Pratt & Whitney Unveils Engine Laser Sensor, Plans Boeing 747 Testbed Flight

A newly developed laser-optical technique for measuring detailed engine thrust and other parameters with unprecedented accuracy will be evaluated in flight by Pratt & Whitney on the manufacturer’s Boeing 747SP flying testbed.

Pratt & Whitney says the device will support the development of more-efficient engine core technologies and, because of its high-fidelity measurement capability, could also enable the detection of non-CO₂ particulate emissions in flight.

Developed in partnership with Virginia Tech, the system depends on a principle known as Rayleigh scattering, in which the light reflecting from air molecules—in this case passing over a laser-illuminated area—can provide information, here about the gas flow field. Pratt & Whitney says the system, which can be used in the inlet as well as the exhaust, could potentially augment and eventually replace the traditional sensors, rakes and probes used in current engine testing.

Known as FRST (Filtered Rayleigh Scattering for Thrust measurement), the system has already undergone test and development at Virginia Tech on small business jet engines including the Honeywell TFE731 and Pratt & Whitney Canada PW300. The device can project a “sheet” of laser light across the entire planar area of an inlet and exhaust. Developers say it offers significant advantages compared to traditional sensors and probes that only measure flow in discrete parts of the gas path.

“We think this is going to change the way we do things in the industry,” says Al Krejmas, Pratt & Whitney’s principle fellow discipline lead for test and validation and lead developer of FRST at the engine manufacturer.

“As we go forward in the development of our products engineers want more fidelity and data. But all of that is diametrically opposed to the direction of business and its desire for cheaper and faster. So, our mindset was to change the culture and move to an optical-based measurement system versus all of these wires and sensors,” Krejmas says. “It’s a move toward what we consider to be a 21st century measurement system where we get more, higher-quality data.”

Describing the system in more detail, Todd Lowe, a professor of aerospace engineering at Virginia Tech, says Rayleigh scattering provides information directly from molecules which are in motion because of flow velocities. “The signal that we get back from Rayleigh scattering puts all that information about velocity, temperature, and density into spectral form and we derive a pressure. That has huge implications because we can get all of that information and do it robustly,” Lowe says.

Light from a high-power laser is transmitted to the limited confines of the inlet or exhaust via a fiber-optic cable system. Notch filter sensors arranged around the circumference of the inlet or exhaust liner also carry the reflected light back to a camera mounted remotely in the nacelle. Optical data from the camera is then transmitted to a processor for spectral analysis.

As well as flows through engines, Pratt and the Virginia Tech developers tested the system in the university’s slow-speed Stability Wind Tunnel on an aerodynamic model of a 737.  “We were able to satisfy ourselves that we can measure not only thrust, but also the drag of the airplane in the flow field in and around it. So that was another building block and monumental step toward getting to where we're headed with this,” Krejmas says.

By eliminating the traditional inlet sensor rakes, developers say future engine tests could be done with zero inlet distortion. “If there’s nothing sitting in the flow, there is also no risk of anything being ingested in the engine. But we also get a much richer data set than we're classically used to getting,” Krejmas says.

The ability to derive data across a broader swathe could also be useful for testing emerging propulsion systems, he adds. “When we get these distributed propulsion concepts, whether they be commercial or military, measuring thrust across multiple engines and fans would be hard using our traditional methods. With the new system you put a sheet of laser across all of it and get a good dataset.”

Pratt aims to evaluate the FRST by piggybacking on an unspecified upcoming engine-test campaign to perform back-to-back testing with the existing data system. “We'll measure both the inlet and exit plane with all of the traditional instrumentation as a validation of the measurement system before testing the FRST in flight and then compare the result,” Krejmas says.