How come a turboprop can't reach speeds as high as a turbofan?

Question

I understand that turbofan powered aircraft can reach much faster speeds than turboprops like the C130 Hercules, and I can't imagine a turbofan turboprop 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.

Answer 1

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.

Answer 2

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.

While a turbofan ingests less air than a turboprop, both do it at essentially the same speed, which is flight speed. The difference is in the exit speed.

Thrust is air mass flow times speed difference between entry and exit speed. Engines of the same thrust will have the same product of mass flow times speed difference, but with the lower mass flow of a turbofan this only means that the speed difference is higher.

It should be immediately clear that a higher exit speed will also allow a higher flight speed. For thrust to be positive, exit speed must be quite a bit higher than flight speed, giving jets a larger margin than what turboprops can muster. In addition, the use of a propeller limits the useable top flight speed to clearly subsonic Mach numbers; a restriction that turbofans do not have.

Answer 3

The main operating limit for any blade is reaching the transonic speed on its tip.

The tip of a blade sees an airflow's speed W which is the composition of the rotating speed U and the speed of flight V, as visible for example in the following plot from this report:

When this sum W reaches the transonic regime, there's a deterioration of the aerodynamic characteristics: lift decreases and drag and pitching moment increase, as visible for example in the following plots from this NASA report:

Note that this is valid for any blade: helicopter blades, propeller blades, fan blades of a turbofan and compressor blades of a turbofan/jet. They all are limited by the onset of the transonic speed on the blade tips (although the fans of some turbofans reach indeed supersonic speeds, but that's the result of different compromises).

Just as an example: the optimal rotation speed of an helicopter rotor is around Mach 0.7. This limits the speed of flight to what remains to reach the speed of sound i.e. the remaining Mach 0.3. That's why helicopters fly so "slowly". For a propeller the same reasoning applies, but the limitation on the flight speed is shifted toward Mach 0.6. And for the compressor's blades of a turbofan/jet that limit is somewhere in-between, around Mach 0.4.

Mach 0.4? But why then a turbofan/jet can fly supersonic, till Mach 3.2 for example for the Blackbird, in spite of that limitation? Because in front of it lies an extremely important component that most of the time passes unnoticed: the inlet. The main job of an inlet is to reduced the speed of the incoming air to Mach 0.4, whatever high the speed of flight is. If the speed of flight is high subsonic (Mach 0.4 to 1), then the inlet simply works as a divergent inlet (diffuser): its section gets bigger and bigger, the pressure gets higher and higher and the speed gets lower and lower (the compressor thanks Bernoulli). If the speed of flight is supersonic, in front of the inlet one or more shockwaves are formed, across which the speed reduces to high subsonic and afterward the divergent portion of the inlet finishes the work reducing again the airspeed till Mach 0.4. This is exactly how the inlets of a jetliner, the Concorde or the Blackbird work(ed) to supply their compressors with a relative slow airflow.