# At what point is a turbine powered ducted propeller considered a turbofan?

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 ?

• I would say when it uses exhaust gas as the primary source of thrust rather than a propeller. – Ron Beyer Jan 24 '17 at 0:09
• @RonBeyer Doesn't most thrust in a turbofan come from the fan? – Steve Jan 24 '17 at 1:19
• @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. – Ron Beyer Jan 24 '17 at 1:55
• @Notts90 Is there not a generally accepted engineering or industry wide definition of what constitutes a turbofan? Just a bit curious. – NZKshatriya Jan 24 '17 at 15:11
• @Notts90 I do believe you mean now posted and not "not" posted lol. – NZKshatriya Jan 24 '17 at 21:13

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 (add-on) to the turbine for a turboprop, but a fan is an integrated component for a turbofan.

After a fair bit of thought and research I hope I've managed to come up with a satisfactory 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 and powered by a gearbox. A fan is an integral part of the engine and cannot really be considered a separate entity, as it actually forms part of the low pressure shaft and is considered the first stage of the low pressure compressor.

• Maybe the engine designation is determined by the tail number. It seems quite fitting in this case. :) – FreeMan Jan 24 '17 at 21:54
• I would say that would'nt definitely be a turbofan if there is no mechanical linkage between the "turbojet" section (the gas generator) and the turbine driving the propeller. But this would be just my own definition of what isn't a turbofan. – Marco Sanfilippo Aug 28 '19 at 7:18

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.

• We don't see many contra-rotating turboprops around, actually. – Koyovis Sep 5 '17 at 7:47
• The Tu-95/Tu-114 and the An-70 are the only two (three) that I can think of, indeed. – Marco Sanfilippo Aug 28 '19 at 7:11

The difference between a ducted propeller and a turbofan is mainly determined by the difference between a propeller and a fan.

• A propeller has relatively few blades, which are relatively long and slender.
• A fan has many blades, with a relatively large chord. Like a household fan.

A parameter to catch blade count and chord relative to blade length, is the disk solidity $$\sigma$$. Area of all blades summed together, divided by the area of the circle defined by the blade tip length:

$$\sigma = \frac{\text{blade area}}{\text{disk area}} = \frac{A_b}{A} = \frac{N_b c R}{\pi R^2} = \frac{N_b c}{\pi R}$$ with $$N_b$$ = number of blades, c = blade chord, R = blade radius.

There does not seem to be a defined transition point of $$\sigma$$, above which we're talking about a fan. The 8-bladed propeller of the A400M with a blade radius of 2.6m has a solidity ration of about 0.3 which is amongst the highest in the world. We instantly recognise it as being a propeller.

Author 1

The fan of a geared turbofan like the PW 1000G is instantly recognisable as a fan, with its 20 relatively fat blades and a $$\sigma$$ close to 1. Note that both are driven by a gearbox for obtaining a beneficial tip speed.

Or are all turbofans technically shrouded propellers with some augmented thrust from the turbine exhaust ?

Indeed, technically a fan is a type of propeller. Turboprops also have thrust directly from the turbine exhaust.

1 By Rafael Luiz Canossa - IMG_9975, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=55662586

• Great answer (+1) though I think the comparison to a household fan is confusing. – Notts90 is off to codidact.org Aug 27 '19 at 17:33