SIKORSKY HAS tested a scaled version of a variable-diameter tilt-rotor (VDTR). The configuration is "extremely stable" in hover, says the company.

The VDTR differs from the traditional tilt-rotor concept in that the rotor blades can be extended during hover and then retracted to about 70% of their full length during cruise flight. The telescoping blade acts like that of a conventional helicopter in hover, with enhanced load carrying and auto-rotation features, while, in forward flight, it will be similar to the propeller of a fixed-wing aircraft.

In aircraft mode, the smaller propellers cut the potential for "prop-whirl" flutter, so that the wing does not need to be so heavy or stiff. They are also less sensitive to roll/yaw coupling, so reducing the size requirement for tail surfaces.

Sikorsky has tested a one-sixth- scale model of the VDTR in the United Technologies Research Center large subsonic windtunnel (LSWT) in East Hartford, Connecticut. Another was tested at the Sikorsky Aircraft Model Hover Test site at nearby Stratford.

The firm has also modified a rotorcraft software code produced by the University of Maryland to analyse the complex data from the tests. This code, known as the UMARC/S, needed modifying because the VDTR has many redundant load paths. The telescoping blades also operate in compression, rather than in tension.

The tests on the 2.5m-diameter three-bladed tilt-rotor at the LSWT concentrated on aerodynamic performance and blade loads for all flight conditions. During cruise, engineers varied the rotor diameter between 100% and 67% of the full hover diameter. Tests at the hover stand focused on blade frequencies, loads and rotor stability in the hover.

Sikorsky engineers discovered that, in hover, with the rotor diameter at 83%, the VDTR's stability "...resembles the classic stability curve for an isolated hingeless or bearingless rotor: increased damping, with increased thrust. Testing proves the VDTR is extremely stable."

They also found that further models will need to be developed to assess vibratory blade loads after finding potentially unsteady effects during the transition from hover to forward flight. With the rotor at 100% diameter, the shaft tilted forward at 10¡, and the advance ratio at 0.15. The test team found that, " this low speed, the rotor wake is not blown away rapidly, and the unsteady aerodynamic effects can be significant".

Source: Flight International