A boundary-layer rake of pitot tubes has been designed and built for installation on a flight-test fixture (FTF) on the NASA Dryden F-15B, which is a two-seat version of the F-15 high-performance, supersonic, all-weather fighter airplane. This boundary-layer rake will be used in flight-research projects in which there are requirements for detailed surveys of the turbulent boundary layer. A design feature unique to this rake is a curved rake body; this feature makes it possible to cluster the pitot tubes in the near-wall region more densely than they can be clustered in conventional rakes. Results of tests have shown that this rake exhibits good aerodynamic performance and that it is operationally rugged.

Figure 1. The Curved Shape of the boundary-layer rake makes it possible to cluster the pitot tubes more densely in the near-wall region.

To give the rake its complex three-dimensional shape (see Figure 1), it was necessary to resort to innovative solid-modeling and machining techniques. Starting from a three-view conceptual sketch, a three-dimensional solid model was constructed by use of the ProEngineer solid-modeling computer-aided design/computer-aided manufacturing (CAD/CAM) software package. This software package was used throughout the entire design and machining process, ensuring accurate machining of the rake from the three-dimensional solid model. After a solid model was created in ProEngineer, a computer-controlled wire electrical-discharge machine (EDM) was used to cut the basic shape of the rake out of a solid block of aluminum alloy 2024-T351.

The rake was then machined on a computer numerically controlled (CNC) milling machine. First, the base of the rake was machined for flush mounting on a flat surface. To make room for the installation of the pitot tubes, a cavity was machined inside the rake body. Pitot-tube-mounting holes were drilled on the leading edge of the rake, then the leading edge was tapered to a sharp angle. To close off the cavity in the rake body, an aluminum cover was created on the wire EDM.

Figure 2. These Flat-Plate Skin-Friction Data were calculated from measurements taken by the boundary-layer rake shown in Figure 1.

After the machining process, all parts were deburred, inspected, and then anodized to provide protection from corrosion and wear. The pitot tubes were machined from 304 stainless steel tubing of 0.04-in. (1.0-mm) outside diameter and 0.0075-in. (0.19-mm) wall thickness that had been annealed to a 1/2-hard condition. The pitot tubes were then inserted in the rake body. The tips of the pitot tubes were chamfered to reduce their sensitivity to local flow angles. A low-viscosity, single-component, anaerobic methacrylate ester adhesive (Loctite 609) was used to hold the pitot tubes in place. To help keep the pitot tubes in place and to protect them from vibrations during flight, room-temperature-vulcanizing (RTV) silicone rubber was used to pot the inside of the rake cavity.

A finite-element stress analysis of the rake design showed very high factors of safety for operation in a supersonic wind tunnel. The rake passed a ground vibration test in which random vibrations of 12 times normal Earth gravitational acceleration were imposed for twenty minutes along each of the three mutually perpendicular directions. A wind-tunnel test of the rake was conducted in the NASA Glenn Research Center 8-by-6-ft (2.4-by-1.8-m) supersonic wind tunnel at mach numbers ranging from 0 to 2. The rake pitot pressures agreed well with data obtained from a conventional rake for the entire range of mach numbers tested. The boundary-layer profiles obtained from the rake data matched the standard log-law profile. As shown in Figure 2, values of skin friction computed from the rake data by use of the Clauser-plot method agreed well with Preston-tube results and with the Van Driest II compressible skin-friction correlation.

The rake will be used in a number of future F-15B/FTF flight experiments. One experiment currently underway is an in-flight evaluation of new skin-friction gauge concepts: the rake data as well as the results from a Preston tube will be used to evaluate the accuracy of new skin-friction gauges. Another experiment is planned to validate the microblowing drag-reduction technique in flight: In this experiment, the net drag reduction caused by blowing an extremely small amount of air through a porous plate will be calculated from the momentum balance of the boundary-layer profiles measured by the rake in the upstream and downstream regions of the porous surface.

This work was done by Trong T. Bui and David L. Oates of Dryden Flight Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to the Patent Counsel, Dryden Flight Research Center; (805) 258-3720. Refer to DRC-98-94.


NASA Tech Briefs Magazine

This article first appeared in the December, 1999 issue of NASA Tech Briefs Magazine.

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