Turboprops

gremlin | 14:42 Tue 24th Jan 2006 | How it Works

9 Answers

I have read that Turboprop engines used in aviation are jet engines whose output shaft drives a propeller. What are the advantages of using a prop rather than a standard turbofan style of jet engine? I can understand their use in smaller aircraft, but why use them in large multi-engine civil airliners and transports like the C130 Hercules?

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JudgeJ

The main advantage is that they are more efficient at lower speeds than their pure jet (i.e. turbo-fan) cousins. They are used in aircraft designs where high speed (i.e above about 350-400mph) is not envisaged.

They are also more flexible in that different propellor designs (depending on the tasks the aircraft has to perform) can be used on the same engine.

Clanad

...Additionally, they provide superior short to medium runway length take-off performance. One of the primary reasons, however, is cost. The typical prop-jet engine, (largest manufacturer of engines used here in the U.S. is Pratt and Whitney) is significantly less expensive than a fan-jet engine. The Pratt is not nearly as susceptible to damage by ingestion of foreign objects on the ground (blowing paper, pebbles, small dogs, etc.)as are fan-jets... JudgeJ's description is certainly correct as well...

KebabMeister

Clanad, would you also say that the jet-prop is more responsive than a turbo-fan, i.e. being able to maintain engine RPM whilst changing propellor pitch to assist take off and landings? What I mean is that I would have thought that propellor response is quicker than a jet engine because the former you can maintain a high RPM whilst on the latter you may be varying the RPM more? Does that make sense? lol

Clanad

Very insightful, KebabMeister... there are, however, some differences in how different engine manufacturers pair their engines with particular propellers. By that I mean, for example the Pratt and Whitney engine I used for an example is known as a free turbine.... quite unique in that the power turbine is not connected mechanically to the gas generator turbine (in fact they turn in opposite directions internally). What this means is that if the engine is allowed "unspool", i.e., be retarded to idle while, for example, on final approach, and then a lot of power is suddenly required, as in a go-around) it takes this engine some 5 to 10 seconds to "spool" up. Whereas other popular makes (Garrett TPE series and GE turboprop engines) are more typical in design in that the "flow through" of the compressed, ignited gasses mechanically drives the power shfat to the propeller. It's more responsive but has some down sides as well (to lengthy to go into here)... Turbofan jet engines, while more responsive that older jet engines are still less responsive that the engines described above... Thanx!

KebabMeister

Thanks for the response. I think I would prefer turbo-prop if I was flying :)

gremlin

Question Author

Excellent info - many thanks to everyone who replied.