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Gridgen Speeds F-35 Design

Lockheed Martin is currently developing the F-35 multi-role fighter aircraft in Air Force, Navy, and Marine variants. Each variant has different key performance parameters that must be met contractually, and many of the performance parameters are driven by aerodynamic considerations.

Figure 1. Contours showing change from original configuration. Red shows the largest displacement. +
Contours showing change from original configuration.  Red shows the largest displacement.

Gridgen has played a significant role in the grids generated for aerodynamic design and drag evaluations of all three JSF variants. The flexibility of this tool for rapid structured grid generation was extremely important because it produced high-quality grids in the shortest time, which enabled tight schedules to be met.

In the case described here, CFD was used to redesign the STOVL variant of the F-35 for reduced drag and increased internal volume.

Figure 1 shows how the outer contours of the F-35 were changed from the baseline configuration. As you can see from the figure, the volume has been increased somewhat by pushing out the upper contour of the aircraft near the lift fan that is used for vertical takeoff and landing. In the CFD study, Lockheed Martin wanted to see how this would affect drag.

Lockheed Martin has refined its CFD analysis process over many years to achieve rapid turnaround times. It starts with a geometry database from CATIA V4 or V5. Gridgen is then used to build structured surface and volume grids which are exported to Falcon, a full Navier-Stokes CFD code developed in-house at Lockheed Martin. The results from Falcon are postprocessed with the Lockheed Martin Aero Loads Code and FIELDVIEW.

The Loads Code was used to determine the increments in lift coefficient, drag coefficient, and moment coefficient at each grid point. The results were then plotted in FIELDVIEW. (Figure 2)

Figure 2. Plots of change in left (left), drag (center), and moment (right) highlight the configuration differences. +
Changes in left, drag, and moment

Looking at the data in this manner shows a lift coefficient increase (blue) on the wing that was not apparent from looking at pressure distributions alone. Examination of the drag coefficient increment showed a problem area of increased drag (red) aft of the shape change that was not noticed before. This resulted in a subsequent modification to the shape change to reduce the problem area. This showed a trimmed drag coefficient reduction of 0.00062 compared to the baseline aircraft configuration at a cruise condition.

Based on AIAA 2006-3663, Use of CFD in Developing the JSF F-35 Outer Mold Lines, Perry A. Wooden and Jeff J. Azevedo, Lockheed Martin Aeronautics Company. This article is also available in PDF format.