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I understand that turbofan powered aircraft can reach much faster speeds than turboprops like the C130 Hercules, and I can't imagine a turbofan powered 737 would be of any use really, but as to the logic behind that, I'm a bit confused.

I've read up that a turboprop draws in more air but at a slower speed and a turbofan draws in less air but at a higher speed. But if you have more air in a turboprop, can't you compress it to great amounts, whereas in a turbofan you have less air so you cannot compress as much? And to my understanding (and I may be wrong) the low pressure compressor in a turbofan regulates not only the pressure but the speed of the air, so how is the entry speed of the air relevant to the engine's performance? It just seems a bit counter-intuitive to me. More air = more to compress, but lower speed = less being sucked into the engine per second?

Somewhere online it said that turboprops aren't great at high altitudes unlike a turbofan, because the air gets thinner and the turboprop can't suck as much in then. But then my question is, if a turbofan sucks in less then how does this statement work?

I'm just completely stuck with this at the moment, and I've had a bit of a mindblock. If someone could help me out with this I'd really appreciate it.

Thanks

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    $\begingroup$ "I can't imagine a turbofan powered 737 would be of any use really" Shhh.... don't tell Southwest Airlines - their entire fleet of aircraft are turbofan-powered 737s. $\endgroup$
    – FreeMan
    Jun 27 at 15:27
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    $\begingroup$ Props must spin very fast to maintain optimal AoA at higher IAS. Once they go supersonic, drag ruins advantages they have over fans as "spinning high Aspect Ratio wings". However, higher airspeeds have been successfully achieved with the Tu-95 contrarotating turbo-prop powered by 2 jet cores. Makes one wonder if that's what they originally had in mind for the B-52. $\endgroup$ Jun 27 at 18:17
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    $\begingroup$ A turboprop 737 would have its blades hit the ground. Engines on 737 are really low. Boeing really painted themselves into a corner by not putting tall gear on the 737 Classic. $\endgroup$ Jun 27 at 20:37
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    $\begingroup$ "I can't imagine a turbofan powered 737 would be of any use really" Did you mean to say "turboprop" here? All 737s ever built have been powered by turbofans (though the -100s and -200s were powered by low-bypass turbofans rather than the more modern high-bypass ones.) $\endgroup$
    – reirab
    Jun 27 at 21:44
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    $\begingroup$ Supersonic propellers are also unbelievably loud (the XF-84H was known as the "thunderscreech" because of this, allegedly audible 40 km away even when it was on the ground), which is not a great attribute for airliners $\endgroup$
    – llama
    Jun 28 at 1:41

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The attributes of the incoming air for combustion matter less than how the thrust is generated. Many turbofans operate past Mach 1, but a turboprop's thrust comes practically entirely from the propeller, which has difficulties at such speeds. Supersonic propellers have been tried (XF-88B and earlier), but they were found to be woefully inefficient because of shock waves. Supersonic flow near the blades can occur well before the airplane itself reaches Mach 1. That's the limiting factor.

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    $\begingroup$ I would add that speed of the propeller tips is the combination of the aircraft speed and the orbital speed of the tips, which is why the fastest turboprops use low RPM propellers. $\endgroup$
    – Jan Hudec
    Jul 3 at 9:52

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