4 ms·

Great question. Turbo props and turbo fans, both produce thrust, but very differently. In a turbofan, the turbine spends most of its effort turning the compre

by subhro 4y ago

Great question. Turbo props and turbo fans, both produce thrust, but very differently. In a turbofan, the turbine spends most of its effort turning the compressor blades. In a modern turbofan (TF) engine like GE 9x, you are looking at a 80-20 bypass ratio. This means, 80% of the air drawn from the front just flows through. Now, as the "FAN" (duh!) rotates, a huge volume of air is drawn in and compressed. As the volume moves deeper into the engine, the air pressure increases because the engine tapers down, as well as the compressor works in "compressing" the voume of air. As the air pressure increases, so does the speed and temperature. Now, as this compressed air exits from the back of the engine, it provides a forward thrust. The 20% air that DID get into the core, carrys on the combustion in the engine that provides the energy required to rotate the compressor blades. Now this is definitely simplified as there are many additional pieces, like stator vanes, equipment to shape and correct the velocity of the air, fuel injectors, igniters, afterburners (not in civilian airplanes of course) and many more contraptions. While not completely comparable, a turbo prop (TP) behaves a lot like its piston counterpart. The turbine is simply replacing a traditional 4 stroke IC engine in this case and just tasked with rotating the blades. Each of the blade is a mini (well not so mini) wing that has the shape of an airfoil. As these rotate, they generate lift but since the rotational axis is parallel to the fuselage, it is actually thrust. > Is it just cheaper to build a turboprop, and turbofans are the ultimate in terms of performance? It is definitely cheaper to build a TP. And TPs can take a lot of abuse and needs miniscule maintanance compared to TFs. But if you are talking of raw performance, turbojets (TJ) actually run (fly?) around in circles around TFs. TJs are just incredibly wasteful, inefficient and needs a lot of maintenance.

Toutouxc 4y ago
It sounds like you know your way around a turbofan, but you wrote some confusing stuff, mainly this: > Now, as this compressed air exits from the back of the engine, it provides a forward thrust. This is something you generally don't want to happen in a reaction engine. If you eject above-atmospheric-pressure out of your engine, the expansion takes place downstream, in the wake, and that energy is mostly wasted (some pressure thrust is still created, but it's minuscule). You always want all the expansion to happen inside your engine, in a properly designed propelling nozzle, which should always result in a flow of low-pressure high-speed air leaving the engine. If the nozzle fails to expand the air (choked flow, nozzle too short, wrong shape), you're losing thrust (and efficiency).
- subhro 4y ago
  You are right. I was mostly simplifying the description for NOT aeronautical engineers. But you are absolutely right.
the__alchemist 4y ago
So, to confirm for the comparison, in a nutshell (Some of the upstream posts in this sub-thread imply the bypass section of a TF provides its thrust directly, which I believe your post is showing is incorrect): - Treat TJ as base model - TF uses the bypass section to provide additional power to the compressor - TP generates its thrust from a prop on the shaft, vice the core's output (is this called exhaust?). TJ and TF both use the core's output for thrust. Is this right?

by subhro 4y ago

Toutouxc 4y ago

It sounds like you know your way around a turbofan, but you wrote some confusing stuff, mainly this: > Now, as this compressed air exits from the back of the engine, it provides a forward thrust. This is something you generally don't want to happen in a reaction engine. If you eject above-atmospheric-pressure out of your engine, the expansion takes place downstream, in the wake, and that energy is mostly wasted (some pressure thrust is still created, but it's minuscule). You always want all the expansion to happen inside your engine, in a properly designed propelling nozzle, which should always result in a flow of low-pressure high-speed air leaving the engine. If the nozzle fails to expand the air (choked flow, nozzle too short, wrong shape), you're losing thrust (and efficiency).

subhro 4y ago
You are right. I was mostly simplifying the description for NOT aeronautical engineers. But you are absolutely right.

the__alchemist 4y ago

So, to confirm for the comparison, in a nutshell (Some of the upstream posts in this sub-thread imply the bypass section of a TF provides its thrust directly, which I believe your post is showing is incorrect): - Treat TJ as base model - TF uses the bypass section to provide additional power to the compressor - TP generates its thrust from a prop on the shaft, vice the core's output (is this called exhaust?). TJ and TF both use the core's output for thrust. Is this right?