RESEARCHERS ZOOM TO CALM, BANISH BOOMS DESIGN IS KEY TO QUIET PLANES.

Byline: Jim Skeen Staff Writer

EDWARDS AIR FORCE BASE - Researchers are working on deadening the noise from supersonic aircraft, setting the stage for a day when jets can travel over homes without nerve-rattling, window-shaking sonic booms.

Following initial flight tests in August in which researchers showed for the first time that changing an aircraft's shape can soften sonic booms, researchers from Northrop Grumman and the National Aeronautics and Space Administration plan to conduct up to 20 flights this month.

``There were people who didn't believe it could be done,'' said Ken Plotkin, an expert on sonic booms from Wyle Laboratories, which is assisting Northrop Grumman. ``We've proven the physics - that's a big step.''

The tests are being conducted with two F-5E fighters, one with a modified nose. The two aircraft are being flown a few seconds apart over the same course and their sonic booms are measured.

The August tests, conducted under a $7 million effort funded jointly by NASA, Northrop Grumman, and Defense Advanced Research Projects Agency, wrapped up without the team being able to collect all the information they wanted. The new set of tests, costing close to $1 million, is being funded by NASA.

``In August it was hot and we couldn't get to design conditions,'' said Peter Coen, manager of supersonic vehicle technology at NASA Langley Research Center in Virginia. ``We want to get more data. The more data you have the better you can understand what you have done and how it will react under different conditions.''

NASA's Dryden Flight Research Center is conducting the flight tests.

During one of the flight tests on Tuesday, the two aircraft were flown at a speed of Mach 1.4, roughly 980 mph, and at 32,000 feet altitude. The aircraft traveled the same corridor 45 seconds apart.

To record the sonic booms, a series of microphones are set up 500 feet apart in a 2.5-mile-long line along the flight path. Two sets of microphones are also set up on either side, said Ed Haering, Dryden's principal investigator on the project.

Instruments to measure the sonic booms are also mounted on a glider, flying at 10,000 feet.

For the sonic boom research, a Navy F-5E was outfitted with a specially shaped ``nose glove'' and the addition of aluminum substructure and composite skin to the underside of the fuselage.

``It's a shape only a father could love,'' David Graham, a Northrop Grumman engineer. ``Yes, we added a canoe to the bottom. Yes, it looks like a pelican.''

The modified shape changes the shock wave generated by the aircraft.

As supersonic aircraft speed along, air molecules are pushed aside with great force and this forms a shock wave much like a boat creates a bow wave. The bigger and heavier the aircraft, the more air it displaces, NASA officials said.

The shock wave forms a cone of pressurized air molecules which move outward and rearward in all directions and extend to the ground. As the cone spreads across the landscape along the flight path, they create a continuous sonic boom along the full width of the cone's base.

The sharp release of pressure, after the buildup by the shock wave, is heard as the sonic boom. The change in air pressure is about the same as climbing up three flights of stairs, Haering said.

It is the rate of change - the sudden onset of the pressure change - that makes the sonic boom audible, Haering said.

A Concorde flying at an altitude of 50,000 feet creates a sonic boom of 1.94 pounds per square foot; a space shuttle returning to Earth causes a sonic boom of 1.25 pounds per square foot.

In tests, the standard F-5 registered a sonic boom measuring 1.2 pounds per square foot while the modified jet recorded a sonic boom at .8.

It might be possible to design aircraft with sonic boom pressures of just .3, researchers said.

``At point three you don't hear the shock wave,'' Plotkin said. ``It's like a dull thud.''

There is debate among those at the scene of the research as to whether the difference between the sonic booms of the F-5 aircraft are actually audible to the human ear. Some observers said they hear a distinct difference, but researchers said the difference might just be from atmospheric changes in the short time between the two booms.

Researchers said if an experimental plane was developed to further advance the technology, such a vehicle would produce a sonic boom clearly softer to the human ear. There are no plans for such a vehicle at this point.

The initial sonic boom research was conducted as an offshoot of an effort called the Quiet Supersonic Platform, an effort by the Defense Department to develop technologies for future high-speed bombers, fighters and reconnaissance aircraft.

Such technologies would allow commercial and military aircraft to fly unrestricted over land rather than being restricted to certain test ranges or - as in the case of the Concorde passenger jet - restricted to trans- Atlantic flights.

For the military, the technologies could lead to a small supersonic strike aircraft that would perform more missions per day, deliver more weapons on target and provide a rapid, long-range response capability, DARPA officials said.

The objectives of the QSP program are to create an aircraft that would weigh 100,000 pounds; cruise at speeds of Mach 2 to Mach 2.4, roughly between 1,400 mph and 1,680 mph; and have an unrefueled range of 6,000 miles.

By comparison, DARPA said, the Concorde weighs 400,000 pounds, cruises at Mach 2, and has an unrefueled range of 3,550 miles.

Jim Skeen, [phone]

james.skeen(at)dailynews.com

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(ran in Valley edition only) NASA and Northrop Grumman will be conducting up to 20 test flights this month in a quest to quiet the noise delivered by supersonic aircraft.