Posted on 08/30/2003 9:30:02 AM PDT by BenLurkin
EDWARDS AFB - After more than 55 years of window-rattling and heart-thumping sonic booms, the familiar "Sound of Freedom" may be in for a change. In flights this week in the same supersonic corridor used by Chuck Yeager, a modified F-5 "Freedom Fighter" jet successfully proved that changing the shape of an aircraft can lessen the impact of breaking the sound barrier for those on the ground.
"We're trying to fix what Chuck Yeager broke," said Northrop Grumman chief test pilot Roy Martin.
In two flights Wednesday, the F-5 with a specially-shaped nose produced a smaller sonic boom than a regular F-5 flying the same route at the same speed one minute later.
The feat was repeated Thursday, with further data gathered in the air Friday.
The historic flights were the culmination of a $7 million project led by Northrop Grumman Corp., the Defense Advanced Research Projects Agency and NASA in the quest for quiet supersonic flight.
DARPA's Quiet Supersonic Platform program is an effort to identify and grow technologies that could allow military and civilian aircraft to operate with reduced sonic booms.
Lessening the effects of sonic booms could allow future supersonic aircraft to fly over populated areas. Such aircraft are currently restricted to specific corridors. For example, the recently-retired Concorde passenger airliner was restricted to trans-Atlantic flights.
As an aircraft flies through the air, it continually creates air pressure waves. Past the speed of sound, these waves combine to make shock waves, much like the wake of a ship moving through water. These shock waves create the sonic boom heard on the ground.
This week's tests set out to flatten the peak of the air pressure wave, thus softening the boom.
"That's what happened," said Jim Hart, Northrop Grumman spokesman. "It's very clear."
A typical F-5 creates a sonic boom that is measured at 1.2 pounds per square foot. The modified F-5 reduced that by about one-third, he said.
The eventual goal of the QSP program is an aircraft that produces a boom overpressure rise of less than .3 pounds per square foot.
Although it was not expected that this reduction would be audibly noticeable, observers said they heard a slightly less sharp boom coming from the test aircraft than the control one.
However, now that the concept has been proven, further reductions may be possible with new aircraft designs.
"To really get a quiet boom, you need to design from the ground up," Hart said.
Now that engineers know that shape does have an effect, designers "can use it with confidence," Martin said.
An F-5 was chosen for this test because the nose of the basic design has been elongated in different models of the fighter in the past, Martin said.
The outer structure of the aircraft nose was removed, and the specially shaped "nose glove" made of composite-covered aluminum was installed, adding about four feet to the length.
The modifications did not greatly affect the pilot's handling of the aircraft, although it was required to fly within a limited envelope, said Martin, who has 30 years' experience in F-5s. In fact, the aircraft's performance was better than predicted.
"The F-5 is very tolerant to changes in the airplane," he said. "You just feel like you're flying an F-5."
This particular Navy F-5 was on its way to retirement when Northrop Grumman was given permission to extend its flying hours and modify the nose.
In an interesting historical twist, the aircraft returned to Air Force Plant 42 in Palmdale for the flight tests, where it was originally built in 1974.
The aircraft took off early Wednesday morning from the Palmdale facility, joined by a regular Navy F-5 from Naval Air Station Fallon, Nev.
The two aircraft were joined at the supersonic corridor near Edwards Air Force Base by an F-18 chase aircraft provided by NASA's Dryden Flight Research Center.
The two F-5s reached almost 1.4 Mach -1.4 times the speed of sound - racing over the instrumentation collecting data at Harpers Dry Lake.
A true collaborative effort, Dryden and NASA's Langley Research Center were among the participants collecting data.
"It was a team effort right from the very beginning," Martin said.
On Friday, additional information about the shock wave off the modified aircraft was collected in the air by Dryden's F-15B research test bed aircraft.
The flight test program not only successfully lessened the sonic boom, the results were a near-perfect match to engineers' predictions.
In fact, the line graphing the predicted effect was used as pin-striping along the side of the F-5. The results are nearly identical to that stripe, Martin said.
"It matched the predictions almost right on the dot," he confirmed.
While the flight tests were historic, the idea itself is hardly new.
"This demonstration was trying to prove a theory first put out there in the late 1960s," Hart said.
While the theory itself has been around for decades, it was never tested outside of lab conditions.
With the successful test flights, this phase of the program is complete, Hart said. The data collected will be analyzed and shared with others in the industry for possible future development of quiet supersonic aircraft.
"BOOM TEST - An F-5E aircraft with a modified forward section takes off Wednesday on a test flight that demonstrated for the first time that changing a supersonic aircraft's shape can reduce the intensity of the sonic boom it produces."Northrop Grumman
So9
No, they are a possibility, but I am refering to the turbofan engines in the new F-22 fighter that can run supersonic without afterburners. They were developed as an incremental improvement on conventional turbofans, and seem likely to provide the power needed at the fuel economy required years before pulse detonation can be sorted out sufficiently to ever be certificated for a commercial aircraft.
So9
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