A rotating but fixed propeller sucks in air from the front. What is the cause for this acceleration of air particles? Since propeller blades act like rotating wings, my best guess is that the blades create a lower pressure zone in front of them which causes higher atmospheric pressure air to flow into this lower pressure zone. This creates the effect if the propeller would "suck in" air from the front.
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3$\begingroup$ I bet your question comes from the standard wrong explanation of how wings work, the explanation that fails to account for downwash. Let me direct you to my favorite site for explaining how wings work. $\endgroup$– Mike DunlaveyMar 26, 2015 at 16:22
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$\begingroup$ @jameslarge: Yes of course there's a pressure difference. The wrong explanation is the one that says the air parcels reunite at the trailing edge - the "equal transit time" fallacy. $\endgroup$– Mike DunlaveyFeb 9, 2016 at 14:59
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2 Answers
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A propeller imparts momentum on the air - when the blades hit the air molecules, it gives them momentum towards the back (high pressure) side; this creates a net velocity towards the rear of the propeller. Normally air molecules in the plane of the propeller have equal probability of moving forward or backwards; the propeller's action makes the situation asymmetrical. Since more air molecules move backwards, a low pressure area is created in front - and air will be pushed towards this region be the atmospheric pressure "far away".
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$\begingroup$ I'd like to improve on this answer and mention that the slants act to push air back, rather than suck air from the front. Pushing of the air backward creates a lower pressure zone at the front as the air has to come from somewhere, to equalise air comes in from the front and so on. Essentially one would want to achieve equilibrium $\endgroup$ Feb 9, 2016 at 13:40
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1$\begingroup$ @RussellYang, It doesn't matter how the propeller works---at least, not for the purpose of answering this question. The point is, when the propeller moves a mass of air, that's going to leave a zone of lower pressure in the place that the air formerly occupied, and the only place where air can come from to fill that void is in front of the the propeller. $\endgroup$ Feb 9, 2016 at 14:58
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$\begingroup$ @RussellYang - I have changed my first sentence a little bit to incorporate what you suggested. Do you like it? $\endgroup$– FlorisFeb 9, 2016 at 15:08
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when you look at an airplane propeller, you will se that parts of the propeller blades are slanted. These slants, when rotating, push the air back, creating a low pressure zone, sucking more pressure in, while at the same time pushing more air back.
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$\begingroup$ As the OP says, a propellor is simply a rotating wing, with a similiar airfoil section. If you look at a helicoper blade close up, only when it's stopped :), you will see the similarity between the blade and a wing. You can imagine how it works in two different ways, giving the same result. 1. By changing the momentum of the air in front of the propellor in a "backwards" direction and in effect pulling the aircraft forward. 2. You can also look it as a low/high pressure zone effect as in the OP, see [link] en.wikipedia.org/wiki/Wing $\endgroup$– user74893Mar 23, 2015 at 13:28