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...

from The Economist: Science and technology http://ift.tt/1G9aQK6