Researchers are exploring rapid prototyping methods and materials for wind tunnel models.

Technicians machine traditional metal wind tunnel models in a process that can span months. Although these models are highly precise, the meticulously slow manufacturing process precludes a quick assessment regarding a new design’s feasibility and thus impedes the ever-increasing need to help today’s warfighter address constantly changing warfare threats. In support of the Integrated Rapid Aerodynamics Assessment program, AFRL has been exploring the impact of rapid prototyping (RP) technology in meeting this escalating need. According to AFRL’s Mr. Gary Dale, an originator of this experimental research effort, “We were looking for a way to quickly generate experimental data that we could use to verify computational fluid dynamics (CFD) results. The CFD researchers were generating solutions in a matter of days or even hours, and they wanted to verify their solutions with [wind tunnel] experimental data.” By producing a model in days—or possibly hours, depending upon model complexity—RP technology enables this concurrent study of air vehicle concepts via computer simulation and wind tunnel results.

Because it produces models in less time than conventional methods permit, RP is also cost-effective and, as AFRL wind tunnel engineer Lieutenant Erik Saladin affirms, is becoming more affordable every day. “Generally, the prices are dropping quite a bit in RP. Fifteen years ago (when RP was new), you paid considerably more. You probably pay 20%-30% of that [original] price today,” Lt Saladin estimates. The reduced cost and increased efficiency of RP are already having a positive effect on AFRL’s wind tunnels. Mr. Bill Gillard, team lead of AFRL’s Experimental Fluid Dynamics group, concurs: “Right now, things are pretty exciting. RP inspires a lot of innovation. You have flexibility as well as speed and lower costs. We should be able to complete 13 experiments this year, and that [number] should be on the average to low side in the future.” Prior to the group’s use of RP technology, a good year would have comprised seven experiments.

Initially, AFRL worked with Bradley University students in the process of conducting their senior design projects on RP. “They surveyed the state of the art for RP techniques and materials and reported back to us,” Mr. Dale explains. “That was our baseline.” Since that time, AFRL has employed RP technology for several wind tunnel tests. In one such effort, engineers used AFRL’s Subsonic Aerodynamic Research Laboratory wind tunnel to test an unmanned combat air vehicle (UCAV) X-45A RP model produced by Johns Hopkins University’s Applied Physics Laboratory. This project earned a National Aeronautics and Space Administration Group Achievement Award. More recently, AFRL tested a strike tanker RP design (see Figure 1). Engineers create RP models using stereolithography or laser sintering, two common RP techniques. Stereolithography (used in the X-45A RP model) uses a laser beam to trace a form on the surface of a container of liquid photopolymer; the process builds plastic parts layer by layer. Laser sintering (used in the strike tanker RP model) uses a high-powered laser to fuse small particles of plastic, metal, or ceramic powders into a three-dimensional form.