Newton's third law to explain lift

[Phrak]

"All airfoils must have adverse pressure gradients on their aft end. The usual definition of a laminar flow airfoil is that the favorable pressure gradient ends somewhere between 30% and 75% of chord."

http://www.aviation-history.com/theory/lam-flow.htm

For non-laminar airfoils, this transition from acceleration+decreasing pressure to deceleration+increasing pressure occurs even sooner along the chord.

I think you may have inverted what you intended to say. As I recall a turbulent boundry layer detaches further into an adverse pressure gradient than a laminar boundry layer due to greater mixing with the general airsteam, thus the interest in tripping the boundry layer toward the leading edge into turbulence with devices such as turbulators.

 

[rcgldr]

"All airfoils must have adverse pressure gradients on their aft end. The usual definition of a laminar flow airfoil is that the favorable pressure gradient ends somewhere between 30% and 75% of chord."

http://www.aviation-history.com/theory/lam-flow.htm

For non-laminar airfoils, this transition from acceleration+decreasing pressure to deceleration+increasing pressure occurs even sooner along the chord.

I think you may have inverted what you intended to say. As I recall a turbulent boundry layer detaches further into an adverse pressure gradient than a laminar boundry layer due to greater mixing with the general airsteam, thus the interest in tripping the boundry layer toward the leading edge into turbulence with devices such as turbulators.

I was just quoting the article in that link. Did I get my post link description of the transition wrong? In the case of laminar airfoils, the ideal is to move the transition point further back on the airfoil. In the case of gliders, the low speeds result in the laminar "bubble" causing more drag than the turbulent flow, so a rough surface, or tubulators below and/or above a wing are used. Turbulators are also used on some powered aircraft, but I'm not sure why (maybe at higher air speeds like above mach .6 or mach .8?).

 

[djeitnstine]

This website does a descent job of explaining it, with a lot of emphasis on Coanda effect, but towards the end of this web site, there's a diagram of a wind blowing over a roof, and although the air downwind of the roof is turbulent, it's also at lower pressure, due to what some call "void" effect: when a solid object passes through a fluid, or when a fluid passes around a solid object, low pressure "voids" are created because the solid object blocks or diverts the fluid flow away from these low pressure areas.

After visiting a large number of web sites, my conclusion is that lift is a combination of Coanda and "void" effects.

Thank you sir, this clears up quite a bit although my current knowledge only carries my understanding half way through that lecture lol.

 

[Phrak]

I was just quoting the article in that link. Did I get my post link description of the transition wrong? In the case of laminar airfoils, the ideal is to move the transition point further back on the airfoil. In the case of gliders, the low speeds result in the laminar "bubble" causing more drag than the turbulent flow, so a rough surface, or tubulators below and/or above a wing are used. Turbulators are also used on some powered aircraft, but I'm not sure why (maybe at higher air speeds like above mach .6 or mach .8?).

Something doesn't add up with that link. Really, I haven't looked at either fluid dynamics or theory of flight in a number of years, but I recall the laminar flow idea is a matter of historical note in so far as the belief that maintaining a laminar boundry layer was either advantages or practically achievable.

A model aircraft with a 4 inch cord and maybe 30 mph flies in the laminar regime. An aircraft wing any much larger or faster is in the turbulent regime except under the most tedious conditions: a polished surface (no rivets of course) and any variations in the surface such as wavyness could trip the boundry layer to turbulent flow.

Like so much of fluid dynamics, it's counter-intuitive, but a turbulent boundry layer is advantageous because of the improved L/D. Any of the upper wing surface past the point at which the boundry layer detatches, generates, effectively no lift. The further along the upper surface you can delay the onset of boundry separation, the better.

The best of both worlds would be to have a laminar boundry layer as far past the leading edge as possible, then trip to turbulent flow somewhere immediately before the point where of laminar separation would occur, but I recall the efforts to obtain this are hampered by practical limitations from varying angle of attach and enviromental conditions like bugs, rain, air pressure.

I couldn't find any links worthy of posting, but one of them places the critical Rynyolds number for aircraft wings at 100,000-500,000 with small aircraft wings having values from 2,000,000 to 20,000,000 -- well within the turbulant regime.

 

[rcgldr]

After visiting a large number of web sites, my conclusion is that lift is a combination of Coanda and "void" effects.
http://user.uni-frankfurt.de/~weltner/Mis6/mis6.html

I should also include direct deflection of the air, which is what happens under a wing (assuming that significant lift is being produced from below a wing). So it's Coanda effect, "void" effect, and direct deflection of air.

A model aircraft with a 4 inch cord and maybe 30 mph flies in the laminar regime.

