Bernoulli, yes or no?

[dmspilot]

Explain how the air buddies meeting at the trailing edge is a myth

Smoke tunnel. The air on top beats the air on the bottom. So the Bernoulli effect is actually stronger than it would be if you used the false "equal transit time" hypothesis.

 

[Bill]

If I was about 50 years older, I might consider it.

Nah, actually you wouldn’t.

 

[Capt. Geoffrey Thorpe]

Oh, and Daniel Bernoulli (along with the other famous Bernoulli's as well as Leonhard Euler who was a student of Daniel't father Johann and is, according to some sources, the person who first derived Bernoulli's equation) were Swiss. So, the fourth option should be about Swiss Cheese which has holes in it that the air can blow through.

 

[DoubleD]

All I know is if I stick my hand out of the car at 60 mph, it's pushed up, not sucked up.

 

[eman1200]

I'll say it for @Kritchlow ..................boobs!!!!

 

[SoCal RV Flyer]

All I know is if I stick my hand out of the car at 60 mph, it's pushed up, not sucked up.

True, but your hand is not airfoil-shaped (or at least most people's aren't ). If it were, it'd be a combination of Bernoulli lift and that "push."

Given proper angle of attack and enough power, you can get a flat board to fly. Lots of Depron foam rc planes out there for proof.

 

[Pilawt]

In flight the fabric on the underside of a Cub’s wing is pushed in between the ribs; the fabric on the upperside is billowed out. Neither Newton nor Bernoulli invented lift; both have their place in describing it.

The Newtonian concept is a lot easier for a pilot to visualize while flying, and is an effective teaching tool (e.g., Langewiesche’s Stick & Rudder). Trying to fly while thinking about the layman’s usual goofy “illustration” of the Bernoulli principle (flat lower camber parallel with relative wind, racing molecules, no downwash aft of the wing) just muddles things up.

Even the academic version doesn’t help much. One of my primary students had a tougher time than most getting the feel of the airplane. He was an aeronautical engineer at McDonnell Douglas across the field and knew the science forward and backward. His problem was he was trying to fly the book instead of the airplane.

There’s a neat little free iOS app called “Wind Tunnel” that animates airflow and changes of pressure around an airfoil. Just looking at that makes more sense to a pilot, who is not an aerodynamicist, than does a “Newton v. Bernoulli” debate.

 

[GRG55]

Bernoulli said it, I believe it, and that settles it.

I'll match your "Bernoulli said it" and raise you one "I read it on the internet"...

 

[Capt. Geoffrey Thorpe]

Given proper angle of attack and enough power, you can get a flat board to fly. Lots of Depron foam rc planes out there for proof.

A flat board is a perfectly reasonable airfoil. Not the best airfoil, but an airfoil none the less.

 

[RyanB]

Nah, actually you wouldn’t.

Hehe! You’re right..I wouldn’t.

 

[flyingron]

NASA has a nice set of slides on lift and what is and isn't correct in explaining it.
https://www.grc.nasa.gov/www/k-12/airplane/lift1.html

 

[Cruiser]

show me a wing that will generate lift (Bernoulli) without an angle of attack (Newton)

 

[Capt. Geoffrey Thorpe]

show me a wing that will generate lift (Bernoulli) without an angle of attack (Newton)

Look at a Cessna, Piper, whatever. Airfoils with camber typically generate lift at a zero angle of attack. If you look at the venerable Clark Y you see that the lift coefficient is somewheres around .3ish at an angle of attack of zero.

 

[Checkout_my_Six]

Capt.....are you really trying to rationalize this?...with data?

 

[Capt. Geoffrey Thorpe]

An airplane wing stalls when the critical angle of attack is exceeded and airflow separates from the top of the wing. I don't see how you can explain that without the Bernoulli effect.

What do you mean by "the Bernoulli Effect"?

 

[dmspilot]

What do you mean by "the Bernoulli Effect"?

I mean the relationship between speed and pressure.

 

[Checkout_my_Six]

I mean the relationship between speed and pressure.

there are a few other variables to it ....than that.

 

[nauga]

there are a few other variables to it ....than that.

...which does not negate the relationship nor his point, which is valid. Note that the bullets under the equation you posted (after editing) even state the same relationship.

