How do airplanes fly with heavy weight and air resistance?

[boneh3ad]

If the shape of the wing by any design causes the air velocity over and under the wing to be dramatically different then you get lift.
It does not have to be the case that the shape of the wing causes the travel time to be different but when it does you still get lift.
You could apply flaps and creat drag in one direction to creat lift.
The distance can be the same or different but as you say it's the difference between air velocity over and under or vice versa that causes a pressure differential which causes lift?

That is reasonably accurate. There are a couple of clarifications or fixes to make to your statement though:

For one, in general, the transit time is not the same for a lift-generating body. You would need a very special case for that to occur, and I am not even really sure that you could design a lifting body to do that. Generally speaking, the air traveling over the wing is much faster than that traveling under it (several times faster), so the top surface would have to be designed to be substantially longer, at which point it would likely stop being a lifting body. So, yes, it does not have to be the case that the shape causes the travel times to be different, but to take that one step further, I cannot imagine a situation where the travel times would be the same.

Second, be careful with correlation and causation. Lowering the flaps to create drag will certainly happen, but it is not the creation of that drag that causes the generation of lift. Lift and drag are two sides of the same coin here. Essentially, in lowering flaps, you are increasing the camber of the airfoil. This allows the airfoil to generate more lift at lower speeds and lower angles of attack (of the plane, anyway), and of course along with lift comes drag. You can't have one without the other. To explain why that is, it is perhaps easiest to think in terms of the net momentum change of the flow around the wing.

Any time you have lift, your airfoil will be necessarily deflecting air downward. If you were to draw a large box around the airfoil and do a control volume analysis, you would be able to integrate the total momentum coming into the control volume, which will be entirely in the horizontal direction, and then integrate the total momentum leaving the control volume, which, due to the deflection, would be partially horizontal and partially vertical. That addition of vertical momentum to the fluid is done by a force coming from the wing whose opposite according to Newton is lift. The flow will also have lost some momentum in the horizontal direction, which occurs due to a force coming from the body whose opposite and equal counterpart is drag. Put simply, with lift and drag, one doesn't causes the other. They both must occur at the same time.

Of course, you could explain this same thing by looking at the pressures over the airfoil and you would get the same answer for an inviscid flow. I simply find it easier to illustrate why and how drag and lift are related using Newton's laws and flow deflection.

 

[rcgldr]

It describes why a sheet of plywood gets lift. It does not explain how a well-designed wing gets lift. That sheet of plywood is pushed by the wind.

Interestingly, even a flat plate airfoil still gets a large fraction of its lift from the upper surface (possibly even a majority, but I don't know off the top of my head). There's no shape you can really make (to my knowledge) that will get almost all of its lift from the lower surface at low speed. At very high (high supersonic) speed, this can change, but that's an entirely separate problem

Even a flat plate airfoil, such as those balsa models posted by Omcheeto or the one in the link below get most of their lift by accelerating air downwards from above the wing. Part of the reason is that the low pressure zone above a wing diverts some of the oncoming flow to over the wing, reducing the flow below the wing.

http://www.4p8.com/eric.brasseur/glider2.html

As a pilot,we are taught that lift is provided by both the pressure difference and the angle of attack of the wing, (angle between the chord line of the wing and the direction of motion) lift and drag both increase as aoa increases, up to the critical aoa, which is when the airflow separates from the top of the wing to cause a stall. Aoa must increase as speed decreases to maintain lift.

Pressure difference and diversion of air are a function of angle of attack, air speed, and the air foil. For a cambered airfoil, effective angle of attack is sometimes used, where effective angle of attack is defined to be zero when the physical angle of attack produces zero lift.

A wing in a stalled state still produces lift, but the lift is reduced, and decreases as angle of attack increases beyond the crictical angle of attack. A stall can occur at high speeds, (assuming the wings can handle a high g force turn), and high speed stalls usually result in snap roll, where excessive elevator input results in a fast roll reaction (one wing "stalls" before the other reducing that wings lift, causing it to roll "downward", the other wing rolling "upwards", reducing the "upwards" moving wing's angle of attack so that it produces more lift than the "downwards" moving wing). Some aerobatic radio control models are deliberatly designed to be able to snap roll.

