Newton's third law to explain lift

[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.