This article focuses on the developments in materials, production methods, and turbine designs that have spawned a new class of jets. Cessna’s Citation Mustang became the first very light jet to reach commercialization. According to market leaders, the development of computerized cockpits with simplified displays has also been huge. Composites also let engineers combine lots of parts—panels, framing, rivets—into a single part. The fewer parts, the easier and cheaper it is to assemble and the more reliable the finished airplane. Every aerospace company is looking for bolt-on components that simplify assembly and maintenance. Adam Aircraft seeks to simplify production by making large composite parts. The second mechanical advance that makes very light jets possible is the compact, reliable turbofan. While Pratt is well-known for huge turbines that power commercial passenger jets, the company produces several small turbines. Pratt uses blisks and other technologies to simplify production. The result is a new type of jet, one that makes jet airplanes far more affordable.
At least one aircraft maker had a lot to be thankful for this past Thanksgiving. The day before the holiday, the Federal Aviation Administration awarded Cessna Aircraft Co. a certificate to begin production of the new Citation Mustang jet. When it rolled its first Mustang out of the hangar at Independence, Kan., later that day, its first customer, Kent Scott, president of Scott Aviation of Fresno, Calif., was waiting on the tarmac to take possession.
The Mustang is Cessna's entry in an entirely new class of aircraft, the very light jet, or VLJ. How light? At 6,500 pounds unfueled, the six-seat Mustang weighs less than many sport utility vehicles. Its twin Pratt & Whitney turbofan engines weigh so little, about 275 pounds each, you could walk into any gym and find a dozen men and women who could bench press one of them. Some might even be able to bench press both at the same time.
Yet the Citation Mustang and other VLJs now nearing commercialization are real jets. Each of the Mustang's PW615F engines generates 1,460 pounds of thrust, enough to reach a top speed of 390 miles per hour. It cruises above the weather at 41,000 feet and can fly more than 1,300 miles- nearly halfway across the United States-without refueling.
The Mustang made it out the door just before the Eclipse 500 from Eclipse Aviation in Albuquerque, N.M. The six-seat Eclipse is even lighter than the Mustang. Empty, it weighs in at 3,550 pounds, or about as much as a Toyota Avalon. Although its twin PW610F turbofans produce only 900 pounds of thrust, the Eclipse cruises at 425 miles per hour with a range of 1,300 miles.
Still more very light jets are on the way, including the Adam A700, Embraer Phenom 100, Diamond D-Jet, Piper PiperJet, and Honda HondaJet. None of them is cheap. The Mustang sells for about $2.5 million, the Eclipse for $1.5 million. The D-Jet costs slightly less than the Eclipse; the HondaJet is more than $3.6 million. Yet with traditional business jets heading north from $6 million, the very lights are generally bargains at the price.
Why the sudden interest in cheap jets? There are really two answers to the question, one economic and the other technical. The first has to do with the changing nature of business travel.
"This has been going on for quite some time," said Dan Breitman, vice president of Mississauga operations and turbofan development for Pratt & Whitney Canada Corp., which makes the company's small engines. "I do a lot of traveling myself, and even before 9/11 it was becoming more difficult . It is a 4Vz-hour drive from Toronto to Detroit. If I want to fly, with getting to the airport early and security checks, the time is the same. For that type of distance, it is very time-consuming."
Breitman would waste even more time if he wanted to go to a smaller community. "Today in North America, there are about 30 big airports that you have to go through," he said. " If! want to go to some small town in Texas, I have to go through Atlanta or Chicago and then change planes. These new VLJs can hit about 5,000 airports," Breitman said. "When you take a five- or six-hour trip and shave that to three hours, or turn a two day trip into a one-day, you're really adding value. This is what VLJs are going to do. They're going to change the way people think about. travel."
Breitman and others envision a world where small corporate jets ferry executives around the country. Smaller companies could even buy a share in ajet through one of the industry's many fractional ownership plans (think time shares for airplanes). Analysts and marketers imagine air taxi services going between smaller cities and communities.
Optimism is running high . Several startups, such as DayJet, Linear Air, and Pogo, have jumped into the air taxi business. Eclipse claims 2,500 orders for its new 500.
There are skeptics, of course. One of them is Richard Aboulafia of aerospace consultant Teal Group Corp. in Fairfax, Va. "The FAA will not let you offer a service with a single pilot," he said. "Even if it did, you would have to pay the pilot hundreds of dollars per day. You'd have all the costs associated with a larger aircraft, but you'd be amortizing them with the handful of people in the cabin rather than 200 passengers in the back. That means you have to provide a first-class service and find people willing to pay for it on 600- to 1,000-mile trips."
