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In the context of producing a pulling force perpendicular to the 'spinning plane' of a propeller/fan,
is it correct to say that a propeller mainly achieves it's force by being aerofoils producing lift and a fan mainly achieves it's force by utilizing Newtons third law?

curious123
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    Martin Beckett gave a better answer than I would have because he mentioned the high-vacuum case. At normal mach numbers, airfoils produce lift by momentum transfer to the fluid, so there is no difference. – Mike Dunlavey Jun 18 '12 at 02:06

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There is no difference in physics between a propeller and fan. In English the distinction is probably that a fan moves air while the propeller moves the vehicle through the stationary air (or water).
edit: Although aerospace engineers call the front stage of a turbine engine a "fan".

There is a difference between an aerodynamic regime at low speeds and higher pressures where the air behaves as a fluid, and low pressure regimes where it is purely mechanical 'billard balls' hitting flat blades - as in a high vacuum turbo molecular pump

Martin Beckett
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  • You are right that there is no difference between the functioning of an airfoil and a fan blade, aside from power, efficiency, etc. The principles are identical and both function by the third law. This is somewhat covered in this question: http://physics.stackexchange.com/questions/290/what-really-allows-airplanes-to-fly – Colin K Jun 18 '12 at 01:12
  • Thanks all! Also read the question in the comment above me and briefly looked through a .pdf found there. Good stuff. – curious123 Jun 18 '12 at 13:27
  • @curious123 - well I suppose all fluid dynamics is Newton's 3rd law! But we normally make a distinction between bulk fluid flow and individual molecules – Martin Beckett Jun 18 '12 at 13:34
  • Probably I should have asked how a propeller differs from a fan in how it displaces air, producing lift (pure pushing, Coanda etc.). But that was all based on props looking more like airfoils and fans looking more like cut out sheets of metal (like in a turbofan engine). But looking closely at a turbofan's blades they are probably a bit airfoil-ish as well. Anyway, surely shape-wise cleverly optimized for their specific use depending on some factors. – curious123 Jun 18 '12 at 16:54
  • @curious: I'd bet that the difference in shape (airfoil versus flat blade) us due mostly to cost considerations. A table fan doesn't operate at the high power and speed of a propeller, so the manufacturer can trade in some efficiency for cheaper assembly. A table fan may also be designed more for quiet operation rather than optimum thrust. – Colin K Jun 18 '12 at 17:04
  • About the edit. Yes that's the thing that set off my confusion. The use of the words in context of aircraft propulsion. The difference in achieving lift I thought distinguished them (which probably was wrong). Also just earlier I saw that Wikipedia's article on propellers has a part about fans saying that "A fan is a propeller with a large number of blades.". But that seems to be contradicted by ducted fans. Anyway that's a lengthy discussion that will lead nowhere pragmatic/productive :) I learned alot anyway. – curious123 Jun 18 '12 at 18:30
  • @curious123: You can get a balsa wood glider with flat wings. The wings are airfoils, just not very good ones. In a turbofan engine, you bet those vanes are airfoils, and very good ones. BTW: the propellers on the first Wright flyer gave around 70-100 lbs of thrust. On a Cessna 172 it is about 400 lbs. On an F4U Corsair it is thousands of lbs. – Mike Dunlavey Jun 19 '12 at 19:01
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Actually, there ARE some differences. You would not like to be in the wake of a propeller, if all you want is gentle, whisper-quiet air movement. Ventilator fans, both cooling fans [i.e., automotive, refrigirators, etc.] are WAY gentler than propellers, or, jets...

Consider your desk-top ventilator and, say, a hair dryer or a leaf blower... you will NOT use a hair drier to cool in a hot summer, right? The same with airplane props. It all revolves around the issue of what flow rate is needed in a given diameter of the device. The more flow that needs to be "squeezed" through a given diameter, the more mayham it creates. Not something that you will welcome in your room...

Ronen A. Yehiav
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Main difference is the pressure differential between up-flow side and down-flow side of the "disk". The "disk" is the virtual circle defined by the rorating blades. In oropulsive devices, such as airplane and helicopter rotors and turbofan engines, this pressure difference, also known as "disk loading", is MUCH higher in propulsive devices than in air-moving devices.

Ronen A. Yehiav
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