Actually it's more like 6 inch chord but 10mph airspeed. In the case of gliders, the laminar bubble creates more drag than turbulent airflow, so these small hand launch (now called discus launch because of the launching method) gliders use turbulator strips.

The best of both worlds would be to have a laminar boundry layer as far along the leading edge as possible, then trip to turbulent flow somewhere immediately before the point where of laminar separation would occur, but I recall the efforts to obtain this are hampered by practical limitations from varying angle of attach, and enviromental conditions like bugs, rain and air pressure.

This is done for model and full scale gliders. Note that the laminar detachement isn't "permanent" except for very low Reynolds numbers. Typically there's a reattachment of the flow once it's gone turbulent, because of what I call "void" effect, a wing produces a moving void of low pressure on it's aft portion, and this detachment is called a "seperation bubble". To control this transition bubble, sometimes just sanding the wing surfaces with 600 grit sandpaper is enough, in other cases. turbulator strips are used below and/or above on a wing.

In the case of gliders, oil flow tests are used to determine detachment of air flow. As noted at this web site, "Partially developed separation bubbles can actually have beneficial effects."

http://www.standardcirrus.org/OilFlows.html

 

[Phrak]

I should also include direct deflection of the air, which is what happens under a wing (assuming that significant lift is being produced from below a wing). So it's Coanda effect, "void" effect, and direct deflection of air.

Actually it's more like 6 inch chord but 10mph airspeed. In the case of gliders, the laminar bubble creates more drag than turbulent airflow, so these small hand launch (now called discus launch because of the launching method) gliders use turbulator strips.

This is done for model and full scale gliders. Note that the laminar detachement isn't "permanent" except for very low Reynolds numbers. Typically there's a reattachment of the flow once it's gone turbulent, because of what I call "void" effect, a wing produces a moving void of low pressure on it's aft portion. Sometimes just sanding the surfaces with 600 grit sandpaper is enough, in other cases. turbulator strips are used below and/or above on a wing.

In the case of gliders, oil flow tests are used to determine detachment of air flow. As noted at this web site, "Partially developed separation bubbles can actually have beneficial effects."

http://www.standardcirrus.org/OilFlows.html

Whoah, you've sure done your homework! I'd completely forgotten about the detachment, reattachment and all that.

From what I get out of that link, it's best to have a turbulator stip somewhere at the quarter cord point rather than a bubble. I think I'll go back to lurking.

 

[Phrak]

Originally Posted by Jeff Reid

For non-laminar airfoils, this transition from acceleration+decreasing pressure to deceleration+increasing pressure occurs even sooner along the chord.

Originally Posted by Phrak

I think you may have inverted what you intended to say...

Originally Posted by Jeff Reid

I was just quoting the article in that link. Did I get my post link description of the transition wrong?

Nope. Sorry. I misunderstood.

Originally Posted by Stan Butchart

I am looking for help IDENTIFYING A NET downflow ...

Nobody has answered this yet?? The upward force on the lifting surfaces is equal to the downward force on the surrounding air where the airplane has zero verticle accelleration. The verticle force on the lifting surfaces would be the integral of the pressure times area normals doted projected in the vertical direction.

F_{airplane}=\int P \hat{k}\cdot d\bar{n}

The force on the airsteam is harder. The downward velocity of each air particle decreases the further back in the airstream you find it as it shares it's momentum with surrounding particles. Is this Navier-Stokes?

 

[Stan Butchart]

I am not bright enough to figure out getting the quotes.
"The upward force"... Well put, but some interpretation (personal?) goes with the words.
"The downward velocity of each"...Having worked with classical aerodynamics I fail to find a downward flow which is the result of a force that contributes to, or results in, the actual creation of lifting force.

 

[Phrak]

I am not bright enough to figure out getting the quotes.

Basically, I was appologizing to JR, as it seems I may have insulted his intelligence.

"The upward force"... Well put, but some interpretation (personal?) goes with the words. "The downward velocity of each"...Having worked with classical aerodynamics I fail to find a downward flow which is the result of a force that contributes to, or results in, the actual creation of lifting force.

I've been searching for an easily understood explanation of what's going on, with as little extra clutter as possible. I think the best way to go about it is to start with the flow of air over an outwardly curved surface.

If you're willing to accept that the airflow doesn't break-away from the surface, leaving stagnant air or a vortex underneath, I think it's an acceptable explanation. As the air flows over the top surface of the wing the overall flow above the wing follows an arc. This means that the air is subjected to an acceleration normal to it's direction of flow--that is, toward the wing surface. The acceleration time the mass of the air exerts a counter-force on the wing--the lift. This is just F=ma. The arcing flow redirects the air slightly downward as it leaves the trailing edge.