Concerning claims of enthusiastic orders, he said, "Nobody's order book in this industry is transparent." Aboulafia said that several firms with supposedly fat order books failed to take flight.
Still, Eclipse is set up to manufacture four planes per day. Given a five-day work week, that's 1,000 jets per year. "World War II was the last time anybody produced that many airplanes in a day," said the company's vice president of engineering, Ken Harness. He said Eclipse hopes to expand to sixjets per day, or 1,500 per year.
Breitman is equally optimistic. "When you look at the VLJ market, you have to think completely differently," he said. "This is a market that is being defined as we speak. When Eclipse came along, ,no one believed it could make a commercially viable jet for that money. Now we think the market is huge, thousands of engines annually, not hundreds.
"In 40 years, Pratt Canada made 40,000 PW6s," said Breitman, referring to the company's workhorse turboprop engine for propeller planes. "I think we will do that many jet engines in a much shorter timeframe."
A second factor behind the boom in very light jets is new technology. New airplanes follow new engines, and that is certainly the case with VLJs. The technology grew out of the cruise missile program, Aboulafia said. Williams International LLC of Walled Lake, Mich., developed the first small, reliable turbofan engine for the military. It then teamed with Rolls-Royce plc to adapt it for commercial use. Cessna's CJl became the first light j et to use the new FJ 44 turbofan in 1993.
Williams stumbled in its attempt to make an even lighter turbofan. This gave Pratt & Whitney an opportunity to break into the market with its PW600 series. Williams later bounced back with its FJ33, a smaller version of the FJ44. Honda and General Motors Corp. have teamed to develop the market's third light turbofan.
The development of computerized cockpits with simplified displays has also been huge, Aboulafia said .
"There used to be steam gauges all over the place," he said, referring to the arrays of pressure and power gauges that once decked the cockpit. Today's cockpits provide even more detailed information, but deliver it in simplified formats on flat panel displays. "That's a draw in and of itself," he said. "It's much better for situational awareness, easier to operate, and safer."
Harness agreed. "We've provided a new level of functionality and safety for a reasonable cost," he said. "What we have in the Eclipse cockpit would cost $1 million on a commercial airplane."
Ail-frames have also changed. "There's been a big push toward nonmetallic airframes," said John Tomblin, executive director of Wichita State University's Aviation Research Institute.
"Composites offer unique advantages," Tomblin said, and then proceeded to tick them off. "Composites have high strength-weight ratios, so you get better fuel economy.
"They show excellent fatigue and corrosion resistance. We took apart five Boeing 737 horizontal stabilizers that had been flying for 18 years, and they showed no corrosion whatsoever. They looked like they had just come off the factory floor. Aluminum sure doesn't look like that after 60,000 hours.
"Newer resin systems used as a matrix with reinforcing carbon fibers to form composites] are much more damage tolerant. You can make very complex curved shapes without investing in expensive metal forming equipment. Composites also let you combine lots of parts panels, framing, rivets-into a single part. The fewer parts, the easier and cheaper it is to assemble and the more reliable the finished airplane."
"World War II was the last time anybody produced that many planes in a day," said Eclipse vice president of engineering Ken Harness. Eclipse hopes to eventually build six jets per' day.
Giving Planes a Lift
The economics that make it possible to use composites in very light jets also owe a great deal to NASA's Advanced General Aviation Transport Experiments, or AGATE. Begun in 1994, AGATE teamed NASA, FAA, industry, and academia to revitalize general aviation. On the composite side, Tomblin said, it helped reduce the cost of using composites and developed standardized procedures for certifying composite aircraft.
This had been a big issue in ' the past. When Raytheon Co. began building the first composite business jet, the Premier I, in the late 1990s, it spent millions certifying the process used to make every composite part as well as every individual resin and fiber used to make the composites themselves.
AGATE changed the game. It allowed materials suppliers, rather than airplane manufacturers, to certify composite materials. It also simplified the rules for certifying composite production processes.
"This was a big advantage for us because we started with a database of materials," said Pierre Harter, director of advanced structures for manufacturer Adam Aircraft Inc. in Englewood, Colo. "Before AGATE, companies that wanted to use composites had to estimate their properties, and you never really know what kinds of numbers you will get until you finish testing. So they had to make very conservative assumptions and wait for months and years to get the test results. We had properties up fi'ont and could use them in our design."
Adam uses composites for its ASOO turboprop and A700 very light jet. Harter is quick to point out that the company's production methods are neither new nor novel. Composites are laid up by hand over a mold, layer by layer, then bagged, placed under a vacuum, and placed in an oven to cure the resin until it solidifies. "If we're delivering 100 airplanes per year, we don't need to automate," Harter said. "It made business sense to keep it simple."