As far as dispelling the equal transit theory, this pre-shuttle lifting body prototype (m2-f2, the glider version) should do the job (the chase jet is a F104 Starfighter).

 

[Oryon]

You ate correct about high speed stalling, in effect the g load increases the weight of the plane, thus reducing the critical aoa. I'm no aero engineer nor physicist, this id just my understanding based on my flight training, including aerobatic flying. Full scale aerobatic planes can be snap rolled quite well too.. , :-)

 

[OmCheeto]

Perhaps my perception of this problem is skewed by the fact that I'm a boater, and not an airplaner(?). When "planing", I seriously doubt that a negative pressure has anything but the most insignificant effect on my boat flying across the surface of my lovely river.



Same effect, different density medium.

hmmm...

Perhaps I should PM B. Elliott and have him do some underwater experiments.

OMG! He met Einstein!

​

And check out Einstein checking out his neckerchief.

Zis knot reminds me of a zimplification of zee Kaluza-Klein poly-dimenshional nature of zee universe... hmmm...

:tongue2:

--------------------------------
Why are people who drive boats called boaters, but people who fly planes called pilots? Though there are river pilots. hmmm... Someone remind me of my ponderings come November. The sun is out, and you know where I should already be.

 

[OmCheeto]

Which begs another question: Why did this cargo airplane, forget how to fly?

No need to watch after 45 seconds into the video.
(The videographer uses "Facebook" language , and then accidentally steps on his dog. )

https://www.youtube.com/watch?v=-MB9JDBe4wA

Angle of attack? Load shift? Old engines? Unexperienced person? People shoot at aeroplanes in Bagram so we need to escape as fast as possible? Photoshop?

 

[A.T.]

As a pilot,we are taught that lift is provided by both the pressure difference and the angle of attack ...

And a car's propulsion is provided by both: the engine and the force at the wheels...

 

[voko]

Load shift?

If I remember correctly, it was transporting a few armored vehicles, and apparently they were not fastened securely. A plane which suddenly becomes tail-heavy at about one thousand feet - let's put it this way, a situation I would rather not be in. Even though one can tell the crew fought till the end.

 

[CWatters]

Which begs another question: Why did this cargo airplane, forget how to fly?

Google is your friend..

http://avherald.com/h?article=46183bb4

On Jun 2nd 2013 accident investigators by the Ministry of Transport and Civil Aviation of Afghanistan reported in a press conference that quickly shifting cargo, consisting of three armored vehicles and two mine sweepers totalling at 80 tons of weight, caused the accident. The cargo slammed so hard at the back of the aircraft, that parts of the aircraft separated and wiring in the back was severed. As result of the shift and loss of aircraft parts the center of gravity moved so far back, that the attitude of the aircraft could no longer be controlled, the nose of the aircraft rose beyond the flying envelope of the aircraft and the aircraft stalled destroying the aircraft and killing all crew in the resulting impact. Parts of the aircraft, that separated as result of the initial load shift, were recovered from the runway. The straps used to tie down the cargo were recovered from the accident site, although charred they provided evidence of having fractured before final impact, it was unclear however, whether the fracture(s) had happened before or after takeoff.

 

[CWatters]

NASA has a series of slides on incorrect theories of how aircraft fly...

The "Longer Path" theory, or the "Equal Transit Time" theory...
http://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html

The skipping stone or planning boat theory...
http://www.grc.nasa.gov/WWW/k-12/airplane/wrong2.html

The Venturi effect..
http://www.grc.nasa.gov/WWW/k-12/airplane/wrong3.html

They conclude there isn't really a simple explanation..
http://www.grc.nasa.gov/WWW/k-12/airplane/bernnew.html

 

[OmCheeto]

Google is your friend..

http://avherald.com/h?article=46183bb4

Thanks! Mystery solved.

