Bernoulli.....or...Newton.......what makes lift???

Bernoulli or Newton??

Is Bernoulli getting way more credit for why a wing lifts than he deserves??

Is the 'reaction theory' (Newton) far less credited than it should be??? I think so.

What do you all think??

As a CFII for several thousand hours, I had to teach Bernoulli. I also, in ground school and in flight, talked about the effect of sticking your hand out the window, tilting the palm a little, and see what happens. I did teach a little Newton. I understand that in England, they barely recognize the Bernnoulli effect, but don't know this first hand.

Does the negative pressure suck the covering up off the top of your Cub wings? Mine, neither. Now I'm sure some physics or aero-engineering pro is going to explain laminar flow and all that. But just tell me one thing: Why do the molecules that go on top of the wing HAVE to go faster so they can join back up with the molecues that were just below them before the split. What do they care, and how do they know, is what I have always wanted to know. I don't think they really do join back up. Of course, this would not be the first time I have been wrong, but I think Newton and care with angle of attack has kept me up for a long time.

In a closed pipe, it is understandable. As an engineering student, I studied all these things, and passed "dams and weirs" which a lot of civil engineering students failed the first time, but I have always believed that open systems like aircraft wings follow different physics than closed containers and pipe systems.

Well, you asked. Now stand by for education!

Chuck Shaw
J-3 N26818

They're the same . . .

They are two different approaches that, when properly applied, lead to the exact same answers. For certain problems one is easier than the other to apply, but they both are "correct".

Hank

Oh. . . one more thing . . . Mother Nature makes the lift . . . these are just techniques man has developed to compute it.

What I say is, "Mr. Newton and Mr. Bernoulli are waiting in heaven for you to keep pulling back on the stick over those power wires. I would personally not like to join you in meeting them just yet so PLEASE PUSH OVER THE NOSE AND GIVE US SOME POWER!"

sj

Of course the air that flows over the top of the wing joins back up with the air under the wing at the trailing edge. If that didn't happen you'd soon be pushing tons of air along in front of you.

The Bernoulli effect does indeed try to "suck" the fabric off of the top of your wing; that's what the rib stitching is for.

Which one makes your Cub fly? They both do. The fact is that they are one and the same; just two different ways to explain the same phenomenon.

...Clyde

Bernoulli on top, Newton on bottom? One of my friends saw a fabric wing from the top when in flight, and swore he'd never get in a fabric airplane again!

Bernoulli was right - the lowly gas cap can tell you that; try a vented cap on an otherwise unvented wing tank. I heard the number "three quarters" for Bernoulli on an arbitrary wing.

I'm going to and have ruffled some feathers with this....but rather than state the many forces actually creating lift on any rigid body:

http://www.lerc.nasa.gov/WWW/K-12/airplane/bernnew.html

If you happen to be unlucky enough to take a fluid mechanics course or be an aerodynamics student in the past 10 years, this is what you learned.

Yes, bernouili's is a component of lift. Yes, Newton's third law applies. The kicker is that most people incorrectly model an airfoil as a half-venturi when it's not (called the equal transit time theory). They also make the action-reaction assumption and come up with the skipping stone theory. Basically...it's the right physics but just the wrong conclusion.

It comes down to a rigid body turning flow. Any rigid body that is impacted with a fluid that has mass will have a force imparted on it. That force acts over an area and you get a pressure. Problem is, is that people point to that and say "see, it's ________(insert favorite physicist)." Again, wrong assumption because there are also resulting forces on the top of the wing too. Modeling an airfoil using the bernoulli venturi is incorrect because we know from testing that the flow over a wing actually gets to the trailing edge first...not at the same time. This by the way contributes a small amount of trailing edge thrust.

Force resulting from turning flow resulting in a net positive force, that's all it takes. This is the reason that symmetrical airfoils, barn doors and golf balls can fly. We see even more examples in aviation....this explains why B-52's can climb with thier nose down and at negative angles of attack.

Some more interesting NASA links explaining the effect of turning flow and why all sorts of non-airfoil like objects acheive lift:

http://www.lerc.nasa.gov/WWW/K-12/airplane/factors.html

http://www.lerc.nasa.gov/WWW/K-12/airplane/beach.html

Sure the air flows over the top of the wing and doesn't pile up until you get too high an angle of attack, but I am not so sure that the molecules on top get to the trailing edge exactly at the same time as the ones just below them at the time they are split apart by the flow at the leading edge, thats all. As for Newton's theory of equal and opposite reaction being the same thing as Bernoulli's theory of decreased pressure with increased velocity of gasses, just a different way of exlaining it, well, not quite. But yes, they both must contribute to "lift", which has sure been defined a lot of ways over the years.

