Tuesday, May 1, 2007
Look Ahead Airframes: Sleeker, Safer, Friendlier
TO SOME IN THE HELICOPTER industry, 40 years can seem like an eternity. In an industry as competitive as this one, it’s easy to focus on matters of day-to-day survival. That’s even more true now, when operators are desperate for aircraft and manufacturers lay awake at night wondering if they will be able to meet the production schedules before them.
So, understandably, we were a bit reluctant to ask the question around which this issue is built: What will airframes, power systems, and avionics look like over the next 40 years? We expected scoffing responses about wasting executives’ and program managers’ time on matters of sheer speculation.
Yet we as an industry have been at this game for a century or so, and designing, building, and flying rotorcraft in earnest for more than 60. As AgustaWestland’s marketing director, Roberto Garavaglia, and others have observed, it can take 10 years for a new helicopter to move from the drawing boards to the field. So 40 years is just a few generations to forecast.
Surprisingly, most we spoke to were game.
Some were willing to weigh the question, but their responses revealed they really were focused on where their products, will be for the next five years or so — the part of the timeline one might label "near-term sales."
Others were consumed by the present day. One top engine executive essentially said his company has more than enough to do delivering and refining current products. Still others wouldn’t touch the question. Eurocopter, for instance, declined to be interviewed. You might sympathize. The technological advances possible in future aircraft lie at the heart of a manufacturer’s competitive standing. Why tip your hand?
Many were willing to discuss the question, and their answers reveal that a great deal of running room remains between the rotorcraft of today and technology’s physical and practical boundaries. There is a great deal of tweaking that can be done to current helicopter designs to make them more efficient and safer, such as getting rid of tail rotors and developing active blade-control systems as primary flight controls.
More importantly, there is much potential for revolutionary advances in the science of vertical flight. Indeed, we seem to have posed the question at a time when some leaders in the industry are growing impatient to pursue those advances.
"There seems to be growing interest in developing a helicopter that can exceed existing performance limits," said Rhett Flater, president of the American Helicopter Society International. The society will hold its annual Forum this month in Virginia Beach, Va., close to the homes of two centers of aerospace research — NASA’s Langley Research Center and the U.S. Army’s Aviation Applied Technology Directorate. The Forum is scheduled to include discussions of how those limits might be exceeded.
A U.S. defense rotorcraft expert aired the frustration that is driving some of that interest. This expert remains anonymous because of bureaucratic restrictions on public comments on such broad an issue. The expert is well qualified to discuss the matter. "Helicopter technology hasn’t advanced a whole lot over the past 30-40 years," the expert said. "We’ve added black boxes to the systems. But when you look at the basic air vehicle performance, it’s pretty much the same today as it was at the end of the Vietnam era."
There is hope that that may change, driven as many rotorcraft developments are by the requirements of the U.S. military. Combat operations in Iraq and Afghanistan, combined with the prospect that they will persist for some time and be coupled with ops elsewhere in the world, are confronting military and industry leaders with a simple fact. The Pentagon needs an aircraft that can get off the ground and land without requiring a runway, but can perform like a fixed-wing airplane in between.
The Bell Helicopter/Boeing V-22 can do that, and this year it is slated to go into use in Iraq. But it’s taken 20 years to get there, and it’s one aircraft type against what is emerging as broad range of operational needs.
"The future war fight is different," said Bruce Tenney, associate director of technology for the Army’s Aviation Applied Technology Directorate. "The battlefield dimensions are just dramatically longer."
While helicopters typically today operate at mission radii of 40 nm or so, he said, tomorrow they will need to cover 160 nm or more on a regular basis.
A number of elements are at work here. The U.S. Marine Corps’ concept of distributed operations is one; it calls for small, highly capable units to maneuver over a large area of operations and gain an advantage over the enemy by deliberately using separated, coordinated, independent tactical actions. A U.S. Navy strategy is to sustain operations ashore from ships at sea, which could call on aircraft to carry heavy loads from vessels 100 nm offshore to points 150 nm inland. The Army’s concept of mounted vertical maneuver calls for mounted, protected forces to move by air across extended distances to strike critical objectives throughout the battle space.
Those extended distances will need to be covered quickly, which will require aircraft free of the 170-kt speed limit of traditional helicopters. Tenney said, "We need dramatically improved range performance, more speed, more payload."
One of Tenney’s duties is to oversee the Joint Heavy Lift program, a multi-service effort to determine viable concepts for the missions outlined above. The Army directorate in 2005 awarded concept design and analysis contracts to Sikorsky Aircraft (to pursue two proposals), Boeing (for one proposal of its own and one with Bell), and Frontier Aircraft. Sikorsky’s proposals are based on its X-2 technology demonstration. Boeing pitched what it called the advanced tandem rotor helicopter and, with Bell, a quad tilt-rotor. Frontier Aircraft proposed a 310-kt "optimum-speed" tilt rotor.
The Joint Heavy Lift proposals underwent final design reviews in March. The directorate is now working toward a technology demonstration. There is no funding for one currently, Tenney said, but he hopes to gain it in budget deliberations later this year.
In thinking of the next generations of aircraft, Tenney said, "you’ve got to change your thought patterns for vertical lift." The question is not what the next helicopters will look like, but what will the next generation of aircraft that take off and landing vertically look like.
