Technology for Pressure-Instrumented Thin Airfoil Models

NASA-CR-3891 19850015493 NASA Contractor Report 3891 i 1 Technology for Pressure-Instrumented Thin Airfoil Models David A. Wigley ., ..... " .... _' /, !..... .,L_. '' CONTRACT NAS1-17571 MAY 1985 ( • " " c _J ._._l._,.. ¸_ - j, ;_.. , r_ '._:i , _ . ; . ,. NIA NASA Contractor Report 3891 Technology for Pressure-Instrumented Thin Airfoil Models David A. Wigley Applied Cryogenics & Materials Consultants, Inc. New Castle, Delaware Prepared for Langley Research Center under Contract NAS1-17571 N//X National Aeronautics and Space Administration Scientific and Technical InformationBranch 1985 Use of trademarks or names of manufacturers in this report does not constitute an official endorsement of such products or manufacturers, either expressed or implied, by the National Aeronautics and Space Administration. FINAL REPORT ON PHASE 1 OF NASA CONTRACT NASI-17571 "TECHNOLOGY FOR PRESSURE-INSTRUMENTED THIN AIRFOIL MODELS" PROJECT SU_IARY The objective of Phase 1 of this research was to identify, then select and evaluate, the most appropriate combination of materials and fabrication techniques required to produce a Pressure Instrumented Thin Airfoil model for testing in a Cryogenic wind Tunnel ( PITACT ). Particular attention was to be given to proving the feasability and reliability of each sub-stage and ensuring that they could be combined together without compromising the quality of the resultant segment or model. In order to provide a sharp focus for this research, experimental samples were to be fabricated as if they were trailing edge segments of a 6% thick supercritical airfoil, number 0631X7, scaled to a 325mm (13in.) chord, the maximum likely to be tested in the 13in. x 13in. adaptive wall test section of the 0.3m Transonic Cryogenic Tunnel at NASA Langley Research Center. The majority of these objectives were achieved in Phase I. Specifically it was shown that; -EDM (Electro-Discharge-Machining) by the wire-cut process was a cost- effective technique for cutting profiled bond planes and airfoil contours in 300 series stainless and 18 nickel 200 grade maraging steels. -Chemical millingwas a highly cost-effective technique for creating a complex network of channels in the surfaces of plates of these two metals. Initial samples were flat plates, but subsequently it proved possible to create matched pairs of channels on both the concave and convex surfaces of profiled bond planes. -Orifices of high definition have been produced by the technique of pre-drilling blind holes from the ends of the chemically-milled channels, such that they subsequently outcrop in the airfoil surface when its profile is EDM wire-cut. -Vacuum brazing has been used to join together the plates with the channels and orifice holes pre-formed into their surfaces. Well bonded plates with unblocked channels were achieved in small samples with flat bond planes, but fixturing problems with samples having profiled bond planes caused cross- leaks between channels. For small samples these problems are readily soluble, but for larger plates warpage, caused by the relief of residual stresses, is more serious. -Grain growth in 18 nickel 200 grade maraging steels occurs at brazing temperatures above IO00C (1830F). Good bonds have been made with this material using a nickel-palladium-silicon alloy, Metglas MBF 1005. -Joints between trailing edge segments and the main airfoil have been investigated using a comb and docking port configuration. Potential commercial applications of this research will occur initially in the construction of highly instrumented airfoil models for use in cryogenic and conventional wind tunnels. The opportunity for quality control at intermediate stages of fabrication should make it a cost-effective technique. It is also probable that some, or all, of these technologies would be usable in other applications requiring accurately formed channel networks. FINAL REPORT ON PHASE I OF NASA CONTRACT NASI-17571 "TECHNOLOGY FOR PRESSURE-INSTRUMENTED THIN AIRFOIL MODELS" INTRODUCTION Optimizationof the choiceof materialand fabricationtechniques for constructionof models for cryogenicwind tunnels such as the U.S. NationalTransonicFacilityand the NASA 0.3-m TransonicCryogenicTunnelhas presenteddesignerswith an almost insuperableproblem.Many of the propertiesrequiredare near the limitsattainablewith state-of-the-art technology,and in many cases improvementsin one directionseem inevitablyto be accompaniedby losses in others. For example,the materialhas to have a yield stresshigh enoughto carry the imposedaerodynamicloadings,yet be tough enough to operate safelyat cryogenictemperatures. The need for adequate strength and toughness for safe operation at cryogenic temperatures severely limits the range of materials available for model construction. In