This is kinda like the discussions about what makes an airplane climb, or how to adjust your rate of descent on the glidescope. They can be endless and somewhat pointless. As someone on here says, something like "keep your speed up lest the ground come up and smite thee". That is the real word.
As soon as the fog lifts (see link) lets just go fly low and sort of slow, at 1,300 feet AGL on the GPS of course, and enjoy life.
http://www.gwcmodela.com/8168j3fog.jpg
Chuck

Thanks N4653B

Thanks, N4653B for the link that really comes close to answering the question at hand, and for verifiying that the transit velocity on top of the wing is not such that the molecules join their buddies at the trailing edge which, is in so many books. Good post!

Chuck Shaw
N26818

Dave Calkins said:

Bernoulli or Newton??

Is Bernoulli getting way more credit for why a wing lifts than he deserves??

Is the 'reaction theory' (Newton) far less credited than it should be??? I think so.

What do you all think??

Click to expand...

I consider it an act of faith; Mr. Piper, Mr. Cessna, Mr. Taylor, and their pals give us lift; Those old-time guys may have thought up the theory, but it was a couple of bicycle mechanics that allow us to rub shoulders with the birds, not them.

.....must...resist....must..not...get..involved...in bernoulii vs newton debate......Arrrggggghhhh...I am weak.

OK, here's the deal:

I don't mean to insult anyone here, but the Bernoulli vs Newton debate is for folks who don't understand the underlying physics.

Bernoulli's principle in absolutely no way, no matter how insignificant, contradicts or is negated by Newtons laws of motion, if fact they are inextricably intertwined in the production of lift.


Yes, the air flows faster over the top of the wing

Yes, as per Bernoulli this results the pressure on top of the wing is less than the pressure on the bottom.

That is lift.

Now, along come the "newton" proponents who say, "but wait the air is accelerated downward, that is where the lift really comes from, it's created by Newton"

well, of course air is accelerated downward, it can't happen any other way if a force is created on the wing, whether it is created by Bernoulii's principle, Coanda Effect or little green bugs, air has to be accelrated downward.

Newton's laws are not creating the force, all Newton's laws are saying are tha if there is a force on the wing (no matter how it is generated) then there is also an equal and opposite force on something else. In this case theonly something else is the air, so that force is exerted on the air. Another one of newton's laws say that if you apply a force to something which is free to move (and the air is) it will accelerate, so in accordance with newton's laws, the air is accelerated downward by a force resulting in part from bernoulii's principal.

Another type of "lift is newtonian" proponent insist that lift is mostly created by air being deflected downward off the bottom of the wing. This might be true if the air flowing aground a wing behaved exacly like a stream of water from a garden hose bouncing off a piece of plywood. The thing is, airflow around an airfoil is not like that. Air doesn't bounce off the bottom of a wing, in order to grasp what's going on, you have to look at the total airflow around the wing, and understand that the airflow is affected by the airfoil well ahead of the leading edge and well above and below the airfoil. It's not a discrete stream, rebounding off a flat surface. FOr those who favor this view, who claim that lift is created by air deflected downward by the bottom of the wing, especially at high angles of attack: how do you explain a stall? Say a wing has a critical AOA of 14 degrees. At 13.5 degrees AOA, the wing is producing a lot of lift, yet at 14.5 degrees AOA, the lift drops to a small fraction. Why? You should still have a lot of air "bouncing off the bottom of the wing" right? and the angle isn't that differnet, only a degree, so that the trignometry is about hte same, so why does the lift drop to very little? This leads us inevitabley to the conclusion that the flow over the top of the wing has much more to do with lift that "air bouncing off the bottom"

To say that lift is created by one or the other is meaningless. A good analogy would be to argue over whether a car is moved by combustion or friction.

Does the negative pressure suck the covering up off the top of your Cub wings? Mine, neither.

Click to expand...

Yeah, it doesn't because the covering is fastened to the ribs. Why do you think so much work goes into rib stitcining?

But just tell me one thing: Why do the molecules that go on top of the wing HAVE to go faster so they can join back up with the molecues that were just below them before the split. What do they care, and how do they know, is what I have always wanted to know. I don't think they really do join back up.

Click to expand...

You're right, they don't join back up. The thing is NOBODY says they do! Ok, that's wrong. In all my reading on aerodynamics, (and it's been a whole bunch) I have found exactly one source which says the molecules join back up the trailing edge. The FAA's flight training handbook. It is wrong. They do not join up. This has been known as long as there have been wind tunnels. However, the airflow on hte top of the wing is faster than the airflow on the bottom, this is an absloute fact. This also has been know as long as there have been wind tunnels.

