How Aeroplanes Fly and Propellers Pull
 

Hi All,

A point of curiosity prompted this 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?

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?

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°.

aerothingies
 

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

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.

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

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

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

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

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.

Smartarse, spoilsport.

It's Magic.
 

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

Wing-walkers seem to manage!

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

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

Pilots are smelly and the earth repels them.

Helos are ugly and the earth repels them.

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.

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.

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

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.

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

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

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

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

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...

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....:{

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

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...

> 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

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:

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

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

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

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

There seem to be two

Navier-Stokes and Napier-Stokes

Are they different or the same or what?

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

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.

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