Going Direct: Coffin Corner for Single-Engine Jets

The progress of Stratos and making light airplanes quieter.

Stratos Aircraft

While at the 2010 NBAA convention, I stopped by the booth of Stratos Aircraft to check up on the progress the company was making with its eponymous single-engine jet. What I discovered was pretty much exactly what I expected to discover. The company was exceedingly optimistic about the prospects for its jet while admitting there were challenges ahead.

In case you haven't heard about it, and chances are you haven't, the Stratos is a single-engine turbofan design - there doesn't yet exist an actual airplane, but the program appears to be in full swing.

The Stratos differs from other single-engine jets in that its engine, a Williams FJ44-3AP, the same as used on the Piper Altaire, is housed inside the fuselage of the airplane instead of being placed atop the fuselage (a la the Cirrus Vision jet) or built into the tail (as in the Altaire). The advantages are obvious: a great deal less drag and very little if any pitching moment with power change. With its exceptionally clean lines, it also looks a little bit like the airplane is flying by the grace of its winning good looks alone.

Of course, there is an engine in there, and there are sizable challenges inherent in situating it within the fuselage. In this four-place jet, your fifth passenger, a big turbofan, is sitting just a bit behind the rear seat. Noise and vibration are very real concerns. Moreover, even when they're hidden away, turbofan engines still run on air, so you've got to deliver some of that atmospherically abundant gas to the engine. In this case, it's done with a pair of huge inlets located on the upper leading-edge wing root. Nothing new there. It's been done for decades, starting in the 1950s with early fighter jets, including the Lockheed P.80 Shooting Star. Keeping those inlets free of ice, for obvious reasons, is also critical. Stratos says it has a special duct design that will prevent ice from building up without extensive, power-robbing use of engine bleed air.

The other challenges have to do with Stratos' lofty performance goals. Whereas Diamond and Cirrus both intentionally downscaled their goals, Stratos and Piper are both shooting for the stars, Stratos even more so than Piper. The hoped-for ceiling — and I say "hoped-for" because no other single-engine airplane has come close to achieving this certification allowance — for Stratos is 41,000 feet. Piper is shooting for 35,000 with its Altaire. In both cases, the companies are going to have to prove to the FAA that they can answer the tough questions about pressurization, such as how they are going to keep people alive if they lose the only engine at 41,000 feet (or at 35,000, for that matter). Both companies claim confidently that they can do it.

The other issue is stalling speed. Both airplanes are slated to go very fast, 360 knots for the Altaire and 400 knots for the Stratos. Piper's strategy is to use a small wing and get a higher allowed stall speed, up in the 70s, says Piper, compared with the usual 61-knot standard for singles. This it plans to do through the use of stronger seats. The FAA, to our knowledge, has never approved anything above the 65-knot stall speed of the Pilatus PC-12, and even that airplane has to stall at 61 knots in Europe, a trick it pulls off through simply and agonizingly reducing its maximum takeoff weight until it will hit that 61-knot mark.

For both Stratos, which is hoping to get the trick done stateside with two knots of credit for very crashworthy seats, and Piper, which is hoping for a great deal of credit, more than 10 knots, using better seats as well, the risks are high. Miss the mark and you need more wing or more flaps or less weight or some difficult-to-achieve combination of those things, all of which almost certainly mean missing the performance goals and having certification slip. Then again, if they hit those goals, they have airplanes that will be very competitive in the marketplace.

Of course, this is all dependent on funding. With what it claims is sufficient funding for the Altaire, Piper seems poised to follow through on its turbofan single program. For startup Stratos, the funding question is much more critical, especially in this economic climate.

Cash might be king, but it's not everything. Unless the two programs can avoid the risks inherent in flying higher than Part 23 singles have ever flown and in landing faster too, the money question will be moot.

Frenemy Noise
When I was a kid, there was a bumper sticker popular among the many Air Force families that lived in our town that stated simply, "Jet Noise: The Sound of Freedom."

The airplanes they operated out of the base were F-4s and F-105s, so there was plenty of noise to go around. And there was no helping it. A Phantom's afterburner is no subtle thing. But the message was: "We know our jets are noisy, but that's the cost of being a superpower." I never heard anyone in our town argue against that view.

I admit, at AirVenture every year, I love hearing the sounds of everything from C-5s to P-51s and even little two-stroke-powered bug smashers doing their thing. For us pilots, power plants are the sound of a different but still very real kind of freedom.

A few years ago at AirVenture, I watched a group of people turn as one to see what was making the sudden, bone-shuddering sound behind them only to witness a pair of F-22s, making one of their first public appearances anywhere, as they blasted by at low level. The spontaneous applause, high-fives and broad smiles told the tale. Not all noise is bad. At least not to us pilots.

Then again, we need to be realistic on several fronts. As sweet as we might find the sound of a really cool airplane flying by, the bottom line is that, in the grand scheme, noise is bad. It's bad for our ears, bad for our neighbor relations and bad for business.

The ears are perhaps the most obvious issue here. My dad, who's 84 years old, and I, age 51, are a good case in point. We're both pilots and we've both flown plenty of high-performance airplanes. But by the time my dad was my age, he already had a substantial hearing loss. My ears are still in very good shape. The difference is obvious. For nearly my entire flying career I've been religious about wearing high-quality hearing protection whenever I'm in an airplane that's at all noisy, which is to say, any piston or turboprop airplane. In my dad's early flying career, he didn't have the option or the knowledge. We didn't know how bad noise was for our ears, and if we had known, there weren't good headsets available. And the macho pilot culture of the time didn't help. My dad's ears, and those of hundreds of thousands of other pilots, suffered for it all.

Still, I wish I didn't have to wear noise-canceling headsets, but I do. Our airplanes are just too loud. While we've made much progress with jets, quieting piston airplanes has proved more difficult. After all, with a high-powered engine or two with their fast-turning props doing their thing very close to where we sit, the wonder is that our airplanes aren't louder than they are.

When I was at NBAA last October, I toured a Gulfstream G650 with a completed interior. I got to talking with one of the project managers about the airplane, and he explained just how difficult it is to track noise down — and even when you succeed in doing that, it's even more difficult to mitigate. Noise comes from so many sources — air movement over the skin of the airplane, the engines, the pressurization system, mechanical systems — that there's no magic bullet to make an airplane quieter.

The good news for G650 buyers is that Gulfstream has succeeded, it says, in making the G650 cabin the quietest cabin in its fleet, and its airplanes are plenty quiet to begin with. Can they be quieter? Only if Gulfstream wants to lead in that area of innovation.

When it comes to 172s and DiamondStars, on the other hand, cutting decibels is even tougher work. Both Cessna and Diamond, among others, have cut noise by limiting prop rpm, which is also a good way to improve engine life, reduce vibration and improve fuel efficiency. It's also an excellent way to cut power. And there are a number of ways we can make our airplanes quieter through passive noise attenuation methods, such as making the glass thicker, installing noise barrier materials, creating aerodynamic designs that cut down on airframe wind noise and paying careful attention to intersections and gaps, places where wind noise rules.

Perhaps the most promising area of potential improvement, however, is through power plant development. At last November's AOPA Summit in Long Beach, California, I heard about an emerging electric modification to the Cessna 172 Skyhawk. The electric 172 would be so quiet that its designers are considering using a horn on the ramp to alert others to its presence.

Making our light airplanes quieter is one area in which there's a very real competitive advantage to be gained. It might be hard for a company to sell the fact that its airplane is less noisy than its competitors', but that's one aviation arms race I'd like to see get started soon.