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I understand that axial-flow turbine engines generally include stator vanes behind every(?) compressor and turbine stage. This makes quite intuitively sense, because without them the rotating blades would mostly swirl the air around through the engine, which would not only prevent the stages from properly transporting/compressing/expanding the air in axial direction, but also put useless rotational kinetic energy into that air. The latter is presumably also the reason why turbofans have such guide vanes behind the fan, too.

All of this applies just as well to unducted fans/propellers. Yet I've never seen stator blades behind an open propeller. There are contra-rotating coaxial propellers of course, the idea of which seems to be similar: get rid of the useless solenoidal air motion. But actively contra-rotating both props adds a lot of complexity, and seems somewhat unnecessary if the goal is mostly to remove large part of the swirling motion. Cancelling all torque is not necessary for a plane.

Especially, I would think stators could also be shaped to help retrieve some energy from the tip vortices like a duct does, without however interfering with the inflow that seems to be the reason why ducts make props inefficient at low speed.

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  • $\begingroup$ Loss of propulsion efficiency is gained back with faster, lower drag, higher flying, and larger (less parasite drag per unit mass) aircraft. The 747 is champ at 80 ton miles per gallon fuel, even though it burns 5 gallons of fuel per mile! $\endgroup$
    – Robert DiGiovanni
    Commented Jan 29, 2019 at 18:21
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    $\begingroup$ The stator is the wing right behind the propfan. The contra-rotating propellers are precisely there to remove swirl and reduce swirl-related losses and asymmetries. And the tip vortices will be there regardless. $\endgroup$
    – Peter Kämpf
    Commented Mar 21, 2019 at 16:46
  • $\begingroup$ @PeterKämpf but contra-rotation requires mechanically demanding gearing, unlike a stator. And, if tip vortices would be created by the prop but most of their energy (or even just some of it) recovered again by the stator, then this would be no problem. $\endgroup$
    – leftaroundabout
    Commented Mar 21, 2019 at 23:15
  • $\begingroup$ You ran only recover swirl losses, but not the energy in the tip vortices. $\endgroup$
    – Peter Kämpf
    Commented Mar 22, 2019 at 6:05
  • $\begingroup$ @PeterKämpf well, a duct can do this. Why would it be impossible with cleverly designed vanes? $\endgroup$
    – leftaroundabout
    Commented Mar 22, 2019 at 8:57

2 Answers 2

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When a compressor rotor stage speeds up the flow, the stator:

  1. Slows it back down, turning kinetic energy into pressure (see image here from the book The Jet Engine)
  2. Presents the flow at the correct angle for the next rotor stage.

A nozzle guide vanes stage (what a turbine stator stage likes to be called) comes before the turbine stage, and it does the same but in reverse (turns pressure into velocity) and adjusts the flow direction for the upcoming turbine stage.

The de-swirl function of the stator stage behind a fan in a turbofan (the outlet guide vanes – OGVs) is beneficial because of how much air the fan has acted on.

The number (and size) of blades tell the whole story. It's often said that propellers move air by a little, and jet engines by a lot. To put that into perspective, the typical pressure ratio rise behind a fan is 1.4–1.6, whereas behind a propeller it's less than 1.02.$^1$

This is where a turbofan's OGVs become beneficial. Adding a static stage of blades behind a propeller (with or without a case) that doesn't accelerate the air by that much to recover meaningful pressure (compared to a turbofan's fan, not much kinetic energy to begin with) in the axial direction won't help. On the contrary it will add drag (air hitting a static object), and weight.

Actively contra-rotating both props adds a lot of complexity, and seems somewhat unnecessary if the goal is mostly to remove large part of the swirling motion. Cancelling all torque is not necessary for a plane.

A propeller is like a wing, it needs to act on (hit) air to create a normal force. If we substitute the second set of moving prop blades with static ones, we've just lost a lot of thrust.

$^1$: https://history.nasa.gov/SP-468/ch10-3.htm

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  • $\begingroup$ So to summarize: because props have so much larger radius, they can afford few blades and low angles of attack, thus they don't much move air circularly in the first place; the motivation for contra-rotating props is mostly torque (again because of the large radius) and sucking through more air, rather than rotational energy. I still wonder how a more fan-like prop would perform without a duct but with outlet guide vanes. $\endgroup$
    – leftaroundabout
    Commented Jan 29, 2019 at 11:16
  • $\begingroup$ @leftaroundabout - Because of the fewer and smaller (chord) blades of a prop, a prop does not move a lot of air, so there isn't much pressure to recover. Remember, turbofans produce a lot more thrust than propellers. $\endgroup$
    – user14897
    Commented Jan 29, 2019 at 14:50
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Quick answer is that it boils down to thrust vs drag. Propellers create thrust as airfoils and mass movers, fans are less efficient mass movers that can move much more mass if sufficient power is available. This is why fans are teamed with jet engines. Brute force.

A stator will not benefit the more efficient propeller, but I would not wish to discourage research.

The "reason ducts make props" link provided above is outstanding in that it shows each more efficient propulsion method falling by the wayside as speed increases due to drag, while the turbojet keeps improving.

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