Bernoulli's equation is BS!

[CobberKane]

Hey, I just watched a lecture on mathematics (yeah, I know - no life) and apparently Bernoulli's equation, the one that explains lift, is plain wrong! Any of you technical types heard this?

 

[Matt308]

Nope. I also am questioning Newton. What a fooking loonie.

 

[CobberKane]

Well, apparently Newton had it right before Bernoulli came up with the idea that the higher speed of airflow over the curved upper side of the wing causes lower pressure that 'lifts' the plane. Truth is, aircraft fly with the leading edges of their wings higher than the trailing edges, thus pushing the air down, and as every action has an equal and opposite reaction (Newton) the aircraft goes up. In short, wings don't pull from the top, they push from the bottom. Bernoulli was full of it.

 

[wuzak]

Since Bernoulli first revealed his principle in 1738 I doubt that he was talking about lifting wings.

The reason it doesn't apply to wing lifting is that Bernoulli's Principle applies to flow along a streamline. It also applies to liquids and gases which are moving at slow speeds (ie not being affected by compressibility).

The people who are full of it are those who taught Bernoulli's principle as the reason for wing lift.

 

[oldcrowcv63]

Yes there is ram pressure on the bottom of a flat plate that appears to be the same as wing lift. However a flat plate cannot generate lift without an angle of attack. If it really were the only source of lift the Wright brothers would not have built a wind tunnel to find the best, most efficient shape for their wing. The airfoil shape adds a level of efficiency above the lift force of a simple flat plate as well as lift at no angle of attack. The way I understand it is this: Relative Air moving at significantly below Mach 1 is incompressible. That means any bounded air acts like water in a pipe. It also means air around it acts like the pipe itself. The 'pipe' prevents the air from moving in any direction away from the airfoil shape; nor is it able to bunch up (compress) or spread out (decompress). Air entering a restricted passage will move faster as it becomes more constricted to conserve mass flow. In other words, the same slug of 'bounded' air must move faster to get through a constricted tunnel than the air ahead or behind it. That higher velocity brings with it a lower pressure.
This is the difficult part. Bernoulli's equation for incompressible flow has three terms: static pressure, total pressure and dynamic pressure. The total pressure is the pressure along any streamline passing by the airfoil. It is a constant along any flow streamline in the airmass above and below the wing. Wing upper surface Camber (curvature) acts like a constriction in a tube and forces the air to move faster (increases v and dynamic pressure) But the total pressure remains the same in the moving airmass so the static pressure has to decrease. The static pressure is the measurable pressure felt by the wing. A difference in static pressure above and below the wing is a separate lift force that can exist at zero angle of attack. Have you ever seen videos of vortices shed by a wing? I believe that is the visible manifestation of the circulation that exists, due to the difference in velocity (dynamic pressure) above and below a wing. I think this is correct and I expect if we have an aero engineer (who, unlike me, actually paid attention and learned aerodynamics) in the forum he will make the appropriate corrections to my rambling babble.

Here is a helpful video:


View: https://www.youtube.com/watch?v=f0tvhjIpjSE

 

[meatloaf109]

Magic.
All done with mirrors.
That is my firm belief.

 

[wuzak]

Having observed the wings of an airliner through the window on occasion, I would suggest that they generate lift by flapping their wings.

 

[oldcrowcv63]

Why do people have such a problem with aerodynamics? Every good aerodynamicist knows bumble bees can't possibly fly.

 

[meatloaf109]

Of course Bumblebees can fly, mirrors, it is all done with mirrors!

 

[GregP]

A gentleman I know had a walking cane that was too tall. I asked him, "why not cut some off the bottom and make it the right length?" He said, "That wouldn't help at all ... It's the top that's too tall!"

I think his name was Newton Bernouli ...

About the Bumblebees and mirrors ... don't forget the smoke! I think it's smoke and mirrors, not just smoke, but I might have had too many beers and maybe it IS just smoke ... I'll have a beer and think about it ...

 

[N4521U]

After all, the difference between a Dog and a Fox is.............. one drink....

Doesn't have anything to do with the conversation but may explain Barnooli and his theory.

And my post.

 

[Marcel]

Anything ugly enough is repulsed by the earth and therefor will fly. This explains why helicopters are able to do so ......

