New helicopter missions in both the commercial and military environment demand newer, faster helicopters. The major problem with this necessity is the absolute upper limit of rotary-wing airspeed. Several designers, including Bell, Boeing, Piasecki, and Sikorsky have brought various ideas for designs to break the helicopter speed barrier.
Since the first powered flight in 1903, inventors and innovators have refined and redesigned existing aircraft in order to make them fly higher, farther, and faster than before. That race is still continuing in the world of fixed-wing aircraft. Since the development of the helicopter in 1913, the focus in rotary-wing development has been stability and stationary power—being able to hover higher and longer or carry more weight. Little thought has been put into rotary speed proportional to power until the last twenty years or so when newer composite materials have been developed to withstand the strains put on them by increased aerodynamic loading. Augusta-Westland has been at the forefront of helicopter speed development since 1986 when an AW Lynx set the world speed record at 249.1 mph. (“Maximum Forward Speed”) As helicopter missions have become more varied and applicable, new designs have been needed to accommodate the growing segment of the industry (Hambling, 2008).
The main problem affecting helicopter maximum forward speed, or VNE, is a phenomenon unique to rotary-wing aircraft known as dyssymmetry of lift. Dyssymmetry of lift describes the condition wherein the advancing and retreating blades, due to their movement compared to the relative wind, have unequal airspeeds and generate unequal amounts of lift. To compensate for this, helicopter rotor systems are articulated in such as way as to allow the blade to “flap” up or down, changing the angle of attack and equalising the lift forces on both sides of the aircraft. Unfortunately, the faster a helicopter travels forward, the more lift must be compensated for (lift increases exponentially as airspeed increases) and the more the retreating blade must increase its angle of attack. In a traditionally-articulated helicopter, the absolute upper airspeed for any helicopter is approximately 250 mph. At this point, even the most advanced rotor systems and composite materials cannot prevent exceeding the critical angle of attack on the retreating side or creating shockwave-induced flow separation on the advancing side, and will, invariably, stall.
The first major foray into attempting to create a high-speed aircraft that maintained the ability to hover was the Bell-Boeing V-22 Osprey, a hybrid tilt-rotor aircraft that could takeoff, land, and hover like an helicopter while maintaining the high-speed and long-range cruise capacity of a turbo-prop airplane. This development was predicated by the U.S. Military’s need for a multi-role transport aircraft that could deploy troops and equipment to remote locations without a traditional runway and do it faster than a traditional helicopter. This project has seen numerous fall-backs since inception, including technical flaws (instability being chief among them) and fatal crashes as result of those flaws.
Seeing the development of the Osprey wrought with problems (and due to the military’s need for a reliable escort for the “Thundering Chicken”), Piasecki Aircraft has begun work on a modification of a Sikorsky YSH-60F Seahawk. Piasecki Aircraft has been a true innovator in the industry, developing the CH-47 Chinook and the CH-46 Sea Knight—reciprocating rotor systems that create twice the lifting power while counteracting torque with two sets of counter-rotating blades. The modified YSH-60F, dubbed “Speedhawk,” takes the traditional tail rotor system and replaces it with a movable “pusher propeller” to generate more forward speed without having to tilt the main rotor disc while still providing anti-torque capabilities at low speeds. This design has been looked at being adapted for the civil market as a faster transport helicopter for offshore oil rig crews, and is the closest to actual implementation as Piasecki has already developed a working prototype that fits all current military specifications.
Another traditional innovator in helicopter design, Sikorsky, is working on designs to also smash the 250 mph speed barrier imposed by traditional design limitations. The Sikorsky X2 helicopter design utilises another unusual design element: a coaxial rotor system. This coaxial system takes the essential anti-torque tandem rotor system used in the Chinook, and stacks the two systems on top of each other, as in the Russian Kamov series attack helicopters. This coaxial design has proved quite successful in Russian military operations, creating helicopters that push the design limitations of speed without sacrificing stability as in the Lynx. Sikorsky plans to pair this coaxial system with a pusher propeller in order to achieve planned speeds in excess of 280 mph.
As rotary-wing design thinking moves more and more “out of the box,” innovative designs will begin take hold and the basic shape of the helicopter, as we know it today, will seem as distant as that of the autogyro. New commuter missions will demand that helicopters be able to get people between city centers faster than terrestrial travel and directly to locations not necessarily adjacent to airports. Air ambulance services will demand faster transport for patients. The oil industry is seeking to adapt the Piasecki designs once they’re approved for production as a faster, more reliable “rig runner”. European transportation markets already make extensive use of helicopters for intercity travel, especially in mountainous or especially remote areas where traditional runways cannot effectively be built. The biggest market for these new “super-speed” helicopters will still surely be the military, where demand for effective, versatile attack aircraft has only increased as warfare has become more surgical and precise, but, like any technology that has developed from government investments, the private sector will find extremely clever uses for the hardware—given the chance.
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