Bell has released a rendering showing updated design concepts for vertical takeoff and landing-capable crewed and uncrewed aircraft that feature proprotors that fold away during cruise. The company is working toward building a flying demonstrator using this technology as part of a program intended to present options for a potential future high-speed, runway-independent special operations airlifter.
The new rendering accompanied an announcement from Bell yesterday about the completion of wind tunnel testing in support of its work under the U.S. Defense Advanced Research Projects Agency’s (DARPA) Speed and Runway Independent Technology (SPRINT) program. Bell is one of two companies currently working on designs as part of Phase 1B of SPRINT, the other being Aurora Flight Sciences, a Boeing subsidiary. Aurora’s SPRINT concept uses multiple internally mounted lift fans for vertical flight, as you can read more about here.
The wind tunnel “test phase follows successful evaluation of the Stop/Fold rotor system using the Holloman High Speed Test Track (HHSTT) in New Mexico in 2023,” according to a Bell press release. “Building upon the previous folding rotor testing, the wind tunnel program validated the stability and control of the aircraft through the rotor fold and unfold sequence in flight. Together, these two critical risk reduction tests prove the concept is ready to move ahead into a flight demonstration as part of the DARPA SPRINT program.”
Bell first released renderings of two crewed design concepts, as well as an uncrewed one, featuring what it is now calling the “Stop/Fold rotor system” back in 2021 in relation to U.S. Special Operations Command’s High-Speed Vertical Take-Off and Landing (HSVTOL) project. SPRINT is directly tied to HSVTOL, which is focused primarily on meeting certain future U.S. Air Force special operations aviation requirements.
At its most basic, the Stop/Fold concept is intended to offer vertical takeoff and landing capability, as well as the ability to hover, but with a tilting rotor system that can be stowed in a lower-drag configuration to allow for higher-speed level flight. A separate traditional jet propulsion system provides forward thrust in the latter mode. For comparison, the V-22 Osprey, which Bell developed together with Boeing, has very large and draggy wingtip proprotors that are used for vertical and forward flight. The V-22 is part of a long history at Bell of work on tilting rotor concepts, which also now includes the V-280 Valor and the Future Long-Range Assault Aircraft (FLRAA) for the U.S. Army that is derived from it. FLRAA is set to replace a significant portion of the Army’s Black Hawk-series helicopters.
The new Bell SPRINT rendering offers updated looks at the larger of the two crewed Stop/Fold rotor concepts and the uncrewed design. The smaller crewed design concept is not shown.
When it comes to the uncrewed design concept, the latest Bell render shows a significant redesign of the side-mounted air intakes. There are two new top-mounted intakes, which could provide valuable additional airflow while in the vertical flight mode or in other low-speed flight regimes. The overall shape of the central fuselage has been streamlined, as have the wingtip rotor pods.
The crewed design concepts show signs of similar streamlining in the shape of its fuselage and wingtip rotor pods. The fairings over the proprotor hubs have notably grown in size. A top-mounted air intake seen in previous renders is now missing, suggesting there could be a new flush intake on top instead.
Bell’s latest SPRINT press release does not include any new details about expected flight performance or other capabilities of any of the Stop/Fold rotor concepts. In the past, the company has said the technology could be scaled to designs with gross weights anywhere from 4,000 to 100,000 pounds, and offer top speeds in the level flight mode of up to 400 knots. For comparison, Air Force Special Operations Command’s (AFSOC) CV-22B version of the Osprey has a stated maximum gross weight of 60,500 pounds and a top speed of 280 knots. You can read more about what Bell has previously disclosed about the fold-away rotor concept in this War Zone feature.
Previous renderings have also pointed to the potential for Stop/Fold rotor designs to be used for mission sets well beyond special operations airlift. This could include combat search and rescue (CSAR), intelligence, surveillance, and reconnaissance (ISR), air-to-air combat, and air-to-surface strike. Significant questions have emerged in recent years about how CSAR will be conducted going forward, especially in the context of high-end conflicts against adversaries with significant air defense networks, and the SPRINT-like platform could be an especially important part of that ecosystem in the future.
Whether or not any of Bell’s Stop/Fold rotor designs become a reality remains to be seen. Tiltrotor designs, in general, are complex and present considerable challenges even before fold-away rotors are added to the mix, as has been evidenced by continued troubles with the V-22. At the same time, Bell has made significant advances in relevant technologies, as demonstrated by the V-280.
There is also no guarantee that any design that emerges from SPRINT or HSVTOL will enter operational service. As The War Zone has explored in great depth in the past, the U.S. special operations community has been at least exploring new vertical takeoff and landing capable transport aircraft and related designs for decades now without fielding any of them that we know of.
“There’s a lot of hard things,” Air Force Lt. Gen. Conley, head of AFSOC, said back in October while speaking broadly about ongoing work on SPRINT and HSVTOL. “Some of it’s just the technological pieces of the engines and trying to get enough lift and enough size.”
“I think, conceptually as an Air Force, we struggle a little bit with how big is big enough … as you look at the developmental models, some of them are, you know, the size of a sedan. And then … another iteration would be the size of about a UH-60 [Black Hawk helicopter]. And then you get up to a little bigger … probably C-130 ‘lite,’” he added at the time. “And I think as an Air Force that’s used to move in a lot of big things, and a lot of people, and a lot of pallets, and big amounts of cargo, there’s a little bit of a mental block there.”
At the same time, there is a clear and growing interest in new runway-independent capabilities across the U.S. military. Runway-independent aircraft, or at least ones with more limited runway requirements, crewed and/or uncrewed, are increasingly seen as particularly key to success in a potential future high-end conflict against China in the Pacific. The designs being explored now under SPRINT offer attributes that could also have broader applicability in operations elsewhere globally.
“I think the high speed/range, high-speed kind of lower profile ability to get in and out of places that don’t require long runways, I think that would be attractive to any combatant commander,” Conley said in October. “It’s certainly a capability we need in the Indo-Pacific, just because of the geography, regardless of an [specific] adversary.”
Large established air bases would also be top targets for an opponent in any future major conflict, which could further limit access to traditional runways. There is a significant debate ongoing now about how and where the U.S. military should invest in new hardened infrastructure and other defenses to shield critical bases from attack going forward, as you can read more about here. Being able to operate from more locations, including far-flung ones closer to active combat areas, would help reduce vulnerability and increase overall flexibility, as well.
Bell is certainly continuing to move forward with work on its Stop/Fold rotor system as one option for meeting growing demands for new vertical take-off and landing-capable special operations airlift and other runway-independent capabilities.
Contact the author: joe@twz.com