Bernoulli vs Newton (again)

Cruiser said:
look at it this way.......

Lift is due to the movement of air over the wing. Both Newton and Bernoulli contribute to the efficiency (i.e. amount of) movement required to lift the weight.

Consider why the airplane will speed up when the AOA is reduced in level flight? Less Newton and more Bernoulli. Each contribute in different combinations.
Click to expand...

Perhaps you could explain what you mean by "Bernoulli" and "Newton".
 
Capt. Geoffrey Thorpe said:
Perhaps you could explain what you mean by "Bernoulli" and "Newton".
Click to expand...

Typical descriptions when used in relation to lift (and the OPs question) would be

Bernoulli = change in pressure
Newton = action vs re-action
 
Cruiser said:
Typical descriptions when used in relation to lift (and the OPs question) would be

Bernoulli = change in pressure
Newton = action vs re-action
Click to expand...


Yes. I know Bernoulli is about restriction creates lower pressure but John and Martha, Jeppesen, and CFIs all make it out to be that this principal is what generates lift.

Bernoulli works as far as pinching the water hose creates lower pressure but this is not enough to suck a plane into the air on its own but we are taught that it is.

Newton, I had always thought of as what MAKG called flat plate reflection or whatever. This made sense to me as far as generating lift because look at a kite. That is totallt equal and opposite correct?

I thought this is what was really keeping a plane airborne.

Then thinking of the stalls as a total loss of lift made that seem incorrect.

I am getting now that a stall is not close to a total loss of lift, Bernoulli and Newton work together and that ping pong ball in the wind is part of it too.

I do think "equal and opposite" has more to do with lift than any of the others but the others play a role too.
 
There is a big misunderstanding here. It's a common one about Newton's 3rd Law.

It's actually useless for understanding lift. What it really says is that the force of air on the airfoil is equal to the force of the airfoil on the air. While that's true, it's not very helpful. It does not mean it's downward (in fact, it rarely is -- you can't separate lift from drag) and it does not mean it's under the wing. It certainly doesn't mean the relative wind bounces off the airfoil -- if that were the case, it would rather obviouly follow that the highest lift would always occur at an AoA of 45 deg. The actual number is closer to 17 deg for common airfoils.

Now, you can solve Newton's 2nd Law on each little parcel of air and get a lot further. Whether you want to call that the Navier-Stokes equation or Bernoulli equation or Euler equations is a function of how you want to describe the flow, largely whether you describe dynamics on a parcel as you follow it through the flow (Bernoulli) or in a fixed frame of reference (Euler). They are physically equivalent.
 
Last edited:
SixPapaCharlie said:
Yes. I know Bernoulli is about restriction creates lower pressure but John and Martha, Jeppesen, and CFIs all make it out to be that this principal is what generates lift.

Bernoulli works as far as pinching the water hose creates lower pressure but this is not enough to suck a plane into the air on its own but we are taught that it is.
Click to expand...

First problem. Forget about "restrictions" - that's just nonsense.

Bernoulli's equation relates a change in velocity to a change in pressure. It works well to model the pressure changes above and below the wing due to circulation. And, those pressure differences integrated over the wing will equal lift. All of it.

Newton, I had always thought of as what MAKG called flat plate reflection or whatever. This made sense to me as far as generating lift because look at a kite. That is totallt equal and opposite correct?
Click to expand...

Second problem. There is no "reflection". No air bouncing off the bottom of the wing like billiard balls bouncing off the edge of the table. Air flows along both surfaces - upper and lower.

To generate the lift, there is a net downward acceleration of the air above and below the wing. F=M*A.

Then thinking of the stalls as a total loss of lift made that seem incorrect.
Click to expand...
There is not a total loss of lift - you just passed the peak in the lift as a function of angle of attack curve and as angle of attack is increased lift decreases instead of increasing.

