Within weeks, the Department of Defense will grant one of four competitors a $95 million contract to build an exotic new aircraft — of the sort 20th century inventors went broke and nearly mad trying to create.
The VTOL X-Plane — Vertical Take Off and Landing Experimental Aircraft — is to do “substantially everything a bird can do in the air,” as revered New York University aeronautical engineer Alexander Klemin described aviation’s Holy Grail back in 1938.
That means take off and land vertically, hover like a hummingbird, and cruise at better than 345 miles an hour — two to three times as fast as most helicopters.
This will be a truly experimental aircraft — a technology demonstrator, not a vehicle the military will actually use.
But they will still have the potential to be useful. The competitors have been required to design an aircraft weighing 10,000 to 12,000 pounds fully loaded, with 40% of that weight “useful load,” meaning what it can lift besides itself — fuel, cargo, passengers. To ease the challenge, the Defense Advanced Research Projects Agency, which is running the competition, let the contestants design unmanned aircraft.
The main objective, though, is how well the bird flies.
These prototypes could revolutionize combat missions, military and civilian search-and-rescue operations, civilian air travel, cargo delivery — you name it.
It’s a dream that’s enticed designers since the 1920s, when New York lawyer and World War I veteran Gerard P. Herrick set out to design a “convertiplane” that could fly like a helicopter and an airplane. Herrick, who began his work before anyone even had made a helicopter that was practical, ended up spending his life and fortune on the convertiplane without real success.
The degree of difficulty is still formidable. The aircraft must have a way to transition between vertical and horizontal thrust without loading the craft up with mechanisms that add so much weight the vehicle can’t carry a useful load or create so much drag it can’t fly fast or far.
“It’s a physics-based challenge,” said Ashish Bagai, VTOL X-Plane program manager at DARPA. “Aircraft speed and hover are diametrically opposing in terms of the technologies that are required. You give up hover time if you want to fly faster; you give up speed if you want to hover.”
Aircraft aficionados even put together a “wheel of misfortune” depicting 45 attempts at a VTOL aircraft in the 20th century. Almost all the designs were abandoned.
“Dozens of different configurations were built and tested,” said Mike Hirschberg, who as a Pentagon engineer in the 1990s updated an original 1960s wheel of misfortune and continues to curate it today as executive director of the American Helicopter Society International.
Only four aircraft on the wheel ever went into production: the Harrier, the Soviet Union’s Yak-38, and the modern F-35B Joint Strike Fighter — all “jump jet” fighter planes — and the V-22 Osprey “tiltrotor” troop transport. The others were too complicated or unreliable or costly.
And even those produced don’t touch what DARPA’s VTOL X-Plane must do. Jump jets can’t hover for any length of time, and the Osprey is clumsy and slow compared to what DARPA envisions.
Why keep trying? “The advancements in our flight control systems, in our design tools, in our understanding of the physics, in various other technology areas, have come a very, very long way from where we were to enable these types of questions to be addressed again,” said DARPA’s Bagai.
But the intent, he stressed, isn’t to come up with a production aircraft. DARPA simply hopes to midwife technologies that will make a dream machine possible.
Four contestants are vying to build this plane-of-the-future technology demonstrator: aerospace behemoth Boeing Co.; helicopter maker Sikorsky Aircraft Co., teamed with the famed “Skunk Works” rapid prototyping division of Lockheed Martin (which is in the midst of buying Sikorsky); and two of the world’s most creative little-guy aircraft designers: Abraham Karem, who in the 1980s fabricated a technology demonstator in his Los Angeles garage that led to the Predator drone, and John Langford, who made his aeronautical engineering bones leading an MIT team that set a world record for human-powered flight in 1988.
They’re just notions now — but they could change how the military, and eventually you and I, fly.
Phantom Swift
Boeing Co. of Chicago (featured image)
Phantom Swift, named after Boeing’s Phantom Works facility in Mesa, Ariz., would get vertical thrust from four lift fans. Two are housed fore and aft of center in a sleek, oblong fuselage and two more are in swiveling ducts on the craft’s wingtips. Two conventional turbine engines power the fans.
Louvers above and below the “body lift fans” will close and those fans will shut down as the Phantom Swift’s “wingtip thrusters” tilt forward for transition to horizontal flight, its wing providing lift.
Brian Ritter, director of the project for Boeing, said this “four post lift approach” will make Phantom Swift especially agile because the thrust on all four fans can be controlled independently. The wingtip thrusters can also rotate independently and the louvers of the body lift fans can be adjusted to provide directional control at low speed as well.
