User talk:Michael Belisle

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Lift

Hi Michael, I’m responding to you edit of Lift. I agree with much of what you say and I like the way your words flow. However I do have some comments that I hope you will find as constructive.

The Article (before your contribution) was a complete mess. However this is inevitable as the debate over the years has not really been about the truth of the mechanism of lift (even though it may often appear to be), rather it is a debate about the best way to describe it. As there are so many ways, so many editors and edits can be continuously made what we have is a dynamic system that occasionally reaches an unstable equilibrium (I’m referring to the drafting of the article here not the mechanism of lift!)

In a way different approaches to describing lift are Original Research. There are arguments to suggest that your piece of work should be better published elsewhere and the Article entirely re-written outlining the various was of describing lift with links to them.

If you look back over the long history (not something I particularly recommend as an enjoyable pastime) you will see that just about everything that can be said has already been said several times (as well as a lot of things that should not be said).

The primary challenge is to explain to the layman how a wing works. Unfortunately we always immediately vanish down rabbit holes about how to mathematically model lift. Most things in this world are a challenge to mathematically model but many are easy to understand and indeed were well understood a long time before they were accurately modelled.

Turning to your edits of “basic explanation”. You are adopting the famous “show higher speed = lower pressure then explain why the speed is higher” approach. It is part of the well-worn cycle of the Article’s continuous evolution, although your approach is more eloquent than most.

While it is not wrong, I suggest there are both what could be called high level and low level challenges. At the high level any explanation that does not placate both the pressure lobby and the vertical momentum lobby will always be subject to substantial modification by those feeling excluded.

At a low level I offer the following observations (I hope I am not being argumentative but just looking for ways to help us improve).

Bernoulli’s principle does not dictate that increasing the speed of a fluid inevitably results in decreasing its pressure – some energy change must also take place but it does not define what that may be. We pragmatically use a simplified version of Bernoulli’s equation in some areas of modelling the flow over a wing. This simplified version is full of inaccuracies but they can be acceptable in controlled application. However, this simplified version is derived empirically and not from principles. Reliance on merely stating the word “Bernoulli” to prove an absolute link between speed and pressure is fundamentally flawed.

While I tend to warm to the streamtube conceptual model it does have a limitation as it implies finite boundaries to the air that is part of the mechanism.

There is a big conceptual leap where you assert that the streamtube is squashed. While I am sure it is true you provide no evidence or explanation of how.

Unfortunately between the "squashed streamtube" and the "speed means low pressure" logical steps there lies another – mass per unit time can be constant in a squashed tube by ways other than changing speed, density can also change. Rolo Tamasi (talk) 13:39, 20 April 2008 (UTC)

I'm not sure what your background is, so I apologize if my responses here are too basic or too technical. I responded to some other stuff on the talk page for Lift.

• At the high level any explanation that does not placate both the pressure lobby and the vertical momentum lobby will always be subject to substantial modification by those feeling excluded.

This is a difficulty in communicating complex but intriguing topics to a general audience. (If an electrician doesn't understand general relativity, then surely Einstein was wrong, right?) I don't think this means we should give up and devote equal time to both camps. In this specific case, viscosity (through complex mechanisms) creates pressure differences that cause lift, but that's hard to explain. So, according to JD Anderson, a reasonable explanation is to say that pressure pushes on the airfoil and creates lift. In steady state, this is correct; viscosity is negligible at that point (but it was responsible for the shape of the flow field). Downwash by Newton's third law is an effect of lift, not the cause. And circulation is a mathematical model.

A good high-level explanation here will explain the most correct answer and explain how everything works together.

• Bernoulli’s principle does not dictate that increasing the speed of a fluid inevitably results in decreasing its pressure....

In inviscid, incompressible, steady flow it does along a streamline and by extension, inside a streamtube formed by the flow between two streamlines. In this sense, the principle as employed here can be derived by integrating the Euler equations along a streamline in incompressible, steady flow. In an discussion of an established flow around a airfoil outside of the boundary layer, it's appropriate.

• While I tend to warm to the streamtube conceptual model it does have a limitation as it implies finite boundaries to the air that is part of the mechanism. There is a big conceptual leap where you assert that the streamtube is squashed. While I am sure it is true you provide no evidence or explanation of how.

There are discernible boundaries in the sense that you can identify streamlines where the local velocity is everywhere tangent to the streamline. By definition, fluid will never cross a streamline. It would be helpful to insert a picture, which I'll construct in a few days. The streamtube and “squashing” comes from JD Anderson, where I omitted too many details because I threw it in there when he derailed my train of thought (now in “Stages of Lift Production“).

