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Skymaster15L (Aeronautics) (OP)
20 Sep 12 6:06
"A stagnation point of a body in a moving airstream is a point where: the velocity of the relative airflow is zero and the surface pressure is higher than the ambient atmospheric pressure"

Could you please tell me why surface pressure is higher and not equal to ambient atmospheric pressure.  As far as I know (for incompressible flows) the total pressure of still air is composed of 100% static pressure with 0 % dynamic pressure if there is no movement/velocity involved.  As soon as there is velocity given to a body of air, static pressure starts drops, and total pressure is now also composed of dynamic pressure.  The sum of these pressures, TP, is always a constant.  Also, since at the stagnation point where local airflow flow velocity is 0, static pressure is highest, meaning no dynamic pressure, shouldn't this pressure be then equal to ambient air pressure of the nearby parcel of air that is unaffected by the wing's passage through it?  How could the surface pressure at the stagnation point be possible higher than ambient (total pressure, which would be composed of 100% static and 0% dynamic as I understand it)when both sums have to be a constant anyway.  Am i possibly:
1)using the terms static pressure and stagnation pressure interchangeably, when they actually mean different things
2)worrying about a question that was written wrongly to begin with
3)not understanding something else that I have failed to mention or learn?

Your advice is well appreciated;

Regards
rb1957 (Aerospace)
20 Sep 12 7:48
total pressure = static + dynamic.

but i think there's a difference between wind (moving air) and moving a plane through the air.

"As soon as there is velocity given to a body of air, static pressure starts drops" ... this is what happens (i think) when there is wind blowing ... the atmospheric total pressure remains constant, the wind converts some static pressure into dynamic pressure.

i think in the case of a plane moving through the air, the plane increases locally the total pressure, that the plane's airspeed doesn't change local static pressure.

so i think the answer is zero velocity and pressure = ambient atmosphere.
IRstuff (Aerospace)
20 Sep 12 10:23
Air is compressible, isn't it? Then the pressure at the leading edge of a wing should be higher than ambient; this is basically the process by which sonic booms are created, isn't it.

TTFN
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rb1957 (Aerospace)
24 Sep 12 12:44
thinking about it some more, i think the total pressure near the wing is higher than ambient (ambient = freestream).

isn't this how a pitot static tube works ? it determines airspeed from measuring total pressure and static pressure. static pressure should be a property of the atmosphere, unchanged by the wing being there or not. dynamic pressure is of course totally due to the wing. then total pressure near the wing (as felt at a stagnation point) is Pst +Pdyn.
berkshire (Aeronautics)
24 Sep 12 13:41
At the risk of getting my face covered in egg, I think the point you are missing is that a fraction of a second ago the air was just laying there, than along along comes a wing at the speed the aircraft is flying , and that air at the stagnation point is suddenly accelerated to whatever speed the aircraft is flying at. It's speed is zero relative to the wing, but it has changed speed and has kinetic energy

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

rb1957 (Aerospace)
24 Sep 12 14:33
that why i think it's the pressure near the wing; the increased pressure near the wing causes(is caused by) the lift the wing creates.

as you say, before the wing the air molecules were sitting there "fat, dumb, and happy". then along comes this wing, rushing through their calm existence.
makes sense that the total pressure has increased (due to the dynmaic pressure of the wing).

it sounds like we're saying the same thing, yes? no??
Helpful Member!  rb1957 (Aerospace)
24 Sep 12 14:36
P static is a propert of the atmosphere (independent of the wing).
P dynamic is the dynamic pressure due to airpseed.
P total = Pst+Pdyn = stagnation pt pressure
berkshire (Aeronautics)
24 Sep 12 16:02
Rb,
I think we are saying the same thing .
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

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