Turboprops v Turbofans

[laffinboy]

As an interested pax I do hope this isn't a dumb question, however...

What are the advantages of turboprops over turbofans as a means of powering certain commercial aircraft? The propellor was obviously appropriate (if not essential) during the days of piston powered engines, but why do we still utilise propellors attached to what are effectively small jet engines in the form of a turbo-prop?

Is it not possible to develop small turbofan engines that would have equal if not greater efficency and performance and are suitable for using on smaller commercial aircraft?

Having travelled on a range of turboprop and turbofan engined aircraft, in terms of overall noise and vibration turbofanned aircraft seem to produce significantly less amounts of these, but there must be some advantage to using turboprops. Thoughts please, and apologies again if I have missed something blindingly obvious here!

[tomcs]

I'm not totally qualified to answer this but I'll have a go....Its 6 of 1 and 1/2 a dozen of the other i think....Turboprops provide much greater low level performance like takeoffs for example....whereas the turbofans are much better in the cruise....I think....please correct me if I'm wrong.

Tom:cool:

[redsnail]

laffinboy,
Not a bad question. The type of operation determines whether a turboprop or a turbofan is used.
For short journeys, ie approx 60 minutes or less, a turboprop is much more efficient. It can (for a similar class of aircraft) get to it's optimum cruise level faster and it is usually much more flexible in the approach segments too. The biggest advantage is that they can operate on much shorter runways.
For a long trip, the turbofan comes into it's own. It can cruise at higher levels for a lot longer (and overall faster).
I wouldn't like to cross the Pacific in a turboprop however, I wouldn't like to try to get into an 800m strip in a turbofan.
There are economies of scale with regard engine size too. With a turbofan, the "engine" itself is reasonably small, it is the "fan" that makes the engine appear large.

Again, this is a very simplistic view.

[Stiff Under Carriage]

To add to Redsnail's comment, for shorter trips, the use of a turboprop is more efficient. One must also look at the efficiency of the engine itself. A turboprop has a much greater thrust efficiency at lower speeds, up to approx 350-400kt from memory, So on a short haul flight a speed higher than that, would be of little use, for the distance flown.

I hope this adds to you answer.

SUF.

[knobbygb]

Not qualified to comment in any way, but...

Turboprop and turbofan engines may look very different, but if you think about it, are more similar than people think.
Take a turboprop, add a load of extra blades, put a shroud around the whole thing to cut the noise down on the noise and it starts to look a lot more life a fan engine.

Most of the thrust in a modern turbofan is generated by the fan at the front, rather than the air actually passing through the core of the engine, so in a way this is acting like a large propellor. I think.

Obviously not that simple in reality - variable pitch/constant speed props etc.

What happened to the 'Unducted Fan'? (Turbojet engine with external 'blades' on the rear). Was this an attempt at 'best of both worlds'?

[Shaker One]

... and just expanding further on the benefits of a propellor (in particularly in terms of field length available) - the prop assists in creating lift by virtue of its prop wash being sent back over the wing whereas a jet must rely solely on its forward movement through the air before any lift is generated.

This equates to a shorter takeoff distance ie; less runway required for the propellor driven aircraft with similar power/thrust available. Two good comparisons would be the Embraer 145 and the Saab 2000. Similar power units but you'd rather be in the Saab when there's a shortage of concrete in front of you.

----

Shaker One

[Mycroft]

The UDF was the subject of a discussion (http://pprune.org/forums/showthread.php?s=&threadid=10357) last year, and I think the final verdict was that it was too noisy, especially at some frequencies and although more fuel efficient was not significantly so to warrant development. There were also perceived problems with blade failure, as such would of course be uncontained.