Rotary-wing maker Bell is at work testing a folding prop-rotor concept for its DARPA-influenced High Speed Vertical Takeoff & Landing concept in New Mexico.

Yesterday, a Bell release touted the delivery of a High-Speed Vertical Takeoff and Landing (HSVTOL) test article to Holloman Air Force Base for demonstration and technology evaluation. But Bell has done more than just deliver, the rig you see above is already there, already undergoing testing.

Folding rotor blades are integral to the HSVTOL concept Bell is exploring with partner Sierra Nevada Corporation for the Defense Advanced Research Projects Agency’s SPRINT (Speed and Runway Independent Technologies) program.

SPRINT, which DARPA has undertaken in cooperation with U.S. Special Operations Command, aims to build aircraft that combine runway independence (i.e. VTOL capability) with levels of speed, altitude and range that surpass existing helicopter and tilt-rotor designs.

The program essentially describes high-speed vertical takeoff and landing aircraft, platforms that DARPA and DoD see as suitable for missions from emergency medical evacuation to helping troops infiltrate very-hard-to-reach areas. Those areas tacitly include the Indo-Pacific where runways are few and far between and in the event of major conflict, will quickly become fewer and farther between.

DARPA’s announcement seeking proposals specified its desire for a scalable X-plane with the ability to cruise at speeds from 400 to 450 knots at altitudes between 15,000 and 30,000 feet. The potential optionally-piloted aircraft should carry a payload of 5,000 pounds, with a substantial 30-foot-long, eight-foot-wide cargo bay capable of carrying a small vehicle or two and a half pallets. The ask includes endurance of 1.5 hours and a 200 nautical mile range.

The three-phase program will see a number of proposals selected, refined and then winnowed to a few players who will undertake further risk reduction and certification approval work before a final downselect picks out one or more firms to build and fly the aircraft. DARPA has not specified a firm timeline.

Bell has thrown its hat in the ring with an HSVTOL concept that “blends the hover capability of a helicopter with the speed [400+ kts], range, and survivability of jet aircraft.” It has already been at work on the technology as one of 11 companies participating in the AFWERX HSVTOL Concept Challenge, a crowdsourcing effort to advance such concepts.

Bell’s concept is scalable the company’s chief innovation strategist, Jeff Nissen affirms, a design that applies to aircraft from small to heavy, from 6,000 to 100,000 pounds takeoff weight. It is effectively a tilt-rotor like the company’s V-280 Valor which the Army recently selected for its Future Long-Range Assault Aircraft (FLRAA) mission.

However, in addition to a pair of tilt-rotors powered by turboshaft engines, it incorporates one or more turbofans (jet engines) which come online to provide thrust as the aircraft transitions from VTOL hover and climb to wing-borne cruise flight. Pratt & Whitney is partnering with Bell on the propulsion side.

The most attention-getting aspect of the design are prop-rotors that fold as the concept transits from VTOL to cruise mode. As the design accelerates out of VTOL flight, its engine nacelles and rotors tilt forward as seen on the V-280. Once they are in the full-forward position, the rotors are stopped, feathered and then retracted aft in line with the nacelle. Stowed in their new position they reduce drag, facilitating high speed and greater range.

The test article now in place at Holloman AFB is on a sled which rides on a rail-track. It includes the major propulsion elements - the foldable prop-rotor, gearbox, a turboshaft engine (which runs the prop-rotor) and the turbofan envisioned as residing in the tail of the design to provide thrust in cruise.

“There’s quite a bit in play,” Nissen says, referring to the test rig and testing. “It represents the key technologies you need to [enable] a high-speed VTOL aircraft... It has additional flight control, ground control, real-time telemetry. It is [essentially] an aircraft that never leaves the ground.”

While testing will assess the interaction of these elements, evaluating the folding prop-rotors will be high on the priority list - a learning exercise in which Bell can figure out when and how to “hold ‘em” or “fold ‘em”.

“What hasn’t been tested before is the transition into the high-speed mode,” Nissen says. “You use the folding rotor [for VTOL flight] then you stop and fold it and rely on a turbofan to propel you to high speed. The hard part, or the one unique thing, is the stopping and folding of the rotor while you’re handing off the thrust to the other propulsion device which is a turbofan.”

As of now Nissen explains that the concept’s flight modes will overlap. As it flies forward like a traditional tilt-rotor, the jet will fire-up and produce thrust at which point the prop-rotors will be stopped, feathered and retracted.

I asked whether this might not all happen in a briefer sequence. Could the rotors be feathered while traveling the last few degrees of forward tilt and stop as soon as full-forward before retracting?

“It’s a concept so we have to go through many technology and development efforts. The sled is just one of those,” Nissen acknowledged. Basically, they’ll experiment with when to fold ‘em.

The idea isn’t new. Folding rotor concepts go back to WWII. The most relatable example from Bell’s past was when it conducted folding rotor tests in NASA Ames’ wind tunnel in 1972. Back then, a folding version of Bell’s 25-foot diameter rotor was tested in the Ames 40- by 80-foot tunnel in a research program with the Army’s Air Mobility Research and Development Laboratory (AMRDL).

The current tests will actually see the folding prop-rotors perform in the open-air rather than in a wind tunnel, generating real-world insights as the sled moves at velocities a bit over 100 knots - the speed at which Bell reckons the VTOL to cruise-flight transition will take place.

“The test is a pretty big milestone that at least answers a lot of the initial assumptions of what we think it’s going to take to fly,” Nissen says. The rig will measure aerodynamic loads, speed, gravitational forces, rotation, vibration and more.

“It’s fully instrumented. We’re measuring everything including some of the track vibrations,” Nissen adds. Bell has had a lot of time, he says, to perfect the general tilt-rotor configuration and to leverage lessons learned from V-280 development.

That brings to mind a couple possibilities. I asked Nissen if Bell envisions variants of its HSVTOL aircraft flying in mission scenarios with the V-280. His answer was refreshingly candid.

“Your guess is as good as mine. We certainly have a lot of work to do to bring the technology to flight.”

In other words, don’t expect new HSVTOL aircraft for the military any time soon. That may perversely be a good thing for Bell. Despite Nissen’s assertion that the platforms will satisfy different mission sets, it’s quite possible an HSVTOL could perform the Valor’s FLRAA role. In that event, Bell would be competing with itself.

DARPA and the Army may realize this, but they’ve probably folded up the idea for now.