Bell Helicopters tests high-speed VTOL propulsion transition on ground…

Bell's HSVTOL concepts with folded rotors on display on 21 September 2021 at the Air Force Association's annual conference. (Janes/Pat Host)

Bell Helicopters tested the high- and low-speed transitions of its High-Speed Vertical Take-off and Landing (HSVTOL) concept on a ground-based sled at Holloman Air Force Base (AFB), New Mexico, the company announced on 8 February.

Bell's HSVTOL concept entails transitioning from rotary propulsion to jet propulsion in mid-air, a mode of flight never tested before. The New Mexico tests, conducted at Holloman AFB's High Speed Test Track, entailed stopping a spinning rotor and folding it into the nacelle while a jet engine took over to provide thrust.

This transition was the most critical untested technology involved in the HSVTOL programme, Jeff Nissen, Bell Helicopters' HSVTOL programme manager, said in an 8 February interview. The tests advanced the transition method to Technical Readiness Level (TRL) 5, or component demonstration in a relevant environment.

While this test series may not be the last risk-reduction testing, “it certainly was the one that moved the [TRL level] the furthest to date, and then next will be the aircraft. It's hard for me to envision, scope- and scale-wise, any other risk reduction that we would do that would even come close to what we accomplished there with the rig,” Nissen said.

The sled-testing demonstrated “everything needed in an aircraft except the airframe”, Nissen said. “The folding rotor system, a flight-representative rotor at full scale, a propulsion system, and the flight-control system that controls it all” were all tested.