NASA has a series of slides on incorrect theories of how aircraft fly...

The "Longer Path" theory, or the "Equal Transit Time" theory...
http://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html

The skipping stone or planning boat theory...
http://www.grc.nasa.gov/WWW/k-12/airplane/wrong2.html

The Venturi effect..
http://www.grc.nasa.gov/WWW/k-12/airplane/wrong3.html

They conclude there isn't really a simple explanation..
http://www.grc.nasa.gov/WWW/k-12/airplane/bernnew.html

Boy those Glenn people are blowhards. And kind of hypocritical too. If one clicks on the "For kids" link, you'll find the following explanation:

How Wings Lift the Plane

Airplane wings are shaped to make air move faster over the top of the wing. When air moves faster, the pressure of the air decreases. So the pressure on the top of the wing is less than the pressure on the bottom of the wing. The difference in pressure creates a force on the wing that lifts the wing up into the air.

Which sounds like the Bernoulli explanation, which they claim is wrong in "Incorrect Theory #1".

Typical government organization. First they lie to you, then they point their finger at you for for spreading their disinformation.



And they don't like my boat analogy... :grumpy: Ok then, explain how a hydrofoil works, Glenns...

 

[boneh3ad]

Did you even read the links? There is nothing inaccurate in there.

 

[OmCheeto]

Did you even read the links? There is nothing inaccurate in there.

No. I skimmed them. They were very long. They struck me as documents written by engineers, edited by management, and then finalized by lawyers.

hmmm...

Were you involved in their creation, bonehead?

 

[boneh3ad]

Boy those Glenn people are blowhards. And kind of hypocritical too. If one clicks on the "For kids" link, you'll find the following explanation:

Which sounds like the Bernoulli explanation, which they claim is wrong in "Incorrect Theory #1".

Only because you didn't read it. Bernoulli absolutely can accurately describe lift. What "Incorrect Theory #1" states is that the oft-stated reason for faster moving flow is due to the idea that two neighboring parcels of air split and moving around opposite sides of the airfoil must meet back up is incorrect. In fact, the parcel of air moving over the top will typically leave the trailing edge of the airfoil long before its underwing neighbor does. The key question there is why the air speeds up over the top. The fact that it does, though, means that you can describe lift with Bernoulli's equation; you just can't explain why the air speeds up over the top directly that way.

And they don't like my boat analogy... :grumpy: Ok then, explain how a hydrofoil works, Glenns...

The difference is that an airplane wing is moving through air. When planing a boat, it rides on the interface between air and water. In that case, the water is so much more dense than the air that its effect on the system is negligible and the real dominant effect for planing is much like "Incorrect Theory #2" in the NASA links. In the boat case, you effectively can ignore the upper surface because of the fact that the momentum of any air turned by the shape is going to be so incredibly less than the momentum of the water deflected by the hull.

No. I skimmed them. They were very long. They struck me as documents written by engineers, edited by management, and then finalized by lawyers.

hmmm...

Were you involved in their creation, bonehead?

So what you are saying is you dismissed an article without actually reading it? Interesting...

 

[OmCheeto]

Only because you didn't read it. Bernoulli absolutely can accurately describe lift. What "Incorrect Theory #1" states is that the oft-stated reason for faster moving flow is due to the idea that two neighboring parcels of air split and moving around opposite sides of the airfoil must meet back up is incorrect. In fact, the parcel of air moving over the top will typically leave the trailing edge of the airfoil long before its underwing neighbor does. The key question there is why the air speeds up over the top. The fact that it does, though, means that you can describe lift with Bernoulli's equation; you just can't explain why the air speeds up over the top directly that way.



The difference is that an airplane wing is moving through air. When planing a boat, it rides on the interface between air and water. In that case, the water is so much more dense than the air that its effect on the system is negligible and the real dominant effect for planing is much like "Incorrect Theory #2" in the NASA links. In the boat case, you effectively can ignore the upper surface because of the fact that the momentum of any air turned by the shape is going to be so incredibly less than the momentum of the water deflected by the hull.

