Skip to comments.Tests aim to silence sonic boom
Posted on 01/14/2004 9:56:38 AM PST by BenLurkin
The future of supersonic flight is taking shape in the very skies where the sonic boom first signaled the conquest of the sound barrier. This time, however, researchers hope to make a much quieter mark on history.
The Shaped Sonic Boom Experiment uses a specially modified F-5E Tiger fighter jet to show that aircraft may be shaped so as to lessen the force of the shock wave created as it goes supersonic, thus producing a quieter sonic boom.
"It is the forerunner of the future of supersonic flight," said Northrop Grumman chief test pilot Roy Martin, who piloted the modified aircraft. "Hopefully we'll look back on it in 10 to 20 years and say 'This was the first to prove we can do it efficiently and quietly.' "
Lessening the effects of sonic booms could allow future supersonic aircraft to fly over populated areas, greatly reducing travel times. Such aircraft are currently restricted to specific corridors. For example, the recently-retired Concorde passenger airliner was restricted to trans-Atlantic flights.
A follow-on to flights using the same aircraft last summer, the NASA-funded experiment will collect data during flights over Edwards Air Force Base in the next two weeks.
While the $7 million initial program began as a joint project with the Defense Advanced Research Projects Agency, Northrop Grumman Corp. and NASA, this follow-on portion is funded by NASA alone, at just under $1 million.
NASA wanted to continue the effort because "it involves creating knowledge about something unknown," said Peter Coen, manager of vehicle technology at NASA's Langley Research Center in Virginia. The agency invests in developing the basic knowledge that can then go on to industry to develop a product.
"NASA still has a charter to develop basic understanding of aeronautics to help American industry maintain its cutting edge," he said.
For this series of tests, researchers placed a series of 41 sensors, about 500 feet apart, in a 2½-mile line in the desert at the north edge of Rogers Dry Lake. Other sensors were placed about two miles on either side of this center line to measure the effects outside the direct flight path.
The modified F-5 flew a precise flight path over the line of sensors, followed about 45 seconds later by a standard F-5 on the same flight path, to provide a basis for comparison.
It was difficult for observers on the ground to determine a difference between the quick, double crack of the first aircraft and the second, but the data collected by the sensitive microphones and other sensors showed that the initial impact of the first shock wave was roughly half that of the unmodified second.
During the 10 a.m. flight on Tuesday, the unmodified F-5's shock wave registered about 1.2 pounds per square foot of pressure, while the modified version recorded about 0.6 psf.
Ultimately, the goal is to achieve a shock wave with only 0.3 psf, the point that researchers have determined is least objectionable to people on the ground.
At that pressure, the listener doesn't hear the familiar crack of a sonic boom, but "more like distant thunder," said Ed Haering , principal investigator for the program at NASA's Dryden Flight Research Center.
Data also was collected Tuesday by an Air Force Test Pilot School glider, hovering under the test aircraft at 10,000 feet. This position put the glider's sensors above the turbulence closer to the ground that can affect the shock wave's signature, Coen said.
Future flights are also planned to use Dryden's specially-equipped F-15 to probe the shock wave near the F-5 to collect data from that perspective.
All together, the data should be able to map the F-5 shock wave from its origination to the ground.
"Then we can mathematically reproduce that and we can predict what will happen," Martin said. "The ability to predict and develop mathematical models is what is so important about this test."
"We painted the answer on the side of the airplane three months before we flew," said Northrop Grumman aerodynamicist David Graham, who designed the modifications. He was referring to the lines along the fuselage that graph the difference in the intensity of the shock waves for the modified and unmodified aircraft.
"That's how confident we were," he said.
Tuesday's test flights measured the effects of the test aircraft flying at 32,000 feet and 1.4 Mach (1,065 mph), the optimum conditions for which the modifications were designed.
Further test flights will deviate from that point to see how well the modifications work in less-than-perfect conditions.
This deviation is important not only to prove that the mathematical models are accurate, but also because any future supersonic aircraft designed using this shaping technology will need to be able to perform in different conditions, such as getting to and leaving that cruising point, Martin said.
The modified airplane first flew in August. While the test data collected then proved the concept and modeling worked, the summer heat made it impossible for the aircraft to fly at the optimum conditions.
"We wanted to take the opportunity while we still had the airplane to get to the proper design point," Coen said.
The modifications to the F-5 added a little more than 3 feet to the length of the aircraft and thickened the nose and front end of the fuselage, adding about 18 inches to the depth. The changes were created using a composite skin over an aluminum frame, Graham said.
The modifications were designed to avoid changing the aerodynamic qualities of the aircraft beyond what was already known. The changes give the aircraft the same basic handling characteristics as a two-seat F-5F with a center fuel tank mounted beneath.
"We didn't want to get into aerodynamic characteristics the airplane wasn't designed for," Martin said.
The F-5 was chosen for the task because it provided the length needed to fulfill the needs of the experiment, while remaining small enough to make the modifications at a reasonable expense, Graham said.
The slender design of the aircraft also is a plus when dealing with supersonic characteristics.
While modifying an existing airplane was the most efficient way to prove the theory and the mathematical models used to predict performance, it is impractical for actual use.
Future supersonic aircraft may be designed from scratch to produce the lesser sonic booms.
The data collected from these flight tests may be used one day to create a supersonic X-plane to demonstrate the capabilities of designing these features in from the beginning, Graham said.
Conservative: "The sound of freedom, cool!"
Liberal: "How dare those warmongers annoy me!"
Liberal: "How dare those warmongers annoy me!"
Back when we were at Code Orange and the fighters were flying combat air patrol, I was sure hoping not to have anything that would justify them going supersonic! That would have been a frightening sound!
Yo, putz writer.
If you can go through it, it ain't much of a BARRIER, is it?