Harley Davidson or Rotax — Sounds can be unique and associated with a certain brand. Harley motorcycles have made their sound a trademark, defending it vigorously against wannabe knock-offs. Sling Pilot Academy found Rotax 912iS engines output a particular sound, too. However, they wanted to shield their neighbors from it. As busy as Sling Pilot Academy has become, keeping good relations is essential.

So, Whisper Sling

Necessity is the mother of invention, so the saying goes. Necessity, in this case, was success. (SPA) at the Torrance, California, airport has grown to become a training behemoth, with their fleet flying more than 4,000 hours per month, the bulk of it in the Sling two-place LSA it uses as a primary trainer. That makes the SPA one of the busiest training companies in the region. And Torrance, like many urban airports, faces increasing complaints from non-pilot neighbors; in fact, Torrance’s very existence has been threatened for decades by those opposed to the presence of airplanes at, get this, an airport.  You can see where this is going. Although SPA worked hard at community outreach, noting in part that it already used aircraft that operated well under the airport’s noise limits and burned relatively little unleaded autogas (thanks to the Rotax 912 iS engines), complaints continued. At one point last year, SPA management had to fight being kicked off the airport entirely. They needed to do something to go “above and beyond” in order to keep the school running and, indeed, growing.

Engineering Challenge

At this point, co-CEOs Matt Liknaitzky and Jean d’Assonville decided to undertake an ambitious engineering solution to reduce not only the actual noise footprint of Sling LSA but the quality of that sound. But we’re getting ahead of the story here. “We started by changing our procedures,” says Liknaitzky. “We then started reminding people that we had the quietest airplanes on the field. We never busted the noise monitor. But in multiple meetings with the community, we started to listen to what they were saying. They started describing the sound of our airplanes. They said it’s not that our airplanes are loud but the nature of the sound was annoying. This was a turning point for us. We told ourselves that we have to look at the sound quality, not just the quantity.” Sling then began working on solutions to making the whole installation — Rotax and Sling — less annoying. “We know that many aircraft have low-frequency sound but ours were being perceived as being buzzy, like a mosquito,” says Liknaitzky. After significant study, Sling realized that it wasn’t just the higher frequency sound the combination produces but that it wavered as well, likely the result of acoustics and the Rotax’s high-rpm nature. And they had to get on with a solution because “we’re adding 30 airplanes this year, and we already have 40 in the fleet,” says Liknaitzky. “The first thing we tried was the exhaust,” says Matt. “We added off-the-shelf glass mufflers and that made a huge difference in the sound. Then we realized you have to either add length or change the direction to reduce noise. Changing the direction has more of a backpressure penalty. We had to keep to the Rotax spec of 2.9 psi of backpressure. The standard muffler is 1.0 psi,” says Liknaitzky, so there’s some room to reduce noise even if it costs some backpressure. 

Keep Improving

“We iterated on the muffler: making it longer or shorter; changing the diameter,” says Matt. “We kept iterating and performing flight tests, but we also used our ears as an instrument.” The goal was to achieve something an instrument might not pick up: the quality of the noise. Eventually, the company settled on a configuration that reduced noise by 10 dbA using a long pipe that extended beneath the cabin and exited behind the wing with a distinctive “kick up.” That kick up helps place the exhaust in the wake of the wing and aim it away from neighbors on the ground. But it’s also effective in other directions. “If you were to stand behind the airplane with the engine fully revved up, you cannot hear the exhaust. All you can hear is the propeller and prop wash,” says Liknaitzky.  The muffler is a simple perforated inner tube with glass packing around it. All told the muffler is 52 inches long with 40% open holes. Surprisingly, the system appears to actually improve the Sling’s performance. The company assumes that the stock Sling-built exhaust actually had too little backpressure and so the new, quieter system gives the Rotax more of what it needs. Still, testing continues. “We’ve been testing with an older airframe,” says d’Assonville, “but we’re going to be starting over with a brand new airframe. Do all the noise testing in standard configuration and then test out what we think is the final configuration. We’ll start with just the exhaust, then add the cowling modifications and then the DUC three-blade propeller.” What cowling mods, you may ask? Sling discovered that some of the noise signature came from the cowling itself, transmitting some of the acoustic energy of the Rotax through the fiberglass. In the test article, they applied a foil-backed foam sound deadening material, which substantially reduced the perceived noise level. Moreover, it helped reduce the perception of high-frequency noise, getting the company back to the goal of giving the Sling a more low-end-biased noise signature they felt neighbors would find less objectionable. “The change was remarkable,” says Matt. “It took out a lot of the jangle…it made a monster difference.” Another seemingly simple change was to alter the air exit path in the lower cowling. But forcing it to follow the lower fuselage, rather than let it vent directly downward, the sound levels were further reduced. All together, the modifications add about 10 pounds to the Sling two-seater. Sling has the advantage of being not only the user but a close relative to the factory itself, so getting approval through the ASTM process only requires a letter of authorization from the factory for testing. A similar path of the factory conducting its own testing of the modifications can lead to approval to retrofit the parts to existing airplanes and potentially to make the configuration a factory option. At this stage, the package looks like it’ll cost $3,000 to $4,000 per airplane. Testing will continue through spring and summer 2024.

ARTICLE LINKS:

• Sling Pilot Academy, flight school website
• Sling LSA, information and content on this website
• The Airplane Factory USA, importer contact info and all content on this website
• The Airplane Factory, website for the South African manufacturer of the Sling series.
• Torrance, California airport, airport information website

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https://www.youtube.com/watch?v=njW_CJBuuJI https://www.youtube.com/watch?v=YXAIKmpwDJ4