Ok then. How planes fly is complicated. Laminar flows, non-laminar flows, mass flow rates, pressure changes, velocity changes, etc.

[STRIKE]And what about my hydrofoil question? Aren't they simply underwater wings? Seems very similar to me. How do they work?[/STRIKE] (never mind, I'm outa here)

So what you are saying is you dismissed an article without actually reading it? Interesting...

You are absolutely correct.

Incorrect Theory #1

{The upper flow is faster and from Bernoulli's equation the pressure is lower. The difference in pressure across the airfoil produces the lift.} As we have seen in Experiment #1, this part of the theory is correct. In fact, this theory is very appealing because many parts of the theory are correct. In our discussions on pressure-area integration to determine the force on a body immersed in a fluid, we mentioned that if we know the velocity, we can obtain the pressure and determine the force. The problem with the "Equal Transit" theory is that it attempts to provide us with the velocity based on a non-physical assumption as discussed above.

I should have read all the lawyer like talk about the theory being both Incorrect and Correct at the same time. hmmm... Perhaps the final edit was done by a quantum physicist. :tongue:

Anyways, this thread strikes me as being a bit too pedantic.

Unsubscribe.

 

[sophiecentaur]

I have kept an eye on this thread. It has been interesting to watch two groups of people arguing that their basic idea is not wrong and concluding that this implies the other argument has to be wrong. False dichotomy, I think.
Bernouili seems to give a reason for a lift force but it basically relies on sky hooks to keep the surrounding air up there, even when it must be pushed downwards - so there is a need for a reaction force - which is what the other lot are basing their argument on.
Anyone who says that Bernouili can't apply (in some form or another) is clearly wrong. It has to be operating when air flow is disturbed and the direction of the effect of pressure difference is to give lift with an aerofoil or even just a plane wing, when tilted. The only heavier than air machine that doesn't rely on Bernouili is a rocket. The only machine that doesn't rely on a constant downward flow of air is a balloon.
Kiss and make up, you guys.

 

[boneh3ad]

Ok then. How planes fly is complicated. Laminar flows, non-laminar flows, mass flow rates, pressure changes, velocity changes, etc.

Laminar and turbulent flows are largely irrelevant. A plane can can remain aloft and generate plenty of lift regardless of whether the flow over the wing is laminar or turbulent.

And what about my hydrofoil question? Aren't they simply underwater wings? Seems very similar to me. How do they work?

You originally simply mentioned planing a boat, which to my understanding simply means that the majority of the force holding the boat up comes from hydrodynamic forces instead of buoyancy. If you are doing this in a boat with hydrofoils, then yes, the forces on that fully-submerged hyrdrofoil operate on the same principle as a wing on an airplane. On the other hand, you can plane a boat without hydrofoils by going fast enough, in which case the genesis of this hydrodynamic force is simply the deflection of water downward by the bottom of the hull.

I should have read all the lawyer like talk about the theory being both Incorrect and Correct at the same time. hmmm... Perhaps the final edit was done by a quantum physicist. :tongue:

What the NASA pages do is not tear down the Bernoulli explanation of lift, but the pair of reasons often used in conjunction with it about why the air moves faster over the top, those reasons being equal transit time and the Venturi effect. What they are saying is that those two explanations do not actually explain why the air moves faster over an airfoil than under it. However, provided that something else does make this happen, Bernoulli's principle can accurately quantify and qualitatively describe the lift on an airfoil. The real reasons for the acceleration of the flow are more complicated.

The pages also describe how the same phenomenon of lift can also be described using Newton's laws and the turning of the flow provided that you do not neglect the effect of the top surface. Many people, in using the flow turning explanation, tend to over-simplify the issue by saying lift is essentially the product of the air stream hitting the bottom of the wing and pushing up on it. The problem is that, as described in the article, the top surface contributes a lot to the flow deflection as well, so if you only look at how the flow turns around the bottom you will greatly under-predict the lift on the body.

