How Aeroplanes Fly and Propellers Pull

[High Wing Drifter]

Hi All,

A point of curiosity prompted this (http://www.pprune.org/forums/showthread.php?t=206959) question in the Wannabes forum.

What is the primary factor that sends us aloft: Is it Bernoulli or Newton's Third Law? I actually took it as read that it was the former. But after reading Stick and Rudder and then thinking about aerfoils such as aerobatic symetrical sections or thin delta wing designs, I'm moved to believe that Newton's Third Law is the primary and possibly the overwhelming influence with the shape and camber doing little more than tweaking the efficiency of the wing.

That's enough of my semi-blind hypothesising. Are there any educated opinions out there?

[balsa model]

What is the primary factor that sends us aloft: Is it Bernoulli or Newton's Third Law?

Let me try:
If you find a wing that flies without causing a vortex, I'll say it's all some Bernoulli magic. There is no escaping Newton's and flow turning action-reaction.
Bernoulli's effect can be used to create the necessary pressure difference at minimum wing incidence angle. It's a matter of wing design efficiency.

Ok. How did I do?

[barit1]

The two are inseparable.

Newton's Third relates [action] = [- reaction]. Bernoulli's effect changes the direction of flow around the airfoil, so that it is deflected (in the case of a wing) downward. This action is reacted by the upward force (lift). One cannot have one without the other.

For a propeller, rotate the above image 90°.

[faultygoods]

I'll go that Balsa, it flys due to Bernoulli's therum and because it does Newtons third Law comes out to play.:8

[18greens]

HWD, its funny you mention Stick and Rudder.

Having just read Kermode and a few other books on aerodynamics its refreshing to read Langewiesche version of flying. 'A plane flies by throwing the air down and the resulting force throws the plane up- forget Bernoulli'. Also referring to the elevators as flippers. Priceless.

I personally always follow the theory that money makes planes fly. Lots of money.

[DFC]

Thrust is created by the engine-propeller combination.

The propeller (for arguments sake) deflects air rearwards.

If the propeller pushed the air back at say 300kt then of course the aircraft should move forwards at 300kt.

Of course the very good propeller can indeed acheive said rearward airflow but the aircraft in the air at cruise will not acheive 300kt.

Ignoring drag.

Taking proeller slip into account, exaplain lift in the same newtonian terms and apply it to the wing.

Regards,

DFC

[Genghis the Engineer]

You're all wrong.

Lift is generated by inverse proportion to the lightening of the operators wallet. The bigger and heavier the flying machine, the more money is required to make it fly.

There's also an attraction between the heavens and pretty flying machines. This is why gliders in particular need relatively little money lightening to make them fly - and conversely helicopters (which are amongst the ugliest machines invented by man) require a great deal - and tanks on the other hand won't fly however much you spend.

Weight also plays a part - since clearly the heavier an aircraft is, the more lift is needed, and thus the more money. So, microlights are cheaper to run than 2-seat light aeroplanes, and proportionally 4 or 6 seat aeroplanes are more expensive again - even if they all look as pretty.



Now my serious point. I've been working in aviation for a couple of decades, have several degrees in it and even more flying licences - and frankly I don't really know what makes an aeroplane generate lift (20 years ago, I thought I knew!). What I do know is how to predict whether it will or not, and how well. Depending upon circumstance, I pick the model that suits the job - even occasionally the rubbish I've just posted above. They are all correct - up to a point.

If you want to see the more complicated ones - such as vorticity theory, Glauert's lifting line theory, or more obscure stuff that requires serious computing power to cope with at-all, I have an office full of books on it that you're welcome to come and read. The problem is, they're all right.

So pick an explanation that does the job and makes sense (N3 or Bernoulli are both fine), and use it for as long as it continues to do so.

G

[PhilM]

I believe (may have the wrong figures here...), but Bernoulli can account for about 80% of the lift generated by a wing of an aircraft, the other 20% is Newtonian lift generated by the downwards movement of air (3rd Law, Every action blah blah).

No idea about props! I try to avoid those at all costs! :O

[Genghis the Engineer]

I believe (may have the wrong figures here...), but Bernoulli can account for about 80% of the lift generated by a wing of an aircraft, the other 20% is Newtonian lift generated by the downwards movement of air (3rd Law, Every action blah blah).
No idea about props! I try to avoid those at all costs! :O

Bernoulli can acccount for all of it.

Newton's laws can account for all of it.

This applies to wings or props (although for props or fast wings you need to switch to the compressible form of the equation).

G

[Farmer 1]

Doesn't Cole's Law have something to do with it, as well, or have I been looking at the wrong tables?

[RJM]

Ja. In Scandinavian aircraft, Coleslaw can have a significant primary effect on the pilot, and a secondary effect on the velocity of the machine if the cockpit is open, depending on the orientation of the pilot at the relevant time - ie heading, gear up or down etc. In closed cockpits (without oxygen masks in use) the effect is on the aircrew only but can be intense depending on time since consumption and the state of the cabbage used in the dish.

So if planning a spin in the Viggen, it may be best to stick to, say, pickled herring. If considering dusting off the DH-82, Stearman etc - most foods are fine.

