I know the propeller does work on the air and thus the flow behind a propeller is higher than the airspeed. However, what is the quality of this flow? If I were doing a heat transfer analysis of an object behind a propeller, other than the accelerated flow speed, the effect of turbulence would matter too, where turbulent flow will contribute towards higher forced convection. I'm not sure how to quantify the added turbulence from a propeller. Is there a simple rule of thumb?
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$\begingroup$ I have a question (and forgive me if I’m wrong), but outside of pusher props, most propellers are immediately in front of things like engines, cowlings, fuselage, wings, etc... wouldn’t these things immediately impart their own turbulence patterns on the airflow driven by the propeller? And if this is the case, then the turbulence would be different from one configuration to another, even provided the exact same engine and prop, right? $\endgroup$– Aaron HolmesJul 16, 2020 at 15:24
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1$\begingroup$ @AaronHolmes yes, you're right. But I think it's possible with larger props, the ratio of the engine aeroshell radius to the prop radius is quite small (otherwise it would impede flow and reduce efficiency as well). And it's quite possible that the "swirl" or patterns generated by the propeller wake hold for sometime until they reach these components that heat transfer is being studied on. I could be wrong and maybe this is an impossible questions without the aircraft geometry like you say, though. Part of the reason I'm asking.. $\endgroup$– spacepropsJul 16, 2020 at 17:14
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1 Answer
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As a rule of thumb, use fully turbulent flow at the elevated speed of the propeller wake on all surfaces wetted by it. The speed increase will result in higher friction drag compared to the propeller-free case and results in a drag increase that looks about right.
A very interesting paper on the efficiency of an installed propeller can be found here. By comparing the gliding qualities of a Luscombe 8E with drag in level flight, the authors measured the drag increase of the airframe caused by the propeller wake. A drag increase of 30% from the propeller effect was measured, resulting in an overall propeller efficiency of only 62% when the propeller alone had an efficiency of 81%. The paper cites another result where the installed propeller on a Bellanca showed only 58% efficiency, but the paper is unavailable on the Web.
For pretty pictures you can consult this paper which analyzed the propeller wake - wing interaction on a C-130 J with 6-bladed Dowty propellers using CFD. The picture below is taken from this paper and shows surfaces of the same vorticity.
CDF simulation of propeller-wing interaction. The tip vortices of the single propeller blades can be seen impinging on the wing surface which suggests that local flow speed will vary with the frequency of the propeller wake hitting the wing. Also, the local turbulence is higher on the side of the down-moving blade. However, most of the wing surface shows only a minor increase in local turbulence, judging from the vorticity of the wing wake. Note that the engine air intake - nacelle intersection contributes a wake of its own which technically must be added to the propeller effects.
However, I doubt those details can be accurately subsumed in a simple rule of thumb; therefore I recommend to use a constant, elevated flow speed on all areas wetted by the prop wake.
answered Jul 16, 2020 at 20:37
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$\begingroup$ Thanks, this is exactly what I'm looking for! $\endgroup$ Jul 16, 2020 at 21:36
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$\begingroup$ Curious if increase of lift from prop wash was factored in, enabling a lower AOA (of entire plane). This would recover some drag efficiency. Data for wing mounted pusher props would make a great addition to this study. $\endgroup$ Jul 17, 2020 at 23:04
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$\begingroup$ @Good point, but lowering the AoA in the linear range does not change non-lift-dependent drag much. On the other hand, the now distorted lift distribution with propeller effects should lower the Oswald factor of the airplane, adding a bit of induced drag. Hard to say what the net effect is – probably negative at low and positive at high AoA. $\endgroup$ Jul 18, 2020 at 6:37