Rapid Model Fabrication for Responsive Aerodynamic Experimental Research

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.

Figure 1. Strike tanker laser sintering model

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.

Despite the success of various wind-tunnel-tested models, researchers needed more information on RP materials. Lt Saladin's comments regarding the materials used for the UCAV X-45A model reflect this pressing need: "There was a strong back steel plate in the middle of the X-45. Without it, the material itself would never have taken the loading. The model had 1,200 lbs of lift force applied to it. In this particular case, since we had the metal, we didn't need to know too much about the materials used." In terms of preparing for future experiments, however, this lack of knowledge is unacceptable. For example, researchers build RP models in material layers, and the orientation of these layers has an important effect on model strength. Lt Saladin offers an analogy: "If you build the model so it is layered like a wedding cake, and the loading is pulling on the top layer, the model's [cake's] overall strength is no greater than that of the cold welds [frosting] between the layers." That is, the strength of a model with material layers configured in this vertically stacked (cakelike) orientation is dependent on the individual strength of each weld bonding one layer to another.

Aligning material layers to produce a 90°directional orientation yields a different result. Consider layers arranged as horizontal material strands— positioned to resemble a bundle of sticks, for example. A pulling force exerted on one end (or both ends) of such a bundle acts not on any bonds holding the layers (sticks) together but, rather, on the material itself. In this case, the RP model's strength equals the sum strength of its layered material, not the bond strength across its layers. In addition, RP models often contain plastic, which has the tendency to distort over time when exposed to heat. Ideally, a model should maintain its shape indefinitely so that engineers can study it again as necessary.

In order to understand these and other RP material properties, an AFRL team initiated an effort both to identify the characteristics of state-of-the-art RP materials available from industry and to determine their applicability to AFRL wind tunnel tests. After extensive research, the team found nine RP materials—six stereolithographic materials and three stainless steel and bronze laser-sintered materials—that seemed capable of meeting wind tunnel requirements. AFRL then subjected these materials to comprehensive evaluation.

Ms. Servane Altman, an AFRL wind tunnel project engineer from the University of Dayton Research Institute (UDRI), summarizes the effort: "We tested at UDRI. They tested the RP materials using the American Society for Testing and Materials standard. We had coupons made in the different shapes required for testing different properties, and UDRI collected their tensile, bearing, flexion, glass transition, and melting temperature data." Test results identified two stereolithographic materials and one laser-sintered material meeting AFRL's specific wind tunnel requirements.

Figure 2. Wing with embedded pressure port

In addition to providing valuable RP material property information, this experimental research effort saved significant time and money over AFRL's alternative: developing RP materials inhouse. Researchers will need to repeat this effort frequently to leverage the RP industry's continuous advancements. RP remains a relatively young technology and has plenty of room for improvement. For example, RP machines currently produce models that are limited in size. To construct larger models, researchers must first fabricate sections and then piece them together. Another area warranting attention involves finding better ways to embed sensors in RP models (see Figure 2). Finally, researchers must explore manufacturing improvements, because the layered construction of RP models renders them less accurate than traditional models.

Until these challenges can be addressed, traditional model building techniques will remain necessary to certain applications. As Mr. Dale indicates, "There is still a need for conventional model machining techniques, because RP has not progressed (and maybe never will) to the point where you have the material strength and temperature capabilities required for many of the wind tunnel testing environments. And while RP may never fully replace existing model machining techniques for these environments, we will take advantage of new RP materials and processes where possible to help reduce the technology assessment timeline." AFRL intends to stay at the forefront of this process by collaborating with industry to support the warfighter.

Ms. Melissa Withrow (Azimuth Corporation), formerly of the Air Force Research Laboratory's Air Vehicles Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn_index.asp . Reference document VA-H-06-01.