Meanwhile a Piper Arrow (4-seater piston, pretty comparable numbers to most Lycoming/Continental singles) takes off at 2700 rpm and has a 74” propeller, for a tip speed of 594mph. You can use a 3-blade propeller to reduce the diameter and cut noise somewhat (among other benefits).
Both of these are just the tip speed relative to the plane, I suppose you’d need to factor the airspeed too and use Pythagorean theorem but I’m no aerospace engineer.
As you noted, this is possible because the constant-speed prop on the turboprop allows it to take larger bites of air.
The Arrow needs that rpm to produce full takeoff power. However after takeoff it can be adjusted to a lower speed with minimal loss in power (e.g., 2500 rpm greatly reduces noise while losing 7-8% power). To reduce RPM like that in a fixed pitch you’d also need to reduce manifold pressure, causing more power loss.
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.
E.g., turbo prop (source https://www.quora.com/How-fast-is-the-tip-of-a-turboprop-bla...)
I double checked this math and got 474mph.
Meanwhile a Piper Arrow (4-seater piston, pretty comparable numbers to most Lycoming/Continental singles) takes off at 2700 rpm and has a 74” propeller, for a tip speed of 594mph. You can use a 3-blade propeller to reduce the diameter and cut noise somewhat (among other benefits).
Both of these are just the tip speed relative to the plane, I suppose you’d need to factor the airspeed too and use Pythagorean theorem but I’m no aerospace engineer.
As you noted, this is possible because the constant-speed prop on the turboprop allows it to take larger bites of air.
The Arrow needs that rpm to produce full takeoff power. However after takeoff it can be adjusted to a lower speed with minimal loss in power (e.g., 2500 rpm greatly reduces noise while losing 7-8% power). To reduce RPM like that in a fixed pitch you’d also need to reduce manifold pressure, causing more power loss.