Bernoulli's Principle and Airflow

[tgrayson]

Aerobatic aircraft are a whole different set of rules, they have wings that have the same camber on the tops as the bottom and rely heavily on the angle of attack of the wing and Newtons laws to maintain flight.

Not true at all. Aerobatic airplanes work identically to any other airplane. The symmetrical airfoil just means a slightly higher AOA for any lift coefficient, assuming the same wing area. Once again, the entire quantity of lift can be explained through the pressure distribution around the airfoil.

As for what "need either a higher wing surface area ratio, or a faster more powerful engine to overcome the decreased dependence on bernoulli and increased dependence on Newton" means, I have no idea, but it is surely wrong. For one thing, the engine has nothing to do with lift production.

 

[Matt777]

I posted the link mainly to provide you with Denker input regarding this book. In his post, he expresses disdain for two of Andersen and Eberhardt's ideas:

1) Coanda effect
2) Downwash

He's pretty much the only knowledgeable person who addresses #1, because he's probably the only one that knows there's anyone that is claiming that flight requires this phenomenon.

Aha! I see. Okay, so the coanda effect is not the cause of the airflow following the wing. Speaking as a total non-expert , that's cool I don't care about coanda- I'm interested in attempting to bebunk the idea of downwash being responsible for lift.

... Problem is, with no viscosity, no lift occurs. Still, it removes one of justifications for bring the Coanda effect into the picture, since it isn't necessary to explain why air follows the contour of the wing.

Couple of things here. First, what causes an acceleration? According to Newton, F = ma, so a = F/m. In other words, an acceleration requires a force. If the air is being accelerated, there is already a force acting on the air. But how can a force already exist prior to the air moving when we're trying to explain the existence of that force by the moving air?

To be honest, you lost me there. Understanding that statement requires me to have an IQ higher than 141 (or to actually have some physics expertise)!


I just had an "aha".
Watch this clip of an A340 (yuck) landing at St. Maarten. http://www.youtube.com/watch?v=Rjaq2pA9BsU&feature=related

If downwash was the cause of lift this clip would show a wake in the water and the people would be thrown to the ground.
Since neither of those things happen, the downwash theory is easily and entirely debunked!!! So there.

You know, I just read the last chapter in "Understanding Flight", which is all about windtunnels... they talk a lot about the calculations that have to be done to correct for errors caused by wing and also stab downwash being interupted by the wall of the wind tunnel. I'm going to speculate that these guys worked and thought about those downwash errors so much that it led them to think that downwash did more than it does.
As you stated, they then made up a hypothesis that they failed to confirm with testing and published a book. Go figure.

Second, downwash is created by wingtip vortices; no vortices, no downwash. When a wing is placed in a wind tunnel, often the ends are allowed to touch the sides of the wind tunnel, which prevents the formation of wingtip vortices and thus eliminates downwash. Yet these wings still generate lift. How could that be?

As a person with absolutely zero college level physics or aerodynamics expertise, do you mind if I ask you a question that happens to involve disagreeing with you? Great!

Okay, based partially upon the misinformatioon I learned from this bogus book, but moreso based logically (at least to me) on the evidence of downwash and vortices shown in this picture http://www.airliners.net/photo/British-Airways/Boeing-777-236-ER/1091105/M/
... Saying that wingtip vortices create downwash & "no vortices, no downwash"
seems, to me, like putting the cart before the horse.

Here's why that confuses me (bear with me if you can): We know that the vortices exist because the high pressure air below the wing combines with the low pressure air at the wingtip...
Because we know that winglets reduce the vortice strength, reducing drag- it seems logical TO A NOVICE that if you were to take a 777 (which doesn't even need winglets because of the efficiency of the wing [it has swept/raked tips]) and increase the wingspan from 200' to 300' and add 30 foot long winglets... there could be possibly no vortices from the wingtips - and the wing would still make lift because of the bernoulli low pressure over the wing. Right? Or am I crazy?

A high performance glider with an extreme aspect ratio can't be producing much of a wingtip vortice at all, yet there is lots a lift because lift is not dictated by how much high pressure spills over the wingtip (due to the lack of a winglet or sufficient span), but by the wing area/chord/AOA. Right?

The picture I linked to above, to the untrained observer, seems to show downwash (though not the cause of lift) eventually turning into a (non wingtip?) vortice... like perhaps there is the presence of wingtip vortices and also the higher velocity downwash at the inboard portions of the wing curling around the lower velocity downwash departing from the outboard portions of the wing. I deduce/hypothesis (invent? imagine?) all of that from that picture and from something mentioned in this bogus book.

