Measuring and testing – Wind tunnel: aerodynamic wing and propeller study
Reexamination Certificate
2002-05-20
2004-09-28
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Wind tunnel: aerodynamic wing and propeller study
Reexamination Certificate
active
06796171
ABSTRACT:
FIELD OF THE INVENTION
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.
BACKGROUND OF THE INVENTION
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®) and fused deposition modeling (FDM®). Both techniques build solid objects layer-by-layer based on data from a computer aided design (CAD) software program. SLA® 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.
FDM® uses a proprietary technology developed by Stratasys, Inc., of Eden Prairie, Minn. It employs a movable nozzle to deposit a thread of molten ABS plastic. The thread solidifies instantly and forms the cross sectional layer of a part. A new thread is then deposited on top to form the next layer.
Significant use of RP components in high-load wind tunnel tests has not occurred, however, because of problems with material strength and fabrication tolerances. A study funded through the NASA Marshall Space Flight Center (MSFC) investigated the feasibility of using wind tunnel models constructed from RP materials and methods for preliminary aerodynamic assessment of future launch vehicle configurations. See A. Springer, “Evaluating Aerodynamic Characteristics of Wind-Tunnel Models Produced by Rapid Prototyping Methods,”
Journal of Spacecraft and Rockets
, Vol. 35, No. 6, November-December 1998. The study concluded that “RP methods and materials can be used only for preliminary design studies and limited configurations because of the RP material properties that allow bending of model components under high loading conditions and the tolerance on the fabrication processes.”
Another study was funded by Bombardier Aerospace, Inc. and conducted by McGill University to determine whether RP techniques could replace CNC machining of wind tunnel model components. See R. N. Chuk and V. J. Thomson, “A Comparison of Rapid Prototyping Techniques Used for Wind Tunnel Model Fabrication,”
Rapid Prototyping Journal
, Vol. 4, No. 4, 1998, pp. 185-196. The study evaluated 22 different RP technologies and concluded that “the current plastic materials of RP models do not provide the structural integrity necessary for the survival of wind tunnel models, especially for thin section parts such as tip fins and flaps.” Further, the study found that the maximum allowable dimensions using RP machines are generally significantly less than traditional CNC machines and therefore larger scale single part models cannot be built.
It is clear that increased use of RP components in wind tunnel models could dramatically reduce the cost and time associated with wind tunnel model fabrication. There is therefore a need for novel wind tunnel model design techniques that overcome some of the difficulties and deficiencies involving the use of RP components.
SUMMARY OF THE INVENTION
The present invention, among other things, presents a solution to some of the aforementioned disadvantages associated with RP wind tunnel models.
It is therefore an object of the present invention to provide a wind tunnel model design having improved strength and stiffness characteristics.
Another object of the present invention is to provide a wind tunnel model design that is less expensive and requires significantly less time to build than conventional designs associated with the prior art.
Yet another object of the present invention is to provide a wind tunnel model design that may incorporate both RP components and conventional metal components so as to optimize cost, construction timing, and strength issues.
Yet another object of the present invention is to provide a wind tunnel model design that includes a reinforcing strongback, that may in some cases be reused, for connecting directly to a balance.
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
Heisler Richard R.
Ratliff Cliford L.
Fasulo, II Albert J.
Jenkins Jermaine
Lefkowitz Edward
The Johns Hopkins University
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