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On WikiPedia, the Progress D-27 is described as a propfan engine and the airplane it powers, the Antonov An-70, is described as propfan-powered.

This Air&Space magazine article contradicts that; it states that the An-70 is turboprop (not propfan) powered, as is the Airbus A400M.

And indeed the basic configuration of the D-27 and the A400M's EuroProp TP400 turboprop are very similar:

  • 4.5 m propeller diameter vs 5.2 m
  • 14,000 hp vs 11,000 hp
  • two-shaft gas generator, third shaft to drive propeller
  • propeller driven through reduction gearbox
  • propeller blades mounted before engine
  • 780 km/h max speed vs 780 km/h cruise speed
  • variabele pitch blades

The only significant difference is that the D-27 has two contra-rotating blade sets while the TP400 uses only one blade set.

Having contra-rotating propellers does not necessarily make a turboprop a propfan though (e.g. the Tu-95 is a turboprop aircraft).

So all things considered, what would make the D-27 really a propfan but not the TP400? Is there really a distinction between a propfan and a turboprop?

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It depends

It seems that currently the term is more a marketing term than an engineering term.

When fears arose among airline officials that the word “turboprop” would meet with consumer resistance, the term “prop-fan” was used in a poll of United Airlines passengers. It worked: 50 percent of the respondents said they’d fly on a prop-fan-powered airliner.

From Air&Space Magazine


It depends which particular features you consider an essential defining element for the term "propfan".

Turboprop?

Many features of propfans are also found on some powerplants described as turboprops by their makers.

  • Contra-rotating blades? - Found on, for example the 1950's NK-12 turboprop.

  • Supersonic tip speeds? - NK-12 turboprop again.

  • More than four blades?

    • The 1945 RB.50 tuboprop had five.
    • The 1984 PW127 turboprop drove a six-bladed prop.
    • The 2009 TP400 turboprop drives an eight-bladed propeller.
  • Curved blades? - TP400 turboprop.

Gearbox?

  • No Reduction Gearbox - A key distinction of a key forerunner, the GE36, was the lack of a reduction gearbox as used in turboprops.

Other names?

  • GE called their GE36 an unducted fan (UDF).
  • Rolls Royce call their design an open rotor engine.
  • The acronym CROR is also in used; it stands for Contra-Rotating Open Rotor.

D27?

The marketing department of its designers call it a propfan.


Dictionaries

Oxford says

  • Turbofan: A jet engine in which a turbine-driven fan provides additional thrust.
  • Turboprop: A jet engine in which a turbine is used to drive a propeller.
  • Propfan: No results found for “propfan”.

Mirriam Webster - same as Oxford essentially.

Aviation terms.com also defines turboprop etc but not propfan.

Aviation Canada, and other aviation glossaries I looked at, also fail to mention propfan.

Dictionary.com says

propfan. Aeronautics. a turbojet having a turbine-driven propeller that operates completely outside the jet engine.

But that definition could equally apply to any turboprop.

Wordsense.eu says

propfan (pl. propfans) A modified turbofan engine with the propeller outside the engine duct of an aircraft.

Which is not any better.

Wikipedia tries to define propfan in terms of what it isn't. The nearest it gets to a definition is to say that a propfan has a propellor with many short twisted blades. However the TP400 has many twisted blades - perhaps they are not short enough to qualify?

Conclusion?

There doesn't currently seem to be a clear and short widely-agreed or widely-used set of engineering criteria for labeling a particular engine a propfan rather than an advanced turboprop, an unducted turbofan, an open rotor engine or any other name.

The D27 is a propfan because its maker's marketing department calls it that.

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    $\begingroup$ I would really like to give you +1, but you keep misspelling its :P $\endgroup$ – Federico Aug 12 '14 at 6:21
  • $\begingroup$ I already suspected marketing was playing a big role in naming a turboprop a propfan. "open rotor" is a commonly found name, as is CROR - contra rotating open rotor. $\endgroup$ – florisla Aug 12 '14 at 6:41
  • $\begingroup$ @Federico: Aargh! $\endgroup$ – RedGrittyBrick Aug 12 '14 at 8:13
  • $\begingroup$ @RedGrittyBrick Do you have a source for your conclusion that there is no clear distinction? I'd look in WikiPedia but the article there states that a propfan is distinct from both a turboprop and a turbofan. $\endgroup$ – florisla Aug 13 '14 at 14:39
  • $\begingroup$ @florisla: I can't give you a single source. I have found that there are many contradictory definitions and that mainstream traditional dictionaries have no definitions. $\endgroup$ – RedGrittyBrick Aug 13 '14 at 15:04
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Yes, it is a Propfan Engine. This explains the increased number of blades in the propeller. Higher Disk Loading requires the use of smaller diameter propellers. The blades are scimitar-shaped, with swept-back leading edges at the blade tips to accommodate the large Mach numbers encountered by the propeller tip at high rotative and flight speeds.