I don't think what is traditionally taught in ground school has anything to do with giving credit to Bernoulli or Newton.

Bernoulli's theory was the easiest to reduce into a simplistic easy to understand, and plausible explanation for lift. This was done so that people with IQ's slightly above the 3 digit floor could pass ground school.

Lucky me.

Jerry

Bernoulli or Newton? Does it really matter?

What makes chocolate so tasty?

Why do we enjoy flying so much?

Why are generic baked beans grey?

There are many questions of life the universe and snakes, only makes your head hurt after a while. I subscribe to the Homer (Simpsons not the greek) school of life, chocolate.......ohhhhhhh, Aircraft........ohhhhh. Just enjoy for enjoyments sake. Now if I can just figure out the quantum physics answer to why dwarf planets didn't reach the size of normal planets I should get some sleep tonight.

It took me years, but I finally figured it out....$$$$$$$$$$$$$$ makes lift!!

All these years I thought it was magic.

Better give Newton more credit.

Your spinning wing, the propellor, puts out a lot of thrust by deflecting large volumes of air. The "airfoil" design of the prop is for drag efficiency, not to gain "accelerated air" thrust. If you had enough power, bolting an angled 2X6 on the prop hub would work.

Don't try to convince the symetrical airfoil flyers that the "Newton theory" isn't valid.

Anyone who spends a bit of time watching a highly loaded transport wing near the ground (at a high AOA), especially in a cool, high humidity environment, will vividly see what the wing is doing. It is deflecting an enormous volume of air, creating the lift to support the aircraft's weight. The engines are merely pulling the wing (in this case, in a high drag configuration) through the air. Same aerodynamics applies to a Cub wing, only not as obvious. One advantage the Cub has, in a three point attitude takeoff, the engine thrust line provides a vertical component of lift, aiding the wing in lifting the aircraft off the ground.

fobjob said:

It took me years, but I finally figured it out....$$$$$$$$$$$$$$ makes lift!!

Click to expand...

No $$$$$$$$$$$$$$$ makes speed, lift is free

I agree that Newton 3rd law (published in 1687) does not get enough credit. The US teaching materials for flight seem to promote Bernoulli (published around 1738) almost exclusively.

As to which force is stronger. It depends on the speed and/or angle of the airflow.

Lets jump into our time machine and take a simple Dutch windmill in 1600 AD.

We modify it and make perfect computer designed wings for the blades.
And then we turn them into the wind like a feathered propeller without any angle. Relying on Bernoulli alone.

Yes the blades will rotate on a very windy day. But they won't have much power and they probably will not pump the water out of flooded low-lands. So we are grabbed up by the locals and burned as witches...

The locals, after and night of celebrating our witch burning, decided to replace our modern Bernoulli blades with flat windmill blades made of wood slats covered in cloth. No airfoil at all. Then they angle the blades until they start to turn in the available wind. The windmill produces power and the country is saved from flooding.

Newton works as a basic yet strong force. But lift alone does not make CONTROLLED FLIGHT.

For an interesting explanation of Bernoulli flaws and false assumptions go here.
http://jef.raskincenter.org/published/coanda_effect.html


Leonardo da Vinci who really worked on aero-dynamics in 1500, should get a lot more credit.



xx

What goes around comes around, and that provides lift.

Circulation ( you can see it in the wing tip vortex), and best defined by approximate soultions to the Navier-Stokes equations.

GR

I have this book. http://www.amazon.com/Stop-Abusing-...ef=sr_1_1/104-0669481-4998301?ie=UTF8&s=books If you really want to get into the details of lift.

HydroCub said:

The "airfoil" design of the prop is for drag efficiency, not to gain "accelerated air" thrust.

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Better write to the prop manufacturers and let them know. They all, without exception, think that the airfoil section of a prop is very important, and have spent considerable effort researching how to chose exactly the best airfoil shape. If minimum drag was all that was sought, they would chose some other cross section other than an classical airfoil shape, as that is not the shape that gives the lowest drag. If what you say is true, don't you think that the propeller industry would have discovered this after almost a century of concentrated research?

HydroCub said:

Don't try to convince the symetrical airfoil flyers that the "Newton theory" isn't valid.

Click to expand...