Let me know if you need any more help.

[Farmer 1]

Smartarse, spoilsport.

[828a]

I've been wondering about wings and lift for years and I still can't understand the magic of it. Let's say that in some fantasy land I am living on the upper surface of a monstrous wing that is generating "lift". Could I breathe normally and would gravity keep my feet anchored down. In other words what is going on in that area directly above the wing?
828a

[BOAC]

Wing-walkers seem to manage!

[hawk37]

828;
In simple terms, lift, or the net force on the wing, is the result of the DIFFERENCE in pressures between the upper and lower surfaces. Hence, the upper surface could be at one atmoshere, and still produce lift if the lower surface is at more than one atmoshphere. But in most locations, the pressure on the top surface is less than the pressure on the bottom surface. So to answer 828's question, standing on this wing could be like standing in a wind slightly greater than the airspeed of the aircraft, at an altitude greater than the aircrafts altitude. On top of Everest, for example, or at Denver, depending on the speed, aerodynamics, pressures, etc. And perhaps on a slope since the wing has camber normally. Breathing as per the wind/pressure situation, gravity has basically the same effect. Hold on.

Hawk

[Basil]

Try Googling 'Coanda'
Theory seems to link in with Newton.
What do you think, Genghis?

[Tony M]

Pilots are smelly and the earth repels them.

[barit1]

Helos are ugly and the earth repels them.

[Keith.Williams.]

Aircraft create lift by accelerating air downwards. The lift created is the equal and opposite reaction that is predicted by Newton's third law.

The wings acelerate the air downwards by creating pressure differences.
Bernouli's theory is one method of explaining how the wings create the pressure differences.

It is worth noting that the lift is not created by the downward velocity of the air, but by the downward acceleration given to it. This might appear to be splitting hairs, but giving the air a large downward velocity means giving it lots of kinetic energy. This loss of energy from the aircraft means drag. If however a very large mass of air is given a very small downward acceleration, it will be given less kinetic energy so there will be less drag.

[Flight Safety]

I've also been pondering this for years, so I'll take a crack at it.

I think that Bernoulli and Newton's third law are both at work in creating lift. When you consider the propeller blade (which is a rotating wing with typical airfoil cross section), it does NOT create thrust by ONLY creating a low pressure area in front of the propeller. If this were so, air behind the propeller would move forward to fill the low pressure area in front of the propeller. From experience we know that propellers move air rearward when producing thrust, and more thrust means more air moving rearward at faster speeds. The fan blade of a high bypass turbofan (another wing) produces thrust with very large movements of air. This illustrates that N3 is at work.

However we also know that Bernoulli is at work in the propeller because of the propeller tip (or wing tip) vortex, which is caused by high pressure air trying to move in and fill the low pressure area on the other side of the wing. We know this can only happen at the tip because the air mass is moving over the wing, and that the wing itself is in the way, blocking the rebalance of the pressures except at the tip.

I think that at very low angles of attack, Bernoulli is mostly at work, but as AOA increases, N3 increasingly does most of the work. I think this explains the shape of the AOA drag curve for airfoils. I also think this explains the AOA stall curve of airfoils. As the stall begins, lift starts to fall off as separation and turbulence begin, but the lift falls off on a curve. I think this shows that as Bernoulli falls away, some N3 remains.

I think Bernoulli adds efficiency to a wing. Even a barn door can fly with enough power, which confirms Genghis's theory of pretty flying machines. However there won't be much Bernoulli at work here, only N3. I think that Bernoulli is very energy efficient because it is only creating pressure differences. I believe N3 takes a lot more energy because with N3 you are not just creating pressure differences, you are moving air. Again, I think this explains the drag curve.

So to summarize, my theory is that at low angles of attack, Bernoulli is mostly at work. But at higher angles of attack N3 is mostly at work, and there is a transition from more of one to less of the other as the AOA changes.

Genghis, please way in as needed. Anyone else, please way in as I would really like to nail this one down once and for all.

[westhawk]

Have a look at this short discussion to add a little fuel to the fire:
http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html

Best,

Westhawk

[Vmu]

Maybe this question is best answered on a more fundamental, but less concrete level?:
Aerodynamic forces are pressure- and shear-forces acting on the surface of a body. Nothing else. Total aerodynamic force is obtained by summing the pressure- and shear-forces over the entire body. The airflow that gives rise to these forces is a result of, or solution to, the Navier-Stokes equations.

[Genghis the Engineer]

Bernoulli works: it describes the pattern of pressures over a shape, and by integrating those over the entire shape, you can accurately predict the total forces and moments acting upon that shape.

Newton works: it describes that for every action there is an equal and opposite reaction. If you are able to measure or calculate the mass flow of air due to the passage forwards of the aeroplane, you'll find it corresponds exactly to the lift.

Now listen carefully: if you know the pressure distribution over the shape travelling through the air, and use this to calculate how much mass of air should be displaced downwards (using, as it happens, Newton's second law). Guess what, you get the right answer, which Newton's laws will then allow you to calculate total forces with.



