Newton's third law to explain lift

[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 boundary layer advancing toward the leading edge with increasing angle of attack. I'm speaking of the permanent separation of the boundary 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 boundary 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 vertical 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 boundary 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 boundary layer to reattach at turbulent/laminar transission; that is, failure to tranite to an attached turbulent boundary 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 acceleration 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]