Musings on Lift

[PT20J]

13 minutes ago, Hank said:

And the airstream impacting the bottom of the wing pushes it upwards, according to Sir Isaac Newton.

If you look carefully at the streamlines in the picture, you will notice that the air does not impact the bottom of the airfoil but curves to follow it's surface. 

16 minutes ago, Hank said:

Oncoming air is split--some goes upward, moving without physical limitations and creates low pressure due to increased flow speed; some is pushed downwards, and because air is compressible, it creates an area of high pressure as flow velocity is reduced by the change in direction.

There are always physical limitations  It takes a force to cause air to increase velocity, so there must be a pressure gradient at work.

At true airspeeds less than 200 kts, the atmosphere acts as if it were incompressible.

[PT20J]

1 hour ago, Utah20Gflyer said:

But for those that don’t believe the wings push air down.  Why does it get so windy when I start my planes engine?

Excellent question. The propeller blades rotate in a plane normal to the axis of rotation and direction of flight. Because of this constraint it is often modelled as a permeable disk that imparts energy to the air flowing through it. The disk creates a low pressure area ahead of it and a high pressure area behind it. This has the effect of creating a stream tube along the axis of rotation that "sucks" air in ahead of the propeller and accelerates it out the back side. The motion of air through the propeller disk is thus different than that of a wing.

[AH-1 Cobra Pilot]

2 hours ago, Utah20Gflyer said:

Approximately half of the air being forced downward is coming from the top side of the wing.  When the air detaches from the top surface it is no longer being forced downward and you lose a lot of your lift. …

But you can’t create high pressure without also creating low pressure so you could say that a plane flies by creating low pressure and that would technically true, but I think it’s more true to say it’s the high pressure that is making the plane fly.   Plane go up, air go down! 

Since I cannot post an image, you will have to follow the link to a bunch of plots that show something a little different.

https://duckduckgo.com/?t=h_&q=plot+of+pressure+around+air+foil&iax=images&ia=images&iai=https%3A%2F%2Fwww.researchgate.net%2Fpublication%2F368462380%2Ffigure%2Ffig9%2FAS%3A11431281120112258%401676377863994%2FPressure-distribution-over-NACA0012-airfoil-for-varying-angles-of-attack-where-Reynolds.ppm

[MikeOH]

32 minutes ago, PT20J said:

Excellent question. The propeller blades rotate in a plane normal to the axis of rotation and direction of flight. Because of this constraint it is often modelled as a permeable disk that imparts energy to the air flowing through it. The disk creates a low pressure area ahead of it and a high pressure area behind it. This has the effect of creating a stream tube along the axis of rotation that "sucks" air in ahead of the propeller and accelerates it out the back side. The motion of air through the propeller disk is thus different than that of a wing.

So, let me get this straight: Airfoils moving through the air operate on different physics?  Sorry, not buying it.

[PT20J]

6 minutes ago, MikeOH said:

So, let me get this straight: Airfoils moving through the air operate on different physics?  Sorry, not buying it.

No, the airfoil and pressure distributions around it are the same whether it is a section of a propeller or a wing. Likewise with tip effects: propellers and wings both shed a vortex sheet along their span and a vortex at their tips. The difference is that the propeller rotates normal to the free airflow and a wing passes tangential to it. 

[MikeOH]

For those that believe lift is NOT caused by air being forced downward, consider this:

An aircraft moves through an arbitrary volume of undisturbed air in level flight.

Level flight DEMANDS that the lift provided by the wing EQUALS the weight of the aircraft (plus any downforce from the tail).

I think we all would agree that lift is being provided by the wing.

The ONLY objects the wing acts against are the stationary air molecules that the wing encounters.

To generate an upward force, i.e. LIFT, the ONLY possible explanation is that the air molecules are ACCELERATED downward in accordance with Newton's 2nd law.  Their simultaneous acceleration forward results in DRAG.

 

[MikeOH]

3 minutes ago, PT20J said:

No, the airfoil and pressure distributions around it are the same whether it is a section of a propeller or a wing. Likewise with tip effects: propellers and wings both shed a vortex sheet along their span and a vortex at their tips. The difference is that the propeller rotates normal to the free airflow and a wing passes tangential to it. 

So, SAME physics.  Thus, to @Utah20Gflyer's question, the 'wind' he feels IS the same thing from the wing as the propeller

[PT20J]

5 minutes ago, MikeOH said:

For those that believe lift is NOT caused by air being forced downward, consider this:

An aircraft moves through an arbitrary volume of undisturbed air in level flight.

Level flight DEMANDS that the lift provided by the wing EQUALS the weight of the aircraft (plus any downforce from the tail).

I think we all would agree that lift is being provided by the wing.

The ONLY objects the wing acts against are the stationary air molecules that the wing encounters.

To generate an upward force, i.e. LIFT, the ONLY possible explanation is that the air molecules are ACCELERATED downward in accordance with Newton's 2nd law.  Their simultaneous acceleration forward results in DRAG.