The research and design experts with whom we spoke agreed that there is much room between today’s technology and its technical limits. Take lift over drag, for instance. The lift/drag ratio, a benchmark of aerodynamic efficiency, is 15 or more for fixed-wing commercial airliners. It’s 3-4 for helicopters. Raising it to 10 or so would put rotorcraft within range of competing with turboprop fixed-wing transports like the C-130. But how do you get there?
The answers aren’t all new.
To achieve greater speed, range, and payload, the experts cited the promise of a compound-helicopter design, using an auxiliary propulsion system to supplement the thrust of the rotors for greater forward speed. Fixed wings can provide extra lift. Piasecki Aircraft, which has long worked on the concept, is preparing its X-49A SpeedHawk for flight tests this year.
A variable-speed rotor like the one Frontier proposed for its Joint Heavy Lift contender (and the one it used on the A-160 Hummingbird vertical-lift unmanned air vehicle it sold to Boeing) also holds promise. The optimum-speed system that Frontier founder Abe Karem developed is designed to allow the helicopter rotor to be operated at an optimal rpm, minimizing the power required to turn it, thereby improving performance efficiency, reducing noise, and extending rotor, transmission, and engine life. It also improves helicopter endurance.
The A-160 "demonstrated the potential for up to fivefold increases in rotorcraft endurance — from 4 to 20 hr — by slowing the main rotor speed to allow rotor blade sections to operate" at their best L/D ratios, said Daniel Schrage of the Georgia Institute of Technology.
Slowed rotors are another option.
The CarterCopter Gyroplane slows its rotor speed to obtain an advance ratio (a measure of the rotor’s efficiency) of almost one. Schrage said it has the potential to obtain much higher advance ratios.
The Heliplane being developed with funding from the U.S. Defense Advanced Research Projects Agency (Darpa) uses a similar approach. The Salt Lake City-based autogiro maker Groen Brothers Aviation is designing a proof-of-concept, long-range, vertical takeoff and landing aircraft intended to cruise at 350 kt. Darpa’s objective is to achieve performance with a rotary-wing aircraft comparable to that of a fixed-wing one.
The work is part of a multi-year, $40-million, four-phase program. Groen Brothers is working on phase one of that program, a 15-month effort funded at $6.4 million. Darpa is scheduled to do a system requirements review of the team’s proof-of-concept aircraft this month.
That concept combines the "gyroplane," varieties of which Groen Brothers has been working on since the late 1980s, with a fixed-wing business jet. The team is using the A700, in the very-light-jet class, which has been developed by Adam Aircraft Industries.
Tomorrow’s helicopter will have no swashplate, if industry can develop a reliable system for individually flying each main rotor blade. A number of initiatives have explored active blade control, including one with Eurocopter, which last year flew a system on a BK-117 at its Donauworth, Germany research center.
The Smart Hybrid Active Rotor Control System (SHARCS) integrates actively controlled rotor blades to reduce helicopter noise and vibration. Performance tests of the 6.5-ft (2m) rotor were scheduled for earlier this year. Wind tunnel tests in Milan, Italy are to begin in a couple of months. SHARCS is led by the Rotorcraft Research Group at Carleton University in Ottawa, Canada, with funding from the Canadian Natural Sciences and Research Council, AgustaWestland, Manufacturing and Materials Ontario, and Sensor Technology, Ltd.
Like SHARCS, most efforts to date have been aimed many at reducing aircraft noise or improving vibration control. The real payoff would come with a system capable serving as a primary flight control.
That would permit designers to get rid of the swashplate in a rotor system, which is a necessity today that increases complexity and rotor hub drag (and, as a result, operating costs and inefficiency).
Developing a low-drag hub alone would be a significant efficiency gain for tomorrow’s rotorcraft. Experts said the vertical drag of a rotorcraft’s hub is roughly equivalent to the entire drag of a fixed-wing aircraft of a similar gross weight. That greatly limits the performance of helicopters.
Tail rotors are another necessary evil on single main-rotor helicopters. They are critical to controlling torque and directional control, but they also add drag, increase the cost and complexity of maintenance, and generate a lot of noise. Most importantly, tail rotors are a safety weakness. They are a critical flight control that is susceptible to a single-point failure. Such vulnerability isn’t tolerated anywhere else in aviation. "We all need to go off and find ways to get rid of tail rotors," the defense rotorcraft expert said.
That would not be the only single-point failure eliminated from the next generations of rotorcraft, if past practice bears out. In the late 1990s, developers and operators of unmanned air vehicles (UAVs) realized they were losing aircraft to single-point flight-control failures. They revised their designs to make them doubly, and today triply, redundant. As a result, it is difficult to lose a UAV to a flight control failure.
Helicopters would benefit from greater use of damage-tolerance practices in their design. These practices dictate that systems and structures are designed in a way that failures are likely to be found in an inspection before they become severe enough to cause loss of the aircraft.
Tomorrow’s aircraft also are likely to be less expensive to operate. A key reason is that military operators are shifting life-cycle costs to aircraft suppliers through long-term support contracts. That motivates suppliers to maximize aircraft reliability. The United Kingdom has led in this contracting area, but the United States is expanding its use of them.