Avoiding high-priced hiccups

  • 10-Nov-2008 02:56 EST
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Cessna deployed LMS Virtual.Lab Motion to analyze dynamic loading of a complete aircraft including system dynamics such as landing gear, wings, and propulsion systems.


The Columbus is a major step forward for Cessna Aircraft Co., not only in terms of size and complexity but also in terms of how the Kansas-based company develops its planes. With business booming across all product lines and staff and facilities at full capacity, Cessna needed to find a structured solution to handle a program of this magnitude. Instead of going about business as usual and designing and manufacturing most of the aircraft in-house, Cessna borrowed a chapter from the makers of large commercial airliners and decided to integrate and assemble co-developed subsystems and supplied parts to create the Columbus.

The real challenge for Cessna was to meet a tight development schedule while simultaneously coordinating design plans for dozens of suppliers and internal groups co-developing major subsystems. It was vital that everything worked together perfectly when finally pieced together.

“In the aircraft industry, ‘hiccups’ in the middle of plane development can cause big problems in terms of time and cost,” explained Robert Howes, Cessna’s Senior Manager of Loads, Acoustics, and Structural Dynamics. “Our job is to avoid these problems with extensive use of simulation and high-fidelity models, especially up front in design.”

He explains that based on these tools, they develop load envelopes for critical-path parts, such as the landing gear, wings, and propulsion system—assemblies that drive the design for most of the plane. Early in development, these elements are combined into a full aircraft model, which is modified and optimized along the way as detailed designs are finalized. When completed, this simulated model eventually becomes an essential element in the aircraft certification process.

As with other Cessna aircraft designs, the loads group counts on LMS Virtual.Lab Motion multibody simulation software to represent the complete aircraft as well as major systems dynamics. LMS Virtual.Lab Motion predicts loads at attachment points for main components, such as the landing gear, wings, tail section, flight control surfaces, and the engines.

A key advantage of LMS Virtual.Lab Motion is that it can exchange data directly with other software packages. Users avoid cumbersome—and often error-prone—data translations and conversions. In this way, LMS Virtual.Lab Motion is tightly integrated not only with other LMS tools and third-party software but also with the in-house codes developed by Cessna for specialized tasks.

“Simulating the entire plane throughout the whole development process is a collaboration between several big pieces of software,” said Howes. “LMS Virtual.Lab Motion is a powerful multibody simulation tool, and the goal is for the software to serve as an essential data-exchange bridge—and often a traffic cop—coordinating the information flow between programs in different disciplines including design, aerodynamics, stress, fatigue, and damage tolerance.”

Howes pointed out that LMS Virtual.Lab Motion load data together with Elfini stress data is fed into LMS Virtual.Lab Durability software to determine component fatigue life. LMS Virtual.Lab Correlation can extract stiffness attributes and mass properties from Nastran. FE models can be validated and correlated via modal tests performed in LMS Test.Lab. LMS Test.Lab is vital to ground vibration testing—essential to the certification process especially when considering aircraft flutter.

Thanks to the close integration between LMS Test.Lab and LMS Virtual.Lab, data can be transferred directly without universal file conversions that often fail to fully represent critical data. Cessna also uses LMS Virtual.Lab Acoustics simulation software to predict interior cabin noise from natural frequencies determined from FEA.

“When designing the individual systems as well as the entire aircraft, it was especially important for us to step up the fidelity of the multibody models to predict true real-world behavior of the Columbus,” said Howes. “In this respect, the flexible body capability of LMS Virtual.Lab Motion was critical to model not only the Columbus but all its flexible components, such as tires, struts, trunnions, and trailing links. If you add in the stiffness matrix for flexible parts, you pretty much have a ‘big win’ case scenario for LMS Virtual.Lab Motion.”

LMS Virtual.Lab Motion lets Cessna engineers change simulations quickly, especially when exploring various parameter variations and sensitivities to design changes. By changing key parameters, engineers can simply rerun the simulation instead of re-creating models from scratch. They can also perform consecutive parametric analyses automatically to quickly explore alternative designs and what-if scenarios. This way, the Cessna engineers can run multiple simulations to predict airframe loads under different flight conditions and different plane weight configurations such as light, heavy, forward-weighted, and aft-weighted.

LMS Virtual.Lab Motion’s ability to run multiple simulations using a flexible multibody model allows the Cessna structures group to cut design time from the airframe timeline—just a few weeks were needed instead of the typical four to five months normally allotted. Likewise, landing gear loads were determined within a week or two fairly early in design instead of several months later in the cycle. Rapid simulation enables engineers to know how landing gears and other parts would behave long before these parts were actually tested.

“In the past, Cessna used simulation mostly for troubleshooting problems we uncovered during testing,” explained Structural Dynamics and Loads Manager Timothy Seitz. “Now we apply simulation to guide the design throughout development so there is less risk of surprises later. With a complex aircraft like the Columbus, we don’t have the luxury of designing plane parts in serial fashion. Rather, multiple disciplines, groups, and suppliers must collaboratively work in parallel from the same full-aircraft simulation model. Everybody is playing in the same sandbox, so to speak. LMS Virtual.Lab Motion is the common link that lets many different areas work together.”

Working in a different manner, Cessna maintains an aggressive schedule for the Columbus. The first prototype flight is anticipated in 2011. FAA certification is expected by 2013. This is one of the most time-consuming aspects of the development process where simulation results must be produced to demonstrate Columbus’ safety and air-worthiness during takeoff, landing, taxiing, braking, and ground-handling. Results must also be provided for static loads on the airframe as well as dynamic flight loads for conditions such as maneuvering, turning, flight into turbulence, and engine failure.

Calculating these various loads at so many points on the airframe can be done quickly and accurately with LMS Virtual.Lab Motion. Moreover, the certification process is facilitated because FAA administrators are assured of the simulation accuracy based on detailed flexible body models. “Because of its overarching use in determining loads throughout the aircraft quickly and accurately, LMS Virtual.Lab Motion is pivotal for our plane certification,” Seitz said.

This article was written for Aerospace Engineering & Manufacturing by Jennifer Schlegel and John Krouse of LMS International.

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