New technology could save airlines billions
By: Russell Smith

New technology developed at WPI uses ultrasonic energy to detect wake turbulence, a hazard created by the wings of moving aircraft. Recent trials of the system at a small airport have shown that it can effectively detect the onset, strength, and decay of the pair of counter-rotating vortices that stream from an airplane's wingtips, information that could help traffic controllers shorten the spacing between landings and take-offs at major airports. Reducing the standard waiting times by 30 seconds could save the airline industry $5 billion annually, according to the Federal Aviation Administration.

The technology, which is simpler and less expensive than other techniques that have been proposed for the real-time detection of wake turbulence, was described in papers presented recently at the 44th AIAA Aerospace Sciences Meeting in Reno, Nevada, and the 12th Conference on Aviation Range and Aerospace Meteorology in Atlanta, Ga.

When airplanes move through the air, their wingtips create rapidly swirling vortices that can linger in the air for several minutes. Another plane flying into this wake turbulence can roll or drop unexpectedly, or even suffer structural damage. The risk is greatest during take-offs and landings, when there is little time or altitude in which to recover. Therefore, air traffic controllers space landings and take-offs by standard intervals, which are based on the longest amount of time it would take for vortices from specific aircraft to dissipate (up to eight minutes for the largest jetliners). In reality, crosswinds often cause the vortices to dissipate or to be blown clear of the runway more quickly.

The patented WPI technology, developed by mechanical engineering professors William Durgin and Hamid Johari and graduate student Rebecca Rodenhiser, uses pulses of high-volume, high-frequency sound waves to measure the speed of rotating air. Two focused beams of sound travel the same triangular path in opposite directions. The path is created by bouncing the sound off two reflectors, one located on the opposite side of the runway from the sound source, and one atop a 30-foot tower.

The difference in the travel times for the two beams is measured. Rotating air will cause the sound beams to either slow down or speed up, depending on whether the sound is traveling with or against the moving air. By monitoring the changing travel times at several points along the runway, the technology will be able to track the position and strength of vortices over time, enabling air traffic controllers to establish a safe separation interval based on the time it takes for the turbulent to subside.

The effectiveness of the technology was first demonstrated in wind tunnel tests at WPI. A prototype, built by Rodenhiser as part of her master's thesis work, was tested under varying weather conditions at an airport in central Massachusetts in the spring of 2005, using a small single-engine plane to generate the vortices. Further tests, with commercial and military jet aircraft, are planned for later this year. The research team ultimately hopes to commercialize the technology.

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