Does any of that make any sense?

The "infinite" wind tunnel wing with no wingtips or vortices makes lift because of the bernoulli low pressure, which doesn't care about vortices because they are a result, not a cause of lift. Right?

Thanks for you patience and for taking the time to answer!
I don't know calculus- so this is as far as I can go with this subject- that is, specifically, to annoy you with nonsensical questions!

1. The best criticism of Andersen and Eberhardt is that the Coanda effect is unnecessary to explain why the air hugs the surface of the airfoil of a conventional airfoil. It does so when the air contains no viscosity and thus the Coanda effect impossible.

2. The irrelevance of downwash is illustrated by the fact that wings in a wind tunnel have no downwash, yet generate lift.

1. & 2: Understood.
2. Also proven by the youtube clip


I think I wont annoy you anymore about this stuff.

 

[Matt777]

The typical wing surface area of a 747-400 is around 5650 sq.ft. that comes out to be 813600 sq inches. Pressure differentials can be measured in psi. so the wing loading comes out to be about 1.069 pounds per square inches. so if the pressure differential is even less than 2 psi, the airplane will fly.

1 pound per square inch. Is that all?
Wow.

Based on this very interesting fact you stated I think you have left us wondering why you don't believe bernoulli and low pressure is 100% responsible for lift (like I didn't several posts ago). Your fact above shows that it just doesn't take that much of a pressure differential. Thanks for that comment.

I wonder if I can find an email address for the authors of "Understanding Flight"? I have some questions/complaints for them!

 

[tgrayson]

Saying that wingtip vortices create downwash & "no vortices, no downwash" seems, to me, like putting the cart before the horse.

That's not possible, because downwash is defined to be the result of vortices.

In brief:

http://www.grc.nasa.gov/WWW/K-12/airplane/downwash.html

Too late to address any of the rest.

 

[Johnsc81]

Not true at all. Aerobatic airplanes work identically to any other airplane. The symmetrical airfoil just means a slightly higher AOA for any lift coefficient, assuming the same wing area. Once again, the entire quantity of lift can be explained through the pressure distribution around the airfoil.

As for what "need either a higher wing surface area ratio, or a faster more powerful engine to overcome the decreased dependence on bernoulli and increased dependence on Newton" means, I have no idea, but it is surely wrong. For one thing, the engine has nothing to do with lift production.

I stand corrected. With symmetrical wings they rely on changes on AOA to adjust of change the point of seperation of laminar flow, and that creates the flow length change and therefore become a "bernoulli wing"

But when I was saida more powerful engine, I was not insinuating that Engine power means greater lift. I was sayinga more powerful engine can make the plane go faster and more airflow over the wing. Prime example: the F-16 is proof that you put enough power behind a brick it will fly.

 

[Matt777]

That's not possible, because downwash is defined to be the result of vortices.

In brief:

http://www.grc.nasa.gov/WWW/K-12/airplane/downwash.html

Too late to address any of the rest.

Wow that book by those , ahem, "Brilliant" aerodynamicists really screwed me up good!

Okay then, I have seen the light... so the "downwash" I see in that picture I linked to above is not coming off the trailing edge of the wing- it is the wingtip vortice that has dissipated its rotational qualities and is simply falling down through the air.... looking like the "downwash" claimed by Anderson.

I should have known from the St. Maarten video I linked to that the downwash I thought was in that picture did not exist!

<Sigh> Why did this damn book get published in the first place?!!!
I want a refund! And I'll sue for damage to my ego!

 

[georgetg]

Airplane displaces air dynamically
Airship displaces air statically

airplane = waterskier
airship = boat

done.

All this other stuff is chaff, minutia...
Its like worrying about how the electrons in the radios interact.

Cheers
George

 

[tgrayson]

I was sayinga more powerful engine can make the plane go faster and more airflow over the wing. Prime example: the F-16 is proof that you put enough power behind a brick it will fly.

True. However, the engine merely means you can achieve that velocity in level or climbing flight. For most airplanes, you can achieve whatever velocity without the engine, as long as you're willing to accept a descent. Most likely, though, supersonic flight is unachievable this way.

 

[tgrayson]

so the "downwash" I see in that picture I linked to above is not coming off the trailing edge of the wing- it is the wingtip vortice that has dissipated its rotational qualities and is simply falling down through the air.... looking like the "downwash" claimed by Anderson.

There is actually a huge vortex sheet that comes off the trailing edge of a wing generating lift, in addition to the wingtip vortices, but it will eventually be rolled up into the wingtip vortices, which is why the vortex strength can be larger a bit further away from the airplane.

