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
It's the end of the flight, and the seat backs and tray tables have been returned to their full upright position. The main landing gear of the Boeing 777, its struts as tall as two-story buildings, unfolds from the wheel wells and swings down until the downlocks engage, which will prevent the sturdy titanium legs from collapsing during the landing, now only moments away.
The runway, stained with streaks of black rubber from the countless tires that have arrived here before these 12 Goodyears, rushes upward, and 500,000 pounds of aluminum, plastic, steel, fuel, passengers, and baggage slam onto the concrete. The main wheels, each 32 inches in diameter with 50-inch radial tires, accelerate from zero to 140 mph in less than a tenth of a second. The tires bulge and shriek, parts of their tread surfaces heated to 500 degrees Fahrenheit. The telescoping struts are compressed several feet under the airplane's enormous weight, and the guy sleeping in seat 36F stirs but doesn't awaken, oblivious to the drama that has just played out below the cabin floor. Of all the punishment an airplane experiences over its lifetime, landings are in a class by themselves: They are sheer torture on the tires and gear, and airplanes must endure them on every single flight. Engineers face some daunting challenges getting airplanes back down on the ground in one piece, and considering all that a landing gear is subjected to, it holds up admirably--largely because of careful design and testing.
One of the centers for such work is the NASA's Langley Research Center in Hampton, Virginia. Here, a team of engineers tests the performance of gear struts, tires, and brake systems on military, commercial and research aircraft, including the space shuttle. ALDF engineers do much of their primary research with a 110,000-pound steel-tube carriage that carries landing gear down two steel rails to a "landing" on a short patch of runway. Propelled by a high-pressure water jet, the carriage, which looks like the kind of modern art sculpture you see in front of government buildings, is capable of reaching a speed of 265 mph and an acceleration of 20 Gs on its short trip down the track. Engineers can even douse the test surface with water to simulate a rain-slick runway. The carriage can capture 28 channels of data, such as cornering and friction loads from the tire or strut being tested, and the carriage itself is arrayed with hundreds of strain gauges that tell its operators if their apparatus is about to turn itself into a pile of water-propelled scrap metal. The researchers are primarily interested in the interrelated effects of different runway surfaces, landing conditions, and tire types.
Sometimes the focus isn't on the tire at all but on testing runway and taxiway surfaces themselves. In recent years, tests have been conducted on surfaces made from paver blocks, which can be used to make a taxiway that fits together like a tile floor and can be easily repaired. Another innovation, the grooved runway surface, which channels water away, was developed and tested at ALDF and is now in use worldwide.
Using the carriage, ALDF engineers have been able to predict tire wear under given steering stresses and crosswind loads. This work started 20 years ago with tests to make sure the space shuttle could land safely. "With the shuttle, the speeds involved and the weight per tire are much higher than any other airplane," says Bob Daugherty, an ALDF engineer. "And there are tremendous wear problems to the extent that there was once a big concern about shuttle tires surviving even one landing." ALDF tests showed that the shuttle could operate on tires that are very similar to commercial aircraft tires, which are a blend of natural and synthetic rubber. As a result, shuttle tire life was increased significantly.
Another airplane that created headaches--but also taught important lessons--was the SR-71 Blackbird. "I'm being partly facetious, but my guess is that Kelly Johnson and his team at the Skunkworks put so much effort into getting the SR-71 from takeoff up to Mach 3 and then back again it was kind of like "How do we get it to the hangar and from the hangar out to the end of the runway?' " says former Blackbird pilot Tom Alison, now a curator at the National Air and Space Museum. The SR-71's landing gear, which is small for a 100,000-pound airplane, was added "as if it was an afterthought," Alison says.
Because the SR-71's gear was perhaps its weakest system, Alison says that the mighty Blackbird had to be treated delicately on the ground. "You could lock the brakes up and skid the tires even at taxi speed if you stepped on the brakes real hard," he says. "You treated it like a large airplane on the ground, even though it had the performance of a fighter-type airplane."
These problems arise because designers of landing gear have always had to manage a host of sometimes conflicting requirements based on the mission and performance of the aircraft their gear will support. The U.S. Air Force, for example, wants the third largest airplane in the world, the C-5 Galaxy, to be able to operate from unpaved fields. The shuttle weighs as much as 240,000 pounds on landing, yet its gear must be capable of touching down either on a dry lakebed or on the Kennedy Space Center runway, which is criss-crossed with tire-shredding half-inch-high channels that allow maximum runoff during a typical Florida rainstorm. And the Navy and Marines like to slam high-performance fighters onto the decks of ships--punishment that would make non-seaworthy gear struts crumple like soup cans.