• Unfortunately between the "squashed streamtube" and the "speed means low pressure" logical steps there lies another – mass per unit time can be constant in a squashed tube by ways other than changing speed, density can also change.

In low-speed flight, it is safe to assume that the flow is incompressible, that if $\rho VA=$constant, then a decrease in area will result in an increase in speed. Assuming otherwise is rightly outside the scope of a basic article on lift. True, there may be problems with this explanation for flow around a 777 at in cruise at $M=0.9$, but a physical description of transonic flow is not in any way accessible to the layman. (Interestingly, the simplest explanation is in hypersonic flow where Newton's third law reasoning provides a reasonable approximation.) Michael Belisle (talk) 22:26, 20 April 2008 (UTC)

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Coanda effect

Hi Michael. On 4 August Ccrummer made some edits to Lift (force). You have challenged some aspects of those edits by inserting a well-placed "disputed" banner. Be aware that I have also challenged this user in a new section in his User talk page. It will be interesting to see what response we achieve. If we achieve nothing, the offending additions should be deleted.

I have recently discovered your comprehensive additions to Talk:Lift (force). I have responded to your most recent request on that page, and I am willing to keep up with the debate and support you in your objectives. Dolphin51 (talk) 03:31, 6 August 2008 (UTC)

Lift and Bernoulli

I'm having problems with the the inclusion of wings and rotors in the real world applications section in the Bernoulli Principle article. My original thoughts on this got too lengthy, so here is the short version.

How close does Bernoulli's equation model the relationship between air velocity and pressure along the surface of a typical wing such as a civilian type aircraft?

It models the relationship between velocity and pressure over a wing in the same way that it models it in any case. Bernoulli for a wing is just a step in determining the amount of lift and drag. Using Bernoulli requires knowledge of the flow field (which could be calculated using Newton's second law or a potential flow solution).

Let's say you know the tangential velocity at the edge of the boundary layer, $\scriptstyle U_{e}$. Starting with the definition of the pressure coefficient, $\scriptstyle C_{p}=(p-p_{\infty })/(\rho U_{\infty }^{2}/2)$, and using the incompressible, steady Bernoulli equation with negligible height change $\scriptstyle p+\rho U^{2}/2={\text{constant}}$, it's possible to obtain a very simple relationship between the velocity and the pressure coefficient in an incompressible flow: $\scriptstyle C_{p}=1-U_{e}^{2}/U_{\infty }^{2}$. You can then integrate the pressure over the surface of the body to get the aerodynamic forces.

Does this relationship hold for both a wing based or an air based frame of reference?

Of course. The physics are equivalent.

How is Bernoulli's equation modified to deal with the actual work peformed on the air, especially for the small component of work done in the direction of lift (which isn't so small in the case of propellers)?

It's modified in the derivation from the equation of energy by including a work term.

My thoughts:

It seems to me that Bernoulli's equations are close, but only because efficent wings generate most of their lift via reduction in pressure, which allows a Bernoulli like conversion of pressure energy into kinetic energy with little work required in the direction of lift. In the case of a propeller the amount of work done in the direction of lift (thrust) is much higher, so Bernoulli applies before and after the virtual propeller disk, but not within the propeller disk (because work is done): propeller analysis.

You might want to look at blade element theory to get an idea of what's going on in the propeller disk.

Since wings also operate in their own induced wash, why aren't similar diagrams and explanations use to explain lift? Jeffareid (talk) 16:01, 27 September 2008 (UTC)

Coanda effect - mostly what I call void theory: a low pressure region is generated on the upper surface of the wing which draws the air above the wing downwards towards what would otherwise be a void after the wing had passed. wing This left out the fact that air is drawn from all directions, except since it can't flow through the solid wing, the result is a net downwards acceleration. Is there a significant surface friction + boundary layer + viscosity effect that allows air to be accelerated perpendicular to a cambered surface that requires less pressure differential to achieve the same downwash?

No.

Newton: 3rd law - forces only exist in pairs, if the air exerts a force on a wing, it coexists with the wing exerting an equal and opposite force on the air. 2nd law - the air responds to the downwards force from the wing by accelerating downwards. Newton always works, low speeds, supersonic speeds, turbulent flows, ... Side note - the downforce is eventually exerted by the air onto what ever surface is supporting the air. In the case of a closed container, there is a pressure differential that decreases with altitude within the container with the result that the net downforce exactly equals the weight of the air and any object hovering or flying in the container (if there is no vertical component of acceleration). In the case of the earth, air pressure is relative to the weight of the air and any objects hovering or flying in the air (again no perpendicular component of acceleration).

