One of the more unique helicopter designs was built in 1940 by Austrian, Bruno Nagler. In an effort to reduce weight, he built the first helicopter with a single rotor blade. Named the NR 55 it was powered by a 40 hp engine that was mounted opposite the rotor blade and acted as a counter weight. Surrounded by an aerodynamic housing, it was located about four feet from the rotor hub. A drive shaft from the engine passed through the hub and powered two small counter-rotating propellers (mounted on the leading and trailing edges) and located just over midway along the 18 foot blade. The single seat helicopter weighed 418 pounds and had a tripod style landing gear. Nagler was able to demonstrate hovering flight (inside with no wind) with a payload as high as 243 pounds.
However, the design had several problems. Gyroscopic precession from the propellers mounted on the rotor blade interfered with the blade’s ability to flap (a rotor system function needed for forward flight). Also, the rotor could spin up to 135 rpm producing large centrifugal forces acting on the engine causing ignition problems and fuel flow issues. Finally, high vibration levels proved difficult to reduce.
Nagler eventually retired the design in favor of a smaller design called the NR 54 V1. This version weighed 176 pounds, had a 13 foot rotor blade and a 24 hp engine. Plagued with many of the same problems, Nagler never got the NR 54 V1 to work and abandoned the single rotor blade concept in favor of a traditional two bladed design. Although, he kept the rotor blade mounted engine design because he felt it offered better weight and anti-torque advantages. Known as the NR 54 V2, it had a small 8 hp engine on each blade. However, it never flew due to centrifugal force issues with the engines.
After World War II, the British took the NR 54 V2 prototype into custody and it is now on display at the National Air and Space Museum. The original NR 55 was put in storage at the Nazi glider club in Vienna until a bombing raid in 1944 destroyed it.