Fear of flying Bernoulli-Style [Archive]

Checkboard

14th Apr 2001, 09:57

This is a typical misunderstanding of the Bernoulli principle. It begins with the simplified explanation that involves drawing two parcels (or molecules or whatever) approaching a wing, one flowing over the top and one underneath, then saying that they have to match up with each other (they don't!) at the trailing edge.

The explanation then follows that the parcel travelling over the top must move faster, as it has to cover a greater distance. Easy to draw and understand, but not correct.

The air flowing over the top of the wing has to increase speed as it has to squeeze the same volume through a smaller gap. Imagain squeezing toothpaste through a tube - the toothpaste in the tube is travelling towards the end rather slowly, but at the narrow nozzle is squirting out quite quickly.

The two airstreams don't meet at the trailing edge at the same speed or matching positions unless there is no lift being created.

[This message has been edited by Checkboard (edited 14 April 2001).]

Genghis the Engineer

14th Apr 2001, 21:04

Bernoulli's theorem and equation are useful models, but they are only models, for explaining how an aircraft lifts. There are lots of others, each useful in its place.

For various reasons, Bernoulli has become the standard theorem for teaching pilots, but don't get hung up on it - thinking too hard about it doesn't help much.

If you take the standard Bernoulli formula it doesn't actually explain lift because it doesn't include viscosity. It also needs modification for compressible flow (that is anything about about M=0.5), but for general explanations it is still fine, which is why you'll find it in most of the textbooks.

G

Oz_Pilot

15th Apr 2001, 06:30

I like the mass flow explanation better i.e. that since air is forced down, the aeroplane is held up. Bernoulli can achieve this result reasonably easily... Circulation theory is really the most correct as it starts with viscosity but is hard to wrap your head around.

Picture this: a cylinder in an airflow. When you rotate the cylinder, one section is moving against the airflow and one moving with it. The first section experiences a higher difference in velocity between skin and air than the second, so the air there is slowed. After some hoop jumping with velocities, pressures and calculus you relate this picture to an aerofoil using a transformation function.

(PS I highly recommend NOT relying on this description!)

The other condition needed is the Kutta Condition, which is that the two airflows over an idealized 2D aerofoil meet with negligible differences in velocity at the trailing edge... which might be where the "two parcels meet again" explanation is derived from. CB, I'm not entirely sure I understand you saying "...unless there is no lift being created" in this condition - if the resulting velocities are the same for upper and lower surfaces, but the direction of flow is now downward, then isn't lift present?

Prof2MDA

15th Apr 2001, 09:22

Don't have time to respond to this in detail, except to note that Chris Carpenter has a nice discussion of this in his Flightwise: Principles of Aircraft Flight, Volume 1.

He does note that while the "mass-flow" theory is something that is happening, it leaves out the fact that the mass-flow is a _result of_ Bernoulli...

Read the section and take your counter arguments up with Professor Carpenter. ;-)

boofhead

15th Apr 2001, 11:25

I was at a symposium on something or other in California and the theory of lift came up. One of the participants used the espression IMPACT LIFT to explain the reason airplanes stay in the air, and it seemed to me to be absolutely right! I have often used the water ski as a good explanation myself, and the idea of impact lift matches perfectly. I don't even have to explain it. Has anyone else heard this idea, or am I just out of the loop? (loopy)

bookworm

15th Apr 2001, 15:31

I like to divide the explanation of lift up into two parts:

1) Why is the pattern of air flow around the wing as it is?

and

2) Given that the pattern of air flow looks like that, what's the resulting lift?

Part 1 is, for me, the tricky part to explain. Why does the air flow faster over the top of the wing?

As Checkboard rightly suggests, air packets don't meet up at the trailing edge, as you can see from John Denker's excellent description (http://www.monmouth.com/~jsd/how/htm/airfoils.html).

(Finding a better explanation involves the Kutta condition mentioned by Oz_pilot and circulation theory -- hand waving is required for much simplification.)

Part 2 is just a computation. The air is travelling faster over the top of the wing therefore the pressure is lower on top, and so there's lift.

Bernoulli's theorem (and despite the name it's no more dubious as a law of physics than Pythagoras's theorem is as a law of geometry) is important in part 2, but it does not help with part 1.

