SoftwareFlap Splitting and Setting of the X31 Wing
The Problem, its Importance, and its Challenges.
A common requirement for fighter planes is the ability to reach high roll angle accelerations, as this parameter is one of most important determinants of the plane's maneuverability.
If a plane has two edge flaps on the wings, deflecting the two independently will cause a rolling moment to ensue, which in turn induces a roll acceleration. Of course, key factors in determining the roll acceleration are the authorities of the inboard and outboard flaps, and the position of the flap split.
Fig.1 - Pressure coefficient distribution for a sample wing design
Unfortunately, these parameters cannot be chosen at will, because during the manoeuvre aerodynamic and mass loads are imposed on the structure, creating stress that the structure must be able to withstand. To guarantee the structural integrity is then necessary to add material, increasing the wing weight and reducing the range.
Accordingly, the solution must be a good compromise between good roll performance and low structural mass.
modeFRONTIER's contribution to its Solution
To address this problem, DASA used modeFRONTIER to optimise the flap splitting (one discrete variable) and the flap settings (two continuous variables) on the X31 wing model.
Fig.2 - The X31 experimental aircraft
Two conflicting objectives were pursued:
- maximum roll angle acceleration
- minimun wing mass
DASA's HISSS-D subsonic and supersonic solver was used for the aeroelastic design evaluation, while the structural weight for a given configuration was minimized, while maintaining structural integrity, using LAGRANGE, another proprietary code. The software run on an SGI Origin2000 parallel computer.
Optimization results using 16 individuals and 8 generations - using the MOGA algorithm - were sufficient to precisely describe the Pareto Frontier, thus singling out the set of optimal designs for different trade-offs.
Fig.3 - Objective space for the designs found by modeFRONTIER.
For a given weight it became then possible to pick the design providing the highest acceleration possible. Conversely, given a desired roll acceleration it was possible to immediately find the design with the lowest weight.
References
[1] Stettner, M. and Haase, W., DASA (Germany)
Multiobjective Aeroelastic Optimisation
NATO meeting on Aerodynamic Design and Optimisation of Flight Vehicles in a Concurrent Multidisciplinary Environment
Ottawa (Canada), 21-22 October 1999