This isn't an automotive issue but I couldn't think of another forum where I could ask it.
Anyway, as far as I can see a tuboprop is basically a jet engine with a turbine at one end and a propeller at the other.
What advantage does this have over a conventional jet engine?
Wouldn't a conventional jet engine be cheaper to produce and more reliable as it doesn't have as many parts?
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Since I work on this sort of stuff, I should be able to answer it. A Turbprop uses a gas turbine core to provide energy to the power turbine which in turn drives the propellor. This is more efficient than a 'jet' or turbofan (as on most big civil aircraft) at low flight speeds. For this reason the turboprop is most suited to low speed a/c such as transport planes, some commuter a/c, etc.
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It's also important to note that the engines are usually matched to a given aircraft, or a given wing profile. A turbojet is typically used on sleek aircraft with wings that only provide lift at high speeds, but also have little drag - ideal for high speed, high altitude aircraft. There is some facility to adjust wing profile, but essentially the design of the wing dictates its operating conditions. however, such aircraft have a minium speed at which they will fly (called the stall speed) and this must be exceeded on the runway in order to actually be able to take off. If the runway is short, large jet planes cannot reach their stall speed and hence cannot use that runway. To address this, aircraft with more 'aggressive' wing profiles were developed, where lift was increased at slow speeds with a resultant lowering of the stall speed (if used on a comparible airplane of similar weight), thus allowing the us eof shorter runways. However, these more aggressive wings suffer from increased drag and thus must be used pretty much exclisively at slow speed, and as SP rightly states the turboprop is more efficient that the turbojet in this respect.
There are also issues with altitude, the turbojet actually improves at higher altitudes, whereas the propellor on the turboprop can only effectively operate up to a certain height.
I believe there might be noise issues as well, the actual thrust of a gas turbine is generated in essence by the action of expanding gas external to the engine body, which is very, very noisy (you will most likely be deafened if you stand near a military jet at take-off). The use of bypass engines both improves efficiency (I forget how) but also shrouds the expanding gas and thus reduces noise. The turboprop only utilises the gas expansion within the turbine, once clear of the engine it doesn't perform any real task (although some might use some slight thrust) and hence it can be baffled or silenced. There will still be noise from both the propellor and the engine but orders of scale lower than for a turbojet.
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Thanks everybody - low speed fuel efficiency and a shorter take-off would explain why British European have replaced jets with turboprops on their shorter routes.
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Turboprop engines are an attempt to combine the best features of a turbojet engined aircraft and a propellor powered aircraft.It did't have much success when the technology first came out as it isn't as fuel efficient as a turbojet and companies were not prepared to gamble with it.But during the economic crisis of the 70's kerosene prices dropped and the design was put in to use in a limited capacity.
Turbojets are more efficient at higher speeds and higher altitudes and props at speeds under 400 mph and altitudes less than 30,000 ft.
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At airspeeds below about 250-300 knots, the turbo-prop has a greater efficiency, i.e. is more fuel efficient than a turbo-jet. It?s more economical to give a small acceleration to a large mass of air (turbo-prop) than a large acceleration to a small amount of air. Basically, I can push a van at walking pace, but it would kill me trying to run pushing a moped!
Also, above about 300 knots the propeller tips start to go supersonic which creates all manner of problems.
So, if the aircraft designer is happy cruising below 300 knots, then he?ll use a turbo-prop for efficiency. There?s also some other benefits with a prop at low approach speeds as you get extra airflow over the wings/flaps.
The Turbo-fan engine, as used on most airliners, has the best of both worlds. It?s basically a Turbo-prop with a duct around the prop.
So the advantages, are basically fuel consumption at the expense of speed.
Hope this helps
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Also:
Turbo props are better at lower altitudes and really useful for short runways and slow speed approach runways (due to sharp turns to get an approach as the raf frequently find hence the Turbo prop Hurcules transport they use) Its not true to say that Turbo props are a more recent thought. Lets just mention the Vanguard and Viscount. Excelent aircraft both of them.
(they tend not to ingest birds either)
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Intersting to compare the self perpetuating power of a turbo jet/prop with that of a turbo petrol/diesel piston engine.
The principal is the same, exhaust gasses developed by a combustion process which drives a turbine which is connected to another turbine which forces air into the combustion chamber which drives a turbine which is connected to another turbine which forces air into the combustion chamber which drives a turbine which is connected to another turbine which forces air into the combustion chamber under incresed pressure which enhances the combustion process which drives a turbine which is connected to another turbine which forces air into the combustion chamber under incresed pressure which enhances the combustion process which drives a turbine which is connected to another turbine which forces air into the combustion chamber under incresed pressure which enhances the combustion process which drives a turbine which is connected to another turbine which forces air into the combustion chamber under incresed pressure which enhances the combustion process which drives a turbine which is connected to another turbine which forces air into the combustion chamber under incresed pressure which enhances the combustion process ....
All happening many thousands of times a minute.
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I always enjoy considering the way that the shaft speed is controlled in an automotive turbo ---- it isn't!
The shaft speed is determined purely by considerations of energy balance. The energy taken from the exhaust gas - largely seen as a drop in its temperature, balances the energy given to the inlet stream by raising its temperature and pressure.
By good design and matching, the shaft never overspeeds.
number_cruncher
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Oh yes they do, having spent my formative years on the C130 I can assure you they can and do overspeed. I have spent many a funfilled day in foreign parts replacing propellors and engines due to this fault.
For those who want a glimmer of understanding of how a gas turbine fuel system is laid out in the schematic form they only need look at the London Underground map. Very similar. Mr Whittle has a lot to answer for.
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We have a couple of Lucas Shaftmoor Lane refugees here, who vouch for the complexity of the fuel systems for gas turbine engines - they are mainly happier with the aroma of hydraulic oil than with that of aircraft fuel!
In my (slightly off topic) post, I was only thinking of the turbochargers which are fitted to automotive engines. These usually expire from poor lubrication, and abuse.
number_cruncher
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Turbine powered helicopters are also turbo-prop it's just that the powershaft goes through a 90 degree gearbox to power a really big prop (rotor) on top (and of course a little one at the back).
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Turbine powered helicopters are also turbo-prop it's just that the powershaft goes through a 90 degree gearbox to power a really big prop (rotor) on top (and of course a little one at the back).
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Unless it is a NOTAR (NO TAil Rotor) then it just has an internal fan and blows air out the side of the tail.
www.kulikovair.com/Notar.htm
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Meant to add, if you want any more info, ask away on the technical questions forum here;
www.airmech.co.uk/forum/ultimatebb.cgi
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Why do I need any other website - all life is here...brilliant.
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It's perhaps interesting to note that most large modern 'jet' aircraft are in fact using turbofans - effectively a turbo prop system. They use oversize first stage compressor blades which perform the two functions of supplying air to the gas turbine core, and also ducting air around the gas turbine core and out at the back. The ducting is more efficient than an open propellor and these engines have the advantages of reduced noise output and improved fuel efficiency.
Find a picture of a 707, for example, and compare the slim turbo jets used there with the very large diameter turbofan units seen hanging from the wings of 747s and the like.
JS
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