I am getting now that a stall is not close to a total loss of lift, Bernoulli and Newton work together and that ping pong ball in the wind is part of it too.
Click to expand...
The real life Bernoulli and Newton are the same thing - just re-arranged a bit. You can use either equation to model 100% of the lift 100% of the time.
 
Wow, read that. Now fascinated by the paddlewing aircraft.

crazy

 
RoscoeT said:
Pulling at a high rate of speed DOES increase lift, right up to the point of stall. Lift is greatest at max AOA.
Click to expand...
That's what the definition of the critical AOA is. It is the point where further increase in AOA does not generate an increase in lift.
Of course there is something going on with the top layer over the wing - it becomes turbulent during a stall, and greatly reduces the amount of air that is washing downward for that Newtonian reaction. Stalled wings still create quite a bit of lift, just not enough to sustain normal flight. Despite what some think (and have argued here), stalled wings do not cause the airplane to "fall like a rock". If you don't think stalled wings create lift, analyze the difference in the descent profile if the wings come completely OFF. ;)
Click to expand...
Precisely. In fact, the AOA vs. Lift curve is pretty symmetrical around the critical angle. Lift does precipitously vanish at the stall point.
 
I always liked the simplification to:

"Newton provides the bulk of lift; Bernoulli just makes it more efficient. "


This can help in the discussion of how symmetrical airfoils and barn doors can still create lift, but why we don't see them often on airplanes.
 
Airplanes fly by pushing air downwards relative to the direction of flight. What we call 'lift' is calculated by multiplying the mass of the downward directed air times the acceleration of the downward flowing air mass. F=MA.

Bernoulli's equation is derived from F=MA.

That's taught in Aero engineering 101 during the first week.
 
The real fun is calculating the mass of the "downward directed air."

It's one of those statements that is literally true, but next to impossible to apply correctly.

Calculating pressures is physically equivalent, but then you only need to know them on the surface of the aircraft (and maybe the ground), not everywhere in space.
 
MAKG1 said:
The real fun is calculating the mass of the "downward directed air."

It's one of those statements that is literally true, but next to impossible to apply correctly.
...
Click to expand...

Not at all. Dr Bernoulli derived his equation from Dr. Newton's handy laws of motion.

If you are a pilot and your name is 'Mr Data', perhaps thinking like Bernoulli would be helpful.

If you are mere 100% organic human pilot, thinking of your airplane's path through the sky in terms of Newton's laws is way more likely to keep you out of trouble!!!!
 
ElPaso Pilot said:
I always liked the simplification to:

"Newton provides the bulk of lift; Bernoulli just makes it more efficient. "


This can help in the discussion of how symmetrical airfoils and barn doors can still create lift, but why we don't see them often on airplanes.
Click to expand...

I just watched a Sporty's video on the Introduction to Flight and it said the opposite.

There is only one source of lift........... When you talk about Bernoulli and Newton it makes you think in terms of two sources. That is misleading. Bernoulli and Newton explain the lifting force in different ways but they are both dealing with the movement of air over the wing. It is the only lifting force there is.
 
Last edited:
Cruiser said:
I just watched a Sporty's video on the Introduction to Flight and it said the opposite.

There is only one source of lift........... When you talk about Bernoulli and Newton it makes you think in terms of two sources. That is misleading. Bernoulli and Newton explain the lifting force in different ways but they are both dealing with the movement of air over the wing. It is the only lifting force there is.
Click to expand...

That's a 2D way of looking at a 3D problem. It's half true to say it's just about air movement.

The relative air movement over the airfoil produces pressure differentials in combination with Newton's 3rd law of motion producing the full coefficient of lift.

Fun to watch people thrash over this subject.
 
Last edited:
JimNtexas said:
Not at all. Dr Bernoulli derived his equation from Dr. Newton's handy laws of motion.

If you are a pilot and your name is 'Mr Data', perhaps thinking like Bernoulli would be helpful.

If you are mere 100% organic human pilot, thinking of your airplane's path through the sky in terms of Newton's laws is way more likely to keep you out of trouble!!!!
Click to expand...

