Chop-chop

HELICOPTERS have clear advantages over fixed-wing aircraft. But they also have a big drawback: they are slow. The fastest struggle to exceed about 320kph (200mph)—less than the cruising speed of a second-world-war-era propeller plane.

At an aircraft-testing facility in Florida, though, a strange-looking helicopter has taken to the skies that may help to close that gap. There are two striking things about the Sikorsky S-97 Raider. The first is its set of two rotors, mounted one on top of the other and turning in opposite directions around a central shaft—much like the radio-controlled helicopters sold in toy shops. The second is the absence of a tail rotor. In its place sits a backwards-facing “pusher” propeller.

Start with the rotors. Such a “coaxial” arrangement has been found in a few helicopters over the years. Kamov, a Russian company, makes one that is often used to lift heavy loads. But the mechanical complexity and the risk of the blades hitting one another has made such machines unpopular. Modern engineering methods, together with the use of stiffer carbon-composite blades that are less likely to flex and collide, promise to reduce such problems.

And twin rotors provide several advantages. They offer inherent stability—which is why children can fly the model choppers equipped with them. The single set of rotors on a standard helicopter produces a rotational force, or torque. Without a tail rotor to counter it, the fuselage would spin in the opposite direction to the rotors. The counter-rotating blades of the Sikorsky machine cancel each other’s torque.

They also allow more speed. Like a fixed wing, a rotor blade provides lift proportional to the amount of air flowing over it. In forward flight an advancing blade moves faster, relative to the surrounding air, than a retreating one. That creates uneven lift that tries to roll the chopper over. To deal with the problem, helicopters can tilt the blades themselves, and thus vary the amount of lift they generate, as they spin—but only up to a point. Since coaxial rotors have an advancing blade on each side of the aircraft, they produce balanced lift at all speeds.

Well, almost all speeds. A helicopter also uses its blades to provide forward thrust. The faster it goes, the faster the tips of its rotors spin. Eventually they will exceed the speed of sound, which causes shock waves that make the machine unflyable. That is where the S-97’s propeller comes in. By providing much of the forward thrust, it allows the rotors to turn at a slower speed, sufficient only to maintain lift. The upshot is that Sikorsky reckons the S-97 will be able to cruise at 440kph.

A second prototype of the S-97 should be in the air later this year. Sikorsky hopes that American’s armed forces will order some. But it may face competition from the V-280 Valor, which is being developed by two other American firms, Bell and Lockheed Martin. This is a smaller version of the V-22 Osprey, which is already used by American forces. Both are “tilt-rotors”, which means they use rotors mounted on the end of a pair of stubby wings that tilt through 90 degrees. This allows the aircraft to take off like a helicopter when the rotors are pointed upwards, but fly like a fixed-wing aircraft when they point forward. It is possible that both technologies will find favour, giving vertical-take-off aircraft a range of new looks.