There is certainly a great deal of downwash coming off a real wing, but the point is that it is a result of lift, not the cause of it. Even if the Coanda were of prime importance, the area of interest would be the interaction of the air and the wing, rather than the downward moving mass of air.

Here's a question: imagine a rocket, shooting stuff out the back to accelerate. Would it still function if we attached a bucket on the rear to catch the ejected material?

 

[sdfcvoh]

Here's a question: imagine a rocket, shooting stuff out the back to accelerate. Would it still function if we attached a bucket on the rear to catch the ejected material?

It would if that bucket was full of emeralds... and maybe blue diamonds, purple horseshoes and pink hearts.

 

[tgrayson]

It would if that bucket was full of emeralds... and maybe blue diamonds, purple horseshoes and pink hearts.

What about green clovers?

 

[sdfcvoh]

What about green clovers?

What??!!! Thats completely out of line! I thought this was a technical thread!!!

 

[Flight81]

So my question is what to teach new students? Or better yet what do you guys (Grayson) teach? I'm gonna be honest, it took about all my brain cells to follow some of those topics. I can't imagine you get that indeepth with new students. Being a new CFI, I want to be teaching the right stuff. Like most I have been teaching the normal bernoulli, newton, stuff for the lift.

 

[tgrayson]

I can't imagine you get that indeepth with new students.

Depends on the needs of the student. There is no FAA-mandated requirement that they have any understanding of this stuff, other than what's needed to pass the knowledge test.

It's not that I would avoid the topic with them, but that they already have so much to learn that anything extra wouldn't stick very easily. Unless I'm willing to present a topic two or three times, I wouldn't have any expectation that they'd remember it.

However, if a particular technical question becomes an obstacle, then I'll address it as accurately as I can. Even if they don't remember it, they'll at least put the question out of their minds so that they can move forward, unlike if I tried to gloss over the subject with an unconvincing FAA sound bite.

 

[JrsyGuy]

after going through this whole thread my only question is why is the aft stagnation point located up on top of the wing? some of the pictures show early on in the thread arent fully clicking right now. and im a mensa member :banghead:

 

[tgrayson]

question is why is the aft stagnation point located up on top of the wing

It isn't. That's where it **would be** if circulation were not established around the wing, moving it back to the trailing edge. In slow motion video, you can actually see this happening.

As for why it wants to be there, isn't it somewhat intuitive that the rear stagnation point will be at the opposite point from where the front stagnation point is? Since the front stagnation point is on the underside of the airfoil, it's reasonable that the rear stagnation wants to be on the top, on the opposite end of the airfoil.

 

[fish314]

So my question is what to teach new students? Or better yet what do you guys (Grayson) teach? I'm gonna be honest, it took about all my brain cells to follow some of those topics. I can't imagine you get that indeepth with new students. Being a new CFI, I want to be teaching the right stuff. Like most I have been teaching the normal bernoulli, newton, stuff for the lift.

The main concept about lift that I really want the student to know is practically, "how do I make it." So I couple a quick brief about the lift equation with a demo in the airplane.

Basically I'll show the lift equation, "L=CsubL*1/2*rho*v^2*S" and I'll define each term. Then I'll say that as the pilot you have no control over the 1/2, since that term is a constant, or the rho (since that term is the density of air, and is determined by the weather that day). "S" is the surface area of the wing, and on most airplanes that term is set as well... you're not going to be changing the size of the wing mid-flight. So that leaves CsubL and V^2.

CsubL is the coefficient of lift, and there are a lot of things that go into it. Basically, though, it's related to the shape of the wing (which we can't change) and the angle of attack (which we can- by pulling back on the yoke or stick).

V^2 is the velocity squared, or the velocity times itself. We can change that too, by speeding up or slowing down.

From there, I'll look at level flight, and say, 'ok, if the airplane weighs 3000 lbs, we need to generate 3000 lbs. of lift to keep it flying level and unaccelerated. If I'm at 100 knots, and I speed up to 200 knots, what do I need to do to my angle of attack to keep producing 3000 lbs. of lift?"

From there I'll talk about slowing down straight and level, which leads naturally to a stall discussion. Then I'll talk about cases where I'm producing more lift than the weight of the aircraft (aerobatics, steep turns, etc.) which leads to a V-g diagram type discussion (or g-available discussion).

Then I fly it. Look, here's 200 knots level flight. Notice it's 1 degree nose high. Here's 120 knots level flight... notice it's 3 degrees nose high. Here's 90 knot slow flight... 8 degrees nose high.

Here's a max g-turn at 120 knots- I can sustain about 1.5 to 2 g's. Here's a max g (3-4 g turn) at 170 knots. Here at 220 knots, I can easily pull 6 g's before stalling.

