The Arrow also has a constant speed prop, the difference is the Arrow is direct drive from a massive 360 cubic inch engine whereas the turboprop has a gear box. It needs this displacement to produce power while at such a low speed for the prop. At almost 6 liters, yet producing only 200hp, it's wildly fuel inefficient, and it's fairly trivial to produce this much power with a modern ~2L engine. This is almost entirely due to a single bad engine design that had resonance issues from several decades ago so the industry (stupidly) considers anything but direct drive to be taboo. This is changing slowly with some newer light sport engines that have gearboxes, however.
If it had even a modest reduction drive, you could run the engine at a reasonable power setting while maintaining a more efficient tip speed but the prop and engine engineers had to meet in the middle resulting in a highly compromised solution.
The IO-360 is actually a fairly efficient engine when run properly. Similar to Ford’s ecoboost line per https://en.m.wikipedia.org/wiki/Brake-specific_fuel_consumpt... (The -720 is basically equivalent design to a -360 just doubled). Airplanes use a lot of fuel because air resistance varies with v^2, not because of poor engine design.
Sure there have been improvements in modern engines, but these primarily target efficiency across power outputs whereas (as sibling noted) planes tend to have stable power requirements.
Not sure what your point is about constant speed or gearing because I already said that.
> At almost 6 liters, yet producing only 200hp, it's wildly fuel inefficient
Is it? I've always thought that while GA engines are in many ways stuck in the 1940'ies, they are actually quite efficient. Wikipedia seems to back that up at https://en.wikipedia.org/wiki/Brake-specific_fuel_consumptio... , with a couple of Lycoming engines producing BSFC numbers in the same ballpark as a modern car engine or Rotax.
Yeah, the thing people always miss is that these engines are flat-rated at whatever hp. I.e., they'll do it for most of their operating lives, vs. the occasional WOT in a car.
Exactly this. Lycoming engineers weren’t stupid, and airplane engines are well-designed within the (admittedly potentially contrived) direct drive constraint.
If it had even a modest reduction drive, you could run the engine at a reasonable power setting while maintaining a more efficient tip speed but the prop and engine engineers had to meet in the middle resulting in a highly compromised solution.