Anyways, this thread strikes me as being a bit too pedantic.

If attempting to answer a question correctly makes a discussion pedantic, then sure, I guess this thread is pedantic.

 

[OmCheeto]

...
Kiss and make up, you guys.

...I guess this thread is pedantic.

resubscribe.

Yes! Bonehead and I agree! :tongue2:

smooches everyone.

-------------------------
ps. In my quest for knowledge, like Feynman, I've found that digging too deeply simply reveals more layers to the onion. I suppose we could analyze this problem at the quantum level, at which point, nothing would make sense, and Drakkith would tell me to go away again.

 

[A.T.]

What do you guys make of the "New Theory of Flight" by Claes Johnson?

https://www.youtube.com/watch?v=t7e_6bkUFzE

http://secretofflight.wordpress.com/

Is this really something completely new, or just more detail? Does his "elegant separation" apply to stalled wings only, or in general?

This sounds like a bit of over-promoting their own work. They really didn't present anything new in that presentation, and I would even argue that some if it is wrong. For example, saying Kutta-Joukowski does not describe real physics is true by definition, as that theory makes the assumption that the flow is inviscid and incompressible, which obviously doesn't describe the real world. Further, pretty much everyone already knew that you had three major factors affecting lift and drag: vertical pressure differences, horizontal pressure differences and separation (viscosity would be another major contributor as well). I really see nothing new here. Further, I disagree with the idea that a large rounded-edge airfoil will necessarily produce lift. At low angle of attack with no separation, it would not. It requires that separation to prevent the flow from simply remaining symmetric.

If I understand correctly, the claim is that on the upper side there is never a clean flow like shown in 2D pictures

but rather always a 3D turbulent flow (elegant separation) which prevents a pressure rise, and thus creates the pressure difference responsible for both : lift and drag.

 

[Oryon]

As i said, I am no aerospace engineer, just a pilot and aviation enthusiast, but, If i recall correctly, one of the big advances with the P-51 mustang was the reduced drag due to a laminar flow airfoil, which to me implies that unless a wing is specifically designed for laminar flow, the air must have some amount of turbulence as it passes over the airfoil.

 

[boneh3ad]

If I understand correctly, the claim is that on the upper side there is never a clean flow like shown in 2D pictures

but rather always a 3D turbulent flow (elegant separation) which prevents a pressure rise, and thus creates the pressure difference responsible for both : lift and drag.

I disagree with that statement. The flow is usually, but not necessarily, turbulent and usually 3D (except on a truly 2D wing), but even a fully laminar, 2D wing will generate lift, as was the case in level flight with the P-51 (thanks, Oryon). I am not saying that the theory in question is wrong so much as it is not new.

As i said, I am no aerospace engineer, just a pilot and aviation enthusiast, but, If i recall correctly, one of the big advances with the P-51 mustang was the reduced drag due to a laminar flow airfoil, which to me implies that unless a wing is specifically designed for laminar flow, the air must have some amount of turbulence as it passes over the airfoil.

For the most part, yes, a wing will be at least partially turbulent unless it is designed to prevent such an occurrence (which can only sometimes be done). The concept is called a natural laminar flow airfoil. Unfortunately the concept is not directly applicable to swept-wing aircraft as most modern planes are designed since they are subject primarily to a different fundamental transition mechanism than are 2D wings.

 

[rcgldr]

Bernoulli

One issue with Bernoulli is that it doesn't deal with energy added to the air. The total aerodynamic force is related to (mass of affected air) x (average acceleration). With respect to the air (zero initial velocity), the total energy added is related to 1/2 x (mass of affected air) x (average exit velocity)^2 (where exit velocity is the velocity of an affected parcel of air when it's pressure returns to ambient). By accelerating a relatively large amount of air by a relatively small amount, the energy added can be relatively small (compared to the total aerodynamic force), but it's still a factor that Bernoulli doesn't account for, since Bernoulli assumes no work is performed on the air (no increase in energy).