If we really want to get complicated, we can come to the Navier-Stokes equations, which are in fact derived from a form of Bernoulli's equation (there are several - the incompressible, compressible, and unsteady forms - in ascending order of difficulty to understand). The N-S equations (together with some clever maths called "Cauchy's integral" and a general principle called "vorticity" are basically a way of taking the shape of something and (guess what) predicting the pressure distribution over a shape. From that, Bernoulli's equation(s) allow us to predict forces, and Newtons laws allow us to predict mass flow effects.


They really are all interlinked and inextricable ways of describing the same thing. It is totally incorrect to say that any portion of an aerodynamic force is due to any one, and not any other.

It is of-course true to say that any of these theories can be described also as simplistic - but rarely so simplistic as to be unuseable. Last week I was busy working out the equations for an obscure type of airspeed indicator - using the simplest incompressible form of Bernoulli's equation, and with accepable accuracy. (I'd rather have been flying, but that's another story!).

G

[TenAndie]

As Keith Said.......:8

[mad_jock]

Navier-Stokes equations

Now that is a very unfair thing to do bringing them into play.

We used to do a exercise in the lab in the wind tunnel.

Various cross sections and angles etc.

And yes you could get a plank of wood to fly but you got lots of votex shedding and it was pretty crap. And virtually impossible to get any trailing edge controls to work in the turbulent flow. But you needed relatively little mass flow to get a reasonable up force.

Now a pure areofoil with zero angle didn't produce much lift but had very good drag characteristics the flow was laminar and trailing edge controls worked a treat.

As with most things looking at nature has given the optimised shape for the most energy effective design.

Personally I think newton does the bulk of the job. The fancy shape lets you control and lower the drag into a cost effective machine.

MJ

[Confabulous]

I've been reading Stick & Rudder lately, and by god it makes more sense then piling Bernoulli on top of Newton and having Navier-Stokes jumping on them, and oooh, don't forget Kutta and... hang on, why do I get the feeling that none of these guys have ever flown an airplane? :}

G is right - they all work. Who cares why, or how? When you're in the air, that is. Have you ever seen a test pilot in a spinning aircraft muttering to himself along the lines of 'that tosser Bernoulli, getting me into all this trouble. Now, how do I calculate my way out of a flat spin?' ;)

I'll be building a Ban-Bi next year, and the most important instrument is going to be a tribute to the most practical pilot ever - Langweische. The instrument? An AoA indexer.

Problem Solved! :p

[screwdriver]

Having watched a c5 Galaxy lift out of an airfield in Jamaica I firmly believe that its down to magic. The truth is that no one REALLY knows--- they only THINK that they know...

[ARINC]

An alternate explanation of lift: "ATTACK ANGLE"

As air flows over a wing, the flow adheres to the surfaces of the wing.
This is called the "Coanda effect." Because the wing is tilted, the air
is deflected downwards as it moves over the wing's surfaces. Air which
flows below the wing is pushed downwards by the wing surface, and because
the wing pushes down on the air, the air must push upwards on the wing,
creating a lifting force. Air which flows over the upper surface of the
wing is adhering to the surface also. The wing "pulls downwards" on the
air as it flows over the tilted wing, and so the air pulls upwards on the
wing, creating more lifting force. (Actually the air follows the wing
because of reduced pressure, the "pull" is not really an attraction.) The
lifting force is created by Newton's Third Law and by conservation of
momentum, as the flowing air which has mass is deflected downward as the
wing moves forward. Because of Coanda Effect, the upper surface of the
wing actually deflects more air than does the lower surface.



Beats the hell out of me....:{

[Flight Safety]

I found this interesting link explaining how boat sails work.

http://www.sailtheory.com/sail.html

There's good basic lift theory in the explanations and it's apparent that boat sails and wings work essentially the same way. Both Bernoulli and N3 are explained, and the relationship between them is explained in a fairly straight forward way. Coanda effect is also included and how it relates.

I found this explanation helpful enough to clear up the issue for me as to how all 3 "effects" contribute to lift, and how they relate to each other.

The sailboat guys seem to have a pretty good handle on this. :8

[apruneuk]

The way I understand the process is that Bernoulli describes the manifestation of Newton 3G in the scenario of an aerofoil passing through the air (or the air passing over an aerofoil). In the case of an aircraft in unaccelerated flight, Lift=Weight and Thrust=Drag. The powerplants create the necessary thrust to propell the machine through the air (and therefore the air over the wing) this thrust is balanced (N3G) by the drag created by the aeroplane in a direction parallel to the thrust line. However, because the wing is curved, the air passing over the top creates a lower pressure than that below resulting in lift (Bernoulli). Because the aeroplane is travelling forwards, the inevitable counter balance to the lift created (3G) is felt at some distance behind the aircraft in the form of wake turbulence. I.E a weight of air equal to that of the air being displaced, rather than being directed vertically downwards is actually deflected downwards and backwards. This process is most easily seen in the form of wingtip vortices at take off and landing when induced drag is at its highest.
Then again, I only fly the things so what do I know? The smelly pilot theory has yet to be disproved...

