ADVANCING BLADE CONCEPT (ABC)™ DYNAMICS

The ABC rotor system, with a pair of counter-rotating, coaxial, very rigid, hingeless rotors represents a significant departure from nearly all predecessor helicopter rotor systems. The demonstrator aircraft, the XH-59A has now completed an initial 67 hour flight test program, and from the dynamicist's viewpoint, several findings of interest have emerged. The system has been notably free from load, stress or stability problems. The only lightly damped mode that has been found is blade edgewise response at its natural, in—plane frequency. Damping of this mode is as low as 0.5% critical on the ground or in autorotation, but it is free from destabilizing coupling and results in very low amplitude, low stress responses. Rotor loads have generally followed analytical predictions. Rotor blade tip clearance was monitored throughout the flight testing. The two rotors are statically 30 inches apart, with clearance reduced to about 20 inches in cruise flight. During the test program, a 10 inch minimum clearance has been enforced. This has not curtailed the flight envelope. For most maneuvers, aerodynamic and gyroscopic effects tend to cancel each other so that tip clearance changes of 3 to 6 inches are typical in the most severe maneuvers flown. Aircraft vibration trends confirm analytical predictions. Levels in the basic aircraft with no vibration control equipment are acceptable but higher than would be desired for a production design. Vibrations are significantly reduced in descending flight at reduced rotor power levels. This result, at speeds up to 193 knots, provides the first confirmation of predictions that vibration levels when flying with auxiliary propulsion will be low. The data bank acquired in the 67 hour flight program has been extensive. Results have followed predictions closely, and the program has been free from unpleasant technical surprises in spite of it involving a totally new rotor concept.