 

As I said in my first post, what happens is that the air changes velocity (direction and speed) due to the presence of the wing. A change in velocity requires a force (Newton's second law), which for a fluid, must necessarily be a pressure (force over an area). A change in velocity also causes a change in momentum (mass x velocity). So mathematically, lift can be derived from either the change in momentum or the change in pressure. But this is not the same as saying it is due to downwash and Newtons third law. The problem with that explanation is that downwash has been shown not to be required for lift, and there is no way to describe the physical means by with the action (downwash) would cause the equal and opposite reaction (lift) that is physically correct.

[MikeOH]

6 minutes ago, PT20J said:

As I said in my first post, what happens is that the air changes velocity (direction and speed) due to the presence of the wing. A change in velocity requires a force (Newton's second law), which for a fluid, must necessarily be a pressure (force over an area). A change in velocity also causes a change in momentum (mass x velocity). So mathematically, lift can be derived from either the change in momentum or the change in pressure. But this is not the same as saying it is due to downwash and Newtons third law. The problem with that explanation is that downwash has been shown not to be required for lift, and there is no way to describe the physical means by with the action (downwash) would cause the equal and opposite reaction (lift) that is physically correct.

I swore I wouldn't let myself get sucked into this type of thread, but here I am, sigh!

This is just a silly discussion of semantics; "downwash" vs. "forcing air downward" where some try to show how 'smart' they are.

IMHO, for a PILOT, sticking your hand out the window of a moving car teaches you everything you need to know about lift and drag (admittedly not ground effect)

You can have the last word telling me how "it's NOT semantics, but an absolutely vital distinction that we all must understand or we're not real pilots."  The fact remains that air molecules are being displaced downward.

I'm done.

[PT20J]

1 hour ago, Hank said:

No, we don't fly at 0° angle of attack.

Maybe not zero, but not much more at cruise. From the attached curves I got long ago from Mooney, the Mooney wing zero lift angle of attack is -1 deg. So, because of the camber, it generates lift at zero. But, we can easily calculate it:

L = 1/2 p V² C_L S

Solving for C_L and converting to convenient units at sea level on a standard day,

C_L = 295 W/V²S

where, W = weight in lb., V = KTAS, S = wing area in ft². For a M20J, S = 174.8 ft². For our example, lets use 2500 lb weight at 150 KIAS:

So, C_L = 295*2500/150^2*174.8 = 0.1875

According to the attached curve, that corresponds to a angle of attack of about +1 deg.

M20K Aerodynamic Coef - Flaps 0.pdf

[PT20J]

I am certainly not trying to argue with anyone and none of the ideas I have presented are my own. In fact, I presented two videos, one by a retired senior Boeing engineer and one by a professor of aeronautics, that essentially say the same thing albeit with much more detail and rigor. Here is a paper (for those who might be interested) written by a (now sadly deceased) well know aeronautical engineer. My only purpose in presenting this was to try to point out that a lot of what is presented in instructional materials for pilots is just plain wrong and it's not really that hard to understand what is really happening, at least at a very basic level. But I do understand the law of primacy and what we first learn sticks with us. But, for those with an interest, I think it is a fascinating subject to explore. So many things I thought I knew in my early flying career have been proven to be wrong and one reason I find aviation continually interesting is that there is always more to learn.

The Facts of Lift - Lissaman.pdf

[GeeBee]

Anything will fly.....given enough thrust. The MD-80 is proof positive. 

 

[PT20J]

1 hour ago, GeeBee said:

Anything will fly.....given enough thrust. The MD-80 is proof positive. 

 

This actually makes a great point. Camber is not necessary to generate lift. A flat plate will generate lift. The purpose of airfoil shapes is to generate lift efficiently which means minimizing the drag created by the wing. Drag takes on two forms: Parasite drag from skin friction which is reduced by minimizing the wing area thus necessitating so-called high lift devices on the leading and trailing edges in order to get lift at lower speeds for takeoff and landing; and, induced drag which is caused by the production of lift and can be reduced by careful airfoil design.

[Pinecone]

19 hours ago, AH-1 Cobra Pilot said:

Not quite correct. 

1.  Think of a symmetrical airfoil at zero angle of attack.  Both sides will have high pressures in equal amounts up to a portion of the chord, and you have no lift or moment, but you do have drag.

2.  How would that explain a stall?

1.  But you would have low pressure after that point in the chord.

2.  Reduction in low pressure on top AND reduction in down wash.

[EricJ]

16 hours ago, PT20J said:

If you look carefully at the streamlines in the picture, you will notice that the air does not impact the bottom of the airfoil but curves to follow it's surface. 

The lines under the wing are further apart, though, which means it's slower there and the pressure is higher.   The higher pressure under the wing helps push up on it.   Likewise the thin spaces between lines means the air is faster there and the pressure is lower, so it pulls up.   

The number of different forces that come into play make it interesting and complex and sometimes non-intuitive.    It's no surprise to me that many people find it a little mysterious or confusing.

[AH-1 Cobra Pilot]

21 minutes ago, Pinecone said:

1.  But you would have low pressure after that point in the chord.

2.  Reduction in low pressure on top AND reduction in down wash.

Take a look at the link I included in a previous comment.  It is surprising just how much of the airfoil surface has low pressure, even the underside, and just how little has high pressure.