This argument is always quite appealing on the surface, beause it seems that the lift "must" be created by air "bouncing off the bottom of the wing" because the distance is the same whether you go over or under the wing, so it can't experience any velocity difference. However the argument completely breaks apart when you examing how air really flows around an symetrical airfoil. When air is flowing around a symetrical airfoil with an angle of attack, the air going "over the top" in fact does travel a longer distance than the air under the bottom, This is because the stagnation point, the point where the airflow seperates and either goes over or under the airfoil is not at the "nose" of the airfoil, but rather well below that point. How far depends on the angle of attack. This can be seen very clearly in a wind tunnel photo of a symetrical airfoil at an angle of attack with smoke streamers inthe airflow. Once you see a photo like this, it is very obvious that airflow around a symetrical foil with an angle of atack is identical to airflow around an asymmetrical airfoil, and the "symmetrical airfoils disprove bernoulli" argument is very obviously a non-starter. [/code]

Here is how I teach my 7th grade technology students. They build a balsa – rubber band powered plane. The wing is flat balsa, 16” X 3”. The outboard 3” are bent up at about 15 degrees. The leading edge of the wing is shimmed with a 1/8” thick piece of balsa. They learn to set the CG and fly the plane with different CG configurations to correct for stall. With the shim, the plane flies great, without it will not fly. The reaction of the air being pushed down at the trailing edge is force up as per Newton. I know this is simplified, but it works. I teach them about Bernoulli with my wind tunnel and a small airfoil shaped wing section.

Marty

Dave,

You been getting into the joy juice a little much?? Or are you starting to look at professorships at local colleges?

Good discussions, and great reminders that there really still is a little bit of magic in all this.

MTV

Alex Clark said:

Lets jump into our time machine and take a simple Dutch windmill in 1600 AD.

We modify it and make perfect computer designed wings for the blades.
And then we turn them into the wind like a feathered propeller without any angle. Relying on Bernoulli alone.

Yes the blades will rotate on a very windy day. But they won't have much power and they probably will not pump the water out of flooded low-lands. So we are grabbed up by the locals and burned as witches...

The locals, after and night of celebrating our witch burning, decided to replace our modern Bernoulli blades with flat windmill blades made of wood slats covered in cloth. No airfoil at all. Then they angle the blades until they start to turn in the available wind. The windmill produces power and the country is saved from flooding.

Click to expand...

While entertaining, your anecdotal comparison really doesn't shed any light on the subject, because it is not a valid comparison. You're comparing the performance of an airfoil turbine at it's least effective position (feathered) to a windmill at it's most favorable configuration.

Of course a featherd prop doesn't turn much, that's why we feather them, so it doesn't turn.


How about we make some sort of a reasonable comparison? let's compare an airfoil "windmill" with it's foil at an optimum angle to an old fashioned Dutch style wind mill with it's sails at an optimum configuration. I think that you'll find that the windmill with the airfoil shaped blades is far more effectiuve in getting power from the wind than the old style windmill.

Now *that* would be a valid and relevant comparison, and it completely contradicts what you are claiming.

Have you noticed that all modern wind turbines have long slender airfoils, much like glider wings, or propellers? Has it ever occured to you to wonder, why, if the force comes from air bouncing off the flat part of the blade, why the wind turbine manufacturers go to all the trouble to design and build carefully calculated airfoil blades when a couple of pieces of plywood, held at the proper angle would be more effective? It's just air, bouncing off the surface, right? so all that counts is getting enough surface area, at the right angle...right? more surface area, more power, big flat surfaces should be best.

But then you look at modern wind turbines and you see that the blades have relatively little surface area.

Maybe there's more to it than air bouncing off a flat surface?

contradicts what you are claiming

Click to expand...

Not really. I intensionally never made any claims in either direction other than Leonardo exploring the subject well before either Newton or Bernoulli, for the actual purposes of flight. And that Newton tends not to get much credit at all in US text.

If you figure the percentage of thrust added to the windmill by the addition of the airfoil blades, then you would be able to figure the percentages.

There is NO DOUBT that the Bernoulli principle creates lift. As does basic impact friction in the case of the windmills. Nor is there any doubt that an airfoil aircraft propeller is more efficient than a bathroom fan blade.

Which adds the greater percentage thrust in this case? The shape of the blades (wings) or the angle of attack?

That is a question, not a conclusion....


xx

How about this question for the aeronautical engineer types. What is more efficient in terms of lift/drag - an airfoil or flat surface? And, as a corelary question, why not an airfoil on the tail of the cub?

Alex Clark said:

Not really. I intensionally never made any claims in either direction

There is NO DOUBT that the Bernoulli principle creates lift. As does basic impact friction in the case of the windmills. Nor is there any doubt that an airfoil aircraft propeller is more efficient than a bathroom fan blade.

Click to expand...

OK, I misunderstood your position then. It seemed that you were taking the same stance as Hydrocub, that a windmill or prop is just flat surfaces deflecting air.

