The retirement of the Concorde four years ago meant the end of supersonic flight - faster than mach one - for airline passengers. NASA's Supersonics Project is working on a new generation of technologies that could enable the return of safe, economically viable, and environmentally friendly supersonic jets to the skies.
Researchers at NASA Langley Research Center's Transonic Dynamics Tunnel (TDT) recently conducted the first round of testing on the SuperSonic SemiSpan Transport (S4T) model.
"It's cutting edge work that hasn't been done before," said James Florance, research scientist and S4T test co-lead.
In one month, the team completed approximately 55 runs and gathered over 2,000 data points. A run takes anywhere from a couple hours to as many as six hours straight.
"It's really a team effort," said Walt Silva, senior research scientist, S4T test co-lead, and Associate Principal Investigator for the aero-propulso-servo-elasticity discipline of the Supersonics Project. "It involves a broad range of technicians, engineers, computer scientists, civil servants, contractors, everybody. It's a huge undertaking done by a big group of people."
This was the first of a number of wind tunnel tests. Results will be used to develop control laws for future studies. Control laws, or control systems, are basically the rules used by an on-board computer to tell the airplane's control surfaces - elevators, rudders and ailerons - what actions to take.
Silva and other team members had one major objective for this test - to determine the characteristics of the S4T model to enable the design of control laws that will be used to suppress flutter and improve ride quality.
"A big part of what we study at the Aeroelasticity Branch is how to predict flutter, how to reduce it and how to suppress it," Silva said.
As an example, Silva compared flutter to what might happen when driving in a car with one's hand out the window.
"Let's say that, when you hit 60 miles per hour, your hand goes unstable. That would be the flutter speed of your hand," Silva said.
The S4T Test Team used the TDT test to learn and predict at what conditions and at what speeds flutter will occur. Once they process the data, the team will give the information to control law designers, who then create a control law to postpone the flutter point.
"We use control systems in a very specific manner that can delay when flutter happens," Silva said.
With the car example, a control law involving movement of the hand at a certain speed will delay flutter of your hand from 60 miles per hour to 70 miles per hour.
The team will also use the data to design control laws to counteract forces from gusts in the atmosphere. A gust can create a great deal of force at the root of the wing, and a control law would reduce that force.
Finally, the team attained data from the test to find ways to improve ride quality for airplane passengers.
"If you're a passenger, you don't want to be in an airplane that is bouncing around and flexing too much," said Silva. "We use control laws in a smart way to move the control surfaces to reduce discomfort."
Although comfort is important, safety of flight is the most critical motive for wind tunnel testing.
Findings reveal that the model appears to be more stable at some conditions than researchers originally thought, although the analysis needs to be double-checked. To ensure the accuracy of those findings as well as to acquire additional data, the S4T Test Team will perform a second set of tests next February.
In the upcoming tests, the S4T model will be used again, but will be examined in different conditions and situations.
The team plans to wind up the research with a third test in February 2009. At that time, they will implement the control laws designed with the results of the first tests to demonstrate improved ride quality, gust load alleviation and flutter suppression.