Business jet for the 21st century

Wichita, KS--At the cavernous Flight Test Center adjacent to Learjet's corporate headquarters here, hydraulic fixtures bend the wings and torque the fuselage of the company's latest business jet, the Learjet Model 45. It's a routine test--one that aircraft manufacturers have performed on prototypes for decades.

But this test aircraft isn't a prototype. It's the real thing, the first new Model 45, straight off the production line.

Unlike business jets of the past, the Learjet 45 never reached the prototype stage. Instead, it jumped straight from the computer screen to production, eliminating all the hand-built mock-ups and validation hardware that serve as a part of traditional aircraft development.

If successful, the 45 could change the way engineers develop business jets. Still two weeks from its first test flight, it already has shown that general aviation engineers can slash costs, reduce scrap rates, and cut time-to-market. It uses half the number of parts of its predecessor, the 35A, and is expected to need minimal re-design. And flight testing, normally a 24-month process, is expected to take 15 months. "We expect flight tests to be merely a validation of the design," notes William W. Greer, Learjet's vice president of engineering and quality assurance.

Customer driven. Learjet executives hope that all of those factors will contribute to a resurgence in the sales of business jets. By simplifying design, cutting costs, and offering more product for less price, they hope to renew customer interest in their products.

Indeed, renewing customer interest may be the goal of the general aviation industry. Prior to 1980, the industry was building and selling nearly 18,000 planes per year. During each of the last three years, that figure has dipped to 1,000 per year. As a result, all of the industry's biggest names--Cessna, Beechcraft, Gulfstream, Piper, Learjet--have struggled, with most changing ownership or nearing bankruptcy.

With the design of the Model 45, Learjet hopes to reverse that trend. Conceived in 1989, the 45 is an all-new aircraft designed from the ground up. This makes it the company's first "clean sheet" design since the Learjet 35 in 1972.

For Learjet, however, the 45 is much more than a new aircraft. In terms of both product and process, it represents a new way of thinking. "For the past 50 years, our industry hasn't changed the way it has designed and built airplanes," notes Brian E. Barents, president and chief executive officer of Learjet, Inc. "But with the 45, we knew we had to design and build it differently."

To start, the company surveyed a broad cross section of general aviation customers, asking about the kinds of features and benefits they sought in a next-generation product. Instead of limiting their survey to pilots, however, they focused on customers. They asked about cabin configurations, seat locations, window spacing, window size, window location, and amenities. They asked about usage habits: typical airport sizes, runway lengths, maintenance habits, and flying ranges. From the beginning, the brunt of their efforts aimed at the people who sat in the cabin.

When they began development, Learjet executives extended the customer-driven approach by forming a New Product Advisory Council, which actually became part of the design team. The panel, consisting of 16 business-jet users from Fortune 500 companies, provided input for engineers at each step of the design process.

For the general aviation industry, the new approach represented a stunning reversal. In the past, most airframe manufacturers designed their products around the pilots. Design engineers, many of whom were pilots themselves, satisfied their own needs. Manufacturing engineers then added their own input to make the designs more easily producible.

On the 45, all of that changed. "Historically, the design of the cabin has been an afterthought," Barents says. "On this airplane, we designed the interior first, then designed the airplane around it."

Learjet's effort to meet customer needs also extended to another key area: value. For more than a decade, lack of value had been the primary cause of the industry's economic woes. During the 1980s, corporate customers had suddenly discovered that they could obtain better value by purchasing used business aircraft. The reason: While costs had skyrocketed, technology hadn't kept pace.

With the 45, Learjet engineers faced a new economic equation. Their goal was to build an aircraft with the features of a medium-sized business jet, but the price tag of a light-model jet. Though medium-sized business jets typically cost $8 million or more, Learjet executives hoped to sell the 45 for $5 million in 1990 dollars. But to achieve that, engineers knew they must change the company's engineering culture and abandon its traditional manufacturing and assembly methods.

Corporate team. To make that happen, Learjet worked with its parent company, Bombardier Aerospace Group, to set up new, automated manufacturing techniques. Using such systems as computer numerical controlled (CNC) machinery and auto-riveters, engineers reasoned they could dramatically lower the cost of production. Their goal: To design the airplane in CAD, analyze it in software, then use the same digital data base to build the tooling. By teaming with Shorts Brothers PLC, Belfast, Northern Ireland, to construct the fuselage and empennage, and de Havilland Canada, Downsview, Ontario, Canada, to build the wing structure, Learjet engineers found they could automate the 45's construction without investing the capital for the equipment. In all, they estimate that the firm saved $40-$60 million by working with sister companies.