The overall downward amount of flow as it gets as far as the tailsection is only a couple degrees or so.

What I would like to know is this: "Is stall a result of the stagnation point migrating toward the leading edge?

 

[rcgldr]

"Is stall a result of the stagnation point migrating toward the leading edge?

As I pointed out in previous posts, it's normal for the air stream to separate and reattach while it transitions from laminar to turbulent flow. As the angle of attack increases, the detachment zone gets larger, but the lift still continues to increase until you reach a crictical angle angle of attack where the lift is at it's maximum. Go beyond this, and the lift decreases, but it doesn't vanish completely, although there may be quite a drop in the amount of lift.

In the case of slow speed flight, the problem is excessive angle of attack reduces lift, causing the aircraft to sink, which increases the effective angle of attack, since the direction is now more downwards than before, this decreases lift more, which causes the aircraft to sink even more so; a viscous cycle, where control is negatively stable.

In the case of high speed flight, the wings aren't identical, so one wing goes past crictical before the other. Since the wing past critical has less lift, there's a viscous roll reaction, downwards on the wing past critical, so it makes even less lift, while upwards on the other wing which reduces it's effective angle of attack but doesn't reduce the lift as much as the wing past critical. The result is a snap roll.

On aerobatic radio control models, with excessive elevator throw, pulling back hard on the elevator results in a fast roll response with no hint of the expected pitch response, and without any aileron control input. It spooked me the first time it happened to me, which was with a friends small aerobatic glider, which I had in a dive out over a tall slope well above the ground below, so I had plenty of time to recover by easing off the elevator input. After that I thought it was cool that a pitch control input would result in a roll response. The owner of the model had set it up that way. Contest aerobatic powered models are setup similar to this, to produce a true snap roll for competion.

Snap rolls are bad during a speed contest, where the snap roll results from pulling too many g's in a turn, while low to the ground. It's a 50/50 chance that the model will roll downwards into the ground and crash, or upwards with no harm done.

For glider being launched via a line drawn by a winch or strong latex tubing, the high loading combined with too aggressive pitch input (excessive elevator trim) can result in a snap roll. The instinct is to try to recover with aierlons but this make the situation worse because the alieron input increases camber and effective angle of attack on the downwards moving wing, reducing it's lift further still. The general rule for gliders is that if something goes wrong, down elevator should be the first control input, to make sure that the glider isn't experiencing a stall or snap roll situation.

If you're willing to accept that the airflow doesn't break-away from the surface, leaving stagnant air or a vortex underneath.

Actually for almost all situations, there's always some break-away "bubble", but it's very small, in the mm range in some cases, during the transition from laminar to turbulent flow. Even if the turbulent flow is composed of small oval eddies, it's still not an issue as long as the pressure is still below ambient in that turbulent flow, which it usually is. In the case of delta wings, they can handle huge angles of attack (over 20 degrees) without stalling, because the shape of the wing (triangular front, flat back) allows it to take advantage of turbulent eddies that flow across it.

 

[Stan Butchart]

Again Phrak states well.
I have been inmeshed in the fuzzy world of logic and symantics. Stuff like -I could not get the arithmetic to work for "centrifical" effects. The force for following the surface came from ambient pressure. Fluid "static" pressure has no directionality, only the receiving surface. This clouds the word "downward". The balloon was pure differential static pressure so did achieving the reduced static pressure by dynamics require "down"?
The tangential accelerations that exist within the curving flow produce the right answers.

Anyway, thanks I can't argue with that but my mind will have to do some smoothing yet.

 

[Phrak]

As I pointed out in previous posts, it's normal for the air stream to separate and reattach while it transitions from laminar to turbulent flow. As the angle of attack increases, the detachment zone gets larger, but the lift still continues to increase until you reach a crictical angle angle of attack where the lift is at it's maximum. Go beyond this, and the lift decreases, but it doesn't vanish completely, although there may be quite a drop in the amount of lift.

Ok. It had occurred to me that I haven't seen any satisfying graphs of pictures of flow fields as a wing section progresses into stall. I've presumed it to be a progressive separation of the boundry layer advancing toward the leading edge with increasing angle of attack. I'm speaking of the permanent separation of the boundry layer, rather than the laminar to turbulent transition. And of course the position of the line of separation isn't necessarily stable but could be oscillatory or even chaotic for all I know.

But I'm given thought to an interesting alternative mechanism (that you may have alluded to--I can't tell.) whereby the boundry transition bubble fails to reattach. Perhaps a reflex wing section, or a high lift system could exhibit this.