On the serious side: yes you can have things fly without curved wings. If the power/weight ratio is high enough, anything will fly, even a barndoor. But the bernouilli effect will help the wing stay into the air.

To see that bernouilli was right, you could try the following: if you curve a piece of paper and put it on a table with the curved side up, you should try to flip the paper by blowing under it. You will not able to do that as the paper will be sucked to the table because of the faster airflow under it (your breath). It clearly proves the principle.

 

[CobberKane]

Aha, but as was pointed out in the lecture I watched, if Bernoulli was correct in respect to wings and lift, how can aircraft fly upside down? Newton's second law explains this, because so long as the aircraft's wings are angled up it will keep flying, but if we apply (missapply) Bernoulli's equation the aircraft should head down!

 

[Timppa]

The clearest (correct) explanation of lift I have seen is from late Richard Shevell, a former aeronautics professor at Stanford:

A simple feeling for lift can be obtained by realizing that the wing is a device for pushing air downward. The wing gives the air a downward momentum. The air 'pushes' back on the wing, producing a lift. The resultant downward velocity of the air is the downwash.
There is nothing wrong with this thinking except that one cannot get any useful information from it. Questions such as "how much lift for a given angle of attack?", how much drag to produce the lift?", " how much change in the local air velocity on the wing ?", cannot be answered except through the use of potential theory combined with the theory of circulation.

Potential theory is beyond the scope of this discussion. It allows the calculation of flow around bodies of various shapes, including airfoils. Forgetting viscosity of the air for a moment, i.e. considering a 'perfect' fluid, the theory is excellent for symmetrical shapes but failed for airfoils at some angle of attack because it showed zero lift and zero drag. For over 150 years this fact was used to show that the theory was useless because it was known that there is lift and drag in experiments. Then in 1902 a German scientist named Kutta realized that the theory showed the flow on the bottom surface of the wing curled around the trailing edge and moving forward for perhaps 5% to 10% of the chord before turning aft in the direction of the freestream. But observation showed that the fluid flowed smoothly to the trailing edge of the airfoil (except of course when the airfoil is stalled at too high an angle of attack, a viscous phenomenon).
Kutta assumed that some correction must be applied to the theory so that the flow continued to the trailing edge.This is the well known "Kutta condition".

The correction was to assume a "circulation", a vortex flow, superimposed on the airfoil. When a circulation is applied of an amount necessary to make the flow go smoothly to the trailing edge on both top and bottom surfaces, (meet the Kutta condition) the theory showed a lift very close to experiment and calculated the velocities along the surfaces very well. A wing can be thought of as a vortex extending from one wing tip to the other. At the wing tips the vortices trail aft as many of you have actually seen on a high humidity day. Having established the local velocities which are generally higher on the top surface, the Bernoulli equation can then be used to determine the local pressures and the lift.

The lift produced by the wing 'bound' vortex (so called because the center of the vortex is bound to the wing) is similar to the forces produced on spinning golf, baseball, or tennis balls to make their trajectories curve.

For further information on lift and circulation refer to "Fundamentals of Flight", R.S.Shevell, Prentice-Hall 1989 , "Aerodynamics of the Airplane",C.B. Millikan,McGraw-Hill, circa 1943, or similar textbooks.

Some people including some authors, think that circulation is a mathematical theory not a physical thing. However, it is truly a physical process and the theory explains not only lift but drag due to lift also known as induced drag, ground effect, why birds fly in an approximate 'V' formation, etc. If you want to see circulation develop on your dinner table, place a soup spoon in soup with fat particles or chopped parsley. Set the spoon at a low angle of attack and move it through the soup, and watch the vortices develop. World's cheapest windtunnel!

 

[oldcrowcv63]

Aha, but as was pointed out in the lecture I watched, if Bernoulli was correct in respect to wings and lift, how can aircraft fly upside down? Newton's second law explains this, because so long as the aircraft's wings are angled up it will keep flying, but if we apply (missapply) Bernoulli's equation the aircraft should head down!

