Bell has released a video showing off its High-Speed Vertical Takeoff and Landing (HSVTOL) technology in action. The two-minute video shows a ground test of the nacelle system used to lift and land a VTOL X-plane before converting to jet flight.
Part of DARPA's Speed and Runway Independent Technologies SPRINT X-plane program, the Bell tests carried out at Holloman Air Force Base in New Mexico are intended to show how a rotorcraft can transition from rotor-propelled lift-off to jet-propelled horizontal high-speed flight.
The video shows the turbine-powered rotors on a track-mounted test bed powering up and applying thrust as they would in a takeoff situation that's similar to what one would see in a tiltrotor craft like the Osprey. In a similar manner to such craft, the X-plane in proper flight would tilt its rotor to the horizontal for forward flight, but there the similarity ends.
As can be seen in the test, once the craft reaches sufficient speed, the jet propulsion system is supposed to take over. That's all well and good, but it now means that the rotors become nothing but a liability, acting like brakes as they increase drag. To prevent this, the rotors are designed to feather and then fold back and lock into place.
When fully developed, the new pilot-optional aircraft is expected to be cable to cruise at high subsonic speeds of up to 450 knots (518 mph, 833 km/h) for a range of 200 nm (230 miles, 370 km) at altitudes up to 30,000 ft (9,100 m) while carrying payloads of up to 5,000 lb (2,300 kg) in a compartment large enough to hold a small vehicle. The purpose of the craft will be to provide runway independence for missions ranging from special forces insertion to disaster relief in inaccessible areas.
"The successful sled test completion is a culmination of Bell’s HSVTOL research and unprecedented technology development," said Jason Hurst, Executive Vice President, Engineering. "The technology demonstration provides Bell with critical experience and knowledge that will inform our X-plane development for DARPA’s SPRINT program. It is a pivotal step in the creation of the next generation of high-speed vertical lift aircraft for future war fighters."
The video below shows the Bell HSVTOL system in action.
Source: Bell
Refining the high power tiltrotor technology has become an acute issue since the multiple accidents of the Osprey. Improving both safety and envelope are the primary goals. Bell’s presented nacelle system has solved the critical envelope-task on a really high level – maybe, having even surpassed the requirements.
Important to note the present ease of managing the new envelope (takeoff/landing, hover and lifting performance, top cruising speed etc.) serves as main source of a good flight safety too. Celebration however would be too early at this stage. The solution has a flaw.
Bell wants to double the number of propulsion systems (!) on board: have one for vertical takeoff, hover and transition. And have another for cruising flight. Aircraft complexity will further increase. So will the weight. (Not to mention engineering elegance, which will suffer even more.) These factors all have a chance to work not only to reduce efficiency, but also against safety. Complexity in particular.
At the same time it is known there is an unexplored filed, which can prove to become the optimal solution for the present situation. For some years the technology of variable blade twist for rotors and propellers has been available. The speed range of Bell’s new testbed can easily be achieved at a greater efficiency and safety by using the rotor blades which can be twisted from feather (zero degrees) to almost 90 degrees at the tips. There is a mathematical support showing how the morphing blades follow exactly the way the airflow keeps twisting around the blades - in 3D - as the flight velocity changes.
Also, maybe it is the icing on the cake, many of the control and actuation systems used today for regulating variable blade pitch of rotors and propellers, can almost 100% be adapted and retained for operating the blades with the new variable twist feature.
Hope, Bell too will start exploration of this topic soon.