-Rolz Flugzeugbau to gain eligibility for government contracts from the increasingly militaristic Nazi government. In 1940, the company completed its first effort, with the designation NR 55, at Nagler's Vienna home. This novel asymmetric design relied on only one rotor blade for lift. The 40 hp engine sat inside an aerodynamic fairing more than a meter from the axis of rotation the other side of the rotor hub to counterbalance the blade. It powered two small counter-rotating propellers mounted on the leading and trailing edges of the rotor blade at a point just over half of the rotor blade's 5.4-meter (17 ft 9 in) length from the hub. The drive shaft for these propellers passed through the rotor hub and the interior of the rotor blade. The entire rotor system was able to turn at up to 135 rpm. Amazingly, this 190 kg (418 lb) machine was able to hover during indoor testing with loads up to 110 kg (243 lb). Nagler did not attempt to fly the aircraft in forward flight, because the primary purpose of the aircraft was to demonstrate the validity of Nagler's unique rotor system. This aircraft was certainly one of the most unusual helicopter designs ever to take to the air, but it had a number of problems that carried over into his later models. The mounting of propellers on the rotor itself resulted in gyroscopic precession that interfered with the flapping action of the rotor blade. This caused considerable vibration when the engine misfired, which occurred quite frequently. The enormous centrifugal forces, which acted on the engine during rotation, caused fuel flow and ignition problems that continued to plague Nagler's later wartime models. The NR 55 went into storage at the Nazi glider club in Vienna, but a 1944 bombing raid destroyed it.
Nagler began to seek ways to reduce the size and weight of the NR 55 to create a truly portable aircraft, which troops in the field could fold and carry in a rucksack, or stow in the conning tower of a U-boat. The result was the NR 54 V1, which Nagler completed in 1941. This 80 kg (176 lb) model shared the same asymmetric engine and rotor configuration as the NR 55, but used a 24 hp engine, balanced against a smaller 4-meter (13 ft 2 in) rotor blade. The NR 54 V1 was ultimately a failure, because of the continuing carburetor troubles caused by centrifugal forces, which caused the engine to run at either full throttle or idle, but nowhere in-between.
Nagler built the NR 54 V2 in response to a new set of design specifications set forth by the German Air Ministry in 1943 for a folding man-portable helicopter. They required one-hour endurance, a 50-kilometer (31 mi) operational radius, a ceiling of 500 meters (1,640 ft), and a climb rate of 2.5 meters (8 ft) per second. The Air Ministry viewed the asymmetric engine and rotor configuration used of the NR 55 and NR 54 V1 as too unconventional, and insisted that Nagler's next design must have a symmetrical rotor system. Nagler did not abandon the rotor-mounted engine design, as it reduced the weight considerably without a drive shaft and transmission system, in addition to being torque-free. Nagler placed a small 8 hp one-cylinder, two-cycle engine at point 1.4 meters (4ft 7 in) from the axis of rotation on each of the two rotor blades. The 4.5 kg (10 lb) single-cylinder two-stroke Argus engines turned at 6000 rpm directly driving 0.6-meter (23 in) propellers, which produced 24 kg (53 lb) of thrust each. Argus had originally designed these engines as an auxiliary power source to start larger engines. A pull rope started the engines, and starting the second engine with the first already going was likely an interesting challenge. Rotor blades were 4 meters (13 ft) in length, with a .3-meter (12 in) chord. Only those portions of the blades outboard of the engine could change pitch collectively. The blades were equipped with flapping hinges but did not incorporate any drag hinges, which, given the minimal blade size and mass, was not a significant issue. The simple frame consisted of a tripod that rested on tiny non-castering wheels. A tiny seat that protruded from the framework, which supported the rotor hub, provided a tenuous means for the pilot to remain attached to the aircraft in flight. Nagler intended for a small fabric-covered vertical stabilizer to provide direction stability in forward flight, but there was not any means to control yaw. The NR 54 V2's conical fuel tank sat on top of the rotor hub, as was the case in Nagler's other wartime designs. Engine controls consisted of two handles, which projected downward from the rotor hub. One lever controlled the throttle, and the other tilted the rotor disc for directional control.
