A New Look at Aircraft ASSEMBLY

Manufacturing Engineering, Mar 2004 by Aronson, Robert B

Friction stir welding is the answer

The recent history of aviation is strewn with designs for lower-cost aircraft. Most have a puff of publicity, then disappear when production realities set in. Meanwhile, established builders like Piper and Cessna continue to move along.

Eclipse Aviation Co. (Albuquerque, NM) may turn out to be the exception. They are building the Eclipse 500, a six-passenger, short-hop air taxi with a maximum speed of 375 knots and radius of 1280 nautical miles. Operating cost is estimated at 69 cents per mile. It will sell for around $950,000 per copy, or about one quarter the cost of comparable competitive aircraft.

The market is certainly there. Short-hop aircraft use is increasing. Corporations find company aircraft a better deal that avoids the time, cost, and hassle of the larger airlines. There is an ohvious need not only for personnel transport, but also replacement part delivery. Such short-hop planes are also needed to fill the void created when airline deregulation left many smaller towns with little or no airline service. Eclipse executives estimate the existing market at 1000 to 1500 airplanes per year. "It's a very encouraging sign that we have over 1300 firm orders today and another 715 options," says Oliver Masefield, the company's vp of engineering. Delivery is scheduled to begin in early 2006. To date, a flying prototype has been built along with the majority of the structures for the first of seven FAA flight test aircraft. Over the past year, company engineers have been concentrating on flight tests and modifying the design to incorporate the new Pratt & Whitney engine.

The entire design and assembly philosophy is based on volume production. "We borrowed some ideas from Mr. Ford for this project," says Masefield. Eclipse plans to build six aircraft per day once full production is reached. According to Masefield, competing designs are made at the rate of 100 per year or less.

The biggest stumbling block to lower-cost aircraft has been labor-intensive assembly. Eclipse engineers think they can minimize this factor with three elements:

*Friction stir welding (FSW)

*Highly integrated digital avionics

*Modem engine design

FSW is a fairly new technique for the aerospace industry, but has already gained acceptance as a costreducing factor in many projects, including the latest Space Shuttle's external tank design. But this is reportedly the first application of stir welding to volume aircraft production.

In the FSW system used by Eclipse, the welding tool or pin is carried by a seven-axis, CNC gantry. The base was made by Cincinnati Machine (Cincinnati, OH) and the welding head and control system are from MTS Systems Corp. (Eden Prairie, MN). Welding speed is currently 20 ipm (508 mm/min). There are 5300" (135 m) of stir weld on the plane.

At present, FSW is used to stir weld the fuselage and wing-panel assemblies. This form of welding has eliminated 60% of the rivets and designers hope to reduce rivet content by 80%. Plans are in the works to apply the technique to more of the aircraft construction.

To test the reliability of FSW, engineers built a barrel-like fixture that holds four fuselage segments and is the same diameter as the Eclipse 500 cabin. Ends of the test segment were sealed to make it airtight. Then the segment was subjected to pressurization cycles that simulate the mission profiles. The test segment survived 460,000 cycles or approximately 23 aircraft lifetimes. In addition, several test panels were notched to determine crack propagation rates.

Copying the auto industry, the Eclipse design outsources much of the aircraft part fabrication. "Our function is chiefly that of a designer and integrator. Many of the elements come from outside suppliers. There are an abundance of qualified shops specializing in aerospace work that handle many of our pre-assembly modules," explains Masefield. "This reduces overhead costs to Eclipse."

Final assembly is done using a master jig. First main fuselage segments are joined, then the cabin module, followed by the wings and tail assembly. This reduces a lot of part handling.

This fixture is machined to an accuracy of 0.001 '' (0.03 mm) and the plane's modules are aligned with a laser system. Critical points on the fixture have "hockey puck" reflector holders. During assembly, a laser beam is bounced off a series of these reflectors to ensure exact alignment.

Company engineers keyed their design to aluminum and avoided composites in primary load-carrying structures, which have been favored in many recent new aircraft designs. According to an analysis by Eclipse engineers, the initial benefits of composites are lost when long-term use is considered. Composites were not as good for high-volume production, and both maintenance and repair are quite costly.

To reduce weight, and to add a crack-arresting feature, pockets are milled into the skin panels. Flat section pockets are machined and curved areas are chemically milled.

Conventional fuselages are built from the inside out. That is, the frames and stringers are positioned first, then the skin is attached. In the Eclipse design, the fuselage is made from the outside in. First the skin is positioned, then the stringers and frames are joined. "This technique," says Masefield, "eliminates fault-inducing skin wrinkling and vastly improves the aerodynamic surface."