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Sonic tonic

NASA and other leading aerospace companies and agencies are determined to develop a low sonic boom aircraft – but what are some of the testing challenges involved?

 

The chief barrier to a new age of supersonic commercial flight has always been the loud complaints from below about the noise. A near total ban on supersonic flight and sonic booms over land put paid to Concorde’s chances of becoming a commercial success and has grounded the industry ever since; and while propulsion technology has continued to advance, the much more difficult challenge of developing a low-boom aircraft has remained unsolved.

But that could be about to change as a crop of research projects around the globe attempts to get to grips with the problem. Airbus recently filed designs with the US Patent and Trademarks Office for a low-boom supersonic aircraft, and the Japan Aerospace Exploration Agency (JAXA) announced that in July this year it completed the world’s first successful flight test of a low sonic boom concept aircraft.

Some of the most intensive work in this area is being carried out under NASA’s Commercial Supersonic Technology (CST) Project, which encompasses many aspects of supersonic flight technology and involves the Armstrong, Glen, Ames and Langley research centers. Brett Pauer, the project support manager at the Armstrong Flight Research Center in California, says the goal was to gather data to enable the likes of the FAA and ICAO to establish a global low-boom standard.

As part of that effort, the team is developing new technologies and test techniques that will lead to a better understanding of sonic booms – in particular, the complex fluid dynamics immediately around the surface of the aircraft. “At Armstrong in particular, we are trying to figure out how sonic booms propagate through the air,” explains Pauer. This includes work with local communities on the ground to find out how they respond to the noise generated by supersonic flight. “In the future, it is our hope, although this is not approved, that we can fly a low sonic boom aircraft over communities that have not been exposed to booms. So we are moving toward designing test techniques that would enable us to accomplish that.”

The Armstrong team recently tested a cockpit display which shows the extent of the sonic boom on the ground. “This takes information from the aircraft, and weather data from below, and predicts where the boom generated by the aircraft will hit the ground. We have just recently tested that in the back seat of an F-18 while going supersonic,” notes Pauer. The boom footprint is superimposed on a moving map, with color-coding to show the intensity.

NASA’s fleets of F-15s and F-18s at Armstrong are used like “a flying wind tunnel”, adds Pauer; items of high technology readiness level are loaded onto the airplanes and tested in actual flight, at speeds of Mach 1.7 and higher. “A recent test involved a laminar flow wing section that we turned sideways and mounted vertically on the underside of the aircraft. And we took it up to speed and were able to see the extent of laminar flow on the aerofoil design. We can do that with all kinds of different fixtures – not just wings, but propulsion items and probes as well.”
Probes are a vital test tool for gathering information about the behavior of supersonic flows immediately around the surface of the aircraft. Pauer says standard probes tend to incorporate a ‘lag’ in their design, because the measuring device was separated from the tip of the probe by a long tube of air. He says researchers are working on a new design, in which the sensor is much closer to the surface of the aircraft and therefore is able to provide a more instantaneous reading.

Further out from the aircraft, the team is working on sonic boom propagation and how that is affected by the weather and other phenomena such as turbulence. As in the JAXA tests, this usually entails an aircraft flying at supersonic speeds over a series of microphones on the ground. However, to measure the affect of low-altitude turbulence on the boom, they plan to record the aircraft from both above and below at the same time and analyze the differences.

Pauer explains, “The way we plan on doing that is to use a motor glider, and we shut the motor down above the turbulent level at about 10,000ft, well above the turbulent layer. This will capture the sonic boom above the turbulent layer and then we’ll have microphones underneath so we’ll capture the influence in the microphone recording.”

Schlieren imaging
Most significantly, however, has been the progress made in improving on a 19th century method of photographing shockwaves. The technique, known as Schlieren imaging, is an important testing tool and has long been used
in wind tunnels to capture the flow around scale models and other test objects.

Five years ago, NASA began a program to develop the technology for imaging shockwaves in close proximity to full-scale aircraft in supersonic flight – a much more challenging task. 

The Schlieren images reveal the presence of shockwaves due to the change in air density and the accompanying change in the refractive index. Originally, this required the use of complex optics and strong light sources; more recently image-processing technology has been used to develop ‘synthetic’ Schlieren techniques, in particular background-oriented Schlieren (BOS). Researchers take a series of photographs of the object in supersonic flow against a speckled background – the shockwave image is derived from analysis of the distortions in the pattern. BOS is light on complex hardware and heavy on computer processing, which makes it an attractive means to obtain high-spatial-resolution imaging of shockwaves in flight.

In April 2011, the first phase of Armstrong’s AirBOS testing took place, to capture images of the supersonic shockwave created by an F-18. A high-speed camera on the underside of a B200 King Air captured 109 frames per second while the supersonic target aircraft passed several thousand feet underneath in straight-and-level flight at speeds up to Mach 1.09. The team took pictures with a relatively simple system consisting of a laptop with a frame grabber and using natural desert vegetation as the speckled background.

