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For thrust reversal after landing, a turboprop engine reverses blade pitch and blows air forward. But the engine itself requires air flow for the turbine.

How can a turboprop engine keep running when the prop blows air away from it?

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    $\begingroup$ upvoting because, while obviously the turbine keeps ingesting air, I'm hoping someone will find a good flow-field visualization. I haven't so far. $\endgroup$ – Erin Anne Sep 9 '17 at 1:26
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    $\begingroup$ most of the blowing happens on the outer part of the blades; their twist distribution means that the inner part is stalled and does litte to suck air away from the intake. $\endgroup$ – Peter Kämpf Sep 9 '17 at 1:36
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The prop is not blowing air away from the intake, atleast in that region. In modern propellers, the twist of the blades is such that the blade angle is smaller at the tip compared to the root.

Propeller twist

Airfoil section in propeller blade; image from FAA Pilot's Handbook

Now, if reverse thrust is applied, the outer regions of the propeller is in the negative angle of attack, producing reverse thrust. However, the case is different in the root region, which won't have a negative angle, thus not blowing air 'away'. The net result is that the compressor can quite easily handle the reverse thrust condition.

Propeller angles

Propeller angles in various flight regimes; image from pilotsofamerica.com

Another thing is that the reverse thrust is quite small compared to that of the forward flight and the speed is also quite small. modern compressors don't have much problem in this regard.

For comparison, consider the turboshaft engines in the helicopters in rearward flight- they can handle a much greater range of speeds in that condition. As an extreme case, consider the Russian aircraft doing tailslide- even the turbofan engines cope up with that, albeit in a much limited manner.

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  • $\begingroup$ Yes the blade tip may have negative AoA while the blade root retains positive AoA. Not convinced about your quantification though: how much reverse thrust in percent can a turboprop generate, and how can it maintain a considerable airflow towards the turbine while reversing? A helicopter turboshaft is designed for hovering circumstances, has no propeller blowing air away directly in front of it,, and reverse speed is never huge due to the negative stability of the tail. And I'm not sure which Russian tailslide you're referring to. $\endgroup$ – Koyovis Sep 11 '17 at 6:02

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