How Aeroplanes Fly and Propellers Pull

[Genghis the Engineer]

Originally Posted by Mac the Knife

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 (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!"

[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.......

[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!

[barit1]

Originally Posted by 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...

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]

Originally Posted by 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?

...
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...

[Mugi]

Originally Posted by barit1

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

[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.

[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

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

Mugi,

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

Good question. This article explains things in a convincing manner. Whether it is true or not...

[balsa model]

momentum conservation

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]

Originally Posted by High Wing Drifter

Good question. This article 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...