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Why is the curvature of pump rotor blades "backwards"?

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beanflying:

--- Quote from: RJHayward on October 17, 2021, 05:35:30 am ---beanflying: I get your point, about compressible fluids / gases, maybe off OP topic, sorry (I'm at about 1/2 year basic physics expertise).

Please see photo, regarding 'pushing' air streams downward, VS. simple lower air pressure, up around the faster moving air over that wing 'hump' shape.
Now, easy to accept, that a downward deflected air stream causing a 'lift' force on the wing, but do course textbooks then talk about TWO lifting force types ?
My experience, they always just talked about 'pressure' is lower, where the air is moving faster, there.
Note the picture features wing totally flat, on bottom, no super curvey.

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Discussing wings and Aerodynamics is way off topic so start a topic maybe in the Mech Eng. section? Meanwhile add this to your reading it is sort of the R/C bible for low (non full sized or high speed) Reynolds number data and research :) https://m-selig.ae.illinois.edu/uiuc_lsat/Airfoils-at-Low-Speeds.pdf or the source page here https://m-selig.ae.illinois.edu/

Also this bootleg copy of Martin's book is well worth a read for basic concepts of Aerodynamics. https://archive.org/details/ModelAircraftAerodynamics/mode/2up

beanflying:

--- Quote from: IanB on October 16, 2021, 03:31:10 am ---
--- Quote from: beanflying on October 16, 2021, 02:43:38 am ---Even if we take your earlier assertion of +-10% Pressure without doing the numbers that equates to about a 25% change in Density.
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If we do the numbers it is about a 7% change in density. It is always good to do the numbers.

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You might want to check your maths unless you are working with air at 200+C and even then it is +-7% at around 20C it is +-12 ish or around 25% https://www.omnicalculator.com/physics/air-density

IanB:

--- Quote from: RJHayward on October 17, 2021, 05:35:30 am ---Please see photo, regarding 'pushing' air streams downward, VS. simple lower air pressure, up around the faster moving air over that wing 'hump' shape.
Now, easy to accept, that a downward deflected air stream causing a 'lift' force on the wing, but do course textbooks then talk about TWO lifting force types ?
My experience, they always just talked about 'pressure' is lower, where the air is moving faster, there.
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As I explained above, it is two sides of the same coin. When you do engineering, there are sometimes different ways of calculating the answer to a problem. Just because a textbook shows one way, it doesn't mean other ways do not also exist.

If you have doubts, keep thinking about a helicopter. Do you think it would ever be possible for a helicopter to lift off the ground without a downdraft? Have you ever been near to a helicopter that was landing or taking off? It produces an absolute hurricane, which is the only way such a heavy craft could get off the ground.

--- Quote ---Note the picture features wing totally flat, on bottom, no super curvey.

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Don't be misled by pictures. Anyone can draw a picture of anything. What counts is what is observed in real world experiments. Be in no doubt, if a wing is generating lift, then air is being deflected downwards.