Preliminary Development of a Trailing Rotor System

Continuing advancements in low-disc loading V/STOL aircraft technology may well lead into the 400+ knot speed range. To achieve this goal it is necessary that the rotor used for low speed lift not compromise the cruise efficiency nor create dynamic loading limitations of the aircraft in high speed flight. This paper presents results of a preliminary analytical and experimental investigation of a rotor stopping and folding process which offers promise as a means of accomplishing this goal. It is based on using the autorotative (zero shaft poser) regime of the rotor during the stopping operation. Further, it is the special autorotation regime wherein the rotor operates at low thrust/weight ratios over an angle-of-attack range of approximately 90 degrees. The extensive research conducted by the NACA in the 30's on autogiro rotors (References l through 6) thoroughly defined the zero power rotor thrust characteristics over a wide range of speed and angle of attack. These investigations, however, concentrated on the high thrust/weight ratio regime. It is shown herein that these two regimes (high and low thrust autorotation) of operation are continuous from one to another and that a typical transition from one regime to the other occurs in the vicinity of maximum L/D for an unpowered rotor.