Reaching the Frontier of Aerospace Engineering 

When charged with designing a state-of-the-art machine, most designers embark with one or two primary objectives in mind, such as an impressive power-to-weight ratio for a sports car. Here, trade-offs are common: in order to keep the wheels on the ground, the suspension is tightened and the ride becomes less comfortable; and a car can only be so lightweight before you must sacrifice engine size and thus power.

So when aerospace manufacturer Alenia Aermacchi was tasked with designing a lightweight, quiet, clean and aerodynamic plane, this issue of sacrificing one goal for another had to be overcome.

The complexity of aerodynamic performance and wing weight led Alenia Aermacchi to seek a targeted solution, which the manufacturer found in modeFRONTIER, an optimization platform offered by ESTECO made specifically for improving multiple features of a system at once.

The CAE tool takes its name from the “Pareto frontier,” a concept found in economics and engineering that represents the best set of solutions for a system beyond which the system would be compromised. In the case of aircraft design, a wing's upward and downward forces have historically been measured through wind tunnels and flight tests, but advanced simulation software is helping to take the “guess and check” out of the aerodynamic equation.

Since creating an aerodynamic wing involves meeting several structural goals in terms of shape and weight, Alenia Aermacchi needed a tool designed to identify the best solution when one goal conflicted with another. According to Enrica Marentino, CFD Specialist at Alenia Aermacchi, the software links easily and effectively with most CAE software, but what made modeFRONTIER stand out from other optimization tools was its ability to meet multiple goals through compromise.

“One of the most important features of modeFRONTIER is its capability to really cope with multi-objective optimization problems. The tool, in fact, can extract the Pareto front from the design table, allowing the designer to trade between different optimum solutions.,” said Marentino.

Company engineers began the design process in 2D by refining the shape of the airfoils used in the wing. Once the 2D components were finalized, 3D features such as the twist, chord and span were adjusted. The result was a 2.5 percent boost in aerodynamic performance in the company's thin wing design.

While this configuration demonstrated improvements in aerodynamic performances at high speed, Marentino commented that “the real achievement was that a shape specifically designed for high speed has been optimized at low speed.”

These low-speed optimizations included decreased fuel consumption at cruise conditions, as well as reduced noise levels at low speed thanks to trailing edge high lift devices that were used in the take off and landing phases.

But the thin configuration wasn't the only design that the manufacturing outfit was looking to optimize. While finding the greatest lift-to-drag ratio to maximize aerodynamic performance was the goal for that case, a thick wing design in development would require different objectives, with the focus being on lowering the wing's weight. The key difference between developing the thin and thick wings ultimately lay in the objectives put forth to modeFRONTIER.

“For the thin configuration it was requested to minimize the thickness of the airfoils, for the thick one a target value was set for each profile. In running this process, modeFRONTIER was very useful for its capability in finding optimum (Pareto front) solutions for the multi-objective problem, and in the analysis of the hundreds and hundreds of possible solutions coming from the 2D optimization loop,” said Marentino.

But modeFRONTIER isn't the only tool used in Alenia Aermacchi's design process. The company relies on both in-house developed tools for its specific input and output data formats as well as several commercial tools. Among those tools were: CATIA V5 for the parametric model of the 3D wing, ANSYS ICEM for the generation of a hybrid mesh, and CFD++ for the computational fluid dynamics computations.

In the 2D post-processing phase, Marentino said that the most useful tool turned out to be MCDM (multi-criteria decision making tool) since it provided a reliable process to get a rank for the Pareto front solutions. In the case of wing weight, modeFRONTIER was able to determine multiple configurations that met the company’s weight goal, and the MCDM was then able to pinpoint the best of those options.

Alenia Aermacchi made use of its in-house euler tool and a boundary layer code to help calculate the Pareto front solutions more quickly. After that, Reynolds-averaged Navier-Stokes (RANS) equations were used to find the optimum solutions.

The aim of this optimization work is to create a Green Regional Aircraft in accordance with Europe's Clean Sky project, whose goal is to create more fuel efficient, less noisy aircraft through public-private partnerships.

Each wing shape will offer a unique advantage to meet with Clean Sky guidelines: the thin configuration increases the cruise lift-to-drag ration by approximately 2.5 percent, while the thick configuration decreases the wing weight by about four percent.

Marentino says that the final decision of which wing will be used for the project will be driven by a careful analysis of overall requirements and costs. Even though aerodynamics can have profound effects on other systems, they are but one piece to a larger puzzle in determining the best design. For example, wing thickness affects the size of in-wing fuel tanks, which then affect the wing weight.

When describing this relationship between a design’s performance versus its overall cost, Marenito remarked, “A good design is a good compromise” – a notion that ESTECO hopes more companies will get behind.