To paraphrase helicopter pioneer Igor Sikorsky: If you’re in trouble, an airplane can fly over and drop flowers, but a helicopter can save your life. It can deftly maneuver through tight spots and alight in remote places. It can float next to a mountain to search for the lost. And the best sound a wounded soldier can hear is that telltale rotor beat, just minutes before being evacuated to a hospital. When roads are impassable, bridges have been destroyed, and the electricity has been knocked out, helicopters can still deliver supplies and rescue people.
What they can’t do is fly fast. The world speed record for a helicopter, claimed by a modified Westland Lynx in 1986, is 400 kilometers per hour. A Curtiss biplane bested that speed in 1923. The Westland Lynx was a good start, but it was more like a race car than a family sedan—impressive but not quite practical for routine missions. Today’s working helicopters tend to top out at around 270 km/h.
At Sikorsky Aircraft Corp., in Stratford, Conn., we decided in the 1970s that we wanted to build a really fast helicopter. The goal was to reach 480 km/h without sacrificing the vehicle’s other strengths. Almost 40 years later, Sikorsky is now close to meeting that goal. In August 2010, a technology demonstrator, known as the X2, reached 435 km/h, unofficially breaking the helicopter speed record. A few hurdles remain before Sikorsky can claim the official record, which is maintained by the Fédération Aéronautique Internationale (FAI), the world’s air sports organization. Having gotten this far, we anticipate that Sikorsky will soon begin producing commercial helicopters using X2 technologies.
To take a crack at the speed record, we had to make some fundamental changes to conventional helicopter design. The reason why becomes clear when you consider the difference between how helicopters and airplanes fly.
When an airplane barrels down a runway, air flows over the wings to produce lift. At a certain speed, the pilot pitches the nose up slightly, increasing the angle of the wing to the air. That creates enough extra lift for the airplane to take off. Once a plane is airborne, its speed is limited only by the amount of thrust its engines can provide.
A helicopter generates lift quite differently. It manipulates the air flowing over its spinning rotor blades, allowing the body of the aircraft to hover. The lift generated by the rotor blades can be angled using the helicopter’s flight controls, allowing it to fly sideways, pivot, or even backward.
Compared with the fixed wings of an airplane, a helicopter’s rotating blades make for a much more complicated design. Each blade must withstand the forces of rotation, which can amount to many times the weight of the aircraft on each blade. A helicopter also needs a powerful engine and a large transmission to reduce the engine’s rotation rate to something appropriate for the large rotors. For example, a U.S. Army UH-60 Black Hawk engine’s output of 20 900 revolutions per minute turns the main rotor only 258 times per minute, a ratio of 81 to 1.
But here’s the catch. When a helicopter flies forward, the rotor blades experience a dramatic variation in airspeed. That’s easy to see if you imagine a miniature version of yourself perched on the tip of a helicopter rotor blade. If the helicopter were hovering, you’d feel a constant 800-km/h wind in your face as the rotor spun around. If the helicopter were to fly forward, you would note that the wind was stronger on what’s called the advancing side, when the rotor was moving in the same direction as the helicopter, but that it would be noticeably weaker when the rotor was on the retreating side. By the time the helicopter reached 150 km/h, you would feel a wind speed of 950 km/h on the advancing side, versus 650 km/h on the retreating side. The relative speed of the wind on the retreating side gets lower and lower the faster the aircraft flies. At 300 km/h, the wind on the advancing side would reach 1100 km/h, while the wind on the opposite side would be 500 km/h.
Eventually, the helicopter would reach a point at which the difference between the lift on the advancing and retreating sides of the rotor could not be balanced and the vehicle wouldn’t be able to maintain level flight. To complicate matters further, portions of the tip of a fast-flying helicopter’s advancing blade can exceed the speed of sound, producing shock waves that cause large vibrations and generate considerable noise. For these reasons, most helicopters just don’t like to go fast.
In the 1950s, aircraft designers began to look at other configurations to achieve vertical takeoff and landing and reach forward speeds greater than 450 km/h. One approach was to design an aircraft whose thrust could be tilted vertically for takeoff and landing and horizontally for forward flight, during which time the vehicle would produce its lift from fixed wings. Some, such as the Bell X-22, used tilting ducted fans, while others used tilting propellers—for example, the Vought-Hiller-Ryan XC-142A and Canadair CL-84. Some had tilting jet engines, like the EWR VJ 101C. All handled poorly when hovering and produced downward air velocities high enough to blow a house down and uproot trees.
A more successful variation on this theme is the tilt-rotor, which uses helicopter-like rotor blades instead of ducted fans or propellers, with the axis of rotation switching from vertical to horizontal after takeoff. But this dual use is awkward: The rotors are too small for the aircraft to hover efficiently and larger than optimal for forward flight. The complicated tilting mechanism and excessive amount of power it requires also make the tilt-rotor option costly and complex.