A footnote: though Gengis is strictly correct when he says that viscosity plays a part, that could be misinterpreted. The Kutta condition mentioned by Oz_pilot relies on there being some tiny residual viscosity to encourage the air flow at the trailing edge to leave in the direction of the trailing edge (usually slightly downwards).

But beyond that, an approximation of air as inviscid (no viscosity) and incompressible using Bernoulli's theorem does a pretty good job of predicting real-life lift coefficients. Look, for example, at lift vs AOA for different Reynolds numbers (as shown in an aerodynamics text like Abbott and von Doenhoff) and you'll see that as Re goes down from infinity (no viscosity) to real flight Re values, the significant difference is only really around stall (which is a viscosity related phenomenon).

Even if you do bring in viscosity in using a more complicated equation, the equation looks very much like Bernoulli's theorem anyway.

[This message has been edited by bookworm (edited 15 April 2001).]

Luftwaffle

15th Apr 2001, 20:47

I would answer the "why does the vacuum not just suck more air in from above?" question by saying, "Yes, good -- IT DOES." When a low pressure area is created above the wing, it sucks everything around it to fill that gap INCLUDING THE WING OF THE AIRPLANE. The continued movement of the airplane through the air serves to maintain the low pressure area. The student who asks that question has just started to understand what is going on.

An analogous question could be, "you say the major airlines are hiring, but won't they just hire people who have more experience than me, from regionals or other majors?"
Well, sure they will, but then those regionals will need to hire new people from the charter outfits and the charters will turn around and hire low timers just like you. Even if the vacancies are being filled by air molecules or candidates from the top, there still needs to be some brought up from the bottom. (Economic downturns and severe downdraughts will be a topic for next week's lesson.)

To answer the impact lift people, yes, I agree it's much easier to visualize that source of lift. I don't dispute that the air incident on the underside of the wing provides lift, but it's not the major contributor. Why do I say that? Consider how much lift is lost when air flow on top of the wing is disrupted by frost or ice.

Prof2MDA

15th Apr 2001, 21:55

This thread is in serious need of some Flightwise quotes!

Still too busy to respond in detail, but with regard to "impact lift", this only applies to hypersonic- very high altitude type vehicles. It does not result in enough force to explain why most airplanes fly.

Prof2MDA

15th Apr 2001, 22:07

Should also include this site:

http://www.lerc.nasa.gov/WWW/K-12/airplane/bga.html

I did take a quick peak at that other thread where the original poster here got the question. Lot's of ignorance there! From their discussion on how jet engines create thrust it is obvious to me that they don't understand how a balloon creates thrust when you untie it and let it go!

Just to quickly state on that last item, recall that the air inside an inflated balloon is under higher pressure, and that pressure is pushing out in all directions. A tied balloon won't move because it is pushing in all directions. If it has an opening (from being untied) the air is still pushing on the side opposite the opening, but now is not pushing on anything where the hole is. This creates a difference of forces, and so the balloon is moved by Newtons laws. Jet and rocket engines work on the same principle.

Squawk 8888

15th Apr 2001, 22:43

Prof, that's why I brought it here- I was kinda hoping someone might be interested in jumping in on the other thread. Lots of smart people on that BB, but aviation & aerodynamics are not their field of expertise.

NIMBUS

16th Apr 2001, 01:37

Squawk,
The whole Bernoulli thing is just the easiest way for a student to understand.
The lower pressure on the upper surface of the wing contributes only a very tiny amount of lift. The majority of lift is produced by the downward deflection of the air mass, as per Newtons principles. The airfoil shape sets up a 'circulatory' airflow, and its the reaction to the downward motion of air that keeps the wing up (lift).
Best demonstration is to stand under (or near) a helicopter!

bookworm

16th Apr 2001, 01:58

The lower pressure on the upper surface of the wing contributes only a very tiny amount of lift. The majority of lift is produced by the downward deflection of the air mass, as per Newtons principles. 

Unfortunately NIMBUS, that's not really correct.

If you throw a rubber ball against a wall, a force is applied to the wall at the impact spot for a short time. Is that due to

a) the increased pressure underneath the compressed ball at the impact spot or

b) the change of momentum imparted to the ball by the wall?

The answer is that these are different ways of looking at (and if necessary calculating the magnitude of) the same phenomenon.

