Scientists seek smoother copter ride

BUILDING A BETTER BLADE UM

March 14, 1992|By Douglas Birch | Douglas Birch,Staff Writer

COLLEGE PARK -- Gut-shaking vibrations make helicopters uncomfortable to ride in, expensive to maintain and hard to maneuver, limiting their use in commercial aviation and restricting their effectiveness in combat.

So a team of engineers at the University of Maryland has set out to eliminate damaging jitters by designing "smart" rotor blades that measure aerodynamic stresses and change shape in flight to reduce them. The team also hopes to build rotor blades that can warn pilots when the blades are about to crack or fail.

"We should be able to see an 80 to 90 percent reduction in vibration," said Dr. Inderjit Chopra, a professor in the Department of Aerospace Engineering at College Park. "We expect that the life of the helicopter may be tripled."

The Army has given Dr. Chopra and a team of 14 College Park engineering students and teachers a $2.3 million grant to develop the next generation of rotor blades -- with an eye toward using them in the RAH-66 Comanche, an attack helicopter under development.

The College Park grant is only part of a major push by both the Army and National Aeronautics and Space Administration to improve helicopter performance and reliability. NASA has already begun a $25 million effort at the space agency's Ames Research Center in Mountain View, Calif., to develop computer-aided helicopter controls that could, for example, enable the aircraft to automatically follow terrain and avoid obstacles.

One Army helicopter expert, who spoke on condition he not be identified, said research on rotor blades is promising but preliminary.

"Nobody's proven anything yet,although in concept it should work very well," he said. "I'm a believer in this, but the proof is in the doing."

Dr. Chopra is more optimistic, predicting that his team will be able to build working prototype blades within the next five to 10 years.

Once that happens, he said, the technology would be transferred to a helicopter manufacturer or a small company, perhaps founded by a student involved in the project, and commercial production would begin.

One of the Army's requirements, he said, is that College Park engineers work closely with the aviation industry. "All the helicopter companies will be watching what we are doing," he said.

Dr. Chopra also predicted that "smart" rotor blades and other "intelligent" parts will someday make helicopters as agile as a fixed-wing aircraft -- permitting them, for example, to turn much more sharply and even fly upside down.

These changes would help the Army realize its long-held dream of a "flying tank," he said, and permit wider use of helicopters as passenger aircraft.

Working in laboratories scattered around the College of Engineering, Dr. Chopra's team is now designing and building by hand a scale-model of a "smart" blade. Engineers hope to begin wind-tunnel and other tests this summer.

The team plans to build four fiberglass rotor blades that are 12

inches wide and 6 feet long. Attached to a hub, the four blades would form one complete rotor blade assembly.

Each blade would be embedded with piezoelectric crystals, a material used in the tweeters in home stereo speakers.

These crystals expand when a positive current is applied and contract when the current is negative. When exposed to pressure, the crystals also generate electricity.

College Park engineers plan to use the crystals, called "actuators," both as sensors and as tiny motors -- with no parts -- monitored and driven by on-board computers.

The computers are supposed to order changes in each blade's shape to subtly "tune" it to changing aerodynamic conditions.

Dr. Chopra cautioned that his team must figure out how to construct the piezoelectric crystals so they are strong enough to change the shape of the blade material.

Dr. Chopra also plans to use the crystals to move flaps on the edges of the blades. The trailing edge flaps, similar to those on the rear of an airplane wing, would further improve a helicopter's performance.

The actuators could also make the blades hum like tuning forks to produce harmonic waves that are designed to cancel damaging ones caused by the mechanics of flight.

Engineers plan to use fiber-optic cable to help warn of blade failure. A computer would pulse laser light through strands of cable embedded in the blade, then measure slight changes in the length and shape of the strands.

Careful measurement of those changes could pinpoint areas where the blade might be weakened or cracked, engineers said.

"The ultimate idea is to maintain the health of the structure," said Dr. James Sirkis, a fiber-optics specialist and an assistant professor of mechanical engineering.

However, Dr. Sirkis said the crystals and fiber optics must be built into the blade in a way that doesn't weaken the part's structural integrity.

He noted that there has been a lot of talk about the value of "smart" materials in aerospace in recent years. But primarily because of the cost, few practical applications have emerged, he said.

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