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 3 of 4)
The 777's designers also faced the perennial problem of where to put the gear when it retracts into the wheel wells. "You're always strapped for space, because they want to put freight, electrical bays, air conditioning packs, and goodness knows what else down there," says John Davies, a Boeing landing gear designer. "I can remember times when [airframe designers] have configured an aircraft, figured out where they want a gear, but not where to stow it.
Sometimes you start working on that process later than you might want. But I think that's turning around with the advent of computer-aided design systems. More people realize the importance--the airplane spends a lot of time on its gear." Today, gear designers are more involved from the beginning of the design process, Davies says.
The actual manufacture of landing gear is sub-contracted to a handful of companies that specialize in building struts, such as Menasco and BFGoodrich Aerospace. Sometimes, gear engineers from contractors are actually detailed to a particular aircraft manufacturer so they can design the gear in-house. Once the design is finalized, the contractor takes over manufacture of the components, says Louis Hrusch, chief engineer for BFGoodrich's landing gear division. Gear legs are made by forging, which offers good strength-to-weight ratios and involves taking a rough cast of the part and essentially compressing the material, usually steel alloy or titanium, into shape. "Forging gives you better properties all the way around, in terms of fatigue and wear. You start with a cast ingot and heat it to a semi-solid and you pound it in that state," Hrusch says. In terms of materials, steel is still the best material for building landing gear since it is stronger than titanium, although titanium wears better and is much less prone to corrosion, he says. After the strut is forged, wheels and brakes are then attached after being designed and built by subcontractors to exacting specifications provided by the aircraft manufacturer. Designing and building brakes and wheels demands "pretty unique expertise," says Davies. That's because in any gear system, the wheels, tires, and brakes take the brunt of the effects of high speeds and the violence associated with even routine taxiing, takeoffs, and landings.
The appearance of a telltale puff of blue smoke that marks the contact of the tire on the runway is not necessarily when most of the wear on aircraft tires occurs, despite the streaks of rubber deposited on those blackened touchdown zones. Tire wear is a complex problem that depends far more heavily on whether the tire is aligned with the direction of the airplane's motion. "If there is a crosswind, you're never really rolling straight down the runway even if your body is going straight," Daugherty says. "You're actually cocked into the wind, and that really tears up tires." An airplane's tire under a crosswind literally gets bent out of shape. The part of the tire that isn't in contact with the runway or taxiway is constantly being pulled sideways as the tire distorts under the load. The combination of crosswinds on rollout and the steering forces exerted during taxiing causes tremendous wear, accounting for 90 to 95 percent of the total.
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.