Another approach to high-speed vertical takeoff used specially designed lift engines that worked only during takeoff and landing. These heavy engines produced large thrust for vertical takeoff and landing; in forward flight they were shut down and covered by doors. One such vehicle, the Dassault Mirage IIIV, was able to reach Mach 2, twice the speed of sound. Unfortunately, if you were on your roof waiting to be rescued, such an aircraft might not only fail to retrieve you, it could set your house on fire.
Aircraft engineers have also tried out jet engines with adjustable exhaust nozzles. The Hawker Siddeley Kestrel, which first flew in 1960, could aim its thrust in different directions, making the aircraft quite agile in forward flight and able to take off and land vertically. But the exhaust was again too hot and fast for the helicopter to be suitable for rescue purposes.
What we at Sikorsky settled on almost four decades ago was a design we called the Advancing Blade Concept. It uses two counterrotating rigid rotors that spin around the same axis, which is why they are known as coaxial rotors. In forward flight, each rotor produces a surfeit of lift on its advancing side, freeing the retreating side from having to do any heavy lifting, all while maintaining good balance. Sikorsky patented the concept in 1964, but considerable engineering was needed to actually get something like this in the air.
The first flight of a demonstrator vehicle using coaxial rotors took place in June 1973. The U.S. Army, which sponsored the flight tests, called it the XH-59A. Sikorsky’s test pilots took the helicopter out on a few low-speed forays to see how it handled. On one of these early low-altitude flights, the helicopter suddenly pitched nose up. The pilot was forced to land the aircraft on its tail, damaging the landing gear in the process. The helicopter rolled about 45 degrees, hitting the tips of the rotor blades on the asphalt. Fortunately, nobody was hurt.
That accident raised some big questions. For one thing, what could have gone wrong at a speed as unimpressive as 46 km/h? Sikorsky engineers launched a yearlong investigation into the incident. What we learned was that the very stiff coaxial rotors produced greater-than-expected nose-up forces when flying forward. The aircraft’s control system also turned out to be inadequate. The designers had worried that the controls would be extremely sensitive, so the pilots weren’t given the usual wide range over which to manipulate the pitch of the rotor blades. In any helicopter, a pilot can maneuver by changing the blades’ angle of attack in a cyclic manner, meaning that the pitch of each blade changes as it rotates around its hub, altering how much lift it generates at different points in the circle. But with less control over how much the pitch could be changed, the pilot was deprived of adequate authority for controlling the helicopter’s nose.
When the second XH-59A aircraft was built, we modified its control system and moved the helicopter’s center of gravity forward. We also built in electronic stabilization mechanisms to dampen the aircraft’s pitch and roll by sensing the motion of the aircraft and feeding the relevant flight parameters—such as airspeed, rotational rates, and attitude—into a computer. If the aircraft rolled slightly to the right, the computer commanded the flight-control servomechanisms to roll it back slightly to the left. The computer’s commands were then mechanically mixed with the pilot’s input.
This new aircraft took off for the first time on 18 September 1975. During the next five years, the XH-59A was tested both as a pure helicopter and with jet engines attached to the sides of the fuselage to provide extra propulsion. It performed deftly at low speeds and managed to reach an impressive 445 km/h in level flight and 487 km/h in a shallow dive. (These speeds weren’t certified by the FAI to claim the record.)
Because it was intended as an inexpensive proof of concept, the XH-59A had features, such as the strapped-on jets, that no production vehicle would likely have. The heavy and fuel-hungry jets pushed the aircraft to high speed—with some significant problems. For example, the helicopter vibrated so much at these higher speeds that its pilots struggled to control it. And when hovering, the vehicle had an annoying tendency to oscillate rather than float smoothly in place.
In May 1980, the XH-59A’s supporting agencies—the U.S. Army, Navy, and NASA—held a conference to discuss the possibility of an “XH-59B.” But in the end, there were no takers, and the XH-59A was retired to the Army Aviation Museum, in Fort Rucker, Ala.
A few years ago, however, Sikorsky decided to review its portfolio of designs to see whether technological advances in avionics and control systems might be able to address the earlier problems. Engineers at Sikorsky were particularly interested in seeing whether the company could rescue its Advancing Blade Concept.
The first thing to tackle was the vibration problem. Sikorsky’s helicopters have been using a system of active vibration control for the last five years. This technique involves a dozen or so vibration sensors distributed around the helicopter, a control computer, and several vibratory-force generators placed at select spots on the airframe. A computer monitors the helicopter’s motions and activates the force generators to cancel out vibrations.
To make the revived helicopter fly more efficiently at high speeds, specialists in aerodynamics turned to computer-aided design tools to craft the airfoils, rotors, and fuselage. As a result, the X2’s blades have specially designed airfoils with a variable width and an unusual twist. The modeling software indicated that this rotor would have significantly less drag and more lift than what was used in the XH-59A.
