Switzerland is not somewhere I normally associate with advanced, entrepreneurial aircraft design, but Ray Research
is a small outfit working on a civil VTOL fan-in-wing concept it believes could be cheaper to operate than helicopters or tiltrotors.Graphics: Ray Research
The Ray has four low disc-loading fans that are mounted in a low aspect-ratio wing to generate lift in vertical flight, and two tilting ducted fans on the tail that provide vertical thrust and pitch control in hover mode and propulsion in forward flight. Longitudinal vanes under the fans provide sideways thrust vectoring. These vanes and roll shutters on top close off the wing fans in forward flight.
CEO David Posva says the Ray is an improvement over previous fan-in-wing designs like the Vanguard Omniplane
and GE-Ryan XV-5A
. These had two smaller, more highly loaded fans in the wing and needed more power. The XV-5A also had an additional fan in the nose for control.
Posva says the Ray's bigger. more lightly loaded fans are more efficient and, mounted one behind the other in the wing, provide better control. The lower disc loading of the fans reduces the engine power required, which in turn reduces cost, he says.
The tail-mounted tilt ducts, meanwhile, provide a small amount of vertical lift but are used prinicipally to vector thrust for pitch control during transition between vertical and forward flight, and for propulsion in the cruise.
Wing fans lose efficiency as speed increases, and cause the aircraft to pitch up in forward flight, Posva says, so the Ray shifts power the rear wing fans and vectors the tilt ducts to provide additional nose-down pitch control during the transition to forward flight.
To accommodate the big fans, the wing has a deep section and large area, but a low lift coefficient and low aspect ratio. While a longer wing with higher aspect ratio usually gives lower drag, Posva says for the Ray a lower aspect ratio reduces cruise drag.
The company's initial design is a small three/four-passenger, unpressurized aircraft to compete with light twin-turbine helicopters like the Eurocopter EC135. It estimates the Ray's speed and range at 195kt and 980nm versus 121kt and 370nm for the EC135, with operating costs less than half those of the helicopter.
The key driver of cost is engine size and fuel consumption, says Posva in a presentation to October's International Powered Lift Conference in Philadelphia. The Ray would require around 1,000kW (1,340shp) compared with 800kW for the EC135 and 2,500kW for the faster but more expensive Bell Agusta BA609 tiltrotor, he says.
The Ray is a long way for becoming reality, but the company recently completed transition flight tests using a small radio-control model (see video below). Transition tests used a simplified model called the M3b, with open fan ducts. A separate model has flown in cruise mode with closed and partially closed ducts (bottom video). Posva says another model, M4, is planned to demonstrate the final part of the transition - stopping the rotors and closing the wing.Videos: Ray Research