[cwatters]

> Is it Bernoulli or Newton's Third Law?

Both if applied to the whole system correctly. The argument/debate only arises because both camps over-simplify the problem..

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

[Woomera]

I'm with Genghis on this.:ok:

But I must add that he has been very careful to keep it simple and not to confuse the issue by invoking the fundamental effect of the triple ganged reverse flow thronomister arrays at the stagnation point. The Navier-Stokes equations of course follow directly from there.:cool:

[barit1]

The argument/debate only arises because both camps over-simplify the problem..

That, in a nutshell, is the heart of it all. :ok:

[samusi01]

...and here I thought that money created lift.

[Farmer 1]

No, it's Cole's Law, I tell you.

[BizJetJock]

You've all missed the really fundamental point - they fly because you believe they'll fly. If you stop believeing it won't work any more, just like Santa and the Tooth Fairy:D

[Mac the Knife]

There seem to be two

Navier-Stokes and Napier-Stokes

Are they different or the same or what?

[BizJetJock]

It was Navier-Stokes in my days at RAE, but maybe they've "modernised" them.:confused:

[italianjon]

Will power, nothing but will power.

That's why flight deck doors are locked now. Pilots have enough to worry about, without passengers breaking their meditation, willing the aircraft to fly.

[Genghis the Engineer]

It was Navier-Stokes in my days at RAE, but maybe they've "modernised" them.:confused:
Modernised certainly, but not replaced. The Navier Stokes-equations are at the core of most modern computational fluid mechanics.

Which, I'm delighted to say is somebody else's problem, so far I've managed to spend most of my time worrying about whole aeroplanes, not the ability of computers to predict fiddly little bits of airflow - important though it is, it fascinates me not at-all.

G

[Genghis the Engineer]

There seem to be two
Navier-Stokes and Napier-Stokes
Are they different or the same or what?

The main difference is that the first version was spelled correctly.

There was a great scientist called John Napier (http://news.scotsman.com/scitech.cfm?id=523542005) (like a great many very clever people, and the majority of inventors, he was Scottish), he was responsible for a lot of the mathematical theory behind Logarithms, and also for the original theory behind spherical trigonometry. He could therefore be said to have had a great deal of influence upon aviation - but a different bit, and so far as I know had nothing to do with anybody called Stokes. He also predicted the Apocalypse would take place between 1688 and 1700, so we can at-least be thankful that nobody's perfect!

G

[italianjon]

The only problem I encountered with Navier-Stokes equations, while studying them at university, was the fact that you could plug the same problem, with the same mesh size into three different CFD packages and get three different answers.

As far as I recall the equations have no cast in stone solution, but a series of opposing variables which are balanced via successive iterations, until they seem to work.

We then had several discussions as to whether they can be used to describe or approximate the lift/flow theories, and the debates ended pretty much the same as this forum. A Newtonian camp, Bernoulli camp, a few abstentions, and a couple of perplexed by-standers.

The only thing we agreed on is that this has been debated for 100 years, and will probably still be debated on in 100 years!

[barit1]

That's not too unusual in iterative solutions.

It is not unlike the old logical problem: If you place an engineer and a mathematician side-by-side at one end of a corridor, and a buxom blonde at the other end, and tell the two guys every minute they can each advance one-half the remaining distance to the girl, there will be two different responses:

Mathematician: "But I'll never get there!"

Engineer: "I'll get close enough for all practical purposes!" :ok:

[fadaknet]

what's happend :}

[BizJetJock]

Personally I think my wife's theory of flight is much closer to the mark:-
There are lots of cute furry invisible creatures called Bernoullis that hang on to the leading edge and pull upwards. Like lots of cute furry things, they're not as nice as they seem at first glance, and are extremely bloody minded. So if you try and dislodge them by flying faster or pulling on the stick, they just pull up even more. However, ultimately you are stronger than they are, so if you pull hard enough you can dislodge them and you haven't got any lift any more.......:eek:

[callsign Metman]

but...you've all failed to point out the obvious....

everybody knows that propellors (and rotor blades) don't actually contribute anything to keeping an aircraft airborne. They are merely there to keep the pilots cool ' cos when they stop turning you will see that the pilots start sweating!

CM

[littco]

If you look at the first planes and especially the wright brothers plane , their wings had little or no camber and as such totally relied on N3 for lift. Admittedly they only had little thrust, but their principle of flight was the same as putting your hand out the window of a moving car!

Whether you like it or not air is a fluid, just the same as water, snow or ice they are all just at different stage. If you look at fluids in motion then a plane flying through the air is the same as a water skiier on water, a skiier on a sky slope and an ice skater on ice, all are acting as per N3 law!

just my pennies worth and as long as what ever it is keeps me in the air I;ll be happy!:hmm:

[barit1]

If you look at the first planes and especially the wright brothers plane , their wings had little or no camber and as such totally relied on N3 for lift...

And contrary to the title of this thread, the Wright propellers pushed, not pulled... :}

[CAT1]

Bernoulli sucks......

[MustangFlyer]

I was once asked this question a few years ago and I replyed with the usual answers. After thinking about it a bit more and read further (including a running debate in New Scientist) I realised that there is a major failing in the standard answers, as regards giving a basic understanding what is actually going on.