For engineers at Learjet's headquarters in Wichita, however, the road to paperless engineering was paved with difficulties. All three of the major facilities--Learjet, de Havilland, and Shorts Brothers--used different software. De Havilland employed CATIA CAD/CAM software for the detail design of the wing. Shorts used Computervision for the fuselage, and Learjet incorporated Unigraphics for the nacelle and pylon design. Engineers jointly developed interface software so that designers in all locations could share a common database and access files though high-speed data transmission equipment.

Communication problems didn't stop with software, however. Although all three companies resided in English-speaking countries, each used different terms to describe the plane's components. While Learjet engineers used the term "wiring bundle," for example, de Havilland used "wiring harness" and Shorts used "loom."

Still, the benefits of complete computerized design outweighed the drawbacks. Engineers combined the advantages of solid modeling with advanced analysis. Then, working in design-build teams, they applied the principles of design for assembly (DFA). Result: They vastly simplified the aircraft's design. In one part of the forward pressure bulkhead, for example, they consolidated 81 parts into a single, machined piece.

The ability to design on-screen also helped to alleviate much of the cost of hardware at various stages of the design process. "We experienced all of the advantages of solid modeling: digital pre-assembly, digital mockups, interference checks, and gap checks," says Wayne Stout, chief of systems for Learjet, Inc.

Digital pre-assembly was particularly helpful in eliminating the need for prototype hardware and mockups. It enabled engineers to check for adequate clearances between push rods and holes in the ribs of the vertical tail assembly, an area that would normally not be checked until the prototype stage. In one case, they spotted an on-screen interference between cables and a bracket in the empennage. By finding it early, they trimmed the edge of the bracket on-screen and eliminated the interference before it reached the hardware stage. "Those are the kinds of problems that can be solved in software," Greer says. "In a flat drawing, problems like those would be very, very hard to find."

The union of solid modeling and analytical software tools also enabled engineers to design for better performance. In one case, they used the Mechanica Motion program from Rasna Corp. to analyze a linkage that stretched from the cockpit to the rudder. The linkage, which passed through seven sets of pulleys, had required months of hand calculations in the past. Even then, engineers had to test it in prototype form, and by that time it was usually too late.

But by analyzing the linkage in Applied Motion, Learjet engineers learned more about its performance under load. In particular, the software program accurately calculated the linkage's cable stretch and the amount of pilot force required to move it. Ultimately, knowledge of the linkage let Learjet engineers more accurately target the minimum control velocities for the rudder and, as a result, the take-off distances for the aircraft. "Using the computer, we could nail the numbers down instead of waiting to see what we had after building the airplane," Greer says.

Engineers also reduced development time by transforming the digital data base for use in computational fluid dynamics programs that ran on NASA supercomputers. Using VSAERO and TranAir programs to analyze drag, shock-wave, and wake characteristics in transonic and sub-sonic zones, Learjet aerodynamicists refined the 45 prior to wind-tunnel testing.

Industry wake-up call. At a development cost of roughly $240 million, the 45 is the most costly plane that Learjet has ever built. Still, the company's executives are confident that the investment will pay off. With a maximum VFR range of 2,200 nautical miles, a maximum cruise speed of 0.81 Mach, and a certified ceiling of 51,000 feet, the 45 offers high performance in a cabin size that approaches those of medium-sized jets. Employing concurrent engineering techniques and paperless engineering also kept the jet's cost down to $6 million--about $2 million below the normal threshold for medium-sized jets.

As a result, the plane has already sold out through its entire production run for 1998. With certification set for mid-1996, its order backlog already exceeds two years.

Still, Learjet executives say that the lessons learned in the development go beyond the plane's immediate success. "This has been a wake-up call for all of us," Barents says. "We knew we needed to do things differently. We've done them on the 45, and we think that this is the product that will take us into the 21st century."

Actuator design improves reliability, maintainability

Designing an aircraft from a clean sheet of paper can have its advantages, but it also lays new burdens on engineers. In the case of the Learjet 45, the "clean sheet" approach meant that they must meet the latest Federal Aviation Regulations (FARs) for actuators and aircraft systems.

Current regulations call for extraordinary levels of reliability. "The latest FARs have redundancy requirements and failure rate requirements the likes of which most manufacturers have never seen," notes Darrin J. Kopala an engineering product manager for MPC Products Corp., Skokie, IL, which worked with Learjet on the design of the actuator for the horizontal stabilizer.

Learjet also added its own twist to the already stringent regulations by calling for a design that would need little maintenance and came with a five-year warranty.