order to meet the minimum acceptable toughness requirements, many high-strength materials have to be heat-treated to a lower strength condition and this can lead to dimensional instability, either as a result of the formation of an unstable metallurgical structure, or due to the stresses induced on cooling from the heat-treatment temperature. The chosen material also has to be capable of being fabricated using available machining and joining techniques to give a dimensionally stable model with a precisely known shape. However, most conventional fabrication techniques create, to a greater or lesser extent, tensile or compressive stresses in the material, and in many cases they also result in a localized modification to the microstructure of the material. Considerable warpage can be caused either by the creation or relief of these stresses during model fabrication or stress relieving heat-treatments, while additional dimensional instability can be caused by the strains induced by differential thermal contraction during temperature cycling between room and cryogenic temperatures. In order to generate valid aerodynamic data the airfoil surface must have a high quality finish which also has to be either intrinsically resistant to, or capable of being protected against, corrosion and degradation. If the airfoil is to be of maximum use in providing aerodynamic information, it must, in general, be fitted with a complex array of pressure orifices, tubes and sensors of various types such as heat transfer gauges, transition detector gauges and temperature measuring devices. One aspect of particular concern is the method of achieving the necessary connections between orifices drilled into the airfoil surfaces to measure the pressure distribution over the airfoil and the measuring equipment generally located remote to the model. Typical current practice is to machine a recess into one surface of the airfoil to expose the bottom of the pressure tappings, braze fine diameter tubes to connect with each individual orifice and then lead a bundle of such tubes out via the wing root or support sting. The aerodynamic profile of the model is then restored by welding a cover plate, usually fitted with its own pressure orifices, over the recess. Alternatively, large channels are routed out of the airfoil surface, bundles of tubes are then laid in these channels and the gap backfilled with some form of filler which is then re-profiled to the airfoil contours. This technique is, however, unlikely to be particularly successful for thin airfoils used in cryogenic wind tunnels because of differential contraction between the different materials. Furthermore, these are labor intensive, and therefore expensive, methods of construction. Mistakes which occur in the latter stages of model fabrication, where the added value is high, are particularly expensive. The problem is, perhaps, at its most severe in thin airfoils typical of advanced fighter configurations, particularly at the trailing edge where thicknesses can be of the order of 0.040 inch. Hitherto, for example, in such models for the 0.3-m TCT, it has not been possible to provide an adequate array of pressure orifices at locations between 80 and I00 percent of chord using conventional construction techniques. Initial research and development of an alternative approach to airfoil fabrication has been carried out by NASA LaRC personnel and the principal investigator for the last 4-5 years. In essence, it involves machining a network of open channels into the surface of two flat plates which are subsequently vacuum brazed together to form closed, pressure-tight passages. These passages form connections between holes pre-drilled into the plates at one end of each channel and a take-off point at the edge of the airfoil. The passages are then pressure tested for cross-leaks before further expense is incurred in profiling and finishing the airfoil surfaces: if the channels are not pressure-tight the model can be aborted at this relatively inexpensive stage. The initial research was conducted using 15-5PII, 17-4PH, type 347 and Nitronic 40 stainless steels with electrodeposited coatings or thin foils of copper as the braze material. Further work was concentrated on the use of a nickel-based alloy, MBF 20 (AWS BNi2, AMS 4777A) and the development culminated in the fabrication of a symmetrical airfoil with a planar bond line made from 15-5PH stainless steel and vacuum brazed with MBF 20. This development was, however, only partially successful as warpage, caused by the relief of residual stresses in the plates, created numerous cross-leaks between adjacent channels. The research and development carried out during Phase I of this program is essentially a continuation of this earlier work, but

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