Unrelated, but somewhat interesting are these curved bottom, flat top lifting bodies which pretty much disprove equal transit theory. It would seem that lift to drag ratio would be low, but the rocket powered version reached a speed of mach 1.6. m2-f2 glider next to F104 m2-f3 rocket powered version

Jeffareid (talk) 06:24, 12 September 2008 (UTC)

It sounds like you're trying to make an argument in the Bernoulli v. Newton “debate”. They are both parts of a bigger picture. Remember that anything that goes into Wikipedia needs to be verifiable using an independent source. Michael Belisle (talk) 06:51, 21 September 2008 (UTC)

Mainly what the NASA article stated, that Bernoulli applies outside the pressure jump zone in the immediate vicinity of a wing or propeller, but not within the pressure jump zone where work done results in a non-Bernoulli like increase in pressure without a corresponding decrease in kinetice energy (propellers), or vice versa (most typical wings). Bernolli's weakness is that it could be mis-interpreted to believe that lift could be generated without downwash.

The inverse is also true: the weakness of only explaining the downwash is that it could be misinterpreted to believe that lift could be generated without differences in pressure. It is necessary to explain whole picture.

OK, I assumed that it would be obvious that downwash could only coexist with pressure differentials, since air isn't going to accelerate without pressure differentials. (Although I've read that at mach 5+, that most of the lift is due to collisions with the bottom surface of a wing). Jeffareid (talk) 12:22, 28 September 2008 (UTC)

Faster moving air over the top can only coexist with downwards acceleration of air in order for lift to be generated. Then there's the talk of streamlines, but it's my understanding that turbulent flow occurs when the air starts to decelerate (flow from a lower pressure to a higher pressure zone), and that turbulent flow causes air to cross streamlines.

The processes in the boundary-layer are not important to understand in a basic explanation of lift. By definition, flow cannot cross a streamline. The streamlines, however, may be unsteady (and they are in a turbulent flow).

Using a wing based frame of reference most of the flow across the wing is turbulent, except for the leading 25% to 30% of the wing. The point here is the mechanical interaction between wing and air produces lift, and a significant part of this interaction is non-Bernoulli like. Bernoulli just explains the reaction of the nearby air to the pressure differentials created by a wing, but not how the pressure diferential themselves are created. The Wiki wing article uses "void" theoy to explain why air follows the upper surfaces of a wing with postive AOA, and I'm also a "void" theory believer. Underneath, simple deflection could work, but it's my understanding that efficient airfoils also reduce pressure below ambient under the airfoil as air approaches the aft part of an airfoil, just not as much reduction as above, but allowing the pressure jump zone to remain below ambient. Jeffareid (talk) 15:56, 27 September 2008 (UTC)

All airfoils reduce pressure below ambient above and below the airfoil. Michael Belisle (talk) 06:01, 28 September 2008 (UTC)

I seem to recall that the M2-F2 (mentioned above) produced higher than abient pressure below the airfoil. This is an unusual case. I've wondered if low lift to drag ratios for re-entry vehicles are due to circumstance or by deliberate design. However in the case of a propeller, the pressure jumps to well above ambient, and maximum air speed occurs well aft of the propeller disk. I'm not sure in the case of helicopter rotors. Even for a normal wing, under high wing loadings, like a F-16 pulling a 9 G turn, with a wing loading of about 77lbs / ft^2, about .535 psi, 9g's translates into a pressure jump of 4.8125 psi, at an AOA at over 20 degrees, and I'm pretty sure that the pressure jumps to above ambient.