The test was run again in October 2014 with better resolution and higher frame-rate cameras, and achieved a dramatic improvement in the images. The use of different lens and altitude combinations and knife-edge aircraft maneuvers by the pilot of the target aircraft provided the opportunity to obtain side-on images.

Further improvements were made for AirBOS 3 tests in February 2015. After each flight, the NASA-developed software was used to remove the desert background and then the frames were combined and averaged to produce clean and clear images of the shockwaves.

Air-to-air photography, however, is not easy. It involves synchronizing the flight paths of a supersonic and subsonic aircraft, which requires meticulous planning and precision flying, and a complex integration of the aircraft’s navigation systems must be performed to ensure both are properly positioned over the background target area. The only alternative is to photograph from the ground and use the edge of the sun as a background.
While this method yields adequate results, it is only possible to make two observations in each pass as the target aircraft crosses the left and right sides of the sun.

Solar harvest
Then Armstrong engineer Edward Haering made a breakthrough when he noticed that the supersonic shockwave also distorted the visible sunspots on the sun’s surface: “When you film the sun with the right filter you see a bright white circle of light with the black sky around it, and without the aircraft there, its just circular; but when the aircraft passes by, it ripples the edge,” he says. “When doing some of those flights we also saw sunspots, and the sunspots would kind of twinkle as the shockwave went past so we thought, well, if we have a lot more sunspots, we could get a lot more information.”

He then used some calcium-K optical filters to reveal more of the granulated texture of the sun’s surface. “When you image it, you see all these little speckles on the sun, and when the shockwave goes across that you have all those speckles scintillating and twinkling, and mathematically when the flight is over we can look at the data and figure out how much each of those little speckles has shifted and extract back out how strong the shockwaves were.”

Haering is the originator of what is now known as the BOSCO concept, Background Oriented Schlieren using Celestial Objects, which potentially offers a practical and safe approach to studying the shockwaves of larger supersonic aircraft, such as a commercial transport demonstrator.

“The nice thing about using the sun is that at sunrise it is level, so you can see shocks directly underneath, which are of most interest to us because those are the ones that hit the ground. For the flights where we have the supersonic airplane under the King Air aircraft, we have the pilot fly a 90° bank at the last second so we get knife-edge flight and we can see relative to the aeroplane the shocks above and below the airplane. That is great for a fighter, but if we ever get a low-boom transport aircraft, we won’t be able to fly knife-edge, so we will probably have to look sideways at the sun.”

The moon can also be used as a background, but Haering hopes that further advances in camera technology will enable his team to use the stars across the entire night sky, which would dramatically boost the amount of data they could capture: “We are limited right now by the size of the sun and the moon; they are about half a degree of the sky and there is no information outside of that. If you were able to use the stars, you would have the entire sky to look against. But for this technique we are looking at hundreds of frames per second, so you need a really bright source at this point. We are hoping that with better cameras in the future we can see dimmer things at high speed, at a high frame-rate, to be able to image that.”

Haering says the next ground-based test, which is scheduled early next year, will employ a camera and telescope capable of producing images of five times the resolution of those already published. The team is also working on new processing techniques, which are constantly bearing fruit. “There are at least three different kinds of math that we are using and it seems like weekly we can come up with new details and features.” Another round of air-to-air tests will be carried out this December.

NASA’s research includes phenomena other than shockwaves, such as wing tip vortices and engine plumes. Haering says the December tests will also involve observations of subsonic aircraft, such as high-lift aircraft, in the expectation that the findings will be of use to designers in that part of the industry.

Number crunching
But what do the Schlieren images obtained so far suggest about the future design of low-boom aircraft, and what will the research team do with the data it has collected?

One of the main objectives is to validate current CFD code and wind tunnel testing technology, says Haering: “Close in to the aircraft, there is a lot of intense CFD that goes on; that part is difficult to do around propulsion systems in a wind tunnel. Once you are further away from the vehicle, the boom is pretty well established and it is pretty easy to compute how it is going to go down to the ground. So we are really trying to focus in, and look at how good our CFD tools are near the aircraft so we can take these existing F-15s, F-18s and, hopefully, later a low-boom demonstrator aircraft, and prove the CFD is right, and have confidence that in future designs we will accurately predict how loud it is on the ground.”

In a conventionally designed aircraft, a shockwave is produced by every change in its surface; the shockwaves tend to coalesce at the front and rear of the aircraft, creating the boom. The low-boom design would have to prevent this from happening, keeping the shockwave separate.

“It’s about distribution,” says Haering. “The designers have a tough job having all the volume and lift changes so that individual booms are about the same level. When you have a bigger boom behind a smaller boom on the front end, it quickly overtakes it – it superimposes. So you would like to have nearly equal booms that stay separated. You can’t change the energy, but what you can do is redistribute it so that it is less annoying to the ear. Instead of an instantaneous bang, you’re going to have a bunch of little puffs or pops.”

Just how quiet does it need to be? That’s a question for the international aviation community, says Pauer, but maybe in the order of 75dB: “Plus or minus five is what we are looking at. Our role at NASA is to enable this as no designers are going to build an aircraft unless there is a set standard.” 

George Coupe is an engineering and technology writer based in the UK

 

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