I think it is important to carefully examine an aircraft in inverted flight to see if the angle of attack is the same when the aircraft is flying upright as opposed to inverted. I believe most to all aircraft have some AOA built in to their design. If you invert the aircraft you probably have to enter a higher negative angle of attack sufficient to obtain the correct amount of wing positive angle of attack for the flight configuration which will probably also increase the vertical component of the thrust vector beyond what it would be in level flight. Also some wings are symmetrical shaped as the video shows and inverting them isn't much of a problem assuming the correct adjustment to angle of attack.

Are you really serious about dismissing Bernoulli's equation or just goofing. if serious, can you relate the source? you mentioned a lecture. You tube? classroom?

 

[Njaco]

After all, the difference between a Dog and a Fox is.............. one drink....

nope, thats a 6 pack....

 

[vikingBerserker]

IMHO it it causes lift, but not enough to lift a plane on it's own, however it makes the lift from the high pressure below the wing more efficient.

 

[drgondog]

The clearest (correct) explanation of lift I have seen is from late Richard Shevell, a former aeronautics professor at Stanford:

This is correct -

One of the earliest math treatments of circulation and relevance to airfoils was the Theodorsen Transformation - in which the flow into and around a rotating sphere (think "curveball") is mapped from the sphere to corresponding points on the airfoil via complex variable transformation. The Stagnation points on the rotating sphere are then mapped to leading and trailing edge of the airfoil where the freestream flow separates at the leading edge and re-converges on the trailing edge. The 'Rotation' if viewed clockwise on the sphere - with freestream velocity moving from left to right - creates a higher velocity on the top surface via faster velocity combination of rotational velocity plus freestream, whereas the lower surface if freestream Minus the rotational velocity. The points on the rotating sphere are mapped to the corresponding transformed points on the airfoil. The corresponsing pressure distribution is then calculated as a function of velocity (related to Freestream).

The 'Lifting line' vortices are continuous along the AC spanwise and reaches a discontinuity at the tip - the location where the tip vortex is created and exists thereafter as a 'tornado' like vortex. This is the Downwash trailing behind the aircraft and is proportional to the Total Lift generated by the wing. Big Lift = Big Downwash between the tips and Upwash outside the tips.

Shevell is a very distinguished aero. Some of his work on Profile Drag for various body shapes as a function of angle of attack (as an example) are excellent.

For an inviscid flow, the vortex theoretically originates at the point in which lift was first generated and extends to the wing tip wherever it may be.

 

[CobberKane]

I think it is important to carefully examine an aircraft in inverted flight to see if the angle of attack is the same when the aircraft is flying upright as opposed to inverted. I believe most to all aircraft have some AOA built in to their design. If you invert the aircraft you probably have to enter a higher negative angle of attack sufficient to obtain the correct amount of wing positive angle of attack for the flight configuration which will probably also increase the vertical component of the thrust vector beyond what it would be in level flight. Also some wings are symmetrical shaped as the video shows and inverting them isn't much of a problem assuming the correct adjustment to angle of attack.

Are you really serious about dismissing Bernoulli's equation or just goofing. if serious, can you relate the source? you mentioned a lecture. You tube? classroom?

I was being a bit flippant, but the general gist of the lecture was that Bernoulli's equation should not be applied to the questiion of why a wing creates lift. My underatanding is that according the equation, airflow separates at the leading edge of the wing and meets at the trailing edge, and as the distance travelled over the curved top of the wing is further than that across the botom of the wing, pressure is lower on top and the wing creates 'lift'.
Unfortunately there are a couple of problems with this. Firstly, what is the imperative that required the two airflows to meet gain at the trailing edge of the wing? If molecules a and b separate at the leading edge what compells a to speed up so it can meet b at the trailing adge. Apparently, nothing. Secondly, as I posited earlier, how can a plane fly upside down? if Bernouli's equation explains lift and inverted aircraft should head down, irrespective of the angle of attack.
To be fair neither Bernoulli or newton ever aplied themselves to the question of lift or created equations to explain that particullar phenomenum. I don't have the link to the particular lecture I was watching, sorry, but a quick search of the web provides heaps of argument on Bernoulli v Newton regarding lift. One point of concensus seems to be that the nice simple diagram of airflow separating over a wing and meeting at the other end is wrong. The actual factors that produce lift are much more complex.

 

[gumbyk]

If you want to prevent a wing from producing significant lift, you attach something to spoil the air over the UPPER surface of the wing. If the action/reaction theory were correct, this wouldn't have much of an effect.