 

[Stan Butchart]

Regaurding normal acceleration: When I look at a pressure gradient using Rhov^2/r = dP/dr it comes up way short of the gradient useing Bernoulli. Am I missing a basic princple here?

 

[Phrak]

Regaurding normal acceleration: When I look at a pressure gradient using Rhov^2/r = dP/dr it comes up way short of the gradient useing Bernoulli. Am I missing a basic princple here?

What geometry are you using to with the equation Rhov^2/r = dP/dr?

 

[rcgldr]

I haven't seen any satisfying graphs of pictures of flow fields as a wing section progresses into stall.

How about a video? It appears to be a narrow wind tunnel, so it's considered a "2d" airflow (equivalent to a 3d wing with infinite wingspan). It's also apparently a small airfoil so the Reynolds number would be quite low, and the air flow is much more laminar and the angle of attack is much higher than it would be if everything were scaled up to a larger size. The transition into the stalled condition is very abrupt. In the segment annotated as "stall", there's virtually no lift, but near the end of the video, that starts off "flow attached", then "stall", there's still significant lift although there is a stall.

http://www.youtube.com/watch?v=6UlsArvbTeo&fmt=18

Assuming this next video isn't a GGI video, it appears to be a flame aimed at various angles over an glowing (from the heat) airfoil at a fixed angle of about 45 degrees. As the flame angle is made more horizontal, the effective angle of attack becomes higher. What I call "void" effect is more evident here, as the flame flow is detaches from the aft end of the airfoil at low effective angle of attack. At higher effective angle of attack, the flame flow detaches from the "upper" surface of the airfoil, but it's stil accelerated (curved) "downwards", while below the airfoil there is significant direct deflection.

http://www.youtube.com/watch?v=hkJaTTIiXSc&fmt=18

The second video looks much different than the first video. I can think of 3 reasons for this. First the behavior of the heated gas is similar to a wind tunnel with a much higher air speed than the wind tunnel video. Second, it's a heated gas instead of normal air. Third, it's an open environment, whereas the wind tunnel is sealed above and below, preventing much downwards flow of the air (resulting in more pressure effects and less flow effects).

I'll keep searching for a more open (larger) and higher air speed wind tunnel.

 

[rcgldr]

More videos:

For this model, the stalling angle of attack is fairly small:
http://www.youtube.com/watch?v=5wIq75_BzOQ&fmt=18

A small wing and slow air speed:
http://www.youtube.com/watch?v=TGUSmdFmXDg&fmt=18

This is so wrong, but it's the classic and incorrect equal transit time "hump" theory from an old film:
http://www.youtube.com/watch?v=mTW4Fdcoyqs&fmt=18

 

[Stan Butchart]

What geometry are you using to with the equation Rhov^2/r = dP/dr?

For the circular cylinder, the min radius at the top of the cursve "e" for the flow relative to the remote still air.

 

[Phrak]

It's a pity that detailed videos are so hard to find. With luck, in a few years, someone will come up with a quality video of a section undergoing stall, complete with the laminar/turbulant transition bubble included. In fact, it could be done by combining both smoke and the oil film you showed us, if need be. This does bring up a question. Do you have a source I could look at that talks about stall proceeding from the transition point?

This Cambride video http://www.youtube.com/watch?v=6UlsArvbTeo&fmt=18 is the best, overall, I think. If you look at the pulsed smoke part of it, over the top of the section the pulses remain in a nearly verticle row; the v_x velocity remains nearly constant. The overall velocity increases. How much of this effect is due to the top of the wind tunnel interferring with the wing is hard to tell. I recall, the top of the box is only about 3/4 cord from the wing.

In this, http://www.youtube.com/watch?v=5wIq75_BzOQ&fmt=18,
video the abruptness of stall is frightening!

In both videos the boundry layer separates at the trailing edge. The greater the angle of attack the sooner the separation, It makes sense of course; the sooner the flow reaches stagnation, the sooner it separates.

In the second video the sudden separation at the leading edge is the most suspicious. Why should the stagnation point transite so suddenly to the leading edge? Do you think it not a result of the stagnation point moving forward, but a failure of the boundry layer to reattach at turbulent/laminar transission; that is, failure to tranite to an attached turbulent boundry layer?

 

[Phrak]

By the way, you both might find this interesting. A high lift foil set quoted at an incredible CL_max=4.81.

http://adg.stanford.edu/aa241/highlift/highliftintro.html"

 

[Stan Butchart]

Three years later! I have to close by saying that these conversations do produce added insite.
Previously I tried to calculate the centripedal acceleration of curved flow from its inertial path. In reality it works just fine when useing the velocity relative to the surface and surface radious. Interestingly, when we multiply v^2/R *Rho times R/2 (which integrates the entire normal column) we wind up with the Bernoulli equation even though "Bernoulli flow" is not present.

