|Kellett KH-15 "Stable Mable"|
This variable stability helicopter was a flying test-bed developed with the Office of Naval Research as sponsor to study the behaviour of a small helicopter in which the significant parameters of the rotor could be altered.
This single-seater was powered by two hydrogen peroxide rocket engines mounted one at each blade tip, and fuel was stored in two spherical tanks situated to the right and left of the pilot.
The fuselage was an open steel tube structure to which was attached a fixed tripod landing gear, the main rotor, rotor controls and an open truss boom supporting the conventional variable-pitch-type tail rotor.
This helicopter was designed to test the new gyro-stabilizing system, a kind of small rotor mounted concentrically and intended to stabilize the main rotor. This stabilizer system, developed by Dr. G.J. Sissingh, should be applicable to all types of helicopter, whether having single, tandem, side-by-side or co-axial rotors. It is, furthermore, independent of the type of propulsion.
Trials were started early in 1954 to demonstrate positive dynamic stability without loss of control throughout the entire speed range, and the XH-15 in fact exhibited excellent stick-fixed dynamic stability.
P.Lambermont "Helicopters and Autogyros of the World", 1958
The open frame KH-15 used a pair of Reaction Motors rocket engines fitted at the rotor tips, and first flew under sponsorship from the Office of Naval Research on 13 May 1954. It was fitted with a patented gyro stabilisation system but, despite showing remarkable flight characteristics, it progressed no further.
R.Simpson "Airlife's Helicopter and Rotorcraft", 1998
Technical data for KH-15
Engine: 2 x Reaction Motors XLR 32 , rotor diameter: 5.49m, gross weight: 292m