To Harter, though, keeping it simple means using composites to their best advantage to achieve parts consolidation. The ASOO fuselage, for example, is roughly 14 feet long and 5 feet high. Adam manufactures the entire section in two halves, complete with metal inserts for attaching wings, landing gear, engines, and doors.
The approach eliminates scores of metal parts and hundreds or even thousands of bolts, rivets, and welds. This reduces labor costs. "We initially looked at doing a one part fuselage, but worried about accessibility. With two halves, we can throw a lot of fuse people at each surface to put in the controls, floors, and wire bundles, then bond the pieces together," Harter said.
Adam bonds the plane with a two-piece epoxy adhesive, a high-performance cousin of the epoxies found on everyone's workbench. "We grit-blast the composite surface, apply the adhesive, and then use fixtures to bring the parts together with some pressure," Harter explained. "After 18 hours at room temperature, the adhesive is strong enough for us to work on the airplane. It reaches full cure after seven days.
"We designed everything with big tolerances so we don't have to be perfect. We design the composite to be accessibily stronger than we need, and we design the bonds so that we didn't need fixtures that control everything down to a few mils. We give the shop leeway. They make fuselages every day and they have learned how to get the most out of the tools."
The result, Harter said, is an advanced airplane that costs less to build.
Tomblin, Harter's former teacher, is ecstatic. "When the Boeing 787 gets into production, you will see the reign of metallic aircraft coming to an end," Tomblin said. "We'll always use metallic parts for landing gear and attachments, but a higher and higher percentage of the airframe will be nonmetallic."
Mark Ahne, Cessna's manager of manufacturing engineering, and Eclipse's Harness are not so firmly convihced.
Bonding vs. Bolting
Cessna has been making aluminum aircraft for a long time, but that doesn't mean it follows conventional assembly practices. Instead of bolts and rivets, Cessna bonds aluminum panels together to form the surfaces of its wings and fuselage.
According to Ahne, metal bonding offers many advantages. One is better load distribution. Instead of concentrating stress at each fastener, adhesive bonding distributes the load over the full footprint of the overlapping aluminum panels. The bonds are so strong, said Ahne, they survive loads equivalent to five times the Mustang's projected lifespan.
"To get the same strength level with fasteners, we would have to use heavier-gauge panels," Ahne said.
Bonding eliminates the need for heavy rivets, which reduce fuel economy and increase drag. Ahne added that adhesive joints inhibit crack propagation.
By rethinking how it builds airplanes, Cessna also uses adhesive bonding to consolidate parts. "For a typical riveted structure, you build the skeleton in a jig .and then rivet on the skin," Ahne said. "In metal bonding, you build the airplane from the outside in."
Cessna wings, for example, start as extruded sheet metal surfaces in a fixture. Workers bond stringers and other metal strengtheners to their surfaces. They weld and mechanically fasten these reinforced parts into the final wing. This all takes place offline. Then workers wheel the wing to the fuselage and bolt it into place.
Eclipse also opted for aluminum, but for a different reason. "When the Eclipse started in 1998, we originally had a composite design," Harness said. "Part of the design challenge in composites was how to make a high- ,'. production-rate airplane. We would have needed a farm of autoclaves running 2417 to do it.
"So we decided to make it with aluminum. Now the design challenge became, 'How do we make it fast without a lot of labor?' "
At least part of the answer was a process called friction stir welding, invented by The Welding Institute in Britain in 1991. The automated process can weld 10 times faster than manual riveting. Like adhesive bonding, it lets Eclipse distribute loads more evenly, eliminate heavy rivets, reduce labor, and weld dissimilar alloys to one another.
Also like adhesive bonding, friction stir welding starts with an aluminum sheet in a female fixture. Eclipse workers then lay down supporting ribs, stringers, window surrounds, and other structural elements. They move the fixture to the welding gantry, where a seven-axis machine runs a rapidly turning fluted pin over the metal parts.
The high-speed pin creates enough friction to plasticize but not melt the metal. As it pushes into the metal, it mixes alloys mechanically from the different parts with one another.
"When the weld is done, it is stronger than conventional welds and has the same strength as parent metal," Harness said. "The bond is two times stronger than rivets."
Eclipse works with large aluminum parts. "The conventional wisdom is to build up structures from aluminum," Harness said. "The forward and aft bulkheads in the cabin, for example, consist of flat plates with extra pieces bolted to it for strength . Here, we machine from solids.
"The advantage is that we don't have to do any machining in-house. Once a company has made a capital investment in multi-axis machine tools, they can run all day and night producing pieces for us at very low cost per piece. At the assembly level, those parts fit perfectly together because they have machine tolerances and not bent metal tolerances. We can install a wing in 30 minutes per side versus eight or more hours. They literally bolt right on."
