Imagine an aircraft that combines a helicopter’s ability to takeoff and land from almost anywhere, with the speed and range of a fixed wing aircraft. That’s precisely what aviation enthusiasts have dreamed of building for well over 50 years. In fact, so many efforts have been made to get a vertical takeoff and landing (VTOL) aircraft off the ground that even advocates of the concept often refer to the “wheel of misfortune,” a diagram that depicts the dozens of mostly failed concepts.
Many never got off the blackboard, and only three have ever been flown operationally by the military – the main customer for these vehicles.
But now, the Pentagon’s Defense Advanced Research Projects Agency (Darpa) is taking another spin at the “wheel”. This week, the agency announced a new X-plane program, which will seek to build a prototype aircraft to demonstrate a better VTOL design. That means more than just brushing off old ideas: the agency is hoping for an entirely fresh approach.
"Strapping rockets onto the back of a helicopter is not the type of approach we're looking for,” Ashish Bagai, Darpa program manager said in a statement announcing the new programme. “The engineering community is familiar with the numerous attempts in the past that have not worked. This time, rather than tweaking past designs, we are looking for true cross-pollinations of designs and technologies from the fixed-wing and rotary-wing worlds.”
Indeed, the Pentagon agency is seeking what it calls the “elegant confluence” of different engineering designs and approaches to a VTOL aircraft.
There are, as the wheel shows, numerous to choose from. VTOL concepts over the years have included various ingenious solutions for powering and lifting these craft including tilt-rotor aircraft, tilt-wing, vectored thrust, tilt prop and tilt jets, just to name a few. A few have even been used operationally, such as the US military’s tilt-rotor V-22 Osprey; the iconic Harrier, which relies on vectored thrust to control its movements; and the Soviet Yak-38, another vectored thrust machine. But many more never got anywhere near the battlefield, such as a class of aircraft that stood on their tails – like rockets – for lift-off (These impractical designs included the Lockheed XFV-1 and the Convair XFY-1 Pogo.)
‘Art of the possible’
The traditional problem with VTOL aircraft has been the tremendous complexity involved in having to transition from horizontal flight to vertical flight. Many of the schemes, which combine rotors and wings, or involve tilting some component of the aircraft, make for technically elaborate designs that are often impractical to operate and fly.
That Darpa would try to tackle the VTOL problem, even after so many failures, is not surprising. It has long been the tradition of the agency to tackle significant engineering challenges, such as robotics and hypersonics, multiple times until something works. Moreover, the appeal of a VTOL aircraft is simple: helicopters offer the unique ability to take-off and land without an airstrip, providing the military with access to places where fixed-wing aircraft can’t land, while fixed-wing aircraft offer greater speed and range.
The question, now, however, is whether Darpa can come up with anything new. And on that point, not everyone is optimistic.
“The number one problem in the aeronautical sense, with VTOL, is you’re trying to have it all, and in aviation, everything is a tradeoff,” says Roger Connor, curator at the Smithsonian’s Air and Space Museum in Washington DC. “This is really, really hard to do, this is harder than rocket science in a lot of ways,” says Connor. “You’re trying to do something that is counterintuitive, and it’s not obvious that you’re going to have success.”
Nor is it even clear that an entirely fresh approach to the problem is what is needed. Richard Hallion, a former senior advisor to the US Air Force and aerospace historian says there have been improvements in established technology over the years - composite materials, fly-by-wire flight controls, and better propulsion - which may make past VTOL concepts now feasible. “Many of those are waiting to be rediscovered or mined,” he says. “Frankly the history of aerospace technology is one of constant reinvention, reinterpretation, and to a certain degree, rediscovery. That’s not a bad thing.”
No one denies that the road to VTOL aircraft is littered with the corpses of past aircraft, including several Darpa programs, but proponents argue that that’s the whole point of the new initiative. Mark Moore, a Nasa engineer who has specialized in VTOL aircraft for three decades, says he’s excited about Darpa’s plans, but cautions that the example included in a graphic announcing the programme, showing a fan-in-wing design, is not likely the solution. “I would be very surprised if the resulting concepts ended up being fan-in-wing concepts as shown in this graphic,” he says. “Fan-in-Wing were pretty much the least successful of all the VTOL approaches attempted by Nasa and the [Department of Defence] over the past 60 years; they have fundamental transition problems [between take-off and forward flight] that the new technologies available do not solve.”
Instead, Moore suggests that advances in electric propulsion and automation will ultimately lead to entirely new concepts.
Mike Hirschberg, executive director of the American Helicopter Society International, is also optimistic. A new X-plane program, like the one Darpa has started, offers a real opportunity – and money - to tackle an age-old problem, says Hirschberg, who previously served as a consultant to Darpa for over a decade.
Hirschberg, who tracks VTOL aircraft design, and updated the famous “wheel,” disagrees that it is in fact a “wheel of misfortune” (that nickname came about years after the original wheel was developed). It’s true that of the dozens of VTOL concepts, only three have been fielded, but that doesn’t mean the idea is futile, he says.
One could use the same wheel to depict ideas for conventional takeoff and landing aircraft, and perhaps come to a similarly bleak picture. “I don’t know if it is as bleak, but it’s probably not that much better,” he says.
Darpa’s project, while high-risk, is not without a solid base: internal studies done by the agency demonstrate that a VTOL aircraft is feasible. That challenge is simply identifying a design that will make it work practically.
“If you start with a clean sheet and ask: what is the art of the possible, and throw open the door to unconventional approaches, you get a lot more creative ideas, and a lot more innovative ideas,” says Hirschberg. “A lot of them, after full analysis may not work, or may not have a compelling capability that outweighs the cost, or penalties, but maybe there will be a breakthrough, maybe you’ll be able to do something. “
There is, he acknowledged, a great deal of disbelief from people about the Darpa project, and some aviation experts may wonder if the agency is crazy. “Maybe it is, but maybe someone is crazy enough out there to come up with a new idea or a new approach to an old concept.”