Through more than eight decades the conventional single-rotor or tandem helicopter has defeated attempts to develop a more efficient means of vertical lift on nearly all commercial applications. Of course, that hasn’t stopped anyone from continuing to try.
One of the most serious attempts within the past decade is currently housed inside the Urban Aeronautics headquarters in Yavne, Israel, a small city perched on the main highway between Tel Aviv and Ashdod.
Last December, Urban Aeronautics’ unmanned AirMule – featuring two internal lift rotors powered by a single Arriel 2 turboshaft engine and two rear-mounted variable pitch thrusters – staged a series of automated flights: rising about 10ft (3m) above the ground, advancing forward 100m, stopping, turning 180˚, then returning to the point of origin.
The demonstration may seem modest – the thrusters were left deactivated, limiting the aircraft’s forward speed to well below a projected maximum cruise of 120kt (222km/h) – but it was a key step in keeping to Urban Aeronautics founder Rafi Yoeli’s ambitious development plan. Though it has been in development for 12 years, the AirMule is edging closer to a make-or-break period. “We want to be there with a vehicle in five years in service,” Yoeli says.
In fact, Urban Aeronautics has already launched two subsidiaries that will sell the aircraft to different markets. Tactical Robotics is the name of the company that will sell the aircraft to military customers, perhaps with the assistance of industrial partners in the USA and Europe. Meanwhile, Metro Skyways is a new subsidiary charged with developing a market for manned air taxis and ambulances.
Its main point of leverage against the proven conventional helicopter is implied in the “internal” description of the rotor system. The unshrouded, spinning blades of a helicopter limit its access and manoeuvrability in some urban and even rural areas. The AirMule’s shrouded, six-bladed rotors – upgraded last June from four blades each – may offer a safer and more useful alternative.
“We’re not worried about this vehicle eventually flying into cluttered terrain,” Yoeli says. “There is no overhead rotor or tail rotor, so we can fly into any kind of place.”
Assuming field tests prove the technology works, the question arises how such a vehicle would compete with a light helicopter, the decades-long champion in the market.
The internal rotor concept has been attempted before. Piasecki tested a roadable vehicle with internal lift rotors in the early 1960s. The US Army-funded Airgeep and twin-engined Airgeep II, however, were limited to 30kt speed, Yoeli says.
By adding the twin-thrusters, the AirMule overcomes this speed limitation, along with achieving “some lift” from the shaping of the roof of the vehicle, Yoeli says.
Wind tunnel results several years ago validated the company’s assumptions about the AirMule, he adds. “People understood this works. They said, ‘You brought the Airgeep’s speed up from 30kt to 120kt you brought the endurance up from 20min to 4h and you fly in windy conditions.'”
But the Urban Aeroanutics concept faces a more fundamental test. Disc loading is a standard measure for vertical lift efficiency. It divides the weight of the aircraft by the area of the rotor disc. A conventional helicopter has an unshrouded rotor disc, so the area can usually be larger than the ducted fans of the AirMule.
Yoeli is aware of the issue. His standard presentation includes a chart showing that even the most inefficient conventional helicopters are nearly twice as efficient as the AirMule at generating vertical lift.
That means “we need to put a 50% more powerful engine on” the AirMule compared with the same size of helicopter, Yoeli concedes. The AirMule, thus, consumes 50% more fuel than a helicopter for the same mission. “I didn’t like this chart,” Yoeli adds.
But Yoeli still believes that the AirMule can compete with conventional helicopters, even beyond the niche market for gaining access to landing zones with cluttered terrain.
To make his case, he inverts the standard measure for vertical lift efficiency. He calculates the “disc area I have to lift one pound versus what horsepower I’ve provided to lift that same pound, so we now we have a common denominator, which is a pound I want to lift. Now it’s a direct competition between power and disc area.”
He then produces a chart with a y-axis showing power per pound and an x-axis for disc area per pound. Not surprisingly, conventional helicopters are stretched along the far end of the x-axis and very low on the y-axis, meaning they produce very little power per pound with a very large disc rotor. On the top of the y-axis are lift-fan-powered fighters, such as the short take-off and vertical landing Lockheed Martin F-35B. It has a very small disc area, but produces tremendous amount of power.
The chart, says Yoeli, reveals the lack of flexibility at both extremes of the y-axis and the x-axis: “The F-35 is in a very, very difficult spot because if it misses its weight by just a little bit the power goes up through the roof. This is why this is such an expensive aircraft, because every extra pound is another 500hp for the engine!”
The AirMule, on the other hand, is nestled in the curved slope directly between the extremes on the x-axis and the y-axis.
“The slope of the curve here is such that if I wanted to increase the payload by 10%, I could do it by increasing the power by 10% – not 500%,” he says. In this sense, the AirMule trades optimal fuel efficiency for optimal flexibility in matching power to the lift requirement.
Transitioning the AirMule from development into operational service is still the largest challenge. Using the Tactical Robotics subsidiary, Urban Aeronautics hopes to sell the unmanned AirMule first to the Israeli Defense Forces, and then perhaps to the US Marine Corps, which has demonstrated autonomous cargo delivery by unmanned helicopters in Afghanistan. Urban Aeronautics is currently looking to team up with an aerospace prime contractor to usher the AirMule through the certification phase, Yoeli says.
The next step is to launched the manned X-Hawk for the commercial market, through the Metro Skyways subsidiary.
“There’s always more research to do but it’s time to get these vehicles out,” he says. “We’re always challenged with funding. Funding is always challenging. Can we sustain this? We’ll try. We’ve come so far, right? I’m optimistic.”