Compromises

Never one to revel in the practical, recently I found myself expounding to a colleague on why the general aviation world moves at 2700 rpm. Don’t ask me how we end up flapping on about such arcane stuff, but it’s the sort of scintillating exposé we geek out with at my hangar.

But you have to admit, the overwhelming number of small airplane engines redline at 2700 rpm. From the time-honored but now long passed Cessna 150, to all the other Cessna singles, the Pipers, Beeches and all our Experimentals repurposing their direct-drive Lycomings and Continentals, 2700 rpm has an almost universal hold on light-plane tachometers.

Explanations on why 2700 is the magic number vary. Early on I believed it was because 2700 rpm was a nice, slow speed aiding piston-engine dependability. But that doesn’t explain why some big-bore geared Lycomings—and others—spin as high as 3600 rpm with no apparent harm when taking off and can do that through TBO. And today we have the small-bore crowd—look no further than the Rotax line—buzzing away at over 5000 rpm for a couple thousand hours between rebuilds.

Turns out 2700 rpm is a compromise, one of those real-world accommodations schoolboys such as my earlier self haven’t yet thought of, along with such mundanes as fuel burn and who’s going to pay for everything. In short, 2700 rpm is fast enough to get some power and efficiency out of a piston engine, but slow enough to avoid over-speeding propellers with all the mayhem that entails. And if you don’t have the heebie-jeebies about overspeeding props, a few minutes talking to a prop engineer should be sufficient baptismal on that account. Besides, the slower a prop turns the greater its efficiency. While I’d have to pester one of my more learned colleagues for the why, let’s stop here and agree 2700 rpm is a fine compromise speed for crankshafts and propellers bound in holy bolt-lock.

To either speed up the engine for more horsepower or slow down the prop for better efficiency requires a gearbox between the two, and that leads to more weight and a devilish mix of harmonic harpies the engineers must expunge through tedious testing. Or, to be quick about it, gearboxes cost a bucketload more money. Money to design and flight qualify, then build and later rebuild the gearbox occasionally. The market spoke on this over 60 years ago and said bolting a solid chunk of aluminum propeller to the crankshaft was good enough. The engines were made sufficiently large to produce locomotive-like torque at some speed low enough to keep the prop happy, which turned out to be 2700 rpm. Big radials and V-12s had to have prop gearboxes—PSRUs if you insist—because their props are of necessity so large to harness their prodigious power, but for our light-duty needs direct-drive was the way to go.

The point is, 2700 rpm is a compromise. Ignoring the money and considering only the engineering involved it’s always better to have a gearbox to maximize both the engine and the propeller, but in the real world we can’t ignore the money. And finally, there’s the point. It’s easy to go down various rabbit holes in search of perfection in one area, only to find all that effort and treasure hardly matter because some other factor negates perfection in the first factor. Or, as the power boaters say when referencing pounding through waves, “It’s a 28-knot boat but a 10-knot ocean.”

Props, Gear and Panels

More endless turbulence in small aircraft is the debate over fixed or constant-speed propellers. Actually this isn’t much of a question with a 100-mph airplane or a 250-mph airplane, but it’s a head-scratcher with a 180-mph airplane. RV owners can drain a keg arguing this one because the benefits of increased efficiency with a constant-speed prop are just about canceled by the increased weight, complexity and, above all, cost of a twisty prop. But some guys really like having a constant-speed and some swear by a trick fixed-pitch.

Almost as contentious is fixed versus retractable landing gear. Today the conventional wisdom is well-faired fixed gear gives better performance than a retractable system up to 250 mph. Certainly no one disagrees that the fixed gear is simpler, lighter, quicker to build and eliminates a ton of maintenance and a whole load of insurance premiums compared to folding gear. But here emotion unashamedly walks right into the middle of the discussion and confuses things. Yes, the fixed gear is just as good, or so close as to remain a no-brain-required decision, but retractable gear looks so much better. To test this concept, consider the inverse, as demonstrated by Silver- and Bronze-level Sport class air racers. It’s not unusual at these levels to see stodgy fixed gear buggies beat sleek, retractable-gear plastic fantastics—but it looks funny. It would be better to be seen with the gear folded up; I mean, ducks don’t fly around with their feet hanging down, and they don’t even go 100 mph.

At the operational level, speed versus range (which magically turns itself back into speed) can extend preflight planning into the wee hours. It’s more fun to go at high power, but then you have to make a fuel stop. For so many flights, slowing to best economy speed and lollygagging like a tourist can eliminate that fuel stop and get you to your destination earlier. Dang it.

And so it goes. You’d think we rational, trained-pilot types could choose sensibly given all the compromises we’ve had to make in our flying lives, but it seems we’re still learning the difference between need and want. For those building a plane there’s likely no better example of burying the CDI needle than populating an instrument panel. Good Lord, we’re gadget freaks.

One lap around any fair-size fly-in will show one after another of multiple screen panels, each screen reading like the Library of Congress. Famously, many of our Experimentals have more instrumentation than airliners, which is fine if you’re drilling through the icy clag on a regular basis. But this is not how we fly these planes for good reasons, such as the lack of a spare engine, deicing gear and so on. So why go for the cost and panel real estate for Mission Control-like displays? In the early days there was the inevitable need to show off, but as glass has become the norm we’re seeing more rational panel fitments. It’s a trend we could all employ to advantage as we make equipment decisions for our planes. There’s no perfect solution, only good compromises, and in the end I wager most of us would rather spend the money on fuel than capabilities we don’t use. It’s something to think about.