The world’s major helicopter manufacturers have been using essentially the same principles of helicopter aerodynamics, control systems and power plants for four decades or more. It is time to develop new solutions based on improving the aerodynamic and environmental characteristics of these useful machines.

Helicopters are remarkably versatile flying machines, employable for a wide variety of tasks. But speed has never been one of their strong points. Pit the Westland Lynx—which holds the official world speed record for helicopters—against China’s Shanghai Maglev train, in a race starting at Shanghai International Airport, and the train would reach its destination (30 km away on the outskirts of Shanghai) well ahead of the Lynx. While the Maglev normally operates at 431 km/h and can even hit 501 km/h, the Lynx has never managed to exceed 216.45 knots (401 km/h).

But why sneer at the Lynx? Since 1942, when Sikorsky’s R-4 became the world’s first mass produced helicopter, the whirly birds have always operated at an unhurried pace and stoutly resisted all attempts to make them speed up. Blame it on a hazardous flight phenomenon called retreating blade stall, where the rotor blade rotating away from the direction of flight stalls. The condition worsens as forward speed increases. So machines capable of vertical flight have to compromise between hover performance and speed. The highly prized attributes of good hover efficiency and hover endurance, low speed controllability and low downwash means that forward speed is severely limited. As speed increases, there is also a dramatic rise in drag caused by the rotor and rotor head. No wonder the cruising speed of most helicopters is limited to a woeful 150 knots (278 km/h).

That could be about to change. Speed is increasingly becoming a key desirable for helicopters, and the world’s leading manufacturers and researchers are gearing up to spur these sluggish machines to go faster. Obviously, further the operational distance, more attractive becomes the speed. Just as obvious is that safety and economics cannot be compromised in the pursuit of speed. The helicopter industry wants to be assured of safe and trouble free operations and low seat mile costs while travelling considerably faster. And optimism is now growing that vertical lift aircraft—military as well as civilian—could some day be freed from their current speed shackles.

The Speedster

Sikorsky has been at the cutting edge of helicopter technology for decades. Its S-69/XH-59A Advancing Blade Concept Demonstrator proved that high speed was feasible with coaxial rotors and auxiliary propulsion. At high speeds, the retreating blades were offloaded, since most of the load was supported by the advancing blades of both rotors. The penalty due to stall of the retreating blade was thus reduced and two advantages accrued. There was no need to fit a wing for high speeds or improved manoeuvrability. It also became possible to eliminate the anti-torque tail rotor. However, the S-69 was subject to high vibration levels, which ultimately led to it being scrapped. Another Sikorsky machine, the Cypher UAV, has enhanced knowledge of the unique aspects of flight control laws for a fly-by-wire aircraft with coaxial rotors. Finally, through the RAH-66 Comanche, the company gained expertise in composite rotors and advanced transmission design. Now Sikorsky is committing its competence to its X2 Demonstrator—an experimental compound coaxial helicopter.

The X2 first flew on August 27, 2008. It is expected to reach 150 knots (278 km/h) soon and 250 knots by the end of the year. It has closely separated counter-rotating coaxial rotors with very rigid blades, active vibration damping and fly-by-wire controls. The cuffs and fairings on the top fuselage are streamlined and the rotor mast is enclosed to cut drag by 50 per cent. The counter-rotating coaxial rotors mean that the traditional tail rotor is no longer required and has been replaced by a high-speed propeller. This propeller, which works much like those found on traditional aeroplanes, pushes the helicopter from behind. And unlike single-rotor helicopters that tip nose-down to accelerate, this remains level even while speeding up.

Though it aims to achieve record speeds, it will still retain excellent low-speed handling ability, efficient hovering and autorotation safety. Sikorsky is working towards three key goals: low vibration, low pilot workload (meaning many processes will be automated, making it easier to fly), and low noise. But getting the helicopter to production won’t be speedy. Because of the numerous certifications required, it could take up to 10 years before these fast machines become common.