The Soviet advance on Vienna in 1945 interrupted the testing of the aircraft. Nagler and Rolz quickly relocated their operations to Zell am See alongside Friedrich von Doblhoff, another Austrian helicopter pioneer evading the Russian advance. The NR 54 V2 was undergoing ground testing at Zell am See when the war ended. However, the centrifugal force issues remained as the major obstacle to a successful flight and the machine never left the ground. In the aftermath of World War Two, allied intelligence experts sought out German aviation pioneers, including Nagler, to identify which technological developments were suitable for further development. The British interviewed Nagler at his home in Vienna in which he had kept the NR 54 V2 after the war's end and took the aircraft into custody. It eventually found its way to Freeman Field, Indiana for evaluation alongside other captured aircraft. The Air Force then tagged the NR 54 V2 for future display at the National Air Museum.
The U.S. Naval Technical Mission, which evaluated German helicopter programs immediately after the end of the war, included the following statement on the NR 54 V2 in its report:
"In the light of present day aerodynamics, the design is not deemed too practical, especially when dealing with troublesome one cylinder engines. They must be perfectly synchronized and matched in weight when subjected to large centrifugal force to avoid fatal vibration and poor carburation."
Nagler was not the only engineer trying to construct man-portable helicopters for the Nazi war machine. Another Austrian, Paul Baumgartl was attempting to construct strap-on gyro-gliders and helicopters in which the pilot's legs formed the landing gear. This design was to use the same engine that powered the NR 54 V2. Baumgartl completed several machines and, amazingly, even managed to fly one of them for short distances after the war.
Other helicopter designers attempted to pick up where Nagler and Baumgartl had stopped. American Horace Pentecost developed his Hoppi-Copters along the same lines as Baumgartl, and was able to achieve some success in developing several flyable ultra-light helicopters in the immediate post-war years, but was ultimately unable to find a suitable market for his designs. Nagler realized that war-ravaged Europe was not an ideal environment for developing new aircraft and sought to emigrate to Britain or the United States. He became a U.S. resident in 1952, and quickly set about starting a company to continue his lightweight helicopter development. The U.S. military funded several Nagler proposals for man-portable helicopters, but as in the case of his earlier designs, a truly practical design did not emerge. One of his first government contracts was for a "strap-on" helicopter in which a series of rockets brought the rotor up to takeoff speed. Nagler intended this design to allow a soldier to hop across a river and then autorotate down on the far side. Nagler then began work on conventional helicopters and gyroplanes and continued to focus on lightweight models for sport use, but achieved only limited success in marketing them. Throughout his designs, Nagler employed unconventional but very promising methods of weight savings, and aircraft control, such as the use of rotor mounted propulsion systems. However, like many inventors, he remained attached to innovative concepts that the technology of the period could not support.
Though Nagler's attempts at building a practical man-portable helicopter were not successful, several companies such as deLackner, Gyrodyne, and Hiller developed practical lightweight helicopters for the military that came close to achieving his vision in the 1950s. However, the stability, payload, range, and safety issues involved with the operation of this type of aircraft limited them to experimental use only.
Rotor Diameter:8 m (26 ft 3in)
Length:2.42 m (7 ft 11 in)
Height:2.18 m (7 ft 2 in)
Weight:Empty, 36.5 kg (80 lb)
Gross, 140 kg (309 lb)
Engines:2 x Argus A.S. 8 ZF 140 One cylinder, two cycle, 8 hp each
References and Further Reading:
Gersdoff, Kyrill von Die Deutche Luftfahrt Hubschrauber und Tragschrauber. Bonn:
Bernard & Graefe Verlag, 1999.
Labermont, Paul. Helicopters and Autogyros of the World. Cranbury, New Jersey:
Cassell & Company LTD., 1970.
Liptrot, R.N. British Intelligence Objectives Sub Committee Overall Report No.8:
Rotating Wing Activities in Germany During the Period 1939-1945. London: His
Majesty's Stationary Office, 1948.
Peter, Ernst. Tragschrauber/Hubschrauber: Osterreichs Pioniere. Graz, Austria: H.
Weishaupt Verlag, 1985.
Prewitt, R. H. Report on Helicopter Developments in Germany. SAE Information
Smith, J.R. German Aircraft of the Second World War. London: Putnam, 1972.
NR 54 V2 curatorial file, Aeronautics Division, National Air and Space Museum
R.D. Connor, revised 11-25-01