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Looking at how aircraft engines evolved, ie. radial to inline to jet, I'm wondering why the quantum leap all the way to the jet engine took place after engines like the supercharged Merlin had their heyday in the '40s.
I understand the inherent limitations associated with propeller driven aircraft, and that a different propulsion system was necessary for increased speed. But given that the concept of the turbine engine was viable during WWII, and that the power to weight ratio and streamlined shape of an engine was always a consideration, why wasn't the turboprop the next logical advancement after the supercharged inlines?
I guess I find it strange we never saw something like a Mustang or ME-109 with a turboprop in its nose before the ME-262 took to the skies.
the turboprop is a much more complex engine than the pure jet. The gearbox is the main issue, you need to get the speed down from about 15000 rpm to about 1500.
It's a good question that someone asked in my turbine powerplant class in college. At its absolute simplest, a turbojet has only one moving part, a rotating shaft with a compressor at one end and a turbine to extract power at the other. The diffuser and combustor section in the middle could theoretically have no moving parts. In practice, the reality of a turboshaft engine requires some moving parts besides the shaft and associated hardware.
The pure jet weighs less than a turboprop, and early jets were notoriously underpowered. Their specific fuel consumption was awful and their thrust to weight ratio was very low, especially in contrast to a modern turbofan. The added weight of a gearbox wasn't welcome; also, the race was on to go fast. There are some turboprops which are relatively simple (PT6, for instance, with its reverse-flow arrangement) but the gearbox and added weight and complexity required the configuration of the turboprop to wait a few years before it was viable. The same goes for the turbofan, and a fan isn't nearly as complex as a prop.
one other reason is that for pure speed, which is what you wanted in a fighter in world war two, the turbojet engine was more promising than the turboprop engine. remember that even though the turboprop is more efficient than a turbojet, it also has the same speed limitations that the piston engines have, IE that big windmill creating an aerodynamic wall in front of the aircraft.
One other thing to remember is that the core of the turboprop, turboshaft and turbofan engines is a turbojet. At the heart of every turboprop is a turbojet core. So the evolution from the core engine to props, rotors and fans was a technological one, just like the evolution that occured with the piston engine, from a simple opposed engine to the radial, and the addition of super- and turbochargers and even turbocompounds. The latter is a true exercise in Rube Goldbergian complexity.
one other reason is that for pure speed, which is what you wanted in a fighter in world war two, the turbojet engine was more promising than the turboprop engine. remember that even though the turboprop is more efficient than a turbojet, it also has the same speed limitations that the piston engines have, IE that big windmill creating an aerodynamic wall in front of the aircraft.
If you every want to see how fast a turboprop can slow down, just select flat pitch (beta range). Like dropping an anchor.
Since the turboprop/turboshaft engines are a variant of the turbojet engine (reduction gearbox and propeller/output shaft added), the turbojet engine HAD to be invented before the turboprop could be built.
Actually, they did experiment with military turboprop attack/fighter aircraft. The A2D-1 variant of the Navy's AD Skyraider series is a prime example. IIRC, there was ONE of those built. While it flew well, the pure jets were better performers, and propeller driven aircraft don't do well at transonic speeds.
There were several turboprop commercial airliners built before the 707 entered production. Again, the pure jets outperformed the propeller aircraft and took over the market.
Turboshaft engines have proven to be very good in helicopters, and turboprops are very common on smaller aircraft.
The main reason for Jet Engines, was the desire for something simpler (to a radial engine) powerful, and designers were wanting to get rid of the propeller. They were also wanting faster speeds.
From there, the idea of turbine spinning a propeller was a logical progression.
Since the turboprop/turboshaft engines are a variant of the turbojet engine (reduction gearbox and propeller/output shaft added), the turbojet engine HAD to be invented before the turboprop could be built.
Actually, they did experiment with military turboprop attack/fighter aircraft. The A2D-1 variant of the Navy's AD Skyraider series is a prime example. IIRC, there was ONE of those built. While it flew well, the pure jets were better performers, and propeller driven aircraft don't do well at transonic speeds.
There were several turboprop commercial airliners built before the 707 entered production. Again, the pure jets outperformed the propeller aircraft and took over the market.
Turboshaft engines have proven to be very good in helicopters, and turboprops are very common on smaller aircraft.
and republic experimented with a turboprop version of the F84. it didnt go over very well as there was a general malaise when the engine was running due to the prop tips breaking the sound barrier, even at low speeds.
one other reason is that for pure speed, which is what you wanted in a fighter in world war two, the turbojet engine was more promising than the turboprop engine. remember that even though the turboprop is more efficient than a turbojet, it also has the same speed limitations that the piston engines have, IE that big windmill creating an aerodynamic wall in front of the aircraft.
If you think about the first oil shock, that begins with the US production peak in late 1970 beginning 1971, followed by the Arab crisis of October 1973, the modern day concerns about efficiency were not really as big an issue before that point.
As you said, a turboprop is about efficiency.
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