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.
"You've got a landing gear community out there that is a bit at the mercy of everybody else involved in aircraft design," says Dave Morris, a senior project engineer at the Air Force's Wright Laboratories at Wright-Patterson Air Force Base in Dayton, Ohio. The labs are a key site for gear and tire testing for all branches of the U.S. military, as well as for manufacturers of commercial aircraft, tires, and gear.
"They have to work with some constraints that are quite severe and that are not expected in other components in terms of weight," Morris says. He points out that as aircraft evolve, they tend to get heavier, and problems often develop as their gear systems start to strain under the load. "The F-16 started out as a lightweight fighter around 25,000 pounds gross weight. Now they're up to 48,000 pounds and the old tires couldn't cut it anymore," Morris says. After trying steadily higher tire pressures, the Air Force finally had to switch to a larger tire, which necessitated design changes in the lower fuselage in later production models.
Increased weight often drives advances in tire and gear design, and not only because of the need to support the airplane without failing under the load. Runway and taxiway surfaces have their limits too, and will crumble under improperly designed gear. "Flotation" is the term describing the ability to spread the weight of an aircraft over a big enough area of ground to support it, and flotation is a direct function of gear and tire placement. Perhaps no aircraft offers a better example of this principle than the monstrous Convair B-36 Peacemaker, a bomber designed during World War II to attack targets halfway around the world from bases in the continental United States (see Max W. Schelper, a former Convair engineer who worked on the B-36. Only three airfields in the United States had the specially built, 24-inch-thick, steel-reinforced concrete runways the XB-36 would have required. "The crying need was to go to a bogey-type [multiple-wheel] gear to spread the footprint out and to allow it to land on any of the heavy-duty runways then in existence," Schelper says.
At one point, the engineers tried to do away with tires altogether and outfit the XB-36 with a tracked system, which made the huge bomber look like it was being carried by two Sherman tanks. Not surprisingly, the large tracks weighed 5,600 pounds more than the improved multiple-wheel gear. "I won't call it a disaster, because the XB-36 [with the experimental tracks fitted] was the only aircraft that could operate out of Wright-Patterson Air Force Base in snowstorms," Schelper says.
Other aircraft also got the track treatment. Experiments were conducted with such aircraft as a Fairchild C-82 Packet and a Douglas A-20, which was even able to traverse mud and sand. In all these installations, extra weight was almost always the downfall, along with the difficulty of keeping a very complex system of rubber-covered tracks operating at the high speeds encountered during landing and takeoff. In the case of the track-equipped XB-36, "every time you hit the runway, rubber would fly off," Schelper says.
Lesson learned: For better flotation, add more tires of reasonable size. The Air Force got its wish on the gigantic C-5 Galaxy, which was at one time the largest airplane in the world. The airplane can (in theory but very rarely in practice) operate on bare soil, thanks to its 28 tires, arranged on four six-wheel struts and a four-wheel nose strut. The problem with all those struts and tires, though, is that they add so much weight. It's tough to provide adequate flotation without incurring a whopping penalty.
The answer for the Boeing 747, still the largest commercial passenger jet today, was to use four main gear struts to support its bulk, which can exceed 700,000 pounds. Other wide-body airliners have used a similar approach, such as the DC-10-30 and the Airbus 340, which have vestigial-looking two-wheel gear struts mounted between their twin four-wheel mains.
When Boeing landing gear designers turned their attention to the new 777, they faced a familiar problem: A big, heavy airplane needs a lot of flotation, but additional landing gear struts add a lot of weight and space is limited. The solution was two six-wheel struts, the largest ever attached to a commercial aircraft. Each 50-inch-diameter tire measures 20 inches across. The six-wheel arrangement prevented the need to add additional struts, and by using titanium extensively, Boeing kept the weight down even further.
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.
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.