The place of vertical Newtonian acceleration is still not straight forward. Normal acceleration produces pressure change against the local surface element. Lift contribution is the vertical component of that surface pressure. The vertical component of that normal mass acceleration was indeed equal to the lift contribution. It satisfies Newtons laws of force ballance. However, to explane lift, the pressure change against the suface element is created by the normal acceleration. The surface element dose not recognize
the direction from which it created. Lift contribution is determined by surface element orientation.

 

[K^2]

Three years later, and still nobody explained to you that neither of these cause lift?

Bernouli Equation cannot be used to derive lift, because flow at the surface is zero. It can be used as an estimate, however, by considering the layer with fastest flow.

Newton's Third cannot be used directly because there is no direct interaction. The flow that's actually deflected is not the flow that had contact with the wing.

Lift is generated by pressure differential across the wing. That pressure differential has to be found by solving the flow equations. In many situations, finding exact pressure at the surface is problematic. In that case, circulation is used. Kutta-Joukowski theorem relates circulation around a surface to the lift generated by the surface. The theorem is derived using Kutta condition, Stokes' Theorem, and momentum conservation. This is probably the most direct link to Newton's Third in the whole deal.

 

[Stan Butchart]

responce to K^2
My biggest disputs I have had were with people who understood the same things I did.

Lift does indeed come from pressure differential. Pressure differential comes only from acceleration. That is inertial mass re sistance to acceleration. Flow equations are but a description of physical physics.
Pressure change originates in the normal accelerations of the turning flow - the change of velocity vector. It is the sum of the all of the change in the near field.
The KJ theorem gives the right answer but explanes nothing.

I look at the Bernoulli equation as emperical. It gives good answers for normal acceleration.
Ref Abbot & von Doenoff, pg 44

 

[rcgldr]

Newton's Third cannot be used directly because there is no direct interaction. The flow that's actually deflected is not the flow that had contact with the wing.

Still there's a mechanical interaction between the wing and the air, with a macroscopic result that air is accelerated downwards (corresponding to lift) and forwards (corresponding to drag). The relationship between the mechanical interaction, and the response of the air in terms of acceleration, pressure differentials, and total change in mechanical (and thermal) energy is related to the qualities of the air, such as density, viscosity, ..., the nature of the wing (shape, size, surface friction, ...), and the relative speed and angle of attack of the wing. I'm not sure that even Navier Stokes equations can take all of these factors into account, although they do produce reasonable approximations. Just like a lot of things in physics, there's no simple answer once the details of the process are examined.

 

[Stan Butchart]

Although everyone must have put this to bed I wanted to lay out what what I have put together from line of thought.
http://svbutchart.com

 

[deepthishan]

Ok so I just read this statement as part of an explanation of list using Newton's 3rd law

"The amazing thing about wings is that because they are flying through air which is a fluid, the top of the wing deflects air down as well as the bottom of the wing."

What I don't understand is how the top of the wing deflects air downwards. Anyone care to explain?

I don't think there is a universally agreed explanation for lift generation but here's one as you wanted:

Google lift curve slope. You can see from there that only airfoils with a positive camber (not flat nor negatively cambered ones) generate lift under normal conditions.

This is because, when you suspend a +vely cambered airfoil in air, air flow gets deflected downwards (upto a critical stalling angle) at the end tips. Therefore according to Newton's third law, this 'downward' force (the direction of this force depends on the angle of elevation of the airfoil) caused by the mass of air pushing 'downwards', results in an equal force acting 'upwards' on the body. This force is called lift.

You can look up wing tip vortices as well if you are more interested.

(I'm an Aerospace Engineer)

 

[deepthishan]

Google lift curve slope. You can see from there that only airfoils with a positive camber (not flat nor negatively cambered ones) generate lift under normal conditions.

Sorry a mistake- I mean they (-vely cambered and flat airfoils) don't generate lift when they are in line with the airflow.

It's easy to visualize linear airflow over an aerofoil or wing tip. It follows the shape of the body it is flowing over..

 

[Stan Butchart]

Would it be possible to find a mentor level contributer willing to critique
http://svbutchart.com ?
An e address is on the site.

 

[DarioC]

As a forty year pilot I am astounded at this discussion. Propellers are airfoils, helicopter blades are airfoils. No downward flow! Wow. Incredible.
DC

 

[Stan Butchart]

As a forty year pilot I am astounded at this discussion. Propellers are airfoils, helicopter blades are airfoils. No downward flow! Wow. Incredible.
DC

We have to note that there is a delicate distinction here in describing "downwash". The massive downwash that we observe, while initiated by lift, is a function of the circulation.
The downward components of accelerations that produce lift ballance the upwash and do not leave a continuous residual downwash.