Bernoulli's equation is derived from Newton's second law (conservation of momentum) for an inviscid, incompressible fluid, a reasonable assumption for a typical GA airfoil. They are not different, they are different ways of stating the same thing given the same assumptions.

Nauga,
and curved air

 

[Clark1961]

Newton requires no assumptions. Bernoulli requires a moving fluid. Is the fluid moving or is the airfoil moving? Does it matter which is moving?

 

[Cooter]

show me a wing that will generate lift (Bernoulli) without an angle of attack (Newton)

I watched a B52 takeoff yesterday and I would almost swear he was nose low on climbout. I don’t know what his AoA was but it wasn’t much.

Blowing across the top of a straw brings the fluid up. That seems kind of Bernoulliish.

 

[Capt. Geoffrey Thorpe]

Does it matter which is moving?

Nope.

That seems kind of Bernoulliish.

It seems that way, but it kinda is and kinda isn't - you are pretty much violating many of the assumptions that are required for Bernoulli's equation to be valid. If you have the top of your straw in a venturi, then it's more valid. If you are blowing across the top in open space, you are more demonstrating that air has some viscosity and that a moving jet of air will tend to entrain and accelerate the otherwise still air around it - there's a name for that effect, but it's not popping into my head right now.

 

[Cruiser]

Look at a Cessna, Piper, whatever. Airfoils with camber typically generate lift at a zero angle of attack. If you look at the venerable Clark Y you see that the lift coefficient is somewheres around .3ish at an angle of attack of zero.

exactly. It will never get off the ground.
Certainly there is a low pressure area created above the wing. Newton does not account for that.
Is it Bernoulli? Don't think so because there is no constriction happening there.

 

[nauga]

Is the fluid moving or is the airfoil moving?

Does the airfoil cause the fluid to move or does the airfoil pass right through without disturbing the fluid?

Nauga,
undisturbed

 

[Cooter]

It seems that way, but it kinda is and kinda isn't - you are pretty much violating many of the assumptions that are required for Bernoulli's equation to be valid.

 

[Cruiser]

what Capt. said ^^^^^^^^^^^
It is called the Coanda Effect

 

[Checkout_my_Six]

It’s also known as the pitot tube equation.

 

[Pilawt]

It’s also known as the pitot tube equation.

Or the Kobayashi Maru scenario, one or the other.

 

[Capt. Geoffrey Thorpe]

Certainly there is a low pressure area created above the wing. Newton does not account for that.
Is it Bernoulli? Don't think so because there is no constriction happening there.

A wing is an ordinary sized object (larger than sub-atomic) operating at non-relativistic speeds. Newtons laws apply above, below, in front of, and behind the wing. No exceptions. People who find exceptions to Newtons laws get Nobel prizes, and no one has gotten a prize for calculating the lift of an airfoil. (OK, you could argue that people like Euiler, Navier, and Stokes did their work before the Nobel Prize was created, but...)
Bernoulli's equation does not require a constriction to be valid.

 

[Let'sgoflying!]

Is there any way to do an experiment where Newton's is completely nullified so that any residual lift could be assigned to Bernoulli? Or the reverse? Time to put neurons to it.

 

[Capt. Geoffrey Thorpe]

Is there any way to do an experiment where Newton's is completely nullified so that any residual lift could be assigned to Bernoulli? Or the reverse? Time to put neurons to it.

Given that Bernoulli's equation is just a special case of Newtons laws, that seems unlikely. But go right ahead. A Nobel prize is worth upwards of a million dollars, so it's probably worth shooting for.

 

[dmspilot]

Newtons laws apply above, below, in front of, and behind the wing. No exceptions. People who find exceptions to Newtons laws

Given that Bernoulli's equation is just a special case of Newtons laws, that seems unlikely.

You keep saying Newtons [sic] laws, plural. But the debate is not whether Newton's Laws explain lift or not. It is whether or not lift can be explained solely by his 3rd law only. Or at least that's what I thought it was about.

 

[PaulS]

 

[Cruiser]

snip....
Newtons laws apply above, below, in front of, and behind the wing. No exceptions.
.....snip
Bernoulli's equation does not require a constriction to be valid.

So you are saying we don't need Bernoulli?
Newton explains it all?