[AH-1 Cobra Pilot]

22 minutes ago, EricJ said:

The lines under the wing are further apart, though, which means it's slower there and the pressure is higher.   The higher pressure under the wing helps push up on it.   Likewise the thin spaces between lines means the air is faster there and the pressure is lower, so it pulls up.   

The number of different forces that come into play make it interesting and complex and sometimes non-intuitive.    It's no surprise to me that many people find it a little mysterious or confusing.

In an effort not to repeat myself, look at my immediately previous comment.  You will probably find the depictions counterintuitive.

[PT20J]

The important requirement for lift is that the average pressure above the wing be lower than the average pressure below the wing. This does not necessarily mean that the pressure below the wing is higher than the static air pressure.

[ajudson]

Think of it from a particle kinetic theory perspective. Pressure is just particle collisions -- and those impart a momentum. This video helps to visualize what is happening at the micro level (with some simplifications). You can derive all of Navier-Stokes from this fundamental approach.

The summation of forces from all of these individual collisions results in a lift vector that is a function of airfoil shape, relative speed, and angle of attack. Whether you capture this on the macro level in the form of pressure, streamlines, or any other method, the fundamental process is momentum transfer and equal reactions (via Newton - a net mass flow downward for vertical lift). This is not necessarily "downwash" that you are thinking of on the macro scale, as Hank and MikeOH allude to (to be a little more clear here: as the particles collide and transmit their momentum, there is an effective downward motion of a parcel of air on the macro scale).

 

Edited February 24 by ajudson

[GeeBee]

[PT20J]

6 hours ago, ajudson said:

Think of it from a particle kinetic theory perspective. Pressure is just particle collisions -- and those impart a momentum. This video helps to visualize what is happening at the micro level (with some simplifications). You can derive all of Navier-Stokes from this fundamental approach.

The summation of forces from all of these individual collisions results in a lift vector that is a function of airfoil shape, relative speed, and angle of attack. Whether you capture this on the macro level in the form of pressure, streamlines, or any other method, the fundamental process is momentum transfer and equal reactions (via Newton - a net mass flow downward for vertical lift). This is not necessarily "downwash" that you are thinking of on the macro scale, as Hank and MikeOH allude to (to be a little more clear here: as the particles collide and transmit their momentum, there is an effective downward motion of a parcel of air on the macro scale).

 

That's a cool simulation. I did not see where it showed that there is a net downward momentum. I believe that Doug McLean addressed that fallacy at the end of his lecture and in more detail in his book. Both McLean's analysis and Lissaman's paper show that there is an updraft before the airfoil and a corresponding downdraft behind the airfoil and each accounts for 1/2 of the lift force.

It occurs to me that there can be confusion with the term downdraft. There is downdraft behind even an infinitely long wing just as there will be an updraft ahead of the wing. But that's not the downdraft that people generally refer to when invoking Newton's third law to explain lift. The "Newton explanation" downdraft is the one that is not balanced by the updraft ahead of the wing and it is the source of induced drag, not lift. It is caused by a vortex sheet shed by a finite span wing. Details can be found in any aerodynamics textbook where induced drag is introduced.

[ajudson]

56 minutes ago, PT20J said:

That's a cool simulation. I did not see where it showed that there is a net downward momentum. I believe that Doug McLean addressed that fallacy at the end of his lecture and in more detail in his book. Both McLean's analysis and Lissaman's paper show that there is an updraft before the airfoil and a corresponding downdraft behind the airfoil and each accounts for 1/2 of the lift force.

It occurs to me that there can be confusion with the term downdraft. There is downdraft behind even an infinitely long wing just as there will be an updraft ahead of the wing. But that's not the downdraft that people generally refer to when invoking Newton's third law to explain lift. The "Newton explanation" downdraft is the one that is not balanced by the updraft ahead of the wing and it is the source of induced drag, not lift. It is caused by a vortex sheet shed by a finite span wing. Details can be found in any aerodynamics textbook where induced drag is introduced.

It is true that there is net zero momentum accumulation across the entire semi-infinite atmosphere -- eventually all of those particle collisions will impart their momentum into the ground and cancel out to zero. Induced drag in this particle kinetic view is simply the momentum transfer in the perpendicular direction (the wing deflecting the particle downward results in the wing gaining some momentum upward -- lift, and losing some of its forward momentum -- drag).

Edit: refreshing myself with McLean's (very good) video, the end touches on this a bit -- look at his pressure integrals for a pancake control volume. In the semi-infinite case (with a ground plane), the particle collisions eventually impart the entirety of the Lift force into the ground. In the upward direction, eventually all of the particle collisions slowly dissipate to a smaller and smaller pressure disturbance, summing to zero at infinity. The net force is still L upward. Although there was zero momentum accumulation in the atmosphere, the forces were still imparted by every particle collision involved.

Edited February 25 by ajudson

[PT20J]

2 hours ago, ajudson said:

the forces were still imparted by every particle collision involved.

Yes, the lift force can be derived as either a change in momentum or a change in pressure because the two are two sides of the same coin.