Flutter has been a known hazard since almost the dawn of flight, and we have learned to live with it -- or, more precisely, to avoid it. Designers do that by building enough stiffness into the structure to ensure its natural frequency is high enough to prevent coupling with aerodynamic forces.
But after almost 110 years of powered flight, flutter remains hard to predict precisely and aircraft must be flight tested to demonstrate they are clear of flutter even at conditions significantly outside the normal flight envelope. Also, the margin built into the structure to prevent potentially catastrophic divergent aeroelastic coupling adds significant weight.
Photo: NASA Dryden
Lockheed Martin Skunk Works has begun flying an experimental aircraft, the X-56A (above), to demonstrate that it can not only accurately predict and sense the onset of wing flutter but also actively suppress the aeroelastic instabilities. The goal is to enable longer, lighter, more flexible wings that can reduce the drag of future transport and unmanned aircraft.
The X-56A, which made its first flight from NASA Dryden at Edwards AFB in California on June 26, is a follow-on to a series of much smaller, more expendable remotely piloted aircraft flown by the Skunk Works from 2005 to 2007 to validate its flutter prediction models and investigate active flutter suppression.
The small 10lb, 10ft-span RPVs flown under the Body Freedom Flutter (BFF) program cost just $20,000 each and were designed to be flown to the limit -- and beyond, as the video above and photos below show. Of the seven built, two survived. But even with the loss of five vehicles, the whole BFF flight-test program was a quarter of the cost of a flutter windtunnel model, noted a National Research Council report released last year.
Photos: Lockheed Martin
The BFF vehicles were too small to have structures representative of full-size aircraft, so the US Air Force Research Laboratory funded Lockheed to build the 28ft-span, 480lb X-56A, which is powered by two JetCat P400 turbojets. Lockheed will test the unmanned aircraft first with a stiff wing then with a flexible glassfiber wing, of which it has built three sets under the $18 million program.
The X-56A is capable of a maximum speed of 150kt, but flutter onset with the flexible wing will be at 110kt -- well within the flight envelope. The goal is to detect the onset of flutter using accelerometers on the structure and send commands to the flight controls that dampen the vibration and allow the aircraft to fly well beyond the onset speed.
If a test goes too far, and a wing fails in flight -- which is considered a strong possibility -- the aircraft is fitted with a fuselage-mounted ballistic recovery parachute that will bring the centerbody, with its expensive avionics and engines, down safely. Lockheed has built two centerbodies.
After a planned 20 flights for AFRL, the X-56A is to be handed over the NASA late this year. Under a five-year program, the agency is developing its own flexible-wing flight control system, advanced sensors to measure wing shape and detect airflow separation, and eventually plans to use these to fly the X-56A with a slender, flexible wing with 50% higher aspect-ratio than on today's commercial airliners.