OK, how much mass of air do you displace in level flight at cruise, when loaded to max gross, on a standard day at 3000, far from the ground?

Hint: it's not equal to the mass of the aircraft for a heavier than air aircraft.

Hint #2: How far do you have to move the air to make it be "displaced?"

As a pilot, I find it a whole lot easier to think about energy, velocity in three dimensions, and angle of attack than forces.
 
Cruiser said:
I just watched a Sporty's video on the Introduction to Flight and it said the opposite.

There is only one source of lift........... When you talk about Bernoulli and Newton it makes you think in terms of two sources. That is misleading. Bernoulli and Newton explain the lifting force in different ways but they are both dealing with the movement of air over the wing. It is the only lifting force there is.
Click to expand...

But that begs the question of 'where does the lift come from'? How does 'air moving over the wing' create an upward force???

There is one and only one way (until anti-grav is perfected): The wing accelerates a mass of air downward relative to the flight path of the airplane.

This downward acceleration of the mass of air creates an equal and opposite force that acts upward relative to the deflected air mass's downward vector.

The force of lift is therefore equal to the mass of the downward deflected air times the acceleration of that air mass. Which is known in the trade as 'Newton's handy second law'.

"But wait!" I heard you cry. "What about Bernoulli?". Well, I'm glad you asked.

Dr Bernoulli wanted a way to predict the pressure of a fluid as it flows along a path. Do that that he derived his handy equation directly from Newton's Second Law.

Here is how he figured this out: At the age of 5 he was riding in his parent's horse drawn carriage (horseless carriages not being available in 1705AD) when he stuck his hand out of the window. He noticed that when held his palm parallel to the road surface there was no force on his arm, but if he tilted his palm he felt a force imparted on his hand by the relative wind.

He considered briefly that air particles have some inner desire to not be separated from the particles near them and so they would rush around his hand to join their brothers, but even at five years old he knew that was ridiculous.

He wondered what was causing this force.

As he got older he learned about Newton's laws and the concept of conservation of mechanical energy. By combining these two concepts with this fancy new tool called 'calculus' he was able to derive an equation that showed that when fluid flowing in a steady state is deflected from it's path a pressure differential will occur. This pressure occurs because the total energy of the system an object deflecting a fluid flow can't change, so the pressure differential turns out to be exactly equal to the mass of the deflected fluid times the acceleration of the fluid.

He got even fancier, expanding his equation to include not only incompressible fluids but compressional fluids as well.

If you are a genius, or at least a person who really groks calculus, then think of Dr. Bernoulli controlling your lift.

But if you are an ordinary Mark I human pilot, it's better to try an visualize a mass of air being deflected downward from your forward velocity vector.

Fly fast enough and you'll need to starting thinking about Dr Bernoulli's work with compressible flows, but we bug smasher pilots don't usually go there.

What we need to keep in mind that if if your airplane stops deflecting air downward then you and your airplane will fall from the sky!

That's a usually a bad thing.
 
Last edited:
Umm, Jim, you've confused the Bernoulli equation with the Navier-Stokes equation.

The Bernoulli equation is about energy. It says that you can increase the kinetic energy (flow speed), at the expense of potential energy (pressure), along a streamline. Still closely related to Newton's laws, but it's the first law, not so much the second (though they don't separate as well as one might like).

Still waiting on your estimate of total air mass displaced in cruise.
 
Last edited:
MAKG1 said:
OK, how much mass of air do you displace in level flight at cruise, when loaded to max gross, on a standard day at 3000, far from the ground?

Let's derive that:

In level flight Lift = weight of the airplane = mass of the airplane x acceleration of gravity.

Your lift is exactly equal to the mass of the air that is being deflected downward by your lifting surfaces.



Hint #2: How far do you have to move the air to make it be "displaced?"

That's sort of irrelevant, you care about how much acceleration you impose on the air mass , not how far it moves.

In level flight you are accelerating downward a mass of air equal to the mass of your airplane at 9.8 m/sec**2.