It's a similar picture with lift. The lift on a wing is:

a) the pressure difference between top and bottom integrated over ("multiplied by") its surface area

b) the rate of change of vertical momentum imparted to the air by the wing

Both account for all of the lift (with a lot of caveats about how you measure that momentum in case b).

In fact, the difference in pressure from ambient is usually rather greater above the wing than it is below. It that sense, it's more suck than push. Perhaps we shouldn't go there... :)

Prof2MDA

16th Apr 2001, 02:24

One more time: The air moving downward with its associated momentum is a RESULT of the aerodynamic forces that are creating the lift, and NOT creating any lift themselves!

Go read some of John Anderson's or Chris Carpenters books!

DeltaTango

16th Apr 2001, 23:19

Im having trouble understanding the newton's part in kicking the wing.

take a wing fly it at the same aoa-note the lift produced.
extend spoilers on that same wing at the same aoa - note the lift.

get my problem?

DT

NIMBUS

22nd Apr 2001, 03:13

The amount of ‘suction’(lift?) on the top of the wing is very small. For the average C-172, for example, a force of maybe 2 ounces/sq.in.
The airfoil shape is mainly to to convert static (atmospheric) energy to kinetic energy. (This reduces the pressure and allows the wing to more easily move in that direction). The airfoil shape also produces a downwash of air. The wing (airfoil), in effect ‘throws’ the air downward. The reaction to this causes the wing to move in the opposite direction (into the low-pressure area). The recoil when a bullet leaves a rifle, or the air escaping a balloon, is the same thing.
The air impacting the bottom of the wing is also deflected down, and contributes to lift. (think of a kite, or water-skis) especially at large angles of attack.

A propeller is an airfoil, too. Basically, a rotating wing turned 90 degrees. Stand two of three feet in front of a prop at high rpm, and you may feel a very slight breeze. Stand a few feet behind, however, and you’ll feel a lot more than a slight breeze!
The prop, like any airfoil, is designed to fling large quantities of air in a particular direction. (If the majority of lift was due to 'suction', then ground effect would not be a problem).

Finally, and one of my best examples, stunningly simple, yet verging on genius….
Birds!
Birds wings flap to force air down, not to create suction.

Thusly do I squash all opposition to my beliefs!!! :) :)

[This message has been edited by NIMBUS (edited 21 April 2001).]

Prof2MDA

22nd Apr 2001, 08:26

Nimbus,

Although Flightwise does a nice job of telling it how it is, the Illustrated Guide to Aerodymics, by HC "Skip" Smith, director of undergraduate studies in aerospace engineering at Pennsylvania State University probably says it best:

"According to Newton's Third Law, every action must have an equal and opposite reaction; thus, the air must exert an equal force upward on the wing. This, indeed, does occur.

Occasionally one might hear, among the hangar crowd, that this is the *true* explanation of lift and that Bernoulli's principle and the lift theory associated with it are just myths. This is not correct. The two different methods of explaining lift are not opposing theories. They are merely different ways of looking at the same actions. Remember that the momentum change comes from the downwash, which is caused by the wingtip vortices. The vortices, in turn, exist because of the pressure differential between the upper and lower surfaces, and this differential is explained by Bernoulli's theory. The two theories mutually support each other, and simply address the same physical phenomenon in different ways."

As to your assertion that there is not enough force through Bernoulli to "explain lift", this is wholly incorrect.

There is plenty of force difference there to explain lift. It is essentially from Bernoulli's equation that we end up with the basic lift equation, and it works!

L=Cl 1/2 rho V^2 S

You can figure out the Cl fairly easy by swapping terms around if you wish, but there is no problem getting enough for various airspeeds, and figuring out how an airfoil might need to be modified to get more lift at lower speeds than higher ones, etc.

bookworm

22nd Apr 2001, 15:04

NIMBUS wrote

The amount of ‘suction’(lift?) on the top of the wing is very small. For the average C-172, for example, a force of maybe 2 ounces/sq.in.


You actually over-estimate it slightly. 2 ounces/sq.in. provides 288 ounces/sq. ft or about 18 pounds/sq. ft. A typical C172 is wing-loaded at about 15 pounds/sq. ft. That 'small amount of suction' is all the lift it needs to support its own weight and fly.