 

[RandomGuy88]

As a forty year pilot I am astounded at this discussion. Propellers are airfoils, helicopter blades are airfoils. No downward flow! Wow. Incredible.
DC

Ummm... Have you ever stood beneath a helicopter while the rotors are spinning?

Propellers don't technically produce downward flow but that is because they aren't normally pointed down. But really it's still the same thing.

Airfoils generate lift because there is a pressure difference and this pressure difference is a result of streamline curvature. When streamlines curve there is a pressure gradient normal to the streamlines.

 

[DarioC]

Stan,
So...the low pressure and downward flow departing the trailing edge requires an upward flow "downstream" to restore/normalize the atmospheric conditions and that flow is equal in overall dimensions to the original down flow.
DC

 

[D H]

Stan,
So...the low pressure and downward flow departing the trailing edge requires an upward flow "downstream" to restore/normalize the atmospheric conditions and that flow is equal in overall dimensions to the original down flow.
DC

That upward flow had better take place well away from the aircraft or the aircraft won't generate lift. You can't lift yourself by your own bootstraps, and a helicopter can't lift itself by its own vortex ring.

An aircraft must turn the airstream downward or there is no lift. It's a necessary prerequisite of lift by Newton's third law. There is a problem with using Newton's third law to describe lift or thrust. The problem is that it Newton's 3rd is indirect. It doesn't explain what distinguishes a good airfoil from a lousy one.

 

[Stan Butchart]

Stan,
So...the low pressure and downward flow departing the trailing edge requires an upward flow "downstream" to restore/normalize the atmospheric conditions and that flow is equal in overall dimensions to the original down flow.
DC

DC
I apologize for ignoring your prime examples. I am troubled by the descriptions of downwash from helos, props and fans so that I have to keep silant on the subject!
Also in this type of thing you never know if you are reading what the other guy is writing!

The moving wing plus circulation produces an upflow "upstream" of the wing. The downward component of the normal accelerations of turning flow returns it to ambient conditions close behind the TE (pressures have been rising on the back half of the wing.)

Circulation is the "unnatural" disturbance by the wing. It is what is left behind as the downward moving vortex ribbon.

 

[Stan Butchart]

An aircraft must turn the airstream downward or there is no lift.

The pressure changes of lift are created by accelerating the fluid. The pressure change is the reaction to the normal accleration taking place when flow turns. Simple geometry shows that (locally) the vertical contribution of the change is equal to the vertical component of the normal mass acceleration . I am not aware of anything that establishes a requirement for a residual downflow from these accelerations.

 

[D H]

The pressure changes of lift are created by accelerating the fluid.

Correct. To get lift the fluid must be accelerated downward. Fail to do that and you don't have lift. Period. That's the Newton's third law explanation of lift.

How to best turn the airstream downward? Newton's 3rd law doesn't have an answer. Answering that question requires fluid dynamics. Do the fluid dynamics right and you will find (not surprisingly) that a lifting body turns the airstream downwards. It's not surprising because Newton's laws are built into the equations that describe fluid dynamics.

 

[DarioC]

So the normalization takes place below the level of the wing (downstream of course), giving a net end flow downward? For a while I was a bit amused that you were arguing against me when I agreed with you, but was just trying to put the concept in the simplest possible terms.

Actually in the non-mathematical world, if I may call it that, it is easy to see some of what is going on, when an aircraft at low speed/hi lift flies through a media that gives a discernible visual display of the wingtip vortices.

Overall I would think that action/reaction is involved, but that it is way too complex of a process to be explained that way.

I think I saw on here some questionable comments to the effect that a wing has to have a positive angle of attack to produced lift? Probably need to read more carefully, but that is not so. Checking the lift data of a Clark Y will show that there is lift, even at negative angles of attack.

I think I should go back and read this entire thread. (Yikes.)

DC

 

[rcgldr]

I think I saw on here some questionable comments to the effect that a wing has to have a positive angle of attack to produced lift? Probably need to read more carefully, but that is not so. Checking the lift data of a Clark Y will show that there is lift, even at negative angles of attack.

This is where things get complicated. Cambered air foils can generate lift at zero "geometrical" angle of lift, with the leading and trailing edges at the same height, as long as the "highest" point is sufficiently forwards. An alternate term sometimes used is effective angle of attack, which is defined to be zero AOA when a wing produces zero lift. Cambered air foils reduce the amount of drag required to produce lift, but the main way to improve efficiency is to use a long wing span so that a larger mass of air is accelerated by a smaller amount, which results in the same momentum change, but less energy change. A flat or nearly flat wing is actually reasonably good at slow speeds such as small models.

