<http://schema.org/articleBody>
" Click here to post this video on your blog or website. Click here to post a photo of this winner on your blog or website. Helicopter performance depends on the length of the rotor blades. For heavy lifting, a large rotor works best, but short blades reduce drag and ultimately allow for higher maximum speeds. Farhan Gandhi, a Penn State University professor of aerospace engineering, has devised an elegant, simple way to achieve both configurations in the same aircraft, using the same rotor. A rotor blade that changes length has been a long-contemplated, never-achieved goal. &amp;quot;You en-counter huge centrifugal forces, requiring large actuators to pull the rotors in,&amp;quot; says Leo Dadone, a Boeing senior technical fellow in rotorcraft tech-nology. &amp;quot;They tend to jam because of the extreme friction created.&amp;quot; Gandhi sidestepped that problem. In his design, a finely calibrated spring controls the blade&amp;#039;s length. When the rotor spins fast, centrifugal force stretches the spring, and the blade tips expand, increasing overall length. When the rpms drop, the spring draws the tips in. &amp;quot;Instead of trying to muscle it with motors,&amp;quot; Gandhi says, &amp;quot;we&amp;#039;re exploiting the forces of nature and getting the work done for free.&amp;quot; In the future, a tilt-rotor aircraft, like the V-22 Osprey, could increase rotor diameter for hovering and decrease it for flying in airplane mode. Shorter blades are stiffer and less susceptible to flutter at low rpms, and will allow helicopters to take off and land on tighter pads--on ships, on rooftops--before they expand for carrying heavy payloads. They could better maneuver through urban corridors. The design hasn&amp;#039;t been used in flight, but in hover-stand tests with small rotors, Gandhi and his team have shown a 25 percent length increase. His goal is a rotor blade that expands by 40 percent, from, say, 22 ft. to 30 ft. If the rotor can transition from the lab to the skies, Dadone says, &amp;quot;It will be a major evolution of the helicopter.&amp;quot;  The most direct application of variable-length rotors may be in dual-speed tilt-rotor aircraft, such as the V-22 Osprey. At takeoff, the Osprey&amp;#039;s 38-ft.-dia. rotors aim upward, spinning at 412 rpm to lift the aircraft. In midair, the Osprey&amp;#039;s rotors pivot 90 degrees to become propellers, as wing lift balances the aircraft&amp;#039;s weight. The rotors turn at 333 rpm.In the future, the 316-mph Osprey could fly even faster with spring-loaded prop blades that shorten to 12.5 ft.  It may be counterintuitive, but higher helicopter speeds require fewer rpms, so conventional rotorcraft--such as this Black Hawk--could also benefit. The rotor would be 54 ft. at 258 rpm (maximum engine power). But the blades could be shortened to 40 ft., allowing the helicopter to fly through urban areas or land in tight quarters. "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.popularmechanics.com/flight/a2150/4224761/>