To the layman, jet screech evokes a particular sonic notion, the high-pitch shriek that accompanies a powerful military aircraft.
But for engineers, screech has a more specific meaning. It’s an audio effect, yes, but one with potentially deadly consequences. It “causes the jet to flap violently at discrete frequencies,” according to a Defense Technical Information Center research document and can be “a source of acoustic fatigue in the tail and nozzle structures of supersonic aircraft.”
In other words, screech can literally shake planes apart.
Now a team of Air Force engineers is working with a combination of sophisticated sensors and advanced algorithms to detect and fix screech before it becomes a problem. It’s testing a system known as Screech Wave Analysis Methodology (SWAM).
“You get strong pressure gradients pushing on the hard surfaces, and those surfaces can be broken, whether it’s a fuel line or a holder or a liner, or any of the other various components,” said Alan Hale, an analyst at Arnold Engineering Development Complex (AEDC), an Air Force Materiel Command facility. “If I have fuel coming in through a fuel line into an augmenter, and that fuel line gets broken, that engine no longer works properly.”
Today, the Air Force has some capability to detect the presence of screech, but better sensors and advanced analytics tools may allow engineers to fine-tune their understanding and deliver targeted fixes.
“Currently, we can detect a screech event. With this analysis tool, we can now see what that event actually did: How it is happening, what is happening. That allows us to come up with different methods to offset that event,” said Josh Osborne, senior engineer for test capabilities integration.
It’s no small trick to tease out a meaningful signal from among the tumult of jet noise.
“When things burn there is a lot of complexity in the chemistry, and when that coincides with pressure pules, then you have an amplifying effect. So it’s hard to get your hands around it,” Hale said. “Things behave very differently at different temperatures and different pressures. There are a lot of unknowns in the equation, and we are trying to put all these together simultaneously.”
In order to get that coherent picture, the engineers have been experimenting with a variety of advanced pressure sensors, paying special attention to the location of those sensors on the aircraft.
“There are certain locations where you need to have the sensors in order for the analysis tool to get the information it needs,” Osborne said. “You want your sensors located at a place that is active, where the waves are strong and are giving you a lot of information. You don’t want sensors at a node where waves cross and cancel out, where there is no new information.”
With the right sensors feeding the right analytics, the team has been able to read into screech with a new level of understanding, giving pilots new ways to avoid potentially hazardous flight conditions.
Certain flight patterns for instance can be shown to trigger screech in certain aircraft, and pilots can be trained to fly around such conditions. If you know the how and the why of a likely screech event, “you can try not to fly in a certain way. You avoid certain flight situations that are known to cause this problem,” Osborne said.
The team envisions implementing its screech analysis as a ground-test capability that could be applied to any new or existing jet.
If SWAM can be proven in the lab, it could have a widespread impact on how the Air Force flies. “We’ll be able to accurately predict screech so that we can design in change and mitigate cost before it ever sees flight,” Osborne said. “We will make the problem not exist by catching it early on and developing solutions early on.”