Affordable wind tunnel tests

The Johns Hopkins University Applied Physics Laboratory (APL) is developing an alternative, low-cost way of fabricating scale models. The researchers believe the new fabrication method will make aerodynamic wind tunnel tests a more affordable way for air defence programs to collect high-quality data on conceptual missile designs.

APL’s research team, led by Richard Heisler and Clifford Ratliff, is investigating alternative ways to create models from non-metallic materials, such as engineering polymers including thermoplastic, and thermoset resins.

‘Program managers can now afford to take some of their early ideas off the drawing board and obtain high-quality data for a fraction of the cost of building metal models,’ said Ratliff.

The non-metallic prototypes are designed using CAD tools and a rapid prototyping, or fused-deposition machine, which builds up a prototype part in polymer threads.

Under the right test conditions, a rapid prototype part could be tested in a wind tunnel. But in most cases, the part is used to create a mould for stronger models. The polymer model is then placed over a steel ‘strongback,’ or backbone, to prevent a model from bending or flexing during a test.

The materials used to create non-metal models depend on the required test conditions. With a myriad of materials to choose from, the team is developing a ‘recipe’ that defines which non-metallic materials they should use to create models for specific test conditions.

The Lab’s alternative method won’t replace testing a more complete design with a higher fidelity metal model at later stages in a program. But Ratliff notes that applying this technology early in a program’s development cycle is a smarter way of doing business.

Due to the high costs of building a model, program managers often rely heavily on analytical tools, such as computational fluid dynamics (CFD), to predict how a missile system might perform. Although CFD can provide valuable data, it typically requires more time to produce final results and has limitations providing data over a full range of flight conditions.

‘There are lots of ways our technique can serve our customers,’ said Ratliff. ‘It can be used in place of CFD or to verify CFD codes. A combination of testing and CFD can be used to acquire a more complete data set.’

Validating the concept against data obtained from a 1997 APL wind tunnel test using a metal model, the APL team demonstrated that non-metal models could be created and tested at subsonic and transonic speeds.