One of the greatest dangers to aircraft—playing a role in numerous destructive and fatal accidents around the world—comes in the form of droplets of water. Clouds are made up of tiny water particles with diameters typically between 10 and 50 microns. In clean air, cloud droplets can exist in liquid form down to temperatures as low as -40 °C. These subfreezing, liquid clouds are referred to as being “supercooled.” As soon as supercooled droplets contact an aircraft ascending or descending through the cloud cover, they form layers of ice on any unprotected surface, including the leading edges of wings and rotor blades, tails, antennas, and within jet engines. This ice accretion can cause engine damage and dramatically affect the aerodynamics of the aircraft. (On the leading edge of a wing, an ice layer about as thick and rough as a piece of coarse sandpaper can be responsible for as much as a 30-percent decrease in lift and a 40-percent increase in drag.) This can lead to reduced performance and even catastrophic loss of control.
As part of its aeronautics research, NASA has extensively investigated the icing problem, leading to numerous spinoff technologies that are helping reduce the threat. Glenn Research Center has led the Agency’s efforts, testing thermal, chemical, and mechanical anti-icing technologies in its Icing Research Tunnel; developing software tools for modeling ice growth and the impact of icing on aircraft performance; and producing pilot training aids for flight in icing conditions.
One way of mitigating the dangers of ice buildup is through the accurate, real-time identification of icing conditions, and researchers at Glenn have studied ways to detect supercooled water droplets in the flight paths of aircraft in and out of airports. One such method involves combining weather radars with devices called microwave radiometers, which measure the energy emission of liquid water and water vapor in the atmosphere at microwave frequencies (between 1–1,000 gigahertz). Combining the ability of the radar to detect cloud and hydrometeor particles—particles big enough to fall, like rain and hail—with the radiometer’s ability to detect liquid and vapor levels, provides a comprehensive picture of particle size, type, and distribution within clouds—essential information for determining icing risk.
Glenn partnered with Radiometrics Corporation, of Boulder, Colorado, to advance microwave radiometer technology for the detection of icing conditions. Supported by Phase I and II Small Business Innovation Research (SBIR) contracts, Radiometrics identified distinct, polarized signatures for liquid and ice cloud particles. These findings instigated further investigation with a narrower beam radiometer, which the company invented through additional Phase I and II SBIR agreements with Glenn. The resulting technology, a pencil-beam radiometer called the NASA Narrow-beam Multi-waveband Scanning Radiometer (NNMSR), is “a pioneering instrument that is seeing things in nature that have never been seen before,” says Randolph Ware, vice president of sales and marketing for Radiometrics. “You can locate this instrument in an airport, look at a narrow beam width along a flight path, and detect the supercooled liquid that creates the icing hazard.”
Testing in conjunction with Colorado State University’s CHILL radar (named after its original location in CHicago, ILLinois) supported the NNMSR’s ability to detect icing conditions, and further evaluation will take place at Glenn. The development of the NNMSR, Ware says, is a credit to the SBIR program.