Image: U.S. Air Force
Flutter kills. When a vibration, usually in an aircraft’s wing or tail, matches the natural frequency of that structure, the results of that “flutter” can be catastrophic. If the vibration isn’t dampened over time, it can grow, causing the structure to flex uncontrollably and potentially fail.
The potential for flutter problems gets worse as engineers try to design planes to fly higher and longer with even more slender wings than are seen on some of the latest unmanned aircraft. So no wonder the Air Force, NASA and Lockheed Martin are teaming up for new ways to fight flutter with a new experimental drone. Meet the X-56A.
Flutter issues have led to the destruction of many airplanes, especially in the early days before it was fully understood. (Check out this NASA video where the horizontal part of a normally rigid aluminum tail flexes and bends as if it were made of rubber during flutter testing performed by Apollo 13 astronaut Fred Haise.) But even some of the most modern aircraft are not immune. A Lockheed F-117 stealth fighter crashed in 1997 after a loose elevation started the vibration that grew into flutter, leading to total wing failure. Even the newest Boeing 747-8 had a flutter issue after computer models showed the potential for the phenomenon during certain fuel-loading scenarios.
So the Pentagon is turning to the same place it always does when it needs to push the limits of new aircraft design: the Mojave desert. The X-56A is the latest ‘X’ plane unveiled by the United States Air Force and NASA. It follows the innovative path blazed by all the research-focused aircraft at Edwards Air Force Base that came before, but without a pilot.
The X-56A, developed with Lockheed Martin’s famed Skunk Works, is a modular drone designed to test an unusual method of alleviating flutter in lightweight aircraft. Rather than relying upon structural strength to keep the wing in one piece, it uses computing power.
While any kind of aircraft can be affected (or even bridges), the long and slender, high aspect ratio wings on airplanes such as the Predator and Global Hawk are particularly susceptible to flutter. The efficient wing design allows the airplanes to fly for long periods of time at high altitude, but engineers are limited by the fine balance between weight and strength. An even more slender wing might be better, and could potentially allow longer and higher flights. But as of now such wings aren’t possible because more strength (and weight) is usually what is used to combat the potential for flutter.
NASA and the Air Force hope to develop designs for future high-altitude aircraft that minimize the risk of flutter while maintaining the absolutely minimal weight of the slender, efficient wings needed to stay aloft for hours or even days at a time. And they want to do this by relying on a computer to control the flutter by moving the control surfaces to counter the vibrations before they increase to a destructive amplitude.
With a wingspan of just 28 feet, the X-56A is a small-scale version of current high altitude unmanned aircraft. Engineers operating the flying test bed will attempt to intentionally induce flutter in the wings and to see if the fly-by-wire flight control computer can eliminate any problems that arise. The ability of a flight computer to control small changes as an airplane flies through turbulent skies is not new — it’s even used in the Boeing 787 Dreamliner. But the Air Force and NASA want to take the test to the “edge of the envelope” and perhaps beyond.
The X-56A research project includes four separate sets of wings. One is relatively stiff to create a baseline for the tests, according to Aviation Week.
The remaining three sets are flexible wings that make it easier to induce flutter and test the fly-by-wire computer’s ability to reduce or eliminate the potentially destructive phenomenon. The airplane is equipped with a parachute in case the wing is destroyed in flight.
When flutter does occur in the X-56A’s slender wings, the on-board flight computer will manipulate the flight control surfaces on the wings in an attempt to reduce it. Although some fly-by-wire aircraft have been able to reduce the oscillations, the typical solution is to simply increase the strength (and weight) of the structure.
If successful, new designs could lead to ultra lightweight structures and extremely efficient wings for future high altitude drones expanding the capabilities of the eyes in the sky.
The research could also make its way into the civilian world. Because strength, weight and efficiency are interlinked, many futuristic designs promoted by NASA and others rarely go beyond the design concept stage because there is no way to safely use lightweight structures that lack the strength to handle potentially destructive things like flutter.
Of course, relying on a computer to augment the structure of a wing might not be on some people’s list of ideas you want to try on a jet airliner. But the same could be said of fly-by-wire control systems in general just a few decades ago. Today people regularly fly on airplanes that rely 100 percent on a computer to keep them flying. Perhaps relying on the 1s and 0s to also keep the airplane in one piece isn’t that far of a stretch.
Flight testing of the X-56A is expected to begin at Edwards Air Force Base this summer.
No comments:
Post a Comment