Furthermore, there are no piston speed or other mechanical reasons most of these engines couldn’t safely turn another 1000 rpm. From a tuning perspective, reaching their native maximum horsepower output would require an extra 1000 rpm or more. Nearly all traditional general aviation engines are tuned to run at their torque peak (point of best fuel economy) during cruise, which is only a couple hundred rpm less than their maximum power rating at takeoff. It’s certainly not because the engines don’t appreciate being turned faster. That’s no accident the 2500- to 2700-rpm direct-drive aviation engine turn is a compromise fast enough to make some power and slow enough for efficient propeller diameters. You may have noticed Continental and Lycoming aviation engines are somewhere in between our example engines in size and rpm. When converting them to constant high output, there’s typically no choice but to turn up the rpm to make power and fit a gearbox to reduce propeller rpm to more efficient levels. Likewise, automotive engines are bred to loaf at surprisingly low power at low or medium rpm and make rare bursts of meaningful power at high rpm. The only way to get workable thrust from such engines is to gear down the propeller, and even then many such props are still on the fast, noisy end of things. Similarly, ultralight powertrains use tiny engines dependent on high rpm to make power. Even with geared engines the still large propeller diameters posed real issues for designers when it came to landing gear witness the inverted gull-winged Corsair or towering Constellation airliner. Trying to harness that much horsepower to the atmosphere demanded huge props, which absolutely had to turn slowly to avoid excessive tip speed. Back when Pratt & Whitney, Allison, and the rest of them were making 2000 hp with pistons, there was no question such engines would incorporate a gearbox. Naturally, prop speed reduction is sometimes a necessity, trouble or not. The centrifugal clutch means the Powerfin gets a freewheeling pass at what must be a somewhat jerky idle the gear drive also provides convenient water pump and starter motor real estate. A 2.8:1 gear reduction turns the 54-inch Powerfin 3-blade just under a familiar 2700 rpm at takeoff and about 2300 rpm at a 6500 rpm cruise. ![]() The single-cylinder 2-stroke looks a lot like a Honda CR bike engine sitting side-saddle in the Belite’s engine compartment (carburetor to left and exhaust to the right) and is rated at 36.5 hp at 7500 rpm. Polini, an Italian motorbike and ultralight engine builder, uses a helical-gear PSRU with centrifugal clutch on their THOR 250 Dual Spark engine. The stakes rarely get higher, or the technical devils seemingly more elusive than dealing with anything other than the most professionally developed PSRUs. That said, there is much to know regarding slowing the propeller down, much of which may directly involve your corpus or wallet’s well-being in rather dramatic fashion. Either way the mechanicals are fairly pre-determined. You either fit a big Continental or Lycoming without a gearbox or you’re set on a smaller Rotax, auto engine conversion, or compact ultralight engine that does use a propeller speed reduction unit. Ironically, as amateur builders gear reduction is not much of a practical concern. And right there with that classic is propeller speed reduction-it has drawn as many of the self-anointed into computer-aided hangar flying sessions as anything. Great debates are born when both sides of an argument have at least one gear leg to stand on, such as if a 180-mph homebuilt should use a fixed-pitch or constant-speed propeller. Don’t forget the metal propellers weigh hundreds of pounds, and the whole lot was reliable through combat aerobatics. Input from vibration specialist Den Hartog of Harvard (and later MIT) plus endless rounds of crankshaft failures in the dyno cell eventually resulted in a reliable engine-gearbox-propeller combination at well over 2000 hp. Imposingly large, the planetary reduction on the front of this R-2800 is the result of painstaking development by Pratt & Whitney.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |