When perusing the Wikipedia article on the upcoming Rolls Royce UltraFan engine, I noticed that it cited an interesting bit of information from an Aviation Week article:

As the fan system is designed to vary pitch in all phases of flight, including landing, the nacelle will not include a thrust reverser.

I understand that a turboprop can enter the beta range in order to vary the blade pitch into the negative range in order to supply reverse thrust.

However, my understanding of the UltraFan was that it is to be a geared turbofan design with the fan and the core ducted inside a more-or-less conventional turbofan nacelle (minus the thrust reverser, apparently.)

My question, then, is how does the airflow work through this engine with negative fan blade pitch?

When the blade pitch goes negative, then the fan will be blowing air out the intake rather than sucking it in through the intake, no? Where then is the air coming from which is getting pushed out the front and where is the air coming from that is still needing to go in the normal flow direction through the core? Does the flow direction through the bypass ducts reverse?

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    $\begingroup$ The Wikipedia article further says "Along with eliminating the thrust reverser, a short slim nacelle would be lighter and less draggy, but in reverse-thrust the flow would be distorted, having to be turned around the nozzle into the bypass duct, and then partly reversed again into the intermediate compressor.", but unfortunately they do not give a source for this. $\endgroup$
    – Bianfable
    Commented Nov 15, 2019 at 7:52

1 Answer 1


The flow direction reverses only through the bypass duct pulling by blades with negative pitch. A portion of it will still be sucked by low pressure compressor making 180° turn into the core. Having to counteract the losses in turn will obviously reduce the performance. As stated in research paper by the team funded by Rolls-Royce plc: "the core mass flow rate was 57% of that at take-off conditions, for bypass duct flow rates of 74% of total take-off mass flow."

Another design by Pratt & Whitney features reversed position of the whole core requiring two turns of the flow for incoming and outgoing air. So, these turns are not a big problem as we see them traded for other pros.


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