Every aerospace company is looking for bolt-on components that simplify assembly and maintenance. Cessna, for example, chose an electric rather than turbine-powered hydraulic system to power the Mustang's landing gear and brakes. While not as efficient, the system is simpler to install and slashes the number of hydraulic lines that need periodic inspection. It also saves 20 to 30 pounds, an issue on a very light jet.
Outsourcing makes it possible for many small aircraft companies to compete. Like Detroit, the American aviation industry has become a system integrator that does assembly. Outsourcing also speeds throughput. "We can build an airplane in four to five days, compared with the typical 30 to 90 days," Harness said. "In terms of our business approach to outsourcing, we're a high-tech company that produces airplanes."
Turbine to Fit the Belt
The second mechanical advance that makes very light jets possible is the compact, reliable turbofan. Its advent began when Sam Williams noticed that the U.S. Army was testing a rocket belt in the late 1950s. He suggested that switching to a small turbofan might enable soldiers to stay aloft for 20 minutes instead of 20 seconds with a rocket.
Turbofans were just coming into their own at the time. Until then, aircraft had used turbojets for power. Turbo-jets burned all the air coming into the engine with fuel. They produced a lot of thrust, but burned through fuel. Turbofans, on the other hand, burned only some of the air, and used some of the energy they extracted to accelerate the rest of the air through the turbines. They were much more fuel-efficient, but also more complex.
"The challenge in the jet belt was to make turbofan tech small enough. to achieve decent performance and good durability," said Matt Huff, vice president of business development at Sam Williams's company, Williams International. "A typical turbofan has 6,000 pieces. You could just hit the shrink button and make a Swiss watch, but that would be way too complex and delicate. The genius that motivated this company was simplification. We learned how to combine the functions of several parts into single parts."
The jet belt debuted in 1969, and its most memorable application was in the "Man from Glad" commercials. By then, Williams began looking at ways to apply what it had learned to cruise missiles, drones capable of flying long distances under enemy radar. Here, Williams perfected the technologies that made small, reliable turbofans possible.
This started with the blisk, which is short for "bladed disk." Traditionally, tapered turbine blades were attached individually to a disk positioned over the turbine's rotating shaft. The attachment provided some wiggle room so that the thermal expansion mismatches between the thin . blades and thick disk did not shatter the blades after hot and cold cycles.
Blisks combined the parts-multiple disks each containing multiple blades-into a single structure. Williams ,characterized fatigue-resistant alloys until it found one robust enough for the application. It unleashed its five axis machine centers on large metal monoliths, shaving them down to perfect blisks with six rows of blades on a single drum.
Granted, cruise missiles need to survive only on one hot-cold cycle and run no more than five hours. Yet the technology became a building block for future engines.
Williams also developed other technologies. Instead of using complicated systems to inject fuel into the combustor from the outside, it created the fuel slinger. This atomized fuel by flinging it from the center shaft through a ring with fine holes. "We got rid of uneven fuel distribution and many maintenance issues," Huff said.
The company shrank the combustor cooling system from a complex series of louvers into a single perforated surface that could move cooling air through the combustor. It ditched the variable vanes that adjust air circulation through large turbofans in favor of fixed vanes. "It was less efficient, but we might have reduced 100 parts to one part," Huff said.
Many of those innovations carried over to the company's first commercial mini-turbofan, the FJ44. "The biggest challenge," Huff said, "was that a man-rated engine had to last thousands of hours between overhauls. We couldn't just change a few materials. We had to take a fresh look at all our materials."
The debut of the FJ 44 in the early 1990s made possible Cessna's CJl and Raytheon's Premier I light business jets. By then, Pratt & Whitney had seen the potential growth in the market for small jets. When Williams stumbled in its first attempt to build a smaller turbofan, Pratt was there to pick up the slack with its PW600 series engines.
While Pratt is well-known for huge turbines that power commercial passenger jets, the company produces a number of small turbines. "Our small helicopter engines are in the same class as the engines used on VLJs," Breitman said. "We've been doing turbines, combustors, and compressors for small turbines for quite some time. They just have not been' in jet application." Like Williams, Pratt uses blisks and other technologies to simplify production.
The result is a new type of jet, one that makes jet airplanes far more affordable. Perhaps one day soon, air taxi services and corporate fleets of very light jets will begin to transform how Americans fly between smaller cities and communities. In the meantime, the new wave of very light jets remains a testament to the ingenuity of the aerospace industry.
Cruise missiles need to survive only on one hot-cold cycle and run for no more than five hours. Yet their engine technology became the critical building block for future very light jet engines.