Rapid prototype wind tunnel model and method of making same

The present invention relates generally to wind tunnel models. More particularly, the present invention is directed to a wind tunnel model design using rapid prototype components and a reinforcing strongback. Whether an airframe is a new design, modification of an existing design, or evaluation of a competing or foreign design, an accurate, high-confidence representation of the airframe aerodynamics is paramount to any low-risk design or evaluation effort. These aerodynamic estimates are used for vehicle and component sizing, performance estimates, and autopilot design and evaluation. The only accepted method of obtaining the high fidelity aerodynamics data needed for these purposes is to build and test a scale model of the airframe in a wind tunnel. Most wind tunnel models are fabricated of all metal components using Computerized Numerical Control (CNC) milling machines. The dimensional accuracy, surface finish and strength of such all-metal models have a distinguished history of providing high fidelity aerodynamics data for both subsonic and supersonic aircraft and rocket designs. However, the fabrication of all-metal wind tunnel models is very expensive and time consuming. Following is a brief summary of the wind tunnel model construction process and of prior art attempts at reducing the costs and time invested in such models. A typical aircraft development program usually needs at least four to five wind tunnel models to adequately test the aerodynamics of a new airframe. The models are generally made of aluminum (for lightly stressed components) or steel (for highly stressed components) and are sculpted using 3 to 5 axis CNC milling machines. The models can require months to manufacture and are often made by small high technology companies that specialize in wind tunnel model manufacture. Wind tunnel models are generally supported in a wind tunnel by a positioning device that is often referred to as a sting. The rear portion of a model is usually hollow to allow the sting to penetrate the model body without affecting the aerodynamic properties of the model. A force transducer called a balance is attached to the inside of the model in order to measure forces and moments acting on the model (often measuring all six degrees of freedom: drag, sideforce, lift, roll, pitch and yaw). The sting is rigidly fixed to the balance and all lead wires from the balance and any other control lines or strain gage leads from the model are routed inside or along the sting and back to the control room of the wind tunnel facility. The cost of fabricating and instrumenting a typical wind tunnel model is on the order of $100,000; however, complex models that include engine simulators, remote controlled control surfaces, numerous rows of pressure taps, etc., can cost over $1,000,000. Companies that are able to reduce the time and costs associated with wind tunnel models therefore stand to gain a significant competitive advantage. For several years Rapid Prototype (RP) materials and methods have been considered as a potential source of improvements to conventional wind tunnel models. RP parts can generally be made much more rapidly and less expensively than conventional machined parts. RP manufacturing is a field of high technology concerning the generation of three-dimensional solids using particles or layers of mostly polymeric materials. Two of the most popular RP techniques include stereolithography (SLA.RTM.) and fused deposition modeling (FDM.RTM.). Both techniques build solid objects layer-by-layer based on data from a computer aided design (CAD) software program. SLA.RTM. equipment is manufactured by 3D Systems, Inc. of Valencia Calif. and employs a laser beam to selectively solidify the surface layer of a photopolymer resin. The solidified surface layer forms a cross section of the prototype part. A supporting table then lowers the part several thousandths of an inch into the resin and the laser solidifies the next layer. These and other objects are achieved in the present invention in a wind tunnel model including a strongback, made of a rigid material and having an exterior axial surface, the strongback being designed to be supported by a balance. The strongback is at least partially inside the interior volume of a jacket section that is made of a rapid prototype (RP) material. The exterior axial surface of the strongback engages an interior axial surface of the jacket section, and the exterior surface of the jacket section defines at least part of an aerodynamic surface. The techniques of the present invention are applicable to any wind tunnel model design intended for use with a balance for measuring forces and moments applied to the model. This includes aircraft such as planes, rockets and missiles as well as ground based vehicles such as high-speed racecars. Other objects and advantages of the invention will become more fully apparent from the following more detailed description and the appended drawings that illustrate in detail one embodiment of the invention. In the following description, all like reference numerals refer to like elements.