Engine exhaust driven rotorcraft wind tunnel tested

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Wind tunnel testing of a one-fifth scale-model exhaust gas driven co-axial rotor helicopter called Sherpa has been carried out by a Belgian consortium.

The Sherpa is a two-person helicopter with a dry mass of 540kg (1,188lb), a rotor diameter of 5.6m (18.3ft), a top speed of 129kt (240km/h), hover ceiling of 7,210ft (2,200m) and a 400km (216nm) range.

Its exhaust gas propulsion system is called Turbine Direct Driven Rotor or the acronym REDT.

Using the Belgian University of Liege's wind tunnel, the first campaign occurred in December 2008 and focused on auto-rotation qualities and fuselage drag.

A second campaign starting later this year will examine flight stability. A remote controlled one-fifth scale-model version of Sherpa has also been flight tested.

REDT uses a centrifugal compressor that feeds air to two piston engines and directly to the turbines that turn the co-axial rotors.

The direct bypass air is fed at 1.31 bar and 120° celsius while the piston engines, which also drive the compressor, also direct their exhaust gases to the rotors' turbines.

The company claims its REDT's benefits are a reduced number of moving parts, with no gearbox or tail rotor, and improved reliability.

Consortium member Sagita's founder and managing director Hubert Antoine told Flight International: "The retreating blade stall effect has been eliminated with our coaxial rotors as we always have one blade advancing forward on each side, like the Sikorsky X-2."

He added that the blades' tip can not exceed 655ft/s (200m/s) as the direct drive's circumferential turbine speed is limited to a maximum of 163ft/s.

The blades are also very stiff allowing for a very narrow distance between the two rotors.

Sherpa is funded by its four corporate partners and Walloon regional government funding. The universities of Liege and Brussels are the two academic partners.