Firstly, except for some military planes, no plane has the power to fly by engine power alone.

Simple test, take a 747, put it on its a*s*, firewall the throttles ... nothing happens.

So where is the free energy coming from?

At its most basic it is N3, but not the way as usually described. For the simplest explenation it is actually better to think of air as particles.

Each particle has mass and hence momentum (due to temperature [=energy]every particle is moving) and every particle repels each other (think of springs coupling every particle).

What happens with a plane (or a skipping stone on water, or a neutrally ballested submarine) is when it moves the wing represents an immovable surface (as it is not a fluid). As it moves air particles bounce off the wing, but they compress against other particles, which push back. Also there is a second effect because of resistance because they have momentum (think of snooker balls).

If the wing was totally flat relative to the air particles and a perfectly even shape then all the forces would balance out, the momentum transfer and the spring forces.

But if the wing is angled (I wont go into shapes at this level of explanation) the forces become uneven.

If you are a Formula 1 car you want the forces downwards, to hold on the road, if you are a plane you want the forces upwards, to lift you into the air.

Now these forces are quite small and evenly balanced at sea level and no air speed. But think of a hurricane (say 100kph+). The momentum and spring reaction from each air molecule becomes large. If you are standing in a hurricane at 200kph you (or your car or house) will fly! In this case the energy gap comes from the sun (somewhere out in the ocean)

The inverse happens with a plane. You go fast (the energy from fossel fuels) and you close the energy gap and (with the right angle of wing to create unbalanced forces ) you lift.

Summarising: You get free energy from the momentum and repulsive forces of each air molecule. Normally these are evenly balanced. But, if you add energy (speed) and unbalance these forces they will lift you, even though the energy you expend in going fast is not enough to overcome gravity. Basically you get free energy from the atmosphere.

Simple test. You are in on a planet in a vacuum. You go very fast .. what happens?

[barit1]

I was once asked this question a few years ago and I replyed with the usual answers. After thinking about it a bit more and read further (including a running debate in New Scientist) I realised that there is a major failing in the standard answers, as regards giving a basic understanding what is actually going on.

Firstly, except for some military planes, no plane has the power to fly by engine power alone.

Simple test, take a 747, put it on its a*s*, firewall the throttles ... nothing happens.

So where is the free energy coming from?

...
The inverse happens with a plane. You go fast (the energy from fossel fuels) and you close the energy gap and (with the right angle of wing to create unbalanced forces ) you lift.

Summarising: You get free energy from the momentum and repulsive forces of each air molecule. Normally these are evenly balanced. But, if you add energy (speed) and unbalance these forces they will lift you, even though the energy you expend in going fast is not enough to overcome gravity. Basically you get free energy from the atmosphere.

...

What free energy are you talking about?

In level flight, no energy AT ALL is required for the plane to hold altitude - it is neither gaining nor losing potential energy. The only energy involved is the drag (induced + parasitic drag) x velocity, and this energy is provided (exactly) by the engines.

In a climb, to gain potential energy, more propulsive thrust is used. In descent, the opposite applies. But in level fight, the potential energy is constant, and there's no "free energy".

And your treatise completely disregards the viscous forces in the air, which would lead you to an appreciation of Reynold's number... :8

[Mugi]

What free energy are you talking about?
I think MF's example has the 747 sat on its tail with the engines pointing vertically downwards. Gravity is not overcome, yet in flight gravity is overcome but without any increase in engine power. I can see what he's getting at.

I find learning Bernouli et al to be somewhat like having to learn creationalism knowing that it'll be on the test.

My Trevor Thom book says:

"Moving the left rudder pedal forward deflects the rudder to the left. This increases the speed of airflow on the right hand side of the fin, reducing the static pressure there and creating an aerodynamic force to the right."

Argh!
Pretty much the same theory is applied to the elevator in that tome.

Problem is. Why do we then have all moving tails? Why would the airspeed increase below the tail because of decreased AoA? (up elevator)

Also, I can make a flat plate wing fly on a model aircraft and near enough on full size (Lightning springs to mind). All about the angle of attack and deflection of air then surely.

Logically when you apply up elevator on an 'all moving tail' then the aircraft will rotate from the leverage of the deflection force until the tail datum realigns with the airstream. Since the plane is now following a curve the airflow direction (relatively) will keep changing and so the aircraft will continue to rotate so long as the stab is misaligned and therefore creating the deflection :E

[barit1]

The problem with all the simplified examples is that they disregard the concept of l/d; The wing (rotor, propeller...) airfoil generates a large amount of lift at a relatively small expense in induced drag. I think this is the "free energy" MustangFlyer refers to.

