On Airplane lift

[rwjefferson]

@Chronos

That’s exactly what the Bernoulli equation is. Newton’s laws are the entirety of classical mechanics; everything else is just special cases. If you apply Newton’s equations to an ideal gas, you get Bernoulli’s equations.

Bernoulli does not come close to predicting lift, much less “accurately”. You are correct that Bernoulli’s formulas are the derivative of F=MA. (Think air pressure perpendicular to air flow.)

However, Bernoulli’s formula does not speak at all of lift associated with the change of vector of an airmass. It is the change of air mass vector that is the main component of lift. Bernoulli is a bonus.

There is so much more to Newton on fluids (fluid dynamics) than Bernoulli. Bernoulli is a small subset of Newton, just as Newton is a small subset of Einstein.

rwj

[Chronos]

We might be referring to different things by the name “Bernoulli”. In my training, we referred to the full set of general fluid equations as the Bernoulli equations, in which case Bernoulli is fully sufficient. You’re apparently using the term for one of those equations in particular, or to some subset of them. In any event, though, all of the fluid equations (whatever you call them) are derived from Newton’s laws, so if there’s any complete model of lift at all, it can accurately be described as “just Newton”.

[rwjefferson]

Chronos: You are exactly right.

This is why I think it is important to make that clear distinction:

@rwjefferson

(edited or clarity)
[hijack]Are you aware of the ”Reverse Magnus Effect”? A slippery ball curves toward the side that is rotating into the airstream. This is opposite to the direction baseballs curve. The Bernoulli Believers have warped the understanding of Bernoulli’s formula to fit this Magnus (Newtonian) Effect of rough balls. The fact that the smooth ball curve is the predicted and actual Bernoulli effect eludes their comprehension. Therefore they call it the “Reverse Magnus Effect”.[/hijack]

Peace
rwj

[Bindlestiff]

I came to the discussion somewhat late but I have to say it’s refreshing to hear someone with credentials say what I’ve been saying for hears (though I didn’t say it nearly so well).

My ground-school instructor just about went ballistic when I suggested that it was about “pushing” air down. He was so closed to discussion that I found another instructor who could discuss the matter intelligently but warned me that whoever tested me for my certificate would be committed to the FAA’s preference for the “Bernoulli” answer.

Happy Landings

Bindlestiff

[RM_Mentock]

@aerodave

Well, rw, that’s because the “Bernoulli believers” are usually very wrong.

If you go back through the historical thread record, I think you’ll find that the “pushing air down” believers are also usually wrong.

[Pleonast]

@aerodave

In fact, it can be safely argued that fixed-wing flight would be impossible without viscosity.

Is any winged flight possible in a superfluid? Or, will we be reduced to using aerostats? Hmm, how would one maneuver?

@Chronos

Newton’s laws are the entirety of classical mechanics; everything else is just special cases.

So, has a relativistic lift theory been developed yet?

[rwjefferson]

@RM Mentock

If you go back through the historical thread record, I think you’ll find that the “pushing air down” believers are also usually wrong.

On the other hand, the fact that the air deflection caused by the seams of the baseball is enough to completely counter any Bernoulli effect clearly demonstrates that the “air accelerated down” (Newton) believers are correct.

rwj

[RM_Mentock]

So, the conclusion is that everybody is usually wrong about something

[aerodave]

rwj, I’m not sure I see where you’re going with the Magnus/Reverse Magnus point. If you apply the Bernoulli principle in the most logical way, it predicts typical Magnus-style baseball curvature. Reverse Magnus is a not the most obvious logical consequence of any of these lift theories…Instead you have to include some peculiar second-order effects to end up predicting a ball curving the “wrong” way.

Also the Reverse Magnus effect doesn’t just happen for any smooth ball. It happens over a very slim range of Reynolds numbers, when conditions are just right. I gave a more in-depth treatment to “Reverse Magnus” in some thread about beachballs a year or two ago.

[Tomecat]

We have all seen the Blue Angels and others fly upside down. How is the steady flight and lift possible when they do this?

[Chronos]

If you go back through the historical thread record, I think you’ll find that the “pushing air down” believers are also usually wrong.

