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Fall 1998

Motorized glider studies Colorado thermals

by Zhenya Gallon
UCAR Communications

A volunteer team of scientists, glider pilots, and technicians spent the spring exploring an elusive atmospheric phenomenon from a high-performance, hybrid aircraft. With support from UCAR, the scientists are now analyzing what they learned about the gravity waves that sometimes form above thermals. Thermals are rising columns of warm air used by eagles and other raptors, as well as gliders or sailplanes, to gain altitude in motorless flight. Although they have eluded thorough scientific measurement until now, thermals may hold one key to better weather forecasts. For glider pilots, the ability to extend flights over greater distances is an extra benefit of the research.

The Stemme glider in a test flight. (Photo by Carlye Calvin.)

The volunteers are from the Soaring Society of Boulder (SSB). Volunteer Phil Ecklund, a senior captain for United Airlines with 27,000 hours of flight time, explains the effort: "It's a labor of love for all of us. And the chance to be involved in a scientific mission was too good to miss."

A powered aircraft can't make the tight turns to maneuver in and around thermal columns, which have diameters of less than half a kilometer (a third of a mile). Conventional gliders can make the turns, but they lose altitude too quickly to get a complete picture within the lifetime of a thermal. To study the fine structure of thermal waves, the researchers needed an aircraft combining the maneuverability of a high-performance glider with a motor to stay airborne and travel long distances. Project director Joachim Kuettner knew that the Stemme S-10VT motorglider developed by his German colleague Rainer Stemme had those capabilities, so he worked hard to bring one to Boulder for the experiment, named THERMEX. Kuettner holds the UCAR Distinguished Chair for Atmospheric Science and International Research, which is funded by NSF.

Tthe motorglider takes off and climbs using its turbocharged engine, draws in its propeller, and becomes a high-performance glider. "It's the only sailplane that retracts its propeller inside the nose while in flight," says Kuettner, explaining the secret of its performance capabilities. During flight, the propeller can be reactivated. This allowed the researchers to gather more data from individual thermals and to travel between and above promising cloud structures in search of the waves. The Rocky Mountains produce their own "mountain waves." But with the craft's 75-foot wingspan and 1,000-mile range, the team was able to explore thermal waves in South Dakota and Nebraska without interference from mountain waves.

Scientific questions

Thermal waves form over flat lands and oceans in the vertical wind shear created when horizontal winds encounter a thermal column. Small cumulus clouds often form as water vapor cools and condenses after being carried aloft by the convection in a thermal. The team hopes to learn more about the interaction of thermal waves with the convective currents in the boundary layer (the lowest kilometer of the atmosphere).

Skilled glider pilots have known about the extra "lift" in thermal waves for some time. But these atmospheric structures haven't been studied systematically until now "because we had no way to do it before the Stemme S-10VT," says Kuettner. To understand how thermal waves form, the team gathered data on their height range, horizontal velocity, tilt with height, and intensity. "Although El Niño provided unfavorable, mostly calm situations, we encountered several interesting cases," including wave conditions reaching as high as 8,500 meters (24,000 feet), Kuettner adds.

According to THERMEX codirector Robert Grossman, better understanding of thermal waves could lead to improvements in the forecasting ability of computer models. Grossman, one of the volunteers, is an atmospheric scientist at the University of Colorado in Boulder (CU) and a visiting scientist at NCAR. He explains that global winds are slowed by friction, or drag, from the earth's surface. Modern computer models used to forecast winds include a way to account for the drag exerted by mountains. However, thermal convective currents caused by solar heating also form "virtual mountains" if they rise into layers of increasing horizontal winds. Global satellite images show fields of small cumulus all over the world, every day. "Even though not every cloud produces a thermal wave, there are probably enough being produced each day to make a substantial contribution to the global drag problem," Grossman says.

A volunteer effort

"In order to pull a research project off, you need money or you need volunteers," says Kuettner, who serves on the Board of Directors of the SSB. When he explained his plan to SSB members, he found many skilled volunteers interested in the project. He notes that, "Although a volunteer effort is, by its nature, less efficient than a fully funded project, the community-building and educational benefits make it very worthwhile."

The SSB volunteers had already logged many hours in conventional gliders contributed by the society. After putting the Stemme through its paces, Ecklund remarked, "It was some of the most interesting and challenging flying I've done. " For this spring's flights, engineering consultant Jim Hauser and software consultant Colin Barry each contributed specialized data-handling software of their own design. Barry, who is also president of SSB, and retired CU physics professor Rod Smythe were scientific observers. They flew in the copilot seat, taking notes and running the data-gathering programs on a laptop computer. Volunteer Dave Campbell, who owns Mile High Gliding and DC Auto Electric, worked on the scientific instruments that gathered the data. Grossman shared forecasting duties with Kuettner and was observer on several missions. Kuettner also brought in one paid participant, physicist Wolf Dietrich Herold, who is also a highly skilled pilot. Formerly a visiting scientist at NCAR, Herold traveled to Boulder from Zurich, Switzerland, just for the mission.

Thermal waves were once a well-kept secret of champion long-distance glider pilots. Mastering them can greatly extend the duration and distance of unmotorized flight. "What is learned from THERMEX is certain to find its way into international gliding circles," says Kuettner.

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