If an 800,000# 747 has an l/d of 16 (just guessing - corrections welcome) then its drag is 50,000#. Therefore 50,000# of thrust (total) is needed to maintain stable level flight. :ok:

[ojay]

hi guys,

read 'Handling the Big Jets for a pilot type description of all moving tailplanes/stabilizers and why swept wing jet transport aircraft have them;going slightly off topic...

b/rgds

[High Wing Drifter]

Mugi,

It is because of platitudinal quotes such as that Thom one you mentioned that I started this thread in the first place. Like you, it just doesn't seem to add up. I am grateful to Ghengis (among others) for helping me clarify this in my mind by more or less confirming what I suspected. I admit when people start talking about Navier-Stokes I totally glaze (sorry Jon!).
Also, I can make a flat plate wing fly on a model aircraft and near enough on full size (Lightning springs to mind). All about the angle of attack and deflection of air then surely.
One of the examples I expressed in my opening post. Concorde is one of example where the Bernoulli effect would seem to be minimal.

I have soothed my damaged brain by simply reducing the problem to pressure differentials: Lower pressure on top or and higher pressure below. It matter not how the pressure on top of the wing came to be lower than underneath. Be that a simple result of N3 or Bernoulli in action.

The lower the angle of attack, the more bernoulli comes into play. The greater the angle of attack, the more N3 comes into play. At the very high angles of attack the delta and swept wings seem to be able to achieve, there must be some kind of blanking effect that must reduce the pressure of the air on top of the wing to some degree. All to the aide of lift in some form or another. Or is the blanking effect (if it exists) just another manifestation of the Bernoulli effect?

All moving elevators look to me to be symetrical (like aerobatic wings). This would make sense to me or else to lower the nose 3 deg you would need less foward stick than to raise the nose 3 deg (or visa versa if the camber is upside down). If assumpations and appearances are correct then they utilise N3. A tailfin and rudder must also be symetrical and the rudder effectively deforms the wing so I imagine at low rudder deflections, Bernoulli effects are the main contributor as that adds camber, at higher rudder deflections more N3 comes into play as the rudder increases its AoA.

[Mugi]

HWD - the Bernoulli effect for low rudder displacement; I can see the logic there. It's acting as a flap does perhaps and increasing lift.
The thing that bugs me about it all is that all through my life - from school, through ATC, to now - is that Bernoulli is taught as the 'magic flying principle'.

It's obviously not the sole factor as we've discussed. I quite enjoyed Mustang Flyer's 'springiness of air' explanation too. Made me think of quicksand. Run across (high airspeed) and you're fine... but slow down (low airspeed/stall) and you'll sink :D

I wonder how aircraft in ground effect are explainable, with regards to any of the 'models'?

[jtt]

You shouldn't try to think of an aircraft flying due to either Bernoulli or N3. Basically, there are two fundamental laws of physics that always need to be obeyed. The first one is conservation of energy and Bernoulli's equation is just another way of expressing this fundamental law. The second law is conservation of momentum, which can be written in the form of Newtons third law. As e.g. ARINC already pointed out both of them must hold at once and you can't just say "it's flying because of Bernoulli" or "it's flying because of N3".

Moreover, Bernoulli's equation is often employed using a wrong model, i.e. that two molecules from the air stream split at the leading edge will meet again at the end of the wing which, if you don't have a flat plate, would require the air above the wing to move faster, thus producing a lower static pressure above it, resulting in lift. This isn't true and, if you do a rough estimate, leads to the result that the lift would be very small (I read somewhere that a Cessna would only take off at about 400 mph's if this model would be true!). But, unfortunately, it's still taught quite often and you will find in lots of text books on this subject.

Actually there are two contributions producing the lift, coming from both the sides of the wing. Already when you have a flat wing with an certain AoA you get lift from the bottom of the wing because the molecules of the air bounce against it and get deflected downwards, resulting, due to N3, in a corresponding upward force on the wing (plus, of course, some drag which you need to compensate using the engine).

On the upper side of the wing things are a bit less easy to visualize, here you have what's often called the Caonda effect. Just imagine again that you have a flat wing with a certain AoA. If the air would move completely straight (i.e. horizontally) above the wing it would pull away some of the molecules behind the leading edge of the wing (on the lee side) due to friction. That would produce a lower pressure where these molecules now are missing, pulling the following air mass coming over the edge of the wind downwards. As a net result the air going over the top of the wing also gets accelerated downwards, adding further lift, again due to N3. Actually, this second contribution seems to produce the major part of the lift.

There's a very simple experiment demonstrating the Coanda effect quite nicely. All you need is a spoon and a faucet. Open it so you get a constant non-turbulent flow and then move the backside of the spoon slowly from the side into the water stream. You will notice that the water won't move straight down anymore when the spoon comes into contact with the water but gets deflected in the direction of the spoon and you also will feel quite a strong force on the spoon, trying to push it further into the water stream. Now turn everything by 90 degree, replace the water by air and the spoon by the wing and you have what happens at the upper side of the airfoil.

If you now want not only a qualitatve picture but want to get into the nitty-grity details of the exact forces on the different parts of the wing of a certain shape then, as Ghengis the Engineer already pointed out, you need the Navier-Stokes equation to get the (more or less) complete picture. But that's left as an exercise to the reader;-)

Just for those curious about conservation of momentum: even in level flight air must be accelerated downwards in order to have enough upward force on the wing to counter gravity. But that means that a downward momentum is created (the air deflected down) while the aircraft stays at the same height, thus no change in its momentum. Naturally one would ask "Where's the upward momentum to satisfy conservation of momentum?" The answer may sound funny: the upward momentum goes to the earth below. While it tries to pull down the aircraft at the same time it gets pulled up to the aircraft with exactly the same force, (it just doesn't move visibly because it has such a higher mass), resulting in a momentum exactly opposite to the one of the air accelerated downwards.

