A New Look at Aircraft Assembly
Friction stir welding is the answer
By Robert B. Aronson
Economic assembly techniques may make the Eclipse the air taxi of the near future
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.
Business end of a FSW system is a simple pin. Later designs may allow the process to be used in more confined areas.
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 obvious 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.
FSW seam is flush and has a minimal heat-affected zone. Smooth surface means less preparation is needed for painting.
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
- Modern engine design
FSW is a fairly new technique for the aerospace industry, but has already gained acceptance as a cost-reducing 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.
Engineers tested fatigue and damage tolerance life of the fuselage sections in this "barrel test rig." The section was subjected to pressurizations simulating cabin pressure cycles.
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.
All subassemblies of the aircraft are mated in this master fixture. Laser beams bounced off reflectors on the fixture ensured accurate alignment.
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."
In the assembly process, stringers are first stir-welded to the skin, then the frames are added.
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.
Banjo spar, a key structural element in the tail, is a single machined part, replacing traditional multipart supports.
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."
Plant design will copy automotive techniques to achieve high-volume production. Tables at the loading stations, each with a fuselage element, are fed onto the transfer line, then onto the FWS gantry pallets.
Another feature that both cuts costs and speeds assembly is combining parts. Instead of assembling complex modules from a series of smaller parts, an entire subassembly is machined. The Eclipse aircraft has 3900 parts as compared to 10,000 for a comparable Citation made by Cessna. This means higher initial cost, but lower unit cost when the part is made in volume through reduction in handling, fixturing, and fastening requirements. Because the machined parts are also more accurately made, assembly is simplified. There is less of the "cut-and-fit" required by conventional designs.
A major cost in many corporate aircraft is interior furnishings, which can run up to $100,000 on some models. Eclipse has simplified this purchase by offering only three interior options using seating supplied by an automotive contractor.
There are no hydraulics. It was not necessary to route a maze of hydraulic lines through the Eclipse. In some aircraft, there are four sets of hydraulic lines running to the control-surface actuators and the landing gear. The Eclipse design uses brushless servomotors for all actuation. "We have found them to be very reliable. They promise 50,000 hr MTBF," says Masefield.
Initially the designers were going to use a powerplant supplied by Williams International. However, as the design progressed, it was found that they needed an engine with fewer parts and higher durability. They therefore elected to go with a new engine design from Pratt & Whitney, the 900-lb-thrust PW610F.
FSW machine automatically secures stringers and ribs in place against the skin.
In traditional commercial aircraft each avionics function (radar, radio, navigation) has its own box and functions independently. This can result in as many as 46 separate boxes. When developing their avionics, Eclipse engineers acted as system integrators, partnering with a few major suppliers. They reduced the number of boxes to 18, designed the systems so individual functions could "talk" to each other, and added a number of features formerly considered options.
This article was first published in the March 2004 edition of Manufacturing Engineering magazine.