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Turboprop Propellers can be ducted for extra thrust. What are the criteria for determining that the engine is no longer a ducted propeller design, but a turbofan? Does there have to be a type of blade design, a fully separate nacelle/pod, number of blades? Or are all turbofans technically shrouded propellers with some augmented thrust from the turbine exhaust ?

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  • $\begingroup$ I would say when it uses exhaust gas as the primary source of thrust rather than a propeller. $\endgroup$ – Ron Beyer Jan 24 '17 at 0:09
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    $\begingroup$ @RonBeyer Doesn't most thrust in a turbofan come from the fan? $\endgroup$ – Steve Jan 24 '17 at 1:19
  • $\begingroup$ @Steve For a high-bypass turbofan, yes, for a low-bypass, no. I guess the distinction may be what is driving the fan at the front. Usually a ducted fan doesn't have any thrust from the exhaust gas, whereas a turbofan combines the two. $\endgroup$ – Ron Beyer Jan 24 '17 at 1:55
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    $\begingroup$ @Notts90 Is there not a generally accepted engineering or industry wide definition of what constitutes a turbofan? Just a bit curious. $\endgroup$ – NZKshatriya Jan 24 '17 at 15:11
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    $\begingroup$ @Notts90 I do believe you mean now posted and not "not" posted lol. $\endgroup$ – NZKshatriya Jan 24 '17 at 21:13
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tl;dr

They're pretty much all considered turbofans with a few exceptions from the '70s such as this experimental Britten-Norman Islander. They key difference is a prop can be considered a separate entity in a turboprop, but a fan is integrated into the engine.

enter image description here

Slightly longer answer

After a fair bit of thought and research I hope I've managed to come up with an answer, though to start with a disclaimer. The aviation industry is far too fond of merging and creating new words to describe new engine architecture. Take Open Rotor vs. PropFan for example.

To get technical about it, you could look at the similar/reverse argument about the difference between an open rotor turbofan and a turboprop, as addressed by the EASA.

If you look at Appendix 1: Open Rotor Definition (Page 86) of that document they outlined the following key differences

Open rotor module that cannot be distinguished as a separate entity

However the following was the agreed definition, which is still rather ambiguous.

A Turbine Engine featuring contra-rotating fan stages not enclosed within a casing

I think the first one is more key, the prop can be considered as a separate entity, attached to the front of a turbine. A fan is an integral part of the engine and cannot really be considered a separate entity, as its usually a part of the actual turbine shaft and has functional turbine component passing through the middle of it.

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    $\begingroup$ Maybe the engine designation is determined by the tail number. It seems quite fitting in this case. :) $\endgroup$ – FreeMan Jan 24 '17 at 21:54
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In general (there are often exceptions to any rule), a turboprop unit is likely to have a gearbox between the turbine and the propeller. The propeller is likely to be capable of varying its pitch. Also, the propeller may be a set of two contra-rotating propellers in the case of a high power turboprop. A turbofan engine is unlikely to have these features, although PW does have a GTF geared turbofan.

enter image description here

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  • $\begingroup$ We don't see many contra-rotating turboprops around, actually. $\endgroup$ – Koyovis Sep 5 '17 at 7:47
  • $\begingroup$ Please post the source for your image, and for those of us who are a little behind on our Russian air force history, please identify this magnificent beast! $\endgroup$ – FreeMan Sep 6 '17 at 17:27
  • $\begingroup$ It's a Russian TU-95 Bear bomber. The photo is from the Russian Federation Defence Ministry press service. $\endgroup$ – Philip Ngai Sep 13 '17 at 22:58

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