Hover or forward speed—helicopter designers have long been plagued by this dilemma. Practically every aerodynamic feature that helps hover hurts forward flight, and vice versa. So designers have toyed with various outlandish ideas to improve the top speed of helicopters.

The sonorous whirr of a helicopter’s rotor unmistakeably heralds the approach of one of the most versatile machines in the sky. The whirly birds’ ability to take off and land vertically frees them of the constraint of elaborate infrastructure support that other aircraft need. Consequently they have proved their worth in a variety of roles in every type of terrain. Over the years, their performance and reliability has improved tremendously, except in one crucial aspect—speed. Indeed, throttling up these tortoises of the air presents substantial cost and engineering challenges. While a cruising speed of 150 knots (277.8 kmph) has long been accepted as a practical limit, 200 knots (370.4 kmph) was considered an insurmountable barrier and rare the attempts to breach it. The last major effort to design nippier helicopters happened in the 1970s, but they quickly fizzled out. The absolute speed record for rotorcraft was set by a specially modified Westland Lynx ZB-500 in 1986. Piloted by John Egginton, the aircraft reached a speed of 216.45 knots (400.87 kmph). This record was recently surpassed by two similar sounding but radically different designs—Sikorsky’s X2 and Eurocopter’s X3. And both manufacturers describe their creations as “potential game changers.”

Tilt-rotors, of course, go a long way towards solving the forward speed challenge. Take the Bell-Boeing V-22 Osprey, operated by the US Marine Corps and Air Force. It is a hybrid—half helicopter and half aeroplane. For take-off and landing, it functions as a helicopter with the twin rotors horizontal. Then, by tilting its wingtip rotors forward through 90 degrees and flying like a plane, the Osprey can cruise at around 250 knots (463 kmph) and fly farther, un-refuelled, than pure helicopters. But it is subject to hover limitations. It can’t hover as most other helicopters and especially above 4,000 ft. In a theatre like Afghanistan, this can be a severe drawback. Then there’s the AgustaWestland AW609 (formerly known as the Bell/Agusta BA609). It is a civil twin-engine tilt-rotor aircraft with a configuration similar to the V-22 Osprey.

Hover or forward speed—helicopter designers have long been plagued by this dilemma. Practically every aerodynamic feature that helps hover hurts forward flight, and vice versa. So designers have toyed with various outlandish ideas to improve the top speed of helicopters. Some even considered using jet engines to spur the machines at a faster clip, using small wings to provide additional lift. But they faced the same problem—the main rotors would develop asymmetric lift and become unstable on account of the much feared “retreating blade stall”. How does retreating blade stall happen? A rotor provides vertical lift as well as forward thrust. In order to fly faster, more thrust is generated by increasing the tilt of the rotor disk. Although this can only be done to a limited extent, it does not present any problem so long as the helicopter is moving forward at low-to-medium speed. But if it continues accelerating to higher speeds, the rotor blade travelling backward, cannot generate sufficient lift because its speed is low compared to the oncoming faster airflow. It stalls. High drag and twisting moments on the stalled blade create vibrations strong enough to be felt on the controls. Further acceleration can result in uncommanded pitch and roll motion. To make matters worse, the tip of the advancing rotor blade may approach supersonic speeds, creating shock waves and more vibration.

The US helicopter major Sikorsky Aircraft claims to have licked the phenomenon of retreating blade stall by the advancing blade concept employed on its X2 technology demonstrator. The X2 has a pair of coaxial main rotors. So there are always advancing blades on either side of the helicopter. As the helicopter’s forward speed increases, the rotational speed of the main rotors is deliberately reduced, but the dual advancing blades generate sufficient lift to keep the helicopter flying. The lower rotor speed also keeps the tips of the blades subsonic. A bonus of the coaxial rotor configuration is that torque effects are eliminated. So this machine does not need a power-hungry tail rotor. The X2 also has a pusher propeller that gives it the capabilities not found in conventional helicopters. For instance, from a level attitude, the pilot can engage the propeller and accelerate or decelerate without changing the angle of the main rotors.

After a series of gradually accelerating test flights, last September, the X2 crossed 250 knots (463 kmph) true air speed in level flight, setting an unofficial record for a helicopter and achieving the programme’s ultimate speed milestone. That made it the fastest helicopter ever without external jet thrust. The X2 design also won Sikorsky one of aviation’s most prestigious prizes—the Collier trophy. Sikorsky spent $50 million ( Rs. 225 crore) to develop this technology demonstrator which is likely to retire soon. But the story doesn’t end there. The company also has plans for the military-focused S-97 Raider using X2 technology. The S-97, an armed reconnaissance type, will be about 30 per cent larger than the X2, with a two-pilot cockpit and space for armament and auxiliary fuel or troops. However, before this cuttingedge technology is applied to a production helicopter, rotor blade design will be critical, since at high speed there will still be a large increase in drag. The more complex transmission and drive systems will require redundancies to limit the possibility of failures. Active vibration suppression will be needed to minimise noise, prolong component life, and ensure crew comfort. Also, fly-by-wire will be essential to seamlessly transfer engine power from the main rotors to the forward propulsion system and back again, while constantly maintaining desirable handling qualities. The first flight of the S-97 Raider is expected in 2014 and the US military is likely to be the launch customer.