 

[tgrayson]

V^2 is the velocity squared, or the velocity times itself. We can change that too, by speeding up or slowing down.

Only briefly and minutely. The static longitudinal stability will ensure that V will change in response to CL changes. There's only one real independent variable in the lift equation and that's CL.

 

[flyinguitar]

Wow, I can't believe I only just now found this thread. I love aerodynamics. It's really difficult to understand well. (I don't.)

Only briefly and minutely. The static longitudinal stability will ensure that V will change in response to CL changes. There's only one real independent variable in the lift equation and that's CL.

I hadn't thought of it that way, but you're totally correct. How do we go faster? Pitch down. How do we go slower? Pitch up. Power as required for desired flight path, but power's not in the lift equation.

Here's a question: imagine a rocket, shooting stuff out the back to accelerate. Would it still function if we attached a bucket on the rear to catch the ejected material?

If the bucket is attached to the rocket, no. If the bucket is attached anywhere else, yes.

For most airplanes, you can achieve whatever velocity without the engine, as long as you're willing to accept a descent. Most likely, though, supersonic flight is unachievable this way.

So if you take an F-16 to FL600, shut down the engine, and pitch straight down, you won't go supersonic?

Seems to me that since there are plenty of supersonic airplanes that have a thrust/weight ratio of less than 1, you would be able to break the sound barrier in a glide at a descent angle somewhat less than 90 degrees. But there's probably something I'm not thinking of.

Couple of things here. First, what causes an acceleration? According to Newton, F = ma, so a = F/m. In other words, an acceleration requires a force. If the air is being accelerated, there is already a force acting on the air. But how can a force already exist prior to the air moving when we're trying to explain the existence of that force by the moving air?

Second, downwash is created by wingtip vortices; no vortices, no downwash. When a wing is placed in a wind tunnel, often the ends are allowed to touch the sides of the wind tunnel, which prevents the formation of wingtip vortices and thus eliminates downwash. Yet these wings still generate lift. How could that be?

The irrelevance of downwash is illustrated by the fact that wings in a wind tunnel have no downwash, yet generate lift.

I think there is confusion between downwash as a technical term for the downward-rotating part of a wingtip vortex and the effects it has on lift, and the simple fact that a lift-producing wing imparts an overall downward momentum change (or acceleration) on the passing air. It has to - if it didn't, the whole system would violate Newton's third law.

Also, with respect to the whole Newton/Bernoulli debate, Realms09 correctly points out that Bernoulli's equation is simply classical mechanics (Newton's laws) applied to a fluid. Here's how I think of it: as long as you make sure to include flow over the top and bottom of the wing (not just the bottom), it is perfectly correct to view lift in terms of Newton's 3rd law. I find this very intuitive and elegant. However, to use a cooking metaphor, it is akin to the following statement explaining how to roast a chicken:

To roast a chicken, arrange for an interaction to occur between a chicken and an oven.

If you want the aerodynamic equivalent of a detailed recipe with step-by step instructions, then you need to talk about the bound and starting vortices, circulation, the Kutta condition, and life gets complicated....

However, as pilots, when it comes to aerodynamics, we don't really cook - we just show up and eat.

 

[tgrayson]

Seems to me that since there are plenty of supersonic airplanes that have a thrust/weight ratio of less than 1, you would be able to break the sound barrier in a glide at a descent angle somewhat less than 90 degrees. But there's probably something I'm not thinking of.

Actually, that's an excellent example of a thought experiment that shortcuts around a lot of calculations. Your argument is compelling. I was basing my statement off of the P-51 dives, but that was hardly a clean airplane by jet standards. Plus, those occurred a relatively low altitudes with a lot of drag and a higher speed of sound. According to Wikipedia, the X-1 had a thrust/weight ratio of about 0.5.

he simple fact that a lift-producing wing imparts an overall downward momentum change (or acceleration) on the passing air. It has to - if it didn't, the whole system would violate Newton's third law.

Not really. Pressure itself is Newton's third law in action; if there is pressure against the wing, we know that momentum has been transferred to the air and Newton's third law is satisfied. There is no need for the coherent flow of the air be deflected downwards.

And downwash isn't merely "downward-rotating part of a wingtip vortex", it's the net deflection of the entire relative wind. According to theory, this does not happen in the absence of wingtip vortices, but it does not consist solely of vortices.

However, even in the absence of vortices and absence of downwash, there is still a pressure difference between the top and bottom of the airfoil, which means there is a reciprocal momentum transfer between the air and airfoil.