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Turbofans and propfans are, in concept, quite similar: both have a turbojet core, plus one or more additional turbine stages that drive a fan(s) which blows air around the core (this is known as bypass air, because it bypasses the core rather than going through it). The single defining difference is that, with a turbofan, the fan(s) is enclosed in a large cowling (hence the alternative name "ducted fan", as the bypass air passes through an enclosed bypass duct), while, with a propfan, the fan(s) is left un-cowled (hence the alternative name "unducted fan").

However, in practice, there is one additional, rather odd, difference between most turbofans and most propfans. Most turbofans operate in the tractor configuration, with the fan(s) placed out in front of the compressors and pulling the engine (and the airplane attached to said engine) through the air, while most propfans operate in the pusher configuration, with the fan(s) aft of the turbines and (surprise, surprise) pushing the engine (and, hopefully, aircraft) through the air. There are exceptions on both sides; the General Electric CJ805-23 turbofan, powering the Convair 990, operated in pusher configuration, while the Progress D-27 propfan, which powers the Antonov An-70, operates in tractor configuration; however, both are exceptions, and the tractor-turbofans\pusher-propfans rule holds true for the vast majority of engines in both categories.

Why is this?

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  • $\begingroup$ The Progress D27 tractor is the only propfan I know of that is in production, so I wouldn't consider it an exception. $\endgroup$ – Pilothead May 27 '18 at 1:00
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    $\begingroup$ Please don't follow the habit of marketing to invent new terms for old concepts and continue calling a turboprop a turboprop, including the Progress D-27. $\endgroup$ – Peter Kämpf May 27 '18 at 5:03
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"Most propfans" is a bit of a strong way to put it. The An-70 is the only production propfan aircraft, making the Progress D-27 the only mass-produced operational propfan. So its configuration is the norm.

As for the reasons for experimental propfans using the pusher configuration, it's generally more efficient in aircraft to begin with, as the airflow is accelerated after passing the obstruction (fuselage). In a propfan, it also avoids the need for a long LP shaft.

As for why turbofans near-universally use tractor, the main reason is that the compression provided by the fan takes part in compressing air for the core. I'm less than 100% sure that it's actually worth the extra components and drag for placing the fan forward, but the alternatives are either having a long suction duct or having a separate ring cowling for the fan, none of which are ideal.

The tractor configuration of turbofans also helps a bit with core cooling. That wouldn't apply to a propfan, which needs a cowling for the core either way.

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