Engineers met those requirements by designing an electromechanical actuator with layers of redundancy. The system, which offers closed-loop velocity control and position feedback, employs two separate power drive subsystems to drive a rotating screw and translating nut. Each sub-system uses a brake, tach-ometer, clutch, motor, and position sensors to drive the screw and nut. When the motor from either subsystem rotates the screw, the nut moves, pushing the Learjet 45's horizontal stabilizer through a maximum angular displacement of 12 degrees.

The two subsystems, which are designated primary and secondary, can both be decoupled by primary or secondary electronic modules. That way, the actuator has an extra layer of redundancy. If one control module goes down, the remaining electronics would decouple the inoperative power drive.

To facilitate that, MPC engineers worked with the American Precision Industries Deltran Div., Buffalo, NY, which devised a clutch that could be operated by either electronic module. The key: a clutch that uses dual windings.

The challenge for Deltran engineers was to pack the dual-wound clutch into a space measuring only 2 x 2 x 2 inches. Packaging difficulties were exacerbated by requirements for the clutch to deliver high torque density (40 oz-inch) and to operate at low coil power (10.4W at 9 Vdc) and temperature extremes (-55 to 150C).

Deltran engineers custom-designed the unit using AutoCad software, then transferred the AutoCad file to a magnetic analysis program. The analysis helped them to find more efficient magnetic flow paths. Result: The clutch met the torque, power, and temperature requirements, yet fit inside a frame measuring 1.75 x 1.75 x 1.375 inches, thereby helping MPC meet weight reduction goals.

Deltran engineers took the clutch design from concept to hardware in just five weeks. Without the use of computer design and analysis tools, such custom design jobs have previously taken from 8-12 months, they say. "We were looking for the most efficient way to get the flux around the circuit," says Brian W. Buzzard, product engineer for API-Deltran. "The analysis tools helped us do that."

The resulting actuator design also meets Learjet's new maintenance requirements. Use of brushless servo motors, position sensors, and tachometers and non-contacting brakes helped raise scheduled maintenance intervals to 6,000 hours, from a previous level of 600.

MPC engineers say that the success of such technologies helped them push electromechanical actuation technology to aviation's forefront, even in the difficult-to-crack military and commercial markets. The reason: Electromechanical units eliminate fear of airframe corrosion caused by leaking hydraulic oil, according to Kopala.

On the Learjet 45, the new actuator reportedly offers unparalleled reliability. "We designed for less than one failure in a billion flight hours, although an aircraft typically flies only about 20,000 hours in a lifetime," Kopala says. "This meets or exceeds all the FAA and Learjet requirements."

TIMELINE FOR DESIGN

1989- Learjet conceives of new business jet to be designed by customers

1992- Market studies collect data from users

September 1994-Wing delivery at Bombardier's de Havilland facility

September 1995- Learjet 45's first test flight

1996- Anticipated certification

CAD cuts design time, parts

Just how critical a role software played in the development of the new Learjet 45 is illustrated in one example. Engineers at design partner Short Brothers plc, Belfast, UK, used CADDS 5 and CAMU (Concurrent Assembly Mockup) from Computervision to reduce parts in the fuselage by 60%, cut design time 40%, and pare first-article rework by 90%.

The parts-count reduction came primarily from using the CADDS 5 hybrid modeler to combine subassemblies containing numerous parts into fewer, more complex parts. For instance, the pressure bulkhead of the fuselage originally was an assembly of 68 parts. Engineers replaced them with a five-axis machined part, plus brackets and supports. Total parts count in the fuselage went from 9,500 to 3,700, says Short Brothers.

Typical first-article parts rework for regional aircraft fuselages averages about 150% of person-days, Short Brothers estimates. But engineers reduced that number to 20% by using CAMU to help the structural, piping, wiring, stress, tooling, manufacturing, inspection, and assembly teams work concurrently. Team members could view their work in the context of the overall assembly, evaluating other aspects of the design in real time.

That meant they could check for design clashes and correct the problems early. For example, engineers minimized piping and electrical paths with the software, ensuring that pipes wouldn't conflict and wiring would connect properly.

"The fit is superb and assembly went more rapidly and smoothly than we have ever encountered on a pre-production aircraft," says William Morris, Short Brothers vice president for the Learjet 45 project.

--Paul E. Teague, Executive Editor

Partners in design

CATIA CAD program from IBM

Unigraphics CAD program from EDS-Unigraphics

CADDS5 program from Computervision

Mechanica Motion program from Rasna Corp.