Depends on how much detail they give. A very simple explanation of lift would be “Airplane wings produce lift by pushing air downwards”. Leave it at that, and it’s completely correct. The question then comes, though, how exactly does the airplane wing accomplish this? A full answer to that question would require all the machinery of fluid dynamics, and would be very easy to get wrong.

[flight]

@Tomecat

We have all seen the Blue Angels and others fly upside down. How is the steady flight and lift possible when they do this?

With any wing you have what is called the lift curve slope (a). This is the slope of the line on the graph of lift (actually C[sub]l[/sub]) versus angle of attack (alpha). For small angles of attack the curve is almost linear.

For a flat plate or symmetric airfoil the curve goes through the origin. Thus, if you have negative alpha, you have negative lift, and with positive alpha you have positive lift. So, with this kind of airfoil you can fly upside down if you have a negative alpha, which produces negative lift (which holds you up if you are upside down).

My attempt at drawing that in ASCII didn’t work.

For most airfoils you will still have positive a (more alpha gives more lift), but the lift curve cross the zero alpha axis at a posititive lift. Thus, if the airfoil is flying straight into the wind it still produces positive lift, and even produces positive lift for some negative alphas. If, however, you keep dropping the alpha (pitching the nose down) the lift will eventually become negative. If you keep dropping it down even more you can get to where the negative lift on the wings has the same value as the weight. OK, now flip the plane upside down. Now you see that the lift cancels out the weight and you are flying! For most aircraft you pay with a great deal of extra drag for doing this.

Please nobody mention my reversing cause and effect in this post (the graph doesn’t force things to happen, it describes what happens) but it seemed the easiest way to explain it.

[rwjefferson]

@aerodave

Also the Reverse Magnus effect doesn’t just happen for any smooth ball. It happens over a very slim range of Reynolds numbers, when conditions are just right.

Understood. It is the interaction between surface and fluid that determines whether the ball curves by Magnus or Bernoulli.

@aerodave

rwj, I’m not sure I see where you’re going with the Magnus/Reverse Magnus point. If you apply the Bernoulli principle in the most logical way…

As I understand Bernoulli’s equation: Air pressure (perpendicular to airflow) decreases as air speed increases. A surface with higher air speed will experience a lower pressure than a surface with lower air speed. (As in the “Airplanes fly because air flows faster over the wing” fallacy.)

Observe a “smooth” ball rolling. The relative speed between the lower surface and the ground is zero (the ball is rolling). The relative speed of the upper surface to the air is velocity plus rotation. This is greater (v + r >0). Therefore, the air pressure on the upper surface will be lower than the air pressure on the lower surface.

This does not change just because the ball is in flight. A surface need not be solid. The ball curves upward as expected, by Bernoulli.

Observe the same ball, but with a roughened surface. Now, as the ball rotates, fluid air mass is carried by the rotating surface. The mass of the boundary layer increases in volume and density on the windward due to compression. As this added air mass rotates past the lee, it is stripped by the oncoming airstream. Mass departs the [del]pump [/del] ball with an upward vector, the ball curves downward, by Magnus. :dubious: Not by Bernoulli.

Here is a wing and curve ball sim for those without access to a wind tunnel.

How do airplanes fly?
The wing is a pump. Pump down, lift up.

rwj

[rwjefferson]

@Tomecat

We have all seen the Blue Angels and others fly upside down. How is the steady flight and lift possible when they do this?

You’ve been taught incorrectly. The shape of the wing is only relevant only to its efficiency. Any surface/object that deflects air downward provides lift.

How do airplanes fly?
As you accelerate air downward, the air pushes back.

rwj

[rwjefferson]

Previous entry edited to not include present company. You guys are doing great!!!

TomecatYou’ve (we) have been taught incorrectly that lift depends on the hump in the wing. The shape of the wing is only relevant only to its efficiency. Any surface/object that deflects air can deflect air downward and provide lift.

How do airplanes fly?
As you accelerate air downward, the air pushes back.

Peace
To All
rwj

[rwjefferson]

aerodave: There still seems to be confusion over what is Bernoulli and what is Newton.

Air pressure is not Bernoulli, air pressure is Newton. Particles of a fluid carry force in their mass according to F=MA. This results in both dynamic and static pressure.

Bernoulli demonstrated that static air pressure falls with an increase in the velocity of air flow. This is Bernoulli’s Principle, this is Bernoulli.