I should go back and read this entire thread.

That and perhaps various web sites on this stuff. Newton third law and action / reaction explains what happens at the macroscopic scale, which answers the original post. Navier Stokes equations get into all the details, but these can't be really solved, so some approximation is used when generating lift and drag data (polars) for airfoils. It's the in between explanations that get a bit murky.

As far as downwash goes, forces don't dissipate. For an aircraft in level flight (or a glider in a steady non-accelerating descent), the force that gravity exerts on the aircraft is exerted by the aircraft onto the air, which generates sort of a continuous impulse that spreads but eventually exerts that force back onto the surface of the earth, which results in the final Newton 3rd law pair as the Earth will react with an equal and opposing upwards force.

 

[DarioC]

"Cambered air foils can generate lift at zero "geometrical" angle of lift,..."

Not to beat a dead horse, but airfoils can and do generate lift at negative angles of attack; some very common ones flying on very common aircraft.

As for the more complex stuff, I'm not overly interested as it is very unlikely that I will start designing airfoils or airplanes at my age. My present understanding is excessively adequate for the type of technical interfacing that I do with aircraft.

It is an interesting thread though and I'm going to read it more thoroughly.

DC

 

[Stan Butchart]

.As far as downwash goes, forces don't dissipate...eventually exerts that force back onto the surface of the earth, which results in the final Newton 3rd law pair as the Earth will react with an equal and opposing upwards force.

Do you know of a good reference that really explains this. I do not for an instant dispute the concept but have never been able to see the mechanism.
Vortex ribbon downwash transmits an equal momentum to (towards) Earth (you can hear it crash!) but it contains no resistance that would be felt by the wing.

If I take a foil such as on a jet liner, I can have "negative' pressure on both sides. The down force at the bottom of the wing is less than ambient. Before the LE and aft of the TE pressure is (basicaly) ambient. The inertial resistance to any downwash aft of the TE is not supporting the wing.

These are not arguments only quandries.

I woul sure appreciate it if DH or rcgldr could do a short review on http://svbutchart.com for me.

 

[D H]

Being a physicist at heart (we oversimplify everything), I like to look at lift as Newton's 3rd law. There's a upward force on the plane by the air, so there must be a downward force on the air by the plane. If you don't bend the airstream downward there is no thrust.

Working an aerospace engineer for 30 years (we overcomplicate everything), I realize that Newton's 3rd alone does not suffice for explaining lift. Not even close! Fluids are hairy. In the sense that the Navier-Stokes equations aren't completely solvable, we still don't quite understand them. Wind tunnel smoke tests are needed to make sure the simplifications made in modeling the flow match reality. Here's one:

mQdR-tnaVfo[/youtube]

 

[rcgldr]

Cambered air foils can generate lift at zero "geometrical" angle of lift

airfoils can and do generate lift at negative angles of attack

Yes, but they still produce downwash at those negative angles of attack. On a cambered airfoil, most of the diversion of flow takes place near the peak of the cambered airfoil and the pressure differential decreases with distance aft of the peak. If the trailing edge is shortened somewhat, the diversion still takes place, but now the wing has a negative AOA due to the trailing edge being shortened relative to the leading edge. This is why effective angle of attack is sometimes used to compare the properties of different airfoils (effective AOA being zero at zero lift).

As far as downwash goes, forces don't dissipate. ... the force that gravity exerts on the aircraft ... eventually exerts ... back onto the surface of the earth.

Do you know of a good reference that really explains this.

Consider the Earth's atmosphere, and any objects "supported" by the atmosphere (aircraft in level fliight, balloon hovering, ...) as a closed system. The average force on the surface of the Earth will correspond to the total weight of the atmosphere and any objects "supported" by the atmosphere.

Reduce this to a very large sealed container. The container weighs 50 lbs, the air inside weighs 49lbs, and there's a model aircraft inside that weighs 1 lb. As long as the center of mass of the system is not accelerating vertically, the weight of the system will be 100 lbs, even when the model aircraft is in level flight (doing circles or figure eights) inside the container. The weight of the air and the model result in a pressure differential within the container, lower near the top, higher near the bottom, and the resultant downforce on the container corresponds to the weight of the air and the model aircraft. The model's affect on the air inside increases the pressure differential so that the downforce increases by the weight of the model. The model increases the pressure differential by inducing downwash as it flies (unless in ground or "ceiling" effect mode).

