Bell has revealed part of its vision for a future High-Speed Vertical Take-Off and Landing, or HSVTOL, aircraft, a type of platform that U.S. Special Operations Command has recently been taking a particular interest in. The company’s initial HSVTOL concepts aim to combine the “hover capability of a helicopter with the speed, range and survivability features of a fighter aircraft,” according to the company. These designs could potentially lead to a replacement for the Air Force’s CV-22B Osprey tilt-rotor aircraft, as well as other types.
Three different, but similar, HSVTOL concepts were unveiled by Bell today, one of which is apparently unmanned. All of them utilize a propulsion concept in which wingtip rotors are used for vertical lift, then these rotors fold away to reduce drag while turbofans provide thrust for the high-speed, forward flight. The same concept is something that we have seen before in earlier Bell patents for a new-generation "convertiplane."
Future “convertible engines” could eventually provide HSVTOL designs with a powerplant that can switch between turboshaft and turbofan modes, eliminating the need for separate lift and cruise engines. With no air intakes visible on the wingtip nacelles, this could also suggest some form of hybrid-electric propulsion for the rotors, as well, which would use the main jet powerplant to provide gobs of electrical power to drive the rotors during terminal flight modes.
The first two aircraft in Bell’s concept artwork appear to be manned, with cockpit transparencies and access doors into the main cabin, while the last seems to be unmanned. While they all seem to share the same propulsion concept, the three Bell designs otherwise differ in more minor details, including engine intake arrangement and tailfins. In addition,, the airframes all look to incorporate basic low-observable features, suggesting that they are designed to offer some degree of stealth, at least when their rotors are folded away.
“Bell’s HSVTOL technology is a step-change improvement in rotorcraft capabilities,” said Jason Hurst, vice president of Bell’s Innovation division. “Our technology investments have reduced risk and prepared us for rapid development of HSVTOL in a digital engineering environment, leveraging experience from a robust past of technology exploration and close partnerships with the Department of Defense and research laboratories.”
Exactly how the aircraft are expected to work, from a propulsion point of view, has not been revealed, but Bell does mention “emerging propulsion technologies,” which could well be a reference to convertible engines. In addition, the company has detailed aspects of the broader capabilities of its HSVTOL design concepts. These will include the ability to hover while producing limited downwash (unlike the CV-22B, for example), jet-like cruise speeds of over 400 knots, runway independence and hover endurance, and scalability to suit a range of missions from unmanned personnel recovery to tactical mobility.
Finally, Bell says that its HSVTOL aircraft are planned to have gross weights ranging from 4,000 pounds to over 100,000 pounds, suggesting that there are other, similar, designs in the works, in addition to these three. By way of contrast, the CV-22B tilt-rotor has a gross weight of 60,500 pounds. The first design on the left appears larger than the Osprey, while the second looks similar in size. The third unmanned concept looks smaller still, which makes sense. So this is clearly a highly scalable concept.
It is interesting to note that Bell's concept art shows the larger of the two manned designs wearing a tail flash of the kind that airlifters, as well as aerial refueling tankers, assigned to Air Mobility Command typically wear. The smaller of the manned concepts has shark mouth nose art and features a sensor turret under the fuselage, and looks overall to be more of an assault transport in the vein of the CV-22B. The unmanned type looks like it could have the ability to carry weapons and could be envisioned as operating in an escort role, accompanying the larger HSVTOLs to a target area, among other mission sets.
In its press release, Bell also mentioned its particular pedigree in the high-speed vertical lift aircraft technology, pointing to a wide range of previous VTOL configurations such as the X-14, X-22, XV-3, and XV-15, which the company developed variously for NASA, the U.S Army and the Air Force. Today, of course, Bell, together with Boeing, is responsible for the V-22 Osprey tilt-rotor.
In April this year, Bell received a $950,000 contract from the Air Force Research Laboratory (AFRL) to research its HSVTOL concept. A month later, Air Force Colonel Ken Kuebler, Special Operations Command’s Program Executive Officer for Fixed Wing, confirmed that the Bell design was among those the command was looking at to meet the emerging HSVTOL requirement.
In addition to Bell, which is perhaps the established high-speed VTOL player, the Air Force, Special Operations Command, and the AFWERX technology incubator are also looking at smaller companies who might be able to contribute “groundbreaking ideas” to HSVTOL. The AFWERX “challenge,” submissions for which closed last month, has been for other concepts that could offer “optimal agility in austere environments.”
While AFWERX wants to explore various different concepts so it can better understand what sort of speed, range, survivability, payload, and size might be required for potential future HSVTOL requirements, it seems Bell is already working on options that offer widely varying sizes, at least. Above all, HSVTOL is likely to embody far more ambitious performance requirements than the Future Vertical Lift (FVL) program, but will have a timeline to match.
In the meantime, AFWERX has already identified what it calls “critical mission profiles” for the HSVTOL — infiltration and exfiltration of special forces and equipment, personnel recovery, aeromedical evacuation, and tactical mobility — but applications could well extend beyond these and incorporate a range of missions currently flown by different fixed-wing and rotary-wing platforms.
Meanwhile, in the background to this is Air Force Special Operations Command’s efforts to start planning a replacement for the CV-22B, one that will be able to fly at speeds in excess of the 280 knots that the Osprey can reach. Indeed, AFSOC’s commander, Lieutenant General James Slife, has spoken of a desire for “jet speed,” which would seem to be a closer fit to Bell’s HSVTOL concepts.
There are other designs, too, dating back as far as the early 1980s, that reveal just how long the Department of Defense has been looking at exotic vertical-lift platforms, including stealthy ones and high-speed ones. The War Zone has looked in detail at some of these in the two-part feature series linked here.
While a short takeoff and landing (STOL), rather than a VTOL concept, the Air Force’s Speed Agile Concept Demonstrator, or SACD, similarly aimed to achieve high speed. It featured a four-engine configuration and was also intended to move larger and heavier payloads.
Some of these earlier concepts have been unmanned, too, like the initial Bell HSVTOL concepts. Lockheed Martin’s VTOL Advanced Reconnaissance Insertion Organic Unmanned System (VARIOUS) is one example of this line of thinking.
Then there is DARPA’s VTOL X-Plane, another program seeking to exploit the advantages of fixed-wing and rotary-wing concepts, albeit with a planned top sustained flight speed of 300-400 knots. That same program yielded the XV-24A LightningStrike, another unmanned VTOL concept developed for the Defense Advanced Research Projects Agency (DARPA) by Aurora Flight Sciences, now part of Boeing, together with Rolls-Royce and Honeywell, but later canceled.
While a high-speed VTOL design would offer more rapid deployment and enhanced survivability, the freedom from conventional runways and infrastructure also aligns with the Pentagon’s increasing focus on so-called distributed operations, especially the kinds of campaigns that might be expected in a peer conflict in the Asia-Pacific region.
It may not be entirely coincidental, therefore, that China, too, is looking at similar kinds of concepts, including an exotic-looking model of an apparent hybrid gas turbine-electric manned combat rotorcraft appearing earlier this year. As in the Bell concept, at least some of the prop-rotors on the Chinese design incorporate a folding mechanism to reduce drag while in forward flight.
Undoubtedly, actually realizing the promise of true high-speed VTOL flight will involve considerable technological challenges ahead. However, it’s becoming increasingly clear that this is a goal that a growing number of branches of the military, in the United States and elsewhere, are now intent on achieving.
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