1
$\begingroup$

I have seen many references on how the wings create lift, most of them overplaying effect of Bernoulli's rule.
But nowhere has there been any consideration given to this important factor:

Let's assume a Plane is flying horizontally, cruising, with a speed of v. And the wings in our case are just strait rectangular thin blades of aluminum attached to fuselage in a fixed angle, called angle of attack.

This wing creates lift many ways but the two most important components of the lift are the lift created by the top face of the wing which is slanted acts like a slope that is constantly running from under the column of air on top of it and causing that column of air spending part of its huge 1 metric ton per square meter pressure to accelerate down and spill into the vacuum created by the forward sliding of its support, the wing top.

The vertical speed of this column of air $\ V(air- rushing- down)= V. sin(angle- of -attack).$

The work needed to accelerate the flow of vertical column if air downward, from zero to V.sin(a), draws from the pressure of the column of the air on top and creates less pressure on the top surface of the wing. And I am thinking if this speed surpasses the critical speed of air we get turbulent flow and lose the lift, stall. I wonder why this major factor is ignored? Am I missing something here?

$\endgroup$
10
  • 2
    $\begingroup$ There is no such thing as negative pressure. What lifts a plane up is the momentum transfer to the air underneath the wing. Flying is simply the exchange of the momentum that the plane gains from falling constantly to initially resting air that is being pushed downwards by the wing and the body. Every other explanation of flight is flat out wrong. $\endgroup$
    – CuriousOne
    Jul 22, 2016 at 22:43
  • 2
    $\begingroup$ Related: What really allows airplanes to fly?. $\endgroup$ Jul 22, 2016 at 23:07
  • $\begingroup$ @CuriousOne I edited my question to clarify by negative pressure I meant less pressure. I am aware of the effect of pushing air down. This is the contribution of top face of wing. I have seen enough chipped paint and scoffs on top surface of my Cessna to make me believe there is vacuum there. If I see any freezing on my wing top face I descend/land. Because a rough top wing face discourages smooth downward flow of vertical column air. Example: in an open end long vertical pipe divided into two chambers by a valve with lower part vacuumed what happens to pressure if we open the valve suddenly? $\endgroup$
    – kamran
    Jul 22, 2016 at 23:29
  • 2
    $\begingroup$ Of course there is a pressure differential, otherwise nothing would happen, at all, but the local pressure is completely irrelevant for the momentum transfer. These details are all relevant for the drag and lift to drag ratios, i.e. they determine how much worse a wind is compared to an ideal momentum transfer device that can't exist because of the continuity equations. What makes the plane fly, though, is none of that. It's just the net momentum transfer to the air. What keeps the plane up is the reaction force to all the air that gets pushed down. That's it. No need to overcomplicate. $\endgroup$
    – CuriousOne
    Jul 23, 2016 at 0:08
  • $\begingroup$ @CuriousOne i was interrupted at work. Going back to my example of a long vertical pipe consider we have chosen the proportion of the pipe and its two chambers weight in a way that it is vertically positioned on its sealed base and the lower vacuumed chamber is separated from the top part by a remote controlled valve. If we quickly open the valve rushing air from the top compartment will act like a jet and the whole pipe will LIFT UP. $\endgroup$
    – kamran
    Jul 23, 2016 at 0:25

1 Answer 1

0
$\begingroup$

Kamran, Don't know if you're still listening, but... You actually are headed in the right direction.

First,

I'm not sure what you mean by "the reduced pressure in top face of wing is discounted", but will do a best guess.

With all due respect to CuriousOne, the "Push air down Newton Reaction lift" explanation is oversimplified and I believe misleading you to think this "push air down" is something only occurring at the lower wing surface.

The fact (true science) is that a larger amount of air is ""pushed down"" from ABOVE the wing (compared to below)! Yep.

What the "Newton Reaction" story omits it the very pressure difference you mention. There is indeed, a lowered pressure above the wing (pushing down) and an increased pressure below the wing (Pushing up).

The DIFFERENCE in those pressures over the wing's area is the lift force, the WHOLE lift force and NOTHING BUT the lift force. [that last part is actually false, but it sounds good and I'll explain]

As you described, the upper surface is, in essence, "running away" from the air above it. This is what lowers the pressure. In addition, you also realized that it also has the effect of creating some of the down-wash and this is also TRUE.

The lower surface (at a noticeable, moderate Angle of Attack) is obviously "advancing" on some air and this is pushing it out of the way by the fact that a push is in increase in pressure.

NOW...

The pressure difference between top and bottom over the wing area is the upward lift force (Wing loading).

DONE... Except...

We also note that this increased pressure under the wing pushes air away from it in all directions. That pushes some air FORWARD ahead of the wing. This creates the up-wash ahead of the wing and because it also wants to get to the lower pressure above it moves slightly upward too. Yes, there is movement of air away from the wing before it arrives.

It also pushes down on the air further below which contributes to the down-wash behind the wing.

It also pushes air around the tips.

....

Summary:

  • The relative motion of wing and air creates pressure changes around the wing.

  • The top-bottom DIFFERENCE IS THE LIFT FORCE.

BUT ALSO...

  • These very same pressures also accelerate air in all the other places we see movement which includes:

  • The down-wash.

  • The up-wash.

  • The tip up-spill vortex.

The fact is that it was Euler, not Bernoulli that realized that it was pressure gradients that accelerate the air in all cases. Pressure differences between two regions are the cause for the acceleration of air from the higher pressure region to the lower pressure region.

SO, we see that the pressures around the wing do two MAIN things: They push UP on the wing AND they push DOWN on some air. Therefore, the Newton Down-wash satisfies Newton's Third Law, but that doesn't CAUSE lift. It is something caused by the lifting-pressures and also satisfies Good-Old Newton-#3.

P.S. The thing I said was wrong about "nothing but the lift force" was the fact that those same pressure ALSO cause ALL OTHER accelerations if air around the wing.

P.P.S. I must add that the "faster air on top" thing is because the pressure ahead of the wing is full atmospheric pressure, but above the wing it is lower, so, that air ahead of the wing pushes (accelerates) the upper air toward the lower pressure region and the trailing edge.

P.P.P.S. There IS such a thing as "negative pressure". We also call it "partial vacuum", or just "vacuum' for short. It is simply pressure below the ambient pressure. This is common terminology in various industries. The manifold pressure gauge in a piston aircraft measures this partial vacuum and it is still called just 'pressure'. It is absolute pressure.

Regards

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.