If for some bizarre reason you want to know how far the mass of air will displace then you need to figure the drag of the stationary air on the moving air and integrate that over time until the acceleration stops. I'll leave that as an exercise for the student.
Click to expand...
 
MAKG1 said:
Umm, Jim, you've confused the Bernoulli equation with the Navier-Stokes equation.

The Bernoulli equation is about energy. It says that you can increase the kinetic energy (flow speed), at the expense of potential energy (pressure), along a streamline. Still closely related to Newton's laws, but it's the first law, not so much the second (though they don't separate as well as one might like).

Still waiting on your estimate of total air mass displaced in cruise.
Click to expand...

Bernoulli's principle can also be derived directly from Newton's 2nd law.
Click to expand...

Wikipedia and the University of Texas Aero Engineering 101 course.

You have to assume that no energy is created or destroyed, which of I course I've already explained.
 
JimNtexas said:
Your lift is exactly equal to the mass of the air that is being deflected downward by your lifting surfaces.
Click to expand...
NO. Only if you fly an airship. And then it doesn't matter which way you "deflect" it.

JimNtexas said:
In level flight you are accelerating downward a mass of air equal to the mass of your airplane at 9.8 m/sec**2.
Click to expand...

You have confused mass and forces.

Forces are in balance in level flight. Masses are not.

The answer is that an isolated airfoil deflects air everywhere in space (well, excepting the stagnation lines, but those are sets of measure zero), just not by very much. That means the total mass is infinite. Of course the total force is not. You can use much lower mass to fly an airplane if you fling it harder.

As another counterexample to mass vs. force equivalence, a rocket would be unable to fly if they were equivalent, as it can never "deflect" more mass than it weighs. Rockets always carry every molecule they need to fly. This stuff is much more obvious in the context of thrust.
 
Last edited:
Bernoulli 24, Newton 21

Newton took an early lead and held it all through most of the game. With seconds left in the ball game, Newton has the ball on the 10 yard line, 1st and goal and was threatening to score again. Newton tries a short pass to the weak side when Bernoulli intercepts the ball on the 5 yard line and runs it 95 yards, crossing the goal line with only 1 second left. The extra point is good, Bernoulli makes a miraculous come from behind win.!!!

After the game, the coach for Newton was heard to say, "It was a tough day, we just couldn't get the extra lift we needed."
 
Late Breaking News from the Newton vs Bernoulli game........

Word has leaked out that the Bernoulli player that intercepted the ball and ran it back for the winning touchdown was actually a Newton player that had secretly joined the team.
The Newton coach had him benched for several games and the frustrated player wanted into the game so bad he switched uniforms to get on the field.
It looks like this player may have a bright future with the Bernoulli team!!
 
I think it sucks they try to make ANY reference to a physics based explanation of lift. As a pilot I need qualitative, not quantitative.

All most (nearly all? All?) pilots need is a model of lift that will allow them to predict what the airplane will do and stay outta trouble. "Stick and Rudder" is a great book....
 
Cruiser said:
... Bernoulli and Newton explain the lifting force in different ways but they are both dealing with the movement of air over the wing. It is the only lifting force there is.
Click to expand...


A flying barn door just might disagree with you. :yes:
 
MAKG1 said:
NO. Only if you fly an airship. And then it doesn't matter which way you "deflect" it.



You have confused mass and forces.

Forces are in balance in level flight. Masses are not.
Click to expand...

So in your world force doesn't equal mass times acceleration? It must be hard to maintain level flight in that universe.
 
JimNtexas said:
So in your world force doesn't equal mass times acceleration? It must be hard to maintain level flight in that universe.
Click to expand...

Nope. In my world, there are accelerations other than g sometimes.

It's not enough to just memorize formulas.

F=ma does work, but you're not understanding what any of those factors mean in this context.
 
:popcorn: Air must be bent (accelerated) to produce lift. Regardless of top or bottom.
 
You must log in or register to reply here.
Back
Top