[High Wing Drifter]

jtt,
ARINC already pointed out both of them must hold at once and you can't just say "it's flying because of Bernoulli" or "it's flying because of N3".
I totally appreicate that. I guess I'm still hunting along the lines of my initial thread starting question related more to the primary reason for lift. The books say Bernoulli, but as a primary reason I am/was not convinced.
f the air would move completely straight (i.e. horizontally) above the wing it would pull away some of the molecules behind the leading edge of the wing (on the lee side) due to friction. That would produce a lower pressure where these molecules now are missing, pulling the following air mass coming over the edge of the wind downwards. As a net result the air going over the top of the wing also gets accelerated downwards, adding further lift, again due to N3. Actually, this second contribution seems to produce the major part of the lift.
Ah yes! That is exactly what I was hopelessly trying to describe when I called it the "blanking effect". Thank you for an incredibley lucid and understandable descripiton :ok:

Mugi,
I wonder how aircraft in ground effect are explainable, with regards to any of the 'models'?
Good question. This article (http://www.avweb.com/news/airman/185905-1.html) explains things in a convincing manner. Whether it is true or not...

[balsa model]

Jtt: I think that we needn't worry about momentum conservation so much. The downward force that the wings exercise on the air doesn't go into creation of an accelerated column of air. Instead, it goes into accelerating two opposite swirls of air: one caused by the left wing, the other by the right. Their moments cancel out, hence conservation of angular momentum is satisfied. There is no net vertical acceleration of air (did I just open a can of worms, with tales of descending vertices and all?), so no concern for conservation of momentum.
Incidentally, in ground effect the swirls can't form fully because the ground is in the way. Consequently, the ground is responsible for the sizable portion of N3 balance of forces.
Once we're finished with this Bernoulli or not to Bernoulli business, we should tackle Relativity. I got some doubts.

[Mugi]

Good question. This article (http://www.avweb.com/news/airman/185905-1.html) explains things in a convincing manner. Whether it is true or not...

Nice link. Thanks, HWD. Convincing but again (and I'm playing devil's advocate here) hints at more magic and mumbo jumbo.

Now I have to believe that because the air in a vortex I left behind in my wake meets the ground, I get a magic drag reduction, therefore lift boost;)
But I left that air way behind - it's 30m gone, history etc...

[chornedsnorkack]

Nice link. Thanks, HWD. Convincing but again (and I'm playing devil's advocate here) hints at more magic and mumbo jumbo.

Now I have to believe that because the air in a vortex I left behind in my wake meets the ground, I get a magic drag reduction, therefore lift boost;)
But I left that air way behind - it's 30m gone, history etc...

OK. Let us imagine it in this way:

Put a lid on a vessel so that the lid is slightly smaller than the vessel.

The lid could sink into the vessel, because the lid is not actually stuck to the walls - there is a gap all around. But the gap is narrow everywhere. Therefore the air in the vessel is compressed until it supports the weight of the lid and will only slowly escape through the gap. It does not take much power to pump the air back under the lid and keep the lid suspended.

Now, imagine that the wing is in a long and narrow ditch, the wingtips nearly touching the banks. The air can escape in front and behind of the wing - but not to the sides. So, you have to do quite some work to fly along the ditch.

But the moment you leave the ditch, air gets the possibility to escape around the wingtips, too. Which means that you have to do somewhat more work to replace the air cushion beneath you.

Is this a meaningful explanation of ground effect?

[Mugi]

Is this a meaningful explanation of ground effect?

Well, if not it's a great explanation of an aeroplane flying in a ditch. :D
Thanks!

[john_tullamarine]

High Wing Drifter,

At the very high angles of attack the delta and swept wings seem to be able to achieve, there must be some kind of blanking effect that must reduce the pressure of the air on top of the wing to some degree.

You are missing another flow pattern which is relevant (especially to deltas) .. leading edge vortex which provides a substantial part of the lift and accounts for the high alpha. Plenty of references around - for instance (http://adg.stanford.edu/aa241/highlift/sstclmax.html). A very common real world flow visualisation in high humidity conditions as in the Concord shot in the previous link. The phenomenon is important in the flight mechanics of many living entities (for instance (http://www.sciencemag.org/cgi/content/abstract/sci;306/5703/1960)).

[LGB]

All these discussions are really a cause and consequence discussion.

Take this example.

Two fellas walking down the street, one keeps snapping his fingers. When asked why, he says, it's to keep the tigers away. But there are no tigers here? He answers, yes, you see, it works.

This IS really what it is all about. So many theories can explain this, and for some reason it is thought by many that one has to choose one of them only.

Cause and consequence is often a chain of consequences. Without mentioning too many theorems, laws, postulates and clever scholars, my natural explanation is this. The only one law I have to fall back to, is Newtons Third.