http://ivchenko-progress.com/?portfolio=d-27&lang=en

In fact the manufacturers of AN-70, SE Ivchenko Progress, themselves describes the D-27 as a Propfan (also here

EDIT :

A propfan consists of swept, rotatable airfoil blades pivotally mounted to a hub for pitch change movement with respect thereto, and having a solidity ratio of 1.0 or greater at the roots of the blades and less than 1.0 at the tips of the blades. The prop fan is operable at or above critical Mach numbers and at transonic or supersonic tip speeds, and is characterized by: each of the blades having a leading edge. The leading edge, from a location thereon at approximately a midportion of the span and the blade, outwardly to the tip thereof, is curved in a chordal direction to define blade sweep while exhibiting no significant offset curvature in a span-wise direction.

Source: http://www.google.com/patents/US4730985

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    $\begingroup$ Just because the marketing department calls it a propfan does not make it true; I'm not convinced yet. The TP400 has only one blade set so its disc loading is higher than that of the D-27. It also has scimitar-shaped blades. Its speed is higher (cruises at 780 km/h which is the An-70's maximum speed). $\endgroup$ – florisla Aug 11 '14 at 14:19
  • $\begingroup$ I have edited my comment to include what type of engines are defined as PropFan. It's not just a marketing department classification , tests were conducted by the country's Ministry Of Defense to test the performance of the SV-27 propfan intended for the D-27 ruaviation.com/news/2013/4/12/1630 $\endgroup$ – D_S Aug 11 '14 at 15:33
  • $\begingroup$ Van you clarify why the tp400 is not a propfan? It also has variable pitch blades although their shape definitely differs from the D-27's. $\endgroup$ – florisla Aug 11 '14 at 19:09
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To decide whether the D-27 really is a propfan, one has to know the mass flow rates through the propeller and through the engine core.

The German wikipedia entry on the turboprop engine wikipedia.de/Turboprop gives the engines bypass-ratio BPR as the criterion for deciding on the engine type:

BPR ~= 9:1 Turbofan

BPR ~= 20:1 Propfan

BPR ~= 100:1 Turboprop

The bypass-ratio is defined as the mass flow rate through fan/prop divided by mass flow rate through engine core.

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    $\begingroup$ Is the German WikiPedia correct? $\endgroup$ – florisla Jun 9 '15 at 12:23
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    $\begingroup$ It makes sense to classify according to bypass-ratio. There is no reference given however. $\endgroup$ – user7241 Jun 9 '15 at 16:05
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An unducted fan or propfan is defined as a fan or fans being directly driven by a set of turbines, that is, no reduction gearbox. The unducted fan is essentially a turbofan with the fan being outside the nacelle, hence the term "unducted." Early unducted fans were directly integrated onto the turbine section of the plant resulting in the fans being in the aft of the engine, however if you run a shaft to the front, as is done on a turbofan, it can be a tractor.

A turboprop is defined as having a gearbox to connect the turbine section with the propeller.

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  • $\begingroup$ Where is it defined like that? And is that definiton generally accepted? $\endgroup$ – florisla Jun 9 '15 at 12:25
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Yes. It's a prop-fan. However, it's not trying to give it a name that's important. It's knowing what it does and how it does it that is important.

So, this is what it does: "Propfan, so called, because it combines the operating efficiency of the turboprop with the higher speed and altitude capabilities of the turbofan." ref Hamilton Standard/ NASA CR 174790. This can be achieved in a variety of ways: single rotation or counter rotation, wing or fuselage mounted, tractor or pusher, geared or gearless. ref "Single rotation and counter rotation prop-fan propulsion system technologies" ICAS-84-5.6.2. So, whatever particular configuration is chosen to bridge the gap between the conventional turboprop and turbofan then it's safe to call it a prop-fan, but only if you're talking to someone with the same understanding.

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