A unique dual-flow, cold-jet facility has been developed and operated by California Polytechnic State University (Cal Poly) at San Luis Obispo for NASA Dryden Flight Research Center. The facility supports flight research on thrust-vectoring nozzles and thrust-vector control systems. To date, the facility has completed tests on subscale nozzles of the F/A-18 High Alpha Research Vehicle (HARV), the X-31 airplane, and the F-15 Advanced Control Technology for Integrated Vehicles (ACTIVE).

The facility contains a nozzle flow bench (see Figure 1) that incorporates unique features for research on single nozzles and on twin nozzles, which afford the ability to evaluate flow-interaction phenomena. Subscale nozzles are typically mounted on top of a thrust stand on the bench and connected to the end of an airflow-supply tube. The thrust stand is of a multiaxis design that affords capabilities for measuring all components of thrust and moment vectors.

Figure 1. The Equipment on the Nozzle Flow Bench typically includes subscale nozzles mounted on top of a thrust stand and connected to a source of pressurized air via a manifold system that suppresses spurious air-supply- related side loads. The thrust stand enables measurement of all components of thrust and moment vectors.

A manifold system that supplies air to each nozzle independently is designed to avoid the introduction of extraneous side loads. The manifold system includes a plenum and bellows. Airflows at approximately equal rates are supplied on opposite sides of the plenum in order to cancel momentum and pressure effects that could otherwise be attributed to the air supply. This manifold design virtually eliminates extraneous forces from the air supply. In the dual-flow configuration, the flow rate or pressure ratio of each nozzle is independently controlled, providing maximum flexibility in testing.

The development of both the thrust stand and the air-supply manifold has made it possible to perform accurate research on thrust vectoring with small-scale nozzles. A capability for color schlieren photography has also been developed, making it possible to obtain visible records of complicated exhaust-flow fields and shock structures (for example, see Figure 2). A color schlieren video apparatus has also been built for use in evaluating the stability of exhaust -flow fields.

To verify the accuracy of the cold jet, a single 1/24-scale F/A-18 HARV nozzle configured with postexit vanes was tested in this facility, and the results of the tests were compared with those of similar tests performed on a larger-scale model at Langley Research Center. These tests also enabled detailed evaluation of a postexit-vane-tip interference effect that was pronounced at higher pressure ratios.

Additional single-nozzle tests were performed on the X-31 nozzle configuration to evaluate the effects of extreme deflections of postexit vanes. Static-pressure ports were added to the divergent section of the nozzle to obtain data pertinent to concerns about operability. The results of these tests supported the implementation of a 10° increase in deflections of nozzle postexit vanes on the X-31 airplane during its flight-test program, helping the aircraft achieve greater maneuverability.

One of the two-nozzle configurations shown Figure 2 was tested to explore the effects of flow interaction during thrust vectoring of the F/A-18 HARV. One of the main objectives of the tests was to validate a superposition assumption used in the design of the F/A-18 HARV thrust-vector control law. The assumption in question is that exhaust plumes from the two nozzles do not interact with each other and thus their total-force vector can be determined by summing the force vectors of the individual nozzles as modeled in single-nozzle cold-jet tests. The aforementioned tests were the first documented tests to validate this superposition assumption. The results of the tests were found to support this superposition assumption, except at extreme vector angles, where one nozzle could impinge on a postexit vane of the other nozzle.

Figure 2. These Schlieren Photographs reveal some aspects of the exhaust flows from scale-model pairs of thrust-vector nozzles in cold-jet tests.

At present, the facility is testing thrust-vectoring nozzles with axisymmetric configurations, similar (except in scale) to full-scale, production like axisymmetric thrust-vectoring nozzles that are undergoing flight testing on the F-15 ACTIVE aircraft. The sub-scale-nozzle tests in the facility support the F-15 ACTIVE flight-test program. The results of in-flight measurements of nozzle vector plume angles have been found to differ significantly from those of corresponding measurements in subscale tests. The source of these differences has not yet been discovered, but finding this source is the primary goal of both the cold-jet (subscale) and the flight tests.

More specifically, 1/32-scale fixed-geometry vectored nozzles are undergoing tests in the facility. The primary immediate objectives of the cold-jet tests are to (1) attempt to reproduce the flight-measured vector plume angles and (2) evaluate and determine the locations and number of internal nozzle pressure sensors needed to measure in-flight pressure distributions accurately.

In addition, the tests accommodate some basic research on flow separation and stability. A unique test of a transparent two-dimensional nozzle has provided insight into the internal flow field of an axisymmetric thrust-vectoring nozzle. Part of Figure 2 presents images of the two-dimensional nozzle and a three-dimensional axisymmetric nozzle undergoing a test side by side at equal pressure ratios, vector angles, and throat and exit-area conditions. These tests have shown that the shocks inside the nozzles exert significant influence on the exhaust-flow field during thrust vectoring. Additional tests on axisymmetric nozzles instrumented with internal static-pressure probes are under way.

The data obtained in the facility have provided significant support to NASA's research on thrust-vectoring nozzles. Its relatively small scale and innovative approach have resulted in accurate, relatively inexpensive, and rapid testing, with such unique capabilities as its dual-flow capability. The schlieren photographic capability provides a valuable insight into flow-field properties, and the schlieren photographs augment the force and moment data traditionally available from cold-jet facilities.

This work was done by Albion H. Bowers, John S. Orme, and Ronald J. Ray of Dryden Flight Research Center and Thomas Carpenter and Jim Gerhardt of California Polytechnic State University. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Physical Sciences category. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Thomas Carpenter
California Polytechnic State University,
San Luis Obispo
Department of Mechanical Engineering
San Luis Obispo, CA 93407

Refer to DRC-97-49, volume and number of this NASA Tech Briefs issue, and the page number.

NASA Tech Briefs Magazine

This article first appeared in the December, 1998 issue of NASA Tech Briefs Magazine.

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