Bernoulli made great achievements in our understanding of Fluid Dynamics well beyond this formula. He deserves credit for this. However, it is incorrect to attribute all interactons of fluid to Bernoulli.

Fluid Dynamics is best understood as Newton on Fluids. The smooth ball/rough ball experiment clearly demonstrates the much greater strength of Newton over Bernoulli. Even a slight air deflection counters Bernoulli.

Is it wrong to invoke the Bernoulli principle when explaining how an airfoil works? No, but it tends to confuse matters for the technical audience. And it is only a small component of total lift.

How do airplanes fly?
Newton

rwj

[RM_Mentock]

@rwjefferson

How do airplanes fly?
As you accelerate air downward, the air pushes back.

but that’s as much or more of an oversimplification as is attributed to the “bernoulli believers”

Do you think that the air deflected by the wing (and body surfaces) accounts for the lift?

[Stranger_On_A_Train]

@RM Mentock

but that’s as much or more of an oversimplification as is attributed to the “bernoulli believers”

Do you think that the air deflected by the wing (and body surfaces) accounts for the lift?

::sigh:: You guys.

Draw a free-body diagram. It is clear that there is a net upward force on the aircraft that keeps it aloft. Therefore, there is a net downward force on the fluid medium (air) as a reaction. Air is, in one manner or another, pushed down. This is Newton.

Bernoulli’s principle, stated in English, merely says that increasing the velocity of a fluid results in a decrease in pressure. Bernoulli’s equation for inviscid, steady-state, incompressible flows relates velocity and pressure; taking into account enthalpy allows you to deal with compressible flows of ideal gases; other methods of approximation allow you to model real-world situations with some hope of accurately predicting the results.

This isn’t an issue of Bernoulli vs. Newton; Bernoulli is just a model that integrates Newton’s laws (and, in the expanded version, the ideal gas law and thermodynamics) into an equation with which you can analytically model the forces (or rather, pressures) upon a controlled fluid volume. It’s not that Bernoulli’s Principle is, er, in principle, incompatible with the phenomenon of lift on an aircraft wing, but as a model that depends upon a bounded volumetric measure, it is impossible to apply it analytically to an open-ended system; talking about the flow above and below the wing inherently splits the volume, and the behavior of each flow is mechanically dependant only on the surface of the wing over which it flows.

Generally, for approximate solutions to flow dynamics problems, some version of the Navier-Stokes equations are used, which apply boundary conditions and assumptions about viscidity to a network of approximately infinitesmal volumes; sort of an analog to structural finite element analysis, for those who savvy such things. (A more general method using energy methods and statistical mechanics gives a more realistic model but with enormous complexity that is beyond the scope of most CFD applications.)

Bernoulli describes flow under (or over) a wing, and the resultant forces on it (i.e. increased pressure on the bottom side of the wing resulting in net upward force) just fine and in fullness, as long as you are prepared to consider the total volume of the air passing under the wing to be in a controlled boundary. This “two particles of air seperating at the leading edge and meeting at the trailing edge” has no theoretical viability; while it is true that the air may appear to do that, it is of no relevence whatsoever, other than as an incidental result of the model, to the lift of their aircraft. In short, Bernoulli describes an effect, from first (Newtonian) principles. There is no conflict, unless you are one of those people who insist that a bubblebee can’t actually fly.

Stranger

[Kevbo]

@Stranger On A Train

This “two particles of air seperating at the leading edge and meeting at the trailing edge” has no theoretical viability;

Not only theoretical, but experimental as well.

Sorry I don’t have a site, but NASA did some wind tunnel testing where the injected near neutral boyancy particles in pairs, and photographically proved that they do NOT meet. The one traveling over the top got there last, pretty much according to the difference in path length. High speed photography showed that both airstreams were moving at essentially the same velocity.

[CurtC]

@Kevbo

Sorry I don’t have a site, but NASA did some wind tunnel testing where the injected near neutral boyancy particles in pairs, and photographically proved that they do NOT meet. The one traveling over the top got there last, pretty much according to the difference in path length. High speed photography showed that both airstreams were moving at essentially the same velocity.

You misremember. The two streams don’t meet at the same time at the trailing edge; the air that goes over the top arrives at the trailing edge in less time than the bottom-air.