Wing loading ≠ fluid pressure! In subsonic flow, neither wings nor lifting bodies nor propellers compress the flow. Supersonic flow (F-16) is different. Hypersonic flow (re-entry vehicle) is also different. Michael Belisle (talk) 22:03, 28 September 2008 (UTC)

Wing loading = pressure differential / area, which is what I was getting at. I don't recall mentioning compression. In the case of propellers, the pressure jumps to above ambient, which is why the air continues to accelerate in the prop wash as it's pressure decreases back to ambient, as mentioned in the NASA and other articles. I recall that at high speeds and high AOA, the pressure jump is also large and jumps to above ambient, but I don't have a reference to cite yet. Jeffareid (talk) 02:58, 29 September 2008 (UTC)

• Thanks for your response to the wiki lift force discussion. My wording was awkward and it's not my intention to start a Newton versus Bernoulli debate, since both princples coexist (except for that nagging bit of work done on the air, which I still haven't found a good explanation for, other than wing tip vortices, but it's more than just that, again referring to propellers as an example). Regarding the artile you linked to: how wings produce lift, it keeps referring to the requirement of circulation for lift, but circulation around the wing chord ceases to exist as speed approaches and exceed the speed of sound, yet lift is still produced just fine. Obviously there has to be some mathematical model for airfoils used on commercial airliners (speeds above mach .7) and supersonic aircraft, but I don't see these explained. Jeffareid (talk) 12:08, 28 September 2008 (UTC)

Circulation does not cease to exist. Transonic flow is yet another flow regime and it's different too. It's a complicated problem for which no universal theory exists. Much of the work done on airliners is empirical. Michael Belisle (talk) 22:03, 28 September 2008 (UTC)

Maybe I'm misunderstaning "circulation" here. Is there air flowing forwards faster than the bottom surface of a wing? I have read that as speed approach Mach 1, the only circulation near the wing are the tip vortices. Jeffareid (talk) 03:02, 29 September 2008 (UTC)

Lift and Bernoulli II

Via email correspondence with: David Anderson - Fermi National Accelerator Laboratory - Ret. - dfa180@aol.com, I got the impression that he felt that Bernoulli shouldn't be used in a description of how lift is generated. Since this guy is an apparent expert, I'm trying to resolve his opinions versus those expressed on the Wiki page. Other AE's I've corresponded with also seemed to be "anti-Bernoulli", mostly because real world flows don't fit well with the typical Bernoulli's "model" which seem to focus on near laminar flows parallel to the surface of a wing rather than the overall (vertical and horizontal components), and somewhat turbulent flow that occurs, and the fact that work is peformed on the air. Jeffareid (talk) 17:34, 17 October 2008 (UTC)

From the article: "the practical explanation of what those equations mean is still controversial, with persistent misinformation and pervasive misunderstanding." (Previously, I had written "even among experience aerodynamicists", but that disappeared at some point.) It is resolved by doing the best we can to provide an inclusive WP:NPOV explanation of lift. Michael Belisle (talk) 19:09, 17 October 2008 (UTC)

I like the approach of starting with a very simplified model, in order to establish the more easy-to-understand principles. If we start right in with friction and turbulence, these principles get obscured, and the reader ends up not really understanding anything.

It would be like introducing capacitance to a student by showing a non-infinite model with end-effects, resistance, and random oscillations caused by stray inductance and resonant amplification of noise.

The experts may not like including Bernoulli, "because real world flows don't fit well with the typical Bernoulli's model".

But for me, that is best addressed by not starting with real flows. For a simple (pot. flow theory) diagram of the flow, Bernoulli is a great, already somewhat familiar concept which helps to take the mind easily from the middle stage: "ok, here is the flow" to the next stage: "so, that's why the pressure above is low".

I personally can't imagine sorting even a simple PFT-based diagram out if I didn't have clearly in mind that narrowing streamlines mean increasing speed and falling pressure. Mark.camp (talk) 21:36, 3 March 2009 (UTC)

Lift (force)

Michael. My apologies for plunging rather rude into the discussions. Only when going along I become acquainted with the references you have used. I understand thinks may be delicate, certainly regarding (fallacious) "explanations" of lift. Best regards, Crowsnest (talk) 04:33, 13 February 2009 (UTC)

Wikipedia:WikiProject Physics/Taskforces/Fluid dynamics

Crowsnest has expressed interest in reviving the Fluid dynamics taskforces (previously a WikiProject). Since you seem to know a thing or two about FD, would you be interesting in joining it? I'm already taking care of some parts of the revamping and overhauling. It would at the very least, give you a go-to place to find some FD-related tools. Headbomb {^ταλκ_{κοντριβς} – WP Physics} 10:18, 26 February 2009 (UTC)

Yes. I was searching yesterday for which task force to add Lift (force) (within WP:AVIATION at the time) to and lamented about about the lack of a suitable task force. I've been meaning to branch out beyond the Lift article eventually. Michael Belisle (talk) 17:35, 26 February 2009 (UTC)

An exciting opportunity to get involved!

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