Designers of the Boeing 747 gear discovered early that making the inboard main gear wheels steerable helps relieve the strain imposed on the gear system. That steerable system was not installed on the first 747, but was added soon after the inboard main tires experienced high wear from scrubbing laterally across taxiways and runways as the aircraft turned. The newer 777's gear has a similar feature: The aft two wheels of each main gear are steerable. On airliners with two or four wheels on two mains, only the nose wheels are steerable since side forces are less of a problem. Wheels and tires also get pretty hot, primarily as a result of braking. Most modern aircraft brakes are enclosed within the wheels of the main landing gear and comprise an alternating stack of smooth metal rotors and stators, which are bonded to friction-producing pads made from metal or carbon compounds. When the pilot pushes on the tops of the rudder pedals to activate the brakes, the whole stack of plates is squeezed together by hydraulic cylinders arranged around the wheel. All that friction, especially at high landing speeds, produces tremendous heat that radiates directly into the tires, which are inflated with nitrogen or air to a pressure of several hundred pounds per square inch. The combination of pressure and heat is sometimes explosive: The air inside the tire gets hot enough to blow apart either the tire or, sometimes, the entire wheel assembly. Even though it dissipates pretty quickly, a quick burst of heat is also produced by the violent contact of tire and runway. "If they have a shuttle landing at night, they'll use an infrared camera," Daugherty says. "When you look at the tires in infrared, they just turn on like lights."
In aircraft that spend a lot of time at extreme speeds and altitudes, aerodynamic friction on the skin creates another type of problem involving heat. SR-71 Blackbird tires have always sported a flashy silver coating to reflect the heat radiated by the hot skin just inches away. "Almost everything on that airplane was designed with heat in mind," Tom Alison says. "People ask me how fast would it go, or what the limiting airspeed was. There wasn't a limiting airspeed, but there was a limiting temperature, and your speed was determined by how hot it got."
Alison says that each of the Blackbird's tires were pressurized with 400 pounds of nitrogen, and at touchdown speeds of around 180 mph, it wasn't uncommon for a pilot to blow a tire when braking because of the tremendous buildup of heat. "You could hear the pop clear up in the cockpit," he says. Pilots and ground crews were cautious around the tires until they had cooled off a bit. Immediately after every shutdown, SR-71 crew chiefs set up large fans to blow cool air over the hot tires and brakes. "You could see the wheels smoking, and that was when you had been doing everything right," Alison says.
The individual components are important, but a landing gear design can succeed only if the wheels, tires, struts, and brakes work in harmony. The wrong combination can spell disaster, which is exactly what happened during tests of experimental electrical brakes on the Republic A-10 Thunderbolt. When the new brakes were installed on a strut, the action of the brakes induced a "gear walk," a rapid fore-and-aft movement that snapped the strut completely off the test airplane. Today, A-10s employ conventional hydraulic brakes.
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.