Aerion to subsonic set: Go with the (natural laminar) flow
The past couple of years have not been the best of times for a would-be developer to sign deals with industrial partners and customer-users for a new supersonic business jet (SSBJ), but Aerion has been hanging in there and is now seeking to make subsonic aircraft aerodynamically slicker.
“As a result of our discussions with potential OEM partners over the SSBJ, a number of OEMs have raised interest in applying natural laminar flow NLF] technology to subsonic aircraft,” Aerion vice chairman Brian Barents told AIN. “Our Aerion SSBJ design has two sweet spots–one at Mach 0.96 to 0.97 and the other at Mach 1.5. There is a lot of interest in subsonic efficiency gains, and this might be a good avenue for us to pursue and establish relationships with OEMs.” To this end, Aerion has formed Aerion Technologies and “we will pursue this in parallel with further development of the SSBJ,” said Barents.
Will NLF require a clean-sheet design or could the technology be applied to rewinged versions of the business jets we’re already familiar with? “The easy answer is, ‘it depends’,” replied Aerion chief technology officer Dr. Richard Tracy.
“If one is looking at a Mach 0.75 or 0.80 airplane, then it can be done with rewinging or wing modifications. If the intent is to take advantage of efficiency gains or speed increases or just simply to operate at 0.85 or 0.90, this definitely requires an integrated design both in terms of fuselage-wing integration and, frankly, in terms of the propulsion system.” Tracy emphasized that the task is “nothing as elaborate as required to go supersonic,” but it has to be done as a unified airplane. Boeing, he noted, is looking at “laminarizing” engine nacelles for its commercial transports; Dassault is talking about a laminar-flow horizontal tail, and Tracy believes “all those things can be done.”
Designers search high and low for a small decrease in drag and it’s hard to come by. “Laminar flow is out there, and it’s available more or less for free. It doesn’t involve complex gadgets. It’s just a matter of designing for the right pressure distributions,” said Tracy.
As Tracy describes it, NLF is achieved by designing for a combination of pressure gradients both chordwise and spanwise that are conducive to maintaining what nature starts with–laminar flow–and finishing it off with a manufacturing approach that can allow the finished structure to maintain that flow. Both the correct pressure distribution and suitable manufactured surface condition are “eminently achievable in high-subsonic and supersonic airplanes,” he said.
“We have almost two decades of working with techniques for rapidly predicting the location of transition [the point where smooth laminar flow disrupts into the otherwise ubiquitous turbulent-flow boundary layer] on a three-dimensional wing surface. What we have are user-friendly codes for predicting the transition location over an entire wing surface so it can be used as part of an optimization process, including shapes for drag minimization. We also bring an understanding of surface imperfections that are tolerable and not tolerable in areas where you are maintaining laminar flow, so we can provide the criteria needed for setting the manufacturing specs.”
Tracy regards composites as a better material than metal for achieving these levels of manufacturing accuracy because of their lower density and the way they’re made. “The skins tend to be thicker and therefore stiffer and less prone to wrinkling and buckling, but metal surfaces can also be totally compatible with NLF,” he said.
Bearing in mind that the NLF Aerion SSBJ design looks not unlike an F-104 Starfighter, with its long fuselage, short-span straight wings and straight empennage surfaces, would a clean-sheet subsonic NLF business jet resemble today’s designs? “NLF is uncomfortable with moderate to high sweep, certainly beyond 20 to 25 degrees,” noted Tracy. “The amount of laminar flow starts to become limited then. The design can incorporate any amount of sweep but if the objective is the minimum amount of drag at some particular cruise Mach number, then the solution might turn out to be a slightly less swept and slightly thinner wing profile.”
NLF vs non-NLF Bizjet
As regards an NLF design versus a non-NLF business jet, Tracy anticipates gains in range or speed or fuel load of 10 percent–a big number in the context of efficiency gains. “Yes, it is a big number. For the technical conditions of a high-subsonic or low-supersonic design, it turns out that the skin friction per unit area that’s covered by laminar flow is about 10 percent of what it would be if you had turbulent flow there, so parts of the wing and tail surfaces can virtually go away in terms of contributing to skin friction. There is essentially a 90-percent drop in skin friction, and this can be applied to empennage surfaces too unless they are sharply swept.”
Tracy regards a 10-percent efficiency gain as a realistic expectation and “probably not the upper limit for a clean-sheet design–especially one seeking more range and fuel efficiency over speed.” The gains would equate to 10 percent more range for a given amount of fuel or 10 percent less fuel to fly a given range. “If a certain range is the primary target, you can apply this increased efficiency to carrying a little less fuel and having slightly smaller engines, and now you’re into this virtuous circle when the gross weight goes down and the gains start multiplying.”
According to Aerion CFO Doug Nichol, the company is “adequately financed to undertake operations for infrastructure going forward,” with strong backers who believe in the company. There have been no changes in board membership, and “we have the same financial complexion this year as last, with about 20 people working full-time-equivalent–a number that has remained stable over the years.”
“Robert Bass [Aerion’s billionaire founder and chairman] is still not only financially involved but also intellectually committed to the technology and ultimately the success of the SSBJ program,” said Barents. “We have had to live with this prevailing economic environment but we are still encouraged and optimistic about the ultimate success of the program.” Projected price of the SSBJ remains at $80 million in 2007 dollars.