 

[sophiecentaur]

One issue with Bernoulli is that it doesn't deal with energy added to the air. The total aerodynamic force is related to (mass of affected air) x (average acceleration). With respect to the air (zero initial velocity), the total energy added is related to 1/2 x (mass of affected air) x (average exit velocity)^2 (where exit velocity is the velocity of an affected parcel of air when it's pressure returns to ambient). By accelerating a relatively large amount of air by a relatively small amount, the energy added can be relatively small (compared to the total aerodynamic force), but it's still a factor that Bernoulli doesn't account for, since Bernoulli assumes no work is performed on the air (no increase in energy).

Exactly. I wish people would see that this is very relevant and stop trying to adopt one stance or another.
There's a great example of the relevance of this when you compare a displacement boat with a planing boat of the same weight. They are both being supported by the water but the planing boat produces a much smaller wash because the amount of water it needs to displace is spread out over many boat lengths.

 

[Buckleymanor]

Exactly. I wish people would see that this is very relevant and stop trying to adopt one stance or another.
There's a great example of the relevance of this when you compare a displacement boat with a planing boat of the same weight. They are both being supported by the water but the planing boat produces a much smaller wash because the amount of water it needs to displace is spread out over many boat lengths.

One of the boats is supported by the water.The planing boat is supported by both the water and air.Donald Campbell and his ill fated record attempt comes to mindhttp://www.youtube.com/watch?v=D6a--7mYpbQ

 

[russ_watters]

One issue with Bernoulli is that it doesn't deal with energy added to the air.

Not in its basic form, but there is no reason why you couldn't put it in there.
http://webpages.eng.wayne.edu/~ah8818/Webpage1.htm

Indeed, you can add an energy gain or loss to pretty much any conservation of energy statement.

I consider the common statement that Bernoulli's equation doesn't/can't deal with energy loss/gain to be a major misconception about it.

 

[OmCheeto]

One of the boats is supported by the water.The planing boat is supported by both the water and air.Donald Campbell and his ill fated record attempt comes to mindhttp://www.youtube.com/watch?v=D6a--7mYpbQ

I will never understand such people. At 276 mph, the 4623 hp jet engine powered K7 was more a sea-plane than a boat. Why he didn't simply add more wing, throw out a fishing line, and call it a boat, is beyond me.

What was it SophieC said the other day about ego?

Laminar and turbulent flows are largely irrelevant. A plane can can remain aloft and generate plenty of lift regardless of whether the flow over the wing is laminar or turbulent.

...

Really? A plane can fly in a purely turbulent state? Perhaps I don't know what turbulence is then.

hmmm... Where's Drakkith when I need him?

 

[Drakkith]

hmmm... Where's Drakkith when I need him?

You rang?

 

[russ_watters]

Thanks! Mystery solved.

Boy those Glenn people are blowhards. And kind of hypocritical too. If one clicks on the "For kids" link, you'll find the following explanation:

Which sounds like the Bernoulli explanation, which they claim is wrong in "Incorrect Theory #1".

Typical government organization. First they lie to you, then they point their finger at you for for spreading their disinformation.

Agreed. Most of what they say is "wrong" is at worst incomplete, a simplification or not completely applicable. Which of course means that by saying it is wrong, they are oversimplifying! One of the more annoying is their criticism of the applicability of the Venturi tube concept. A wing is basically an inside-out Venturi tube. They criticize the analogy essentially for being an analogy (paraphrase): a wing isn't a Venturi tube because Venturi tubes aren't inside out.

ORLY? Then why is NASA developing an inside-out rocket engine?:

The spike forms one side of a virtual bell, with the other side being formed by the outside air...

http://en.wikipedia.org/wiki/Aerospike_engine

With the aerospike, the ramp serves as the inner wall of the virtual bell nozzle, while atmospheric pressure serves as the "invisible" outer wall. The combustion gasses race along the inner wall (the ramp) and the outer wall (atmospheric pressure) to produce thrust.

http://www.nasa.gov/centers/marshall/news/background/facts/aerospike.html

 

[russ_watters]

The only heavier than air machine that doesn't rely on Bernouili is a rocket.