The Phantom Swift’s wingtip thrusters harken back to the Doak Model VZ-4, which the Army tested in the late 1950s but never produced. But Ritter says it will avoid the problems of previous attempts.
“We are definitely offering something unique, exciting and captivating,” he said.
Lightning strike
Aurora Flight Sciences Corp. of Manassas, Va.
Aurora founder John Langford proved he was an innovator in 1988 when his MIT team built an aircraft that an Olympic cyclist pedaled 71.5 miles through the air. So it’s perhaps no surprise that Aurora’s VTOL X-Plane entry is the most avant garde. Instead of conventional engines, the LightningStrike would rely on “distributed electric propulsion,” a concept NASA and others are also pursuing.
“We still have a turbine engine, but instead of driving thrust directly, it drives electric generators,” Langford explained. Three one-megawatt generators, to be exact, producing the combined equivalent of 4,023 horsepower.
Those electric generators distribute power to 24 lift fans, 18 of them measuring 31 inches in diameter and built into a wing near the tail of the aircraft. Six slightly smaller fans, 27 inches in diameter, are built into a canard — a smaller wing-like structure — near the nose. Both wing and canard tilt upward for vertical flight and forward for horizontal.
The use of individual electrical wires rather than a shaft to transmit power to the fans allows complex algorithms run by triply redundant flight control computers to adjust the amount of power sent to each fan independently. Langford says this will allow unprecedented control over the aerodynamics of the wing and canard and thus unprecedented performance.
Langford concedes it’s a “strange-looking” concept, but in the competition, “The biggest challenge we face is that nobody ever got fired for buying from IBM” — i.e., going with a big and famous company instead of the little guy.
Rotor blown wing
Sikorsky Aircraft Co. of Stamford, Conn.
The Rotor Blown Wing — so-named because its two rotors would blow air across its wing in forward flight, adding lift — is a tail-sitter. That feature harkens back to two 1950s aircraft (one being the Convair) known as “pogos,” as in pogo stick.
The pogos had counter-rotating propellers on their noses and the pilot had to land backward, looking over his shoulder as if parallel parking. The design didn’t catch on.
“We have some technologies that are available now and have some uniqueness that we think will allow it to proliferate,” Sikorsky’s program manager, Linda O’Brien, said of the tail-sitter concept.
For one thing, the Sikorsky/Lockheed tail-sitter is unmanned, so landing backward isn’t an issue, and the aircraft would be largely autonomous, landing itself. The craft’s two rotors are centered on a 36-foot wing that holds a pod rather than a fuselage to house computers and other innards. Two vertical tails sporting two stilt-like appendages each extend rearward, one straight behind each rotor and perpendicular to the wing, providing “legs” on which to land.
The major advantage of a tail-sitter, O’Brien said, is the simplicity of transitioning from vertical to horizontal flight. The aircraft simply takes off straight up, then pitches forward like a conventional airplane coming out of a vertical climb and flies horizontally.
“I believe our concept without a doubt is the most efficient way to do this job,” O’Brien said.
TR36XP
Karem Aircraft of Lake Forest, Calif.
Karem’s entry, named for the 36-foot diameter of its two wing-mounted rotors, is the same basic configuration as the V-22 Osprey — a tiltrotor, which by definition swivels its rotors upward to take off and land like a helicopter and forward to fly like an airplane.
But the TR36XP is far sleeker, with long, slender wings and an upward V-shaped tail on a svelt, 40-foot fuselage.
Karem’s rotor design also promises to make his TR36XP fly far more efficiently than the Osprey. The TR36XP’s rotor diameter is large enough compared to the weight of the aircraft to avoid the hurricane-force downwash generated by the Osprey’s undersized “proprotors.” More importantly, the TR36XP would use an adaptation of Abraham Karem’s patented Optimum Speed Rotor, an invention the former Israeli engineer came up with after designing the unarmed version of the Predator drone for privately held General Atomics.
The Optimum Speed Rotor, which Karem originally put on an unmanned helicopter design he sold to Boeing, can adjust its revolutions per minute to the most efficient rate at all speeds.
The TR36XP is probably the most thoroughly designed of the VTOL X-Plane entries. Karem has been working on his Optimum Speed Tilt Rotor concept, as he styles it, for years, designing manned versions at aircraft weights from 10,000 to 250,000 pounds.
Richard Whittle is the author of “The Dream Machine: The Untold History of the Notorious V-22 Osprey” and of “Predator: The Secret Origins of the Drone Revolution,” both now available in paperback