Then there was the problem of the jet engines. Simply adding jet engines to the helicopter does speed it up, but they are noisy and fuel hungry. So the X2 design team decided to use a variable-pitch pusher propeller at the aft end of the fuselage to provide forward thrust when needed. That propeller is good not only for acceleration but also for rapid deceleration.
Another critical advance was in the controls. The XH-59A had used a mechanical flight control system that was both heavy and extremely complex. It commanded six hydraulic servoactuators to change the pitch of the rotors (three actuators per rotor), operated the rudders, and controlled the two auxiliary jet engines. The X2 control system uses digital computers to perform all these functions, combining the pilot’s inputs with feedback from various flight sensors to ensure that the aircraft executes smooth, well-controlled motions even in turbulent conditions. The system contains three redundant flight-control computers just to be safe. If one of them fails, it instantly shuts down, allowing the aircraft to fly normally on the remaining two computers. The different sensors on which the flight-control system relies are also installed in triplicate. The output commands for the rotors, the control of the servos that cancel out vibrations, and the other complexities of the XH-59A’s mechanical system are easily handled in software, something that would have been nearly impossible to do in the early 1970s.
The engine Sikorsky ended up choosing for the X2, an LHTEC T800 turboshaft, is a modern, state-of-the-art design that might well serve for a production vehicle. The rotors, propeller, and engine are coupled together with gearboxes and shafts. This approach allows the engine’s power to be distributed between the rotors and propeller as needed. While hovering or moving at low speeds, the rotors consume the lion’s share of the power, while at high speeds the pusher propeller gets a boost. Computer simulations indicate that the X2 should be able to reach between 465 and 490 km/h with this engine.
The X2 prototype made its first test hop on 27 August 2008, in Elmira, N.Y. Several more low-speed flight tests also went off without a hitch. To venture to higher speeds, Sikorsky sent the test team to its flight center in West Palm Beach, Fla., which became the X2’s new home. We’re now on a course to push the X2, in stages, to its maximum speed. Most recently, we set our sights on 463 km/h.
On the day of that flight, in August of this year, the test team got started at the crack of dawn. To ensure that the pilots would be flying in smooth air, the crew had to be on site by 5:00 a.m., before the sun had a chance to heat the air enough for the wind to pick up. The crew rolled the aircraft out onto the runway, where a dozen safety officers in bright orange jumpsuits and noise-canceling headsets were on patrol. Two chase vehicles were there to observe the test flight—another helicopter and a fixed-wing turboprop. The latter would be needed to keep up with the X2 as it accelerated to higher speeds. The test team was on high alert as it orchestrated flight activities to keep the three vehicles a safe distance apart.
With everyone’s nerves on edge, the X2 started up its engine at 6:30 a.m., and the helicopter took off. Within a few minutes the X2 had reached a speed of 350 km/h. A dozen people watched from the ground as the airspeed crept up, first to 400, then 410, and finally topping out at 435 km/h—not quite the goal we’d set, but good enough for this round. Cheers and applause broke out on the ground. The pilot slowed the X2, turned it around, and flew back to land on the runway.
Everyone involved was jubilant, but most of all relieved—especially the pilot. To train pilots to fly this brand-new vehicle, Sikorsky built a simulator that lets them preview the controls and the aircraft’s responses. The real X2 has only about 14 flight hours on it, but its engineers and pilots have spent hundreds of hours flying the simulator. In part, that’s because it provides valuable feedback to the designers, who devoted its first few sessions to tuning the feel of the control stick, modifying the displays on the instrument panel, and checking the X2’s performance at low speeds. One of the simulator’s key roles now is in evaluating higher flight speeds and maneuvers that are more aggressive than those the actual X2 has accomplished.
We’ll continue to pursue higher speeds. Assuming all goes well, Sikorsky engineers are planning to adopt some of the technologies used on the X2 for the new helicopters they have on the drawing board. So expect some dramatic shifts in the way helicopters are designed.
The development of those technologies has been unusual in the helicopter industry, especially given the company’s decision to pursue a once-abandoned idea with its own funding. For engineers like us, it’s a thrill to see new ground broken in an industry that’s usually considered mature. And it’s certainly rewarding to see how new technology can benefit a four-decade-old idea for solving the helicopter’s fundamental shortcoming: a need for speed.
About the Author
Thomas Lawrence, a technical fellow at Sikorsky Aircraft Corp., has made a career of peculiar projects. First he helped design an airship lifted by four conjoined helicopters. Next came the XH-59A, a futile effort to break the helicopter speed record. He then focused on the X-Wing, an aircraft that could take off like a helicopter but switched midair to fly like a fixed-wing airplane. None succeeded. Before turning to the very successful X2, Lawrence says, “I wasn’t sure what my career path was.”
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