 

[Stan Butchart]

Reduce this to The model's affect on the air inside increases the pressure differential so that the downforce increases by the weight of the model. The model increases the pressure differential by inducing downwash as it flies QUOTE]

I hope that this response does not mark me as overly dense!

I agree with the principle but still missing the mechanism. If you get a chance could you exspand a bit on the last/

For DH
Again density. You are obviously making a point with that vidio which I missed. Could you give a hint? I see a classic stalled upper surface and the lower surface looks like Newtons original thoughts!

 

[DarioC]

"Yes, but they still produce downwash at those negative angles of attack."

Well yeah. You are preaching to the choir here. Why the but? Of course it does. Did I say somewhere that I thought different? That was my whole point.

I suddenly have this impression that what is actually happening at the leading edge is what is giving Stan problems. I do not think the air is doing what he thinks it is doing at the leading edge. If it did, the wing would not work.

I'm getting the impression that he thinks that the wing is sucking up a huge volume of air from FAR below the leading edge of the wing and throwing it over the top and then down with the equivalent amount of energy behind the trailing edge. Same up energy as down energy. That cannot be what is going on at the leading edge.

DC

 

[rcgldr]

Reduce this to The model's affect on the air inside increases the pressure differential so that the downforce increases by the weight of the model. The model increases the pressure differential by inducing downwash as it flies

I agree with the principle but still missing the mechanism.

See my reply to Dario C below about pressure and acceleration of air. The closed system example is one way to help explain Newton's third law and lift.

video ... a classic stalled upper surface and the lower surface looks like Newtons original thoughts!

Even though the upper surface is stalled, it's still producing some downwash.

"Yes, but they still produce downwash at those negative angles of attack." Why the but?

For anyone reading this thread that might think that downwash wouldn't occur in this case.

I'm getting the impression that he thinks that the wing is sucking up a huge volume of air from FAR below the leading edge of the wing and throwing it over the top and then down with the equivalent amount of energy behind the trailing edge.

I've seen this claim made for "2d" flows, but not for a real wing. The fact that pressure is lower above and higher below a wing is going to lower the flow separation point in front of the leading edge as some air will be diverted upwards to the low pressure zone above, but the net effect of a low pressure zone above a wing is that air accelerates towards that low pressure zone from all directions, except upwards through the wing, resulting in a net downwards acceleration of air from above the wing. A higher than ambient pressure below the wing would similarly result in downwards acceleration of air below the wing.

 

[DarioC]

That pulsed smoke video from Cambridge really got my attention, particularly the reduction in velocity of the flow in relationship to how close it is to the bottom surface of the wing.
There are some "strange" things going on there, Chuckle.

OK, so maybe I am interested in some of the details of what is going on--curiosity will get you every time.

Added after looking at the pulsed flow several more times.

It appears that the net velocity on top of the airfoil departing the trailing edge is about the same as the lower pulses in the bottom stack. Due to the slant of the bottom stack still existing at the lowest pulse I would surmise that the air below it is moving even faster. Which means that the top flow at the trailing edge is moving at the same speed as the "ambient" flow in the tunnel, but the bottom flow, near the wing surface, is slower.

That is not at all what I would have expected and the implications are really interesting.

DC

 

[Stan Butchart]

What we are missing in much of this is the effects of circulation.

On the lift from a neg AOA ~ as long as the last about 1/3 of the mean camber line is pos to the direction of motion, there will be some circulation and therefore lift.

Starting at the beginning ~ At the instant that the wing, with AOA, starts to move the displacement flow attempts symetrical distribution where the forward stagnation streamline originates on a level where the aft stagnation would finish on a lin midway through the airfoil profile. At the TE discontinuity, with the formation of the starting vortex forms the aft stagnation streamline moves down to the area of the trailing edge with an circulation adgustment around the entire foil. Given.

We now have a an additional volume at the back of the wing that must come from somewhere. At the "top" of the wing we can see the added volume of flow from circulation and it had to come from somewhere. At the front of the wing, circulation has moved the stagnation streamline and stanation point downward to direct more of the air over the top.
The origin of the fwd stagnation streamline and the tail of the aft stagnation streamline are at a common level. (rough and theo)

The energy of the displacement flow is almost uncomprehensible. It is required to establish the original motion then remains with an object as it moves forwards. Like ocean waves.

Local motions caused by differences in pressure need care in understanding.
For the flow next to the surface, inetially, the flow is accelerated to the velocity of the wing. As it flows along the surface it actually slows down over the top then speeds up.
( The speed relationship with the surface is fine for normal acceleration)

The "inflow" of motion over the top is from the pressure release of the curving fluid boundary (wing surface) and is a far greater gradiant than one caused by the fluid dynamic motion.