Sometimes the hardest design challenge isn't getting aircraft into the air but getting them back on the ground.
- By John Sotham
- Air & Space magazine, March 1998
(Page 4 of 4)
The need to eliminate adverse movement and vibration in a landing gear system becomes increasingly important as aircraft age. "After 10 or 20 years, the various joints and pieces get a few thousandths of [an inch of] wear here and there, and pretty soon you've got a system where things are shimmying," Daugherty says. "And in fact, there are gear snapping off out there in the commercial world too."
In response, ALDF engineers are developing devices that can monitor the health of a landing gear system and detect minute vibrations and movements that could cause significant problems later on. The data can be stored and downloaded later at certain intervals in an airplane's service life to help predict patterns of potential failure.
Sometimes gear problems appear before the airplane is even built, as is the case with the High Speed Civil Transport, a supersonic aircraft envisioned to cut transoceanic travel times dramatically in the 21st century. The airplane's unique size and shape make it a prime candidate for problems, even when taxiing. The HSCT features a long, slender forward fuselage, with a nose gear located farther back than it is on current airliners. That arrangement can create a situation whereby the up-and-down motion of the nosewheel, as it rolls over even the smallest irregularities in the pavement, can translate into exaggerated pendulum-like movements by the time they reach the forward fuselage and cockpit.
This nasty phenomenon was first experienced by the XB-70, a Mach 3 bomber test flown in the late 1960s. "The XB-70's cabin was 65 feet out in front [of the gear], so it was a big, limber nose sticking out there," says former North American test pilot Al White. The taxiways at Edwards Air Force Base in California, with seams about 20 feet apart, played the airframe as if it were a guitar string. The frequency of the potential vibration present in the forward fuselage corresponded exactly to the spacing of the seams at certain taxi speeds. "It was about two cycles per second and it worked out that at about 20 miles per hour, every time you hit one of them it was amplified. If you sped up or slowed down a little bit, it stopped right away."
The answer for the HSCT may lie in building a smarter gear. Researchers at NASA's Langley Research Center are working on an active control system that could dampen the motion by quickly adjusting the amount of hydraulic fluid in a strut in response to vibrations caused by taxiways and runways. The system will employ sensors placed in the nose of an aircraft that can detect irregularities in the pavement ahead and then send that information to a control system that will direct the strut to respond accordingly to dampen the shock. Tests of the system are being conducted using an old A-6 Intruder strut attached to a hydraulic shaker table, and results have been promising. The new technology could also be applied to other aircraft, such as tactical airlifters, which sometimes operate from unconventional surfaces.
In the small world of landing gear design, engineers are working on new materials, active control struts, and computer monitoring systems. No matter what future aircraft look like, those unsung legs with all the wheels hanging off them will be there, tucked into wings or fuselages, doing their job with heat, smoke, and noise but little fanfare.