 

[GRG55]

I watched a B52 takeoff yesterday and I would almost swear he was nose low on climbout. I don’t know what his AoA was but it wasn’t much.

Wouldn't that have something to do with the wing angle of incidence?

 

[Cooter]

Wouldn't that have something to do with the wing angle of incidence?

Yes, I don’t know what it is but it looks really flat.

 

[Tarheelpilot]

Wouldn't that have something to do with the wing angle of incidence?

And flaps

 

[Brad Smith]

Psst...don't tell pilots of single-surface ultralights about Bernoulli and that they're doing it wrong. They'll never have the confidence to leave the ground again!

 

[2Airtime2]

There is hi and low pressure above and below an airfoil with air moving across it, but...the combination of angle of attack and speed determines the amount of lift and without an angle of attack there is no lift (at least not enough to get any airplane off the ground). I used to think it was simply the airfoil that created lift, then I started thinking about inverted flight and wondered why an inverted plane didn't slam into the ground.

An airfoil makes a wing more efficient at creating lift. For regular flight, at a given speed, a certain (smallish) angle of attack is required to maintain altitude. Take that same plane with same conditions flying inverted and the angle of attack has to be somewhat increased. The fuselage of most planes is level during cruise because most planes have an angle of attack (an angle between the longitudinal axis and chord line) designed in.

There is an excellent book about all this. I believe it was written in the forties but physics hasn't changed since then. It's called Stick and Rudder.

 

[jstro]

You keep saying Newtons [sic] laws, plural. But the debate is not whether Newton's Laws explain lift or not. It is whether or not lift can be explained solely by his 3rd law only. Or at least that's what I thought it was about.

I like how Capt Thorpe is explaining this whole thing. I'll add my bit. Newton's 3rd law is always valid. Newton's 3rd law is not a competing explanation, but a consistent explanation.. Of course, just focusing on Newton does not illucidate the aerodynamics which gives rise to the pressure field around the wing and the resulting momentum changes of the air, so while Newton provides a gross explanation of lift it leaves a lot of the detail out. It's not wrong, it's just a general explanation using a physical law that's always valid. Put another way -- Newton's 3rd law provides a phenomenological explanation for lift, while the Navier Stokes equations (which include Bernoulli) provide the mechanistic explanation. (Disclaimer - I have mechanical and chemical engineering degrees, but not aeronautical)

 

[Capt. Geoffrey Thorpe]

So you are saying we don't need Bernoulli?
Newton explains it all?

Define "Need".

If you want to argue only the third law of Newton applies (action reaction) that does not explain anything. Where does the force that the airplane reacts to come from and how does it get applied to the wing?

That leads us to all of Newton's laws, air has inertia (first law), so that requires a force to accelerate it downwards (second law), and lift is the reaction to the force (third law) required to accelerate the air. But, how does that force get applied?
That leads us to Bernoulli's equation which models how the pressure changes as the air is accelerated / decelerated. And it is the pressure difference between two surfaces that keep the airplane in the air.

Now, where this gets fun...

The basic Bernoulli equation assumes an inviscid (no viscosity) fluid. But, without viscosity, a fluid (air) wouldn't follow the contour of the airfoil and there would be no lift. oops. (You can write extensions to the equation to account for viscosity, but no need to go there for now.)
Well, this gets down to what I tell my students over and over "All models are wrong, some are useful." Bernoulli's equation falls apart in the boundary layer where viscosity dominates. However, the boundary layer is very thin and the pressure at the surface is going to be nearly identical to the pressure "outside" the boundary layer. And, once you get outside, you can blissfully apply (V^2/2)+(P/rho) = constant along a streamline (neglecting the gravity term) all day long and get reasonable results. Ta-da!

Also, as one should be aware, **** flows downhill. But when you look at the pressure distribution over a wing you see that air flows from a lower than the free stream (static) pressure area (say around 1/3 ish of the distance along the chord) uphill back to about the free stream pressure at the trailing edge? How does it do that? How can a fluid flow from a low pressure to a high pressure? Yes, that's right. You guessed it:

But, in the end, give thanks to God that we don't have to integrate the Navier-Stokes equations to fly an airplane. Because if we did, we would all be skrewt.

Airspeed is alive, rotate!