The measurement of local flow angle is critical in many fluid-dynamic applications, including the aerodynamic flight testing of new aircraft and flight systems. Flight researchers at NASA Dryden Flight Research Center have recently developed, flight-tested, and patented the force-based flow-angle probe (FLAP), a novel, force-based instrument for the measurement of local flow direction. Containing no moving parts, the FLAP may provide greater simplicity, improved accuracy, and increased measurement access, relative to conventional moving-vane-type flow-angle probes.

Figure 1. The Prototype FLAP was a fin instrumented with simple electrical-resistance strain gauges.
Forces in the FLAP can be measured by various techniques, including those that involve conventional strain gauges (based on electrical resistance) and those that involve more advanced strain gauges (based on optical fibers). A correlation is used to convert force-measurement data to the local flow angle. The use of fiber optics will enable the construction of a miniature FLAP, leading to the possibility of flow measurement in very small or confined regions. This may also enable the "tufting" of a surface with miniature FLAPs, capable of quantitative flow-angle measurements, similar to attaching yarn tufts for qualitative measurements.

The prototype FLAP was a small, aerodynamically shaped, low-aspect-ratio fin about 2 in. (≈5 cm) long, 1 in. (≈2.5 cm) wide, and 0.125 in. (≈0.3 cm) thick (see Figure 1). The prototype FLAP included simple electrical-resistance strain gauges for measuring forces. Four strain gauges were mounted on the FLAP; two on the upper surface and two on the lower surface. The gauges were connected to form a full Wheatstone bridge, configured as a bending bridge.

Figure 2. The Air-Data Boom of the F-15B FTF was used to carry the FLAP in a flight test.
In preparation for a flight test, the prototype FLAP was mounted on the air-data boom of a flight-test fixture (FTF) on the NASA Dryden F-15B flight research airplane. The FTF is an aerodynamic fixture for flight-research experiments that is carried underneath the F-15B fuselage (see Figure 2). Measurement data were collected as the FLAP was flown on the F-15B at subsonic and supersonic speeds up to mach 1.7 and altitudes up to 45,000 ft (≈13.7 km). FLAP data were also collected under high-angle-of-attack and high-vertical-acceleration flight conditions. The flight data analyzed to date have verified the feasibility of the FLAP concept.

In a second-generation FLAP now under development, the electrical-resistance-strain-gauge force-measurement system of the prototype FLAP is replaced with a fiber-optic-strain-gauge force-measurement system. This FLAP will also be flown on the NASA Dryden F-15B airplane.

This work was done by Stephen Corda and M. Jake Vachon of Dryden Flight Research Center. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to Dryden Flight Research Center, Commercial Technology Office, (661) 276-3689. Refer to DRC-01-09.


NASA Tech Briefs Magazine

This article first appeared in the June, 2003 issue of NASA Tech Briefs Magazine.

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