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 2 of 4)
"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.