Oh, I think I can find application of Bernoulli's principle in a rocket...

 

[OmCheeto]

You rang?

This thread is over my head. You need to tell me to get lost again.

It's over your head, Om. Go away.

We've gone from airfoils, to flat plates, to planing hull boats, to hydrofoil boats, to dirigibles, and now to rockets, and I'm afraid this thread will next delve into how Bumblebees fly.

Please Drak, you are my only hope...

 

[sophiecentaur]

One of the boats is supported by the water.The planing boat is supported by both the water and air.Donald Campbell and his ill fated record attempt comes to mindhttp://www.youtube.com/watch?v=D6a--7mYpbQ

Is the air under the boat supported on skyhooks? It surely rests on the water. Of course the pressure may be low but you can hardly argue it's not there.
We seem to have the big and little endians at work in this thread. "My theory, right or wrong".

 

[Buckleymanor]

Is the air under the boat supported on skyhooks? It surely rests on the water. Of course the pressure may be low but you can hardly argue it's not there.
We seem to have the big and little endians at work in this thread. "My theory, right or wrong".

Well you could remove it, the air that is.
Would an electric driven propellar boat placed in a vacuum still plane as well as one that is not.
Would the wash be the same?

 

[sophiecentaur]

Well you could remove it, the air that is.
Would an electric driven propellar boat placed in a vacuum still plane as well as one that is not.
Would the wash be the same?

Of course not,but what has that got to do with the fact that the supporting force is spread over a bigger area when planing? With a purely displacement boat, it's Archimedes at work and no hydro / aerodynamics. When it's planing, just because there's some fluid flow involved doesn't mean there is not enough force acting on the water to keep the boat from sinking.

I can't imagine another topic that would have people implying that reactionless forces actually exist.

 

[sophiecentaur]

Oh, I think I can find application of Bernoulli's principle in a rocket...

I should have expected that! But I am sure you could design a rocket based on firing ball bearings out of the back which would not rely on aerodynamics to provide the ejecta with momentum.

Now, can you suggest a helium balloon design that involves reaction (please don't go into the kinetic theory of gases for the answer)?

 

[rcgldr]

One issue with Bernoulli is that it doesn't deal with energy added to the air.

Not in its basic form, but there is no reason why you couldn't put it in there.
http://webpages.eng.wayne.edu/~ah8818/Webpage1.htm
I consider the common statement that Bernoulli's equation doesn't/can't deal with energy loss/gain to be a major misconception about it.

That web page is about losses of energy (conversion of mechanical energy into heat due to friction). The issue I have with Bernoulli and how a wing generates lift is the increase in energy (wrt air). If you modify Bernoulli to take into account the increase in energy, I don't consider that Bernoulli anymore, since Bernoulli doesn't help in calculating the increase in energy. I think the reason the energy increase is often ignored is that it's usually small compared to the change in momentum for most wings.

 

[russ_watters]

That web page is about losses of energy (conversion of mechanical energy into heat due to friction). The issue I have with Bernoulli and how a wing generates lift is the increase in energy (wrt air).

A gain and a loss are the same thing with opposite signs. They involve exactly the same change to the equation (addition of an energy change term). If the energy change is a friction loss or a pump, the same new equation is used (in the form KE1+PE1=KE2+PE2+E). The impact to the equation is basically the same as adding or deleting the head term in the equation.

You can even go a step further and apply it to a heating or cooling coil (where there is now a change in volume of air caused by the change in energy).

If you modify Bernoulli to take into account the increase in energy, I don't consider that Bernoulli anymore...

Well, that's that overly pedantic thing again. If you derive a new equation from Bernoulli's equation, Bernoulli still helped you -- you needed Bernoulli to get the new equation!

...since Bernoulli doesn't help in calculating the increase in energy.

Huh? Since you can't generate the new equation without using Bernoulli's original equation and everything about it is the same except the one additional term, it most certainly does help!