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According to Bernoulli’s principle, airflow speeds up over an airfoil which decreases the pressure, and I read somewhere that when airflow speeds up over an airfoil, this creates a vacuum. Is this statement true?

How would a vacuum above an airfoil be different from airflow separation?

Ryan Mortensen
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Ismail El-Shaarawy
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  • See Chapter 3 here http://www.av8n.com/how/#contents. True, there may be some vacuum formed to suck the plane up in the air, but I think that is more than overcom by the air being pushed down by the wing, and conversely the air pushing the wing up in elevation. And of course, the many related topics over on the right. – CrossRoads Nov 13 '18 at 16:09
  • @CrossRoads Okay now if there is a vacuum over the wing, wouldn’t be the same as airflown separation ? – Ismail El-Shaarawy Nov 13 '18 at 16:48
  • See 3.7 and Figure 3.11. The low pressure (vacuum) area can be seen to be pretty small: The second type of stall-warning device (used on the Cessna 152, 172, and some others [my 177], not including the 182) operates on a different principle. It is sensitive to suction at the surface rather than flow along the surface. It is positioned just below the leading edge of the wing, as indicated in the right panel of figure 3.11. – CrossRoads Nov 13 '18 at 16:56
  • At low angles of attack, the leading edge is a low-velocity, high-pressure region; at high angles of attack it becomes a high-velocity, low-pressure region. When the low-pressure region extends far enough down around the leading edge, it will suck air out of the opening. The air flows through a harmonica reed, producing an audible warning. – CrossRoads Nov 13 '18 at 16:57
  • There are other pictures showing the air flowing up and over the wing, with separation occurring at the later part of the wing, moving forward on the wing as the angle of attach increases, until eventually there is insufficient flow to maintain lift. – CrossRoads Nov 13 '18 at 17:01
  • I don’t think a vacuum in its pure sense is created. A low-pressure area, yes, but no vacuum (as in: absence of air and air pressure). – Cpt Reynolds Nov 13 '18 at 17:44
  • Also, could I suggest rewording the title into a question, e.g. „Does lift generation create a vacuum above the airfoil?“ or similar? – Cpt Reynolds Nov 13 '18 at 17:47
  • @CrossRoads Thank you I will check the figures. – Ismail El-Shaarawy Nov 13 '18 at 17:52
  • @Cpt Reynolds I editted the question. that is what I thought as well. even if there is a vacuum, it doesn’t contribute to generating Lift. I will check the figures suggested by CrossRoads. – Ismail El-Shaarawy Nov 13 '18 at 17:56
  • @CptReynolds you have a partial vacuum, although a very weak one. The fact that there is a partial vacuum causes a much larger package of air above the wing to be induced to move down than without. The lift generated by a curved airfoil is so much more than a flat sheet of plywood because the airoil influences a much larger package of air as the bit of "suction" helps induce air well above the wing to move down and contribute to the action/reaction force. – John K Nov 13 '18 at 18:45
  • @JohnK I‘m not sure I follow. A vacuum in the physical sense is the absence of matter, similar to what is found in space. I don’t think that happens on the upper surface of an airfoil. Clearly there is a low pressure area there, and of course it does massively contribute to lift. Nonetheless, there are air molecules present in that low pressure area in great numbers, so it is not a vacuum. – Cpt Reynolds Nov 13 '18 at 19:27
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    I'm just using "partial vacuum" as shorthand for lower pressure in one spot than in an adjacent one. Probably not good as a scientific or engineering term. – John K Nov 13 '18 at 19:52
  • This is not a duplicate of the question it is marked as a duplicate of. Duplicate answers do not equal duplicate questions. – Ryan Mortensen Apr 05 '19 at 04:29

2 Answers2

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I think this is simply a terminology issue.

There is no vacuum in the physical sense on the upper surface of an airfoil. There is an area of low pressure, though, which in certain context is also called a vacuum.

A great example for this linguistic imprecision is my vacuum cleaner, inside of which there is no vacuum in its physical sense by far, but clearly it sports considerably lower pressure than the atmosphere in the rest of my living room.

Cpt Reynolds
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How much of a vacuum there is depends on flight speed. At subsonic speed the amount by which the static pressure on the top forward part of the airfoil is lowered is only a few percent of the static pressure. However, at hypersonic speed the pressure difference is substantial.

How much pressure is reduced can be seen by the lower effectiveness of conventional vertical surfaces when speed is increased. Part of that is aeroelasticity - the loads deform the structure, making the vertical less effective. But when compared with ventral fins, the pressure difference becomes obvious: The ventral fins deform as well, but their contribution to directional stability becomes dominant at higher Mach numbers because they enjoy the luxury of working in air of higher density.

Peter Kämpf
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