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Does the lift of that section of the wing which is in the wake of the propeller increase (because the airspeed in that section is higher)?

If yes, is this fact used actively to improve the lift capability?

Or does it decrease significantly because of turbulence and an overall disturbed airflow on the wing surface?

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    $\begingroup$ or decreased because of the turbulence of the blades? $\endgroup$
    – ratchet freak
    Nov 6, 2014 at 23:27
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    $\begingroup$ Any reason to limit the answers to turboprops? $\endgroup$
    – Skip Miller
    Nov 7, 2014 at 1:34
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    $\begingroup$ @SkipMiller : Turbojets and turbofans will guide the air entirely through a nacelle shroud through a passage without the fluid coming in contact with a wing surface (ignore interference drag). This is outside the premise of my question. You may use a piston engine if you want! All I need is that there's a blanketed airflow on the airfoil. $\endgroup$
    – Raj
    Nov 7, 2014 at 3:42
  • $\begingroup$ @Raj I think he meant why not also consider internal combustion prop engines $\endgroup$
    – ratchet freak
    Nov 7, 2014 at 9:14
  • $\begingroup$ @ratchet freak : as I mentioned earlier, piston prop IC engines too. $\endgroup$
    – Raj
    Nov 7, 2014 at 9:17

3 Answers 3

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It both increases and decreases, depending on the local direction of the propellor (e.g. on the upgoing side, or the downgoing side) . See for example this PhD thesis: Propeller Wing Aerodynamic Interference.

This thesis shows the following image: Prop interaction

Simply said, the velocity of the propellor locally changes the angle of attack, and thus the lift generated by the wing. It is interesting to know that it is possible to locally optimize the shape of the wing to take advantage of this effect. Again the thesis shows the result, and it looks like this:

enter image description here

If you look closely, you can see that the wing is changed at the propellor location to accomodate the adjusted flow.

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  • $\begingroup$ An excellent answer! Answers my first question well. But is this different in velocity actively used anywhere? Like, are the airfoils in the wake different from the ones in the outboard section (say, they are optimised for a higher flow velocity)? Shouldn't they be? $\endgroup$
    – Raj
    Nov 7, 2014 at 23:33
  • $\begingroup$ I have to say that these vertical velocities resulting from the prop generally do not increase the total lift. It increases the lift on the prop-upward side, and decreases on the prop-downward side. This thesis optimized the wing such that the final lift distribution was closer to the elliptical lift distribution. (e.g. trying to cancel out the effects of the prop). $\endgroup$
    – ROIMaison
    Nov 8, 2014 at 19:59
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    $\begingroup$ Further more, I think that the $\Delta V$ of a prop is kept as small as possible (to maintain a high propulsive efficiency). In the thesis (page 19) I found a picture that (I think) states velocity increases of 6%. I don't think this will have a very large influence on the performance in the low mach range that props typically serve, but I'm not sure. $\endgroup$
    – ROIMaison
    Nov 8, 2014 at 20:00
  • $\begingroup$ Can't open the attached "PhD thesis on propellor interaction". Can you plz provide another link. Thanks. $\endgroup$
    – Dcode
    Jun 8, 2017 at 9:22
  • $\begingroup$ I don't have a link, I've added the full title so you can search for it yourself $\endgroup$
    – ROIMaison
    Jun 8, 2017 at 10:10
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According to this NASA Document the lift after the propeller is higher and it is used on purpose to create more lift.

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Wing-mounted propulsion systems have significant effects on the wing aerodynamic characteristics, and these effects are more pronounced when the highlift components are deployed. Various aerodynamic components contribute to the rise of these effects. Some of these effects are external to the wing performance and affect the measurement of the aerodynamic characteristics of the combined assembly. Examples of these effects are the propeller thrust, the location of the thrust line, tile size and location of the exhaust nozzle, and the thrust from the exhaust nozzle alone. Another group of effects are pure aerodynamic effects, such as the propeller slipstream and the flow past the nacelle and nacelle attachments.

Page-7

Results indicate that the lift coefficient of the powered wing could be increased by the propeller slipstream when the rotational speed (disk loading) was increased and high-lift devices were incorporated.

It is also stated, that the exhaust of the turboprop can also create lift.

For example: An exhaust of a free-turbine "PT6A-67" turboprop of a converted Conair Firecat doesn't generate any lift. Conair Firecat

But an exhaust of an Direct-drive "Rolls-Royce RB.53 Dart" turboprop YS-11 which is nearly on the wing, can generate additional lift and acceleration. ys11 enter image description here

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There is a lot of work going on at NASA right now to take advantage of the additional lift created by propellers on the wing, which allows the wing to be thinner for the same amount of lift. Look for a series of papers coming in June of 2015 on distributed electric propulsion where this is being exploited as a way to achieve additional efficiency.

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    $\begingroup$ A thinner wing doesn't by itself produce more lift, more camber does. Thin, unswept wings have a smaller useful AoA region and are heavier than a comparable, thicker wing (or need bracing). Thin wings (with sweep) are most useful for supersonic flight. $\endgroup$
    – Peter Kämpf
    Nov 18, 2014 at 16:27
  • $\begingroup$ Perhaps if thinner wings can be made lighter you have an advantage? $\endgroup$
    – Jonny
    May 20, 2015 at 5:55

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