We generate thrust by accelerating air opposite the way we want to fly.

Speed picks up, until it balances out drag - as hitting the air molecules takes our momentum away. The energy does not dissapear, it is just transferred to these molecules we hit. No atmosphere -> no drag, but also no lift.

As speed picks up, air is deflected downwards by the wing. Any wing will do, as long as there is 1. Enough thrust 2. Stability/control. I would say even a tank can fly, given enough thrust and stability/control surfaces.

From generating thrust to obtaining the lift, so may therories/postulates/laws can be used, like the Bernoulli law causes the pressure difference, which in turn causes downwash. Or "the molecules are forced down" (like waterskis, mentioned above). How much, and why ... it matters mainly to aerodynamic engineers when the design and build aircraft, so they can predict the characteristics before they even test the first aircraft.


So even if I believe that Newtons Third, there are events that must happen before we get that far. It is not a religious you-have-to-pick-one-law-only thing!

Another similar cause and consequence discussion is the one about ground effect. I have seen about 5 different explanations to it. The one I thing best describes it (though they are probably all true), is that the wake turbulence is reduced by the ground stopping the turning motion at the wingtips, and when less molecules are deflected the wrong way (up instead of down), less molecules must be accelerated down. Knowing that, you can call it an air cushion, tilting the lift vector, reducing the downwash, air that cannot escape as easy etc.

[MustangFlyer]

The question is "why planes fly".

There are only a few things to understand:

(1) There are no pulling forces in explaining how a plane flies.

Unfortunately much of the models and mathematics developed came before the atomic theory of matter was finally accepted (thanks to Einstein) and developed. As a crude approximation fluid equations seemed to answer most questions and deliver reasonable results.

This has continued to this date. Aerodynamics is a series of approximations and cludges. Mixtures of different mathematical models (which sort of work .. mostly) and empirical results (such as certain aerofoils are better than others for a particular flight regime).

However misconceptions continue to this day .. such as:

(a) The Coanda effect. This is a symptom of something else that is more fundamental, not a thing in itself. It has no contribution to flight whatsoever (despite what NASA says).

(b) Low pressure "pulls" something up (see NASA site for this explanation ... they should know better). Air does not suck. It has no attactive force whatsoever (well I suppose there is a little gravity or some interactions between oxygen atoms and the metal of the wing .. sod all of anything).

(2) You can explain everything by understanding 2 things:
(a) Air is full of molecules, each with mass and speed and there are a lot of them.
(b) Each molecule is moving, but at different speeds and different directions.

Therefore, air exert a force in all directions, up, down, sideways, at an angle.

A wing (or barn door) when stationary has all the forces balanced. Gravity pulling down, air pressure pushing up, air pressure pushing down. What we call pressure is simply the sum of all the forces of all the air molecules bouncing off the wing. Straight foward Newton.

When we start moving and the wing (or barn door) is angled up, we start bouncing off the air molecules, pushing a lot of them down. The net transfer creates a force upwards (lift).

We also push some molecules forward (remember they are coming in from all directions and at different speed, i.e. different momentums). This is what we call induced drag. Because the wing is angled we can't help pushing some of them forward rather than down, thus wasting energy.

What happens above the wing is a little more subtle. The air is pushing down at all times by molecules (M for short) hitting the wing. But remember, not all air M travel at the same speed (it actually follows a normal distribution). The average speed of an air molecule (at 0C and sea level is about 1600 km/hour. However some are far faster, some are slower, some are virtually stationary.

The wing is angled, as it moves forward the molecules have further to travel to hit the top of it (a simple diagram will show this, basically the surface moves away from it) and hence bounce off it and exert a force downwards. The fastest ones will hit quickly (which is why there is still air there) the slowest will take longer, so overall there is less air there and hence less momentum transfer (=force) at the beginning of the upper part of the wing. At sub sonic speeds and a reasonable sized wing most of the molecules will eventually hit the wing, exerting a force downwards (some will miss it entirely).

So we have a pushing upwards from below, but less of a push downwards from above (particularly at the beginning of the wing).

Sum up the forces .. and a plane flies if you go fast enough.

This explains why the majority of the lift comes from the beginning of the wing (or barn door), there are more net upward forces (upwards + less downwards).


Next post(s):

- Why there are wingtip vortices, why wingtips have less lift, why vortices have nothing to do with loss of lift or drag.
- Why temperature is important to describe why there is less l/d drag issues at height (= why can we go faster the higher we go).
- Why does air flows (= sum total movements of air M) seem to curve.

[bookworm]

(b) Low pressure "pulls" something up (see NASA site for this explanation ... they should know better). Air does not suck. It has no attactive force whatsoever (well I suppose there is a little gravity or some interactions between oxygen atoms and the metal of the wing .. sod all of anything).

I'm not sure it's helpful to be too precious about the use of the word "suck". In its original meaning, it's about creating lower pressures within the body to cause forces towards the mouth on objects outside the body. No attractive forces are involved.

[MustangFlyer]

Well somone has to be attracted to you!