In Layman terms the turbojet engine coverts a small amount of air through a large change in velocity whereas the propeller converts a large amount of air through a smaller change in velocity.
The Turboprop engine is just about identical to the turbofan engine but the prop is on the outside with the turboprop. It works something like this – The intake air is ducted to the low pressure (NL) axial compressor and then on to the high pressure (NH) centrifical compressor for a further stage of compression, it then enters internal ducts and is discharged into the combustion chamber where the fuel is added & ignited. Gases exiting this section impact onto a single stage NH turbine, the turbine extracts energy from the flow & drives the shaft directly connected to the NH compressor. Mounted behind the NH turbine is a single stage NL turbine, which also extracts gas energy. It drives a shaft connected directly to the NL compressor. As the combustion gases continue to flow rearward they are directed towards the 2-stage power turbine assembly. The power turbines turn as a single unit extracting the majority of gas energy remaining to rotate a shaft connected to the reduction gearbox at the front of the engine, it is through this reduction gearbox that power is transmitted to the propeller.
Turboprops have their limitations in the fact that they are limited to how fast they can spin, the limitation comes in the diameter of the blades, simple physics will tell you that with any “wheel” the outside of the wheel travels faster than the inside of the wheel. The same applies then with the prop, if you either spin them too fast, or their diameter is large then the outside of the blades, or tips will go through Mach1, this then produces drag as mach buffet takes over eventually leading to mach stall. Think of a propeller blade as an airfoil, or wing, because that’s exactly what it is, so it behaves in exactly the same way.
The reduction gearbox that bdn12 talks about is there to reduce the effect of the relative shaft speed of the turbine when linked to the prop, as you can expect the core shaft will be turning at a high velocity, in my aircraft if my memory serves me correctly this is about 17,000prm, the max we can turn the props is 1020rpm.
With all modern turboprops (& most complex light twins) the propellers are governed by a CSU, or Constant Speed Unit, so the shaft output is kept at a set speed, the thrust then comes from the propeller pitch angle, again with this comes limitations.
In simple terms with turbojet engines the air is sucked in through the first set of compressor blades, then enters a combustion chamber where fuel is added, the mixture is then ignited which then expands very quickly indeed, the hot gas is then exhausted out the back of the combustion chamber where it passes over another turbine which is driving the shaft that rotates the compressor, so it’s a continuous cycle.
The thrust element comes from Mr Isaac Newton’s 3rd law of motion – for every action, there is an equal and opposite reaction. Herein lies the answer to your question about how does a turbofan produce more thrust than a turboprop, imagine all that air being exhausted out the rear of a turbofan (Newton again) compared with the amount of “thrust” being produced by a constant speed variable pitch propeller. Incidentally the turboprop turbine engine does produce about 10% of thrust from the residual hot exhaust exiting the rear of the engine, but the majority of the heat energy is lost when turning all those turbines.
Clear as mud? thought so 😉
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