Alex Clark said:

Which adds the greater percentage thrust in this case? The shape of the blades (wings) or the angle of attack?

Click to expand...

Well, while it is tempting to make this distinction, I don't think that it is a valid distinction. I don't beleive you can legitimately seperate these. Even a piece of plywood, when examined in a wind tunnel has a much more complex circulation pattern than one might expect. The stagnation point, for significant angles of attack, is not at the edge of the pywood, which is counterintuitive. and the airflow is affected for a significant distance around hte plywood, just as it is for any airfoil. So I don't think you can legitimately determine how much of the force is "air deflected off the bottom" as that doesn't accurately describe the airflow.

This view is probably more authoritatively stated by H.C. Smith former professor of aerospace engineering at Penn State University in his "Illustrated Guide to Aerodynamics" : "It would be difficult to seperate the effects of the top and bottom surfaces because the pressure distribution on both surfaces depends on the overall shape of the airfoil"

Broknbone said:

How about this question for the aeronautical engineer types. What is more efficient in terms of lift/drag - an airfoil or flat surface?

Click to expand...

Airfoil

Broknbone said:

And, as a corelary question, why not an airfoil on the tail of the cub?

Click to expand...

Because flat surfaces are cheaper to manufacture and the cub was not optimizd for speed (I know this comes as a shock to many)

Alex Clark said:

contradicts what you are claiming

Click to expand...

Not really. I intensionally never made any claims in either direction other than Leonardo exploring the subject well before either Newton or Bernoulli, for the actual purposes of flight. And that Newton tends not to get much credit at all in US text.

Click to expand...

Actually, Bernoulli and Newton never made any arguments for or tried to explain aerodynamic lift. They were just trying to account for the obvious energy (Bernoulli) and momentum (Newton) that a moving gas must have.

It wasn't until later when Euler and Navier-Stokes (independently) really set about combining what makes lift. It's not just energy and momentum, it's mass too. Euler was the first to describe the three in concert and then Navier-stokes take it a step further and include viscocity.

Lastly, I'm not sure what texts you are refering to in your post. Just about any serious aerodynamics text on the subject published in the last 25 years accounts for all of the physics behind it.

Again, like I posted earlier....forget who the physicist was and just accept the fact that lift occurs due to the net force resulting from changing the velocity of flow. This isn't just my opinion...this is what the entire aerodynamics industry has concluded. Talk to an aerodynamicist at Lockheed, NASA, and any research laboratory and they will tell you the same thing. Go to the NASA links I've provided above, they have interactive simulators that show why lift is generated by turning flow.

The important fact is that a force causes a change in velocity; and likewise, a change in velocity generates a force. A velocity has both a magnitude, the speed, and a direction associated with it. So, to change either the speed or the direction of a flow, you must impose a force. And if either the speed or the direction of a flow is changed, a force is generated. We know this due to the second law, F=m*(dv/dt).

Stick your hand out of the car window.... arm forward and palm down and angle your hand at 10 degrees up. Now, accelerate from zero to sixty mph and you will "feel" how much lift that deflected air can produce from a very imperfect airfoil. I would suggest that the vertical force your arm experiences is at least 90% caused by the Newton effect.

How about the thin blades of a turbine fan (a ducted rotating propellor)? They are in essence flat plates curved for their optimum rotation velocity and design horizontal airspeed. Stand behind one at idle and you will feel the deflected air of an engine capable of producing an enormous amount of thrust by simply increasing its rotational velocity to deflect more air.

Another factor to consider. Lift is dramatically affected with relatively small pitch control imputs that change the wing's AOA, especially at higher airspeeds. My simple mind tells me that there is little change in the upper air acceleration versus the huge amount of deflected air increase/decrease from the AOA change, thus the very positive control of lift by varying AOA with the same wing design.

Another thought.... there are numerous airfoils that will not fly without an AOA established. Yes, any AOA on even a symetrical airfoil will cause the "over the top accelerated air" lift... but it's the deflected air under the airfoil that is causing the majority of the lift to the airfoil. It's all about the mass of air deflected and to achieve that mostly relates to how AOA and airspeed interact.

Follow a "heavy jet" wing in the sky and it's not unusual to "feel" it's turbulence over 100' below it's flight path. Keeping 500k of weight in the air deflects a lot of air.

Lastly, there's a local guy who delights crowds flying his RC lawnmower. Next time I see him, I'll ask him to tell me his airfoil secret. My guess is that he's a Newton fan. As I recall, he once told a crowd he could bolt an engine to a piece of plywood and fly it through the air if he would take the trouble to figure out how to control it.