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Aeroelastic and Trim Analysis

Patran and MSC Nastran application for aeroelastic analysis and tailoring - Sapienza University of Rome
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In 1984 a very peculiar aircraft flew for the first time: the Grumman Northrop X-29. Its wings were swept forward, contrarily to common airplanes. This design was considered a tremendous increase in the aerodynamic efficiency of the wing, as this particular shape produces the lowest drag coefficient achievable and an improvement in the wing's stall response. Moreover this design reduced the size of the wings by 20% (resulting in a weight gain of almost 40 %). Unluckily all this benefits are obtained at the expense of structural strength of the airplane; in fact when normal (backwards) wings are bent upwards, they also rotate in order to diminish the angle of attack α, while forward swept wings (FSWs) tend to increase this angle, resulting in high stress at the wing's root, coupled with aeroelastic instability at low speed.

The solution of this problem, and also the main argument of this project, consists in the use of composite materials, to augment the structural integrity. In fact by placing fibers in the right direction, it is possible to forbid the rotation of the wing along its span axis, eliminating the structural problem that simple aluminium plates were not able to withstand. This procedure is called aeroelastic tailoring.

With Patran it has been possible to create a 3D representation of the original aircraft, with all its structural properties, in a very simple and effective way. The possibility of designing all the details of the wingbox structure, like the stringers shape, has been fundamental in order to achieve accurate results. Moreover the composite material builder tool is amazingly simple to use and produces high quality results; subsequently the requested analysis has been implemented directly from MSC Nastran, by using a very flexible programming style.

In addiction to the simplicity and directness of the graphic pre-processor, the possibility of performing complex tasks as aeroelastic and trim analysis allows engineers to test their ideas quickly and with accuracy.

When discussing the structural behavior of an aircraft, the static analysis is not accurate enough, because it doesn't consider the mutual interaction between lifting surfaces and the air flow. In fact the aerodynamic load alters the wing geometry, which, in turn, alters the surface met by the air, therefore modifying the applied load. This kind of analysis is called aeroelastic analysis, which can be static or dynamic, and is performed by a specific solver, which is called solver 144.

In addiction to this feature, MSC Nastran allows us to created command surfaces, as the canards or the rudder, to achieve different balance conditions. In our case study we wanted to study the aircraft in a leveled flight conditions so the canard can be use to achieve longitudinal balance, by using the TRIM command.

Subsequently the post-process provided an intuitive visual representation of the results, with graphic display of the requested outputs.

In the picture you can see how the stress is directly represented on the design, allowing the developers to quickly determine and locate the structure's critical point.

"We choose to use Patran-MSC Nastran, because of its high programming flexibility and powerful graphic interface, which allowed us to perform complex analysis while keeping the possibility of quickly understanding, and therefore adjusting, the design. The result is the best trade off between structural strength and weight reduction," stated Dario de Angelis, engineering researcher at Sapienza University of Rome.

About Sapienza University of Rome- Founded in 1303, is one of the oldest universities in the world, and currently the biggest Athenaeum in Europe, with more then 145.000 enrolled students. With 11 faculties and 63 departments, Sapienza excels in many different fields such as phisics, astrophysics, nanotechnology and aerospace engineering. Recently the Vega space vector and the Lares satellite, both developed by Sapienza researchers and professors, have been succesfully lunched.

Author:
Dario de Angelis
Sapienza University of Rome