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.