22nd Apr 2001, 15:23

NIMBUS,

I'm very sorry but that's not correct. You need to find a pressure diagram of a wing and have a look at it yourself - or better yet, sneak yourself into an aerodynamics course at a uni with a wind tunnel. It's fascinating and once you've made a few such diagrams yourself you will be convinced and have gained much understanding. You'll see that the main differences from ambient pressure is on the top of the wing. The pressure on the bottom of the wing is in fact often LOWER than ambient presure, only less lower than the pressure on top of the wing.

Yes, Newton's law still stands. There is a downwash and change of momentum of the air matching the lifting force. Only, this downwash is not generated by air being beaten down by the underside of the wing but rather by air being SUCKED down on the top of it. The air wants to move straight ahead past the airfoil due to it's inertia. However, this forms empty space along the surface - a pressure gradient. This will force the air to turn downwards. No fluid will change its velocity without a pressure gradient and no pressure gradient in a fluid without velocity change. Chicken or egg? :)

Helo rotors are just wings with a different way of creating airflow. Same thing. Ditto for birds.

The reason that you don't feel much of a draft in front of a prop is that the air is sucked in through a hemisphere but blown out through a "pipe". Much larger area of inflow three feet away from the prop than the area of outflow ten feet behind -> much higher velocity ten feet behind. It's the same volume of air passing through - nothing to do with where the lift is generated.

Ground effect is a very interesting thing, I have to admit I'm still struggling to get my head around it. However, it doesn't depend on the rotor beating the air down any more than any other regime of flight. It has more to do with reducing the upwash in front of the wing generated by the low pressure area above it.

As a previous poster pointed out, if lift IS generated on the bottom of the wing, why are spoilers ON TOP of the wing so efficient in killing the lift? Why do we let all the draggy stuff hang out from the bottom of the wing while keeping the top as clean as we can? There is a reason... :)

Cheers,
/ft

NIMBUS

22nd Apr 2001, 20:29

Fascinating topic! You Guys are Good!

Ft,
Maybe I did not get my point across very well. I’m not saying that the air deflected down by the bottom of the wing generates lift, it only adds a little.
I contend that the majority of lift is due to the air ‘sucked’ by the top and ‘thrown’ down past the trailing edge (more or less as you said!).
The low pressure area on top serves to get the air moving in the right direction. In addition, as the wing moves forward, this low pressure attracts a greater quantity of air to flow above the wing, where it can be tossed down. Spoilers on the top destroy the whole airflow pattern.

Ground effect gets to me too, but it actually has little or nothing to do with lift. It reduces drag. You’re right about reducing the upwash, which lessens the effective AOA and decreases induced drag.

Good point about standing in front of the prop.!

As for going to a University, I was always too thick to understand fancy diagrams and numbers with x and y in them! However, I used to pass by one on the way to work! Does that count? :)

Prof.,
Not sure about your statement that the downwash is caused by wingtip vortices…!
I would have thought that the wingtip vortices are an undesirable result of the whole up-/downwash action, not the cause!

I agree that both Bernoulli and Newtonian principle's are at work, but I think the Newton idea is more influential.
Consider this….
Put enough power behind a brick and it will fly. Air deflected down will keep it up. Adding a nice cambered surface on top will make it easier, but its not essential.

Park an aircraft with stall speed of, say, 60 kts, facing into a 65 kt. wind. If Bernoulli is more influential, this aircraft should take off…..(?)

Go on…Poke holes in my beliefs and make me feel bad! :) :)

bunyip

22nd Apr 2001, 21:47

I think your theories make more sense than most, Nimbus. At least I can understand it the way you present it.
The spoiler on top of the wing does its job by increasing drag as well as by disrupting the flow of air. Since the air is faster (?) on top, it is more affected by drag. Some airplanes do have spoilers on the bottom of the wing (gliders, Vampire etc).
The theory boys ignore the fact that without wingtip vortices there would be no lift. A wing of infinite span creates no lift (Kermode). So wind tunnel experiments do not explain lift unless the wing is complete (ie has wing tips in the flow).
A flat plate creates lift, even at a small angle of attack, when I would assume no flow separation would occur, so the difference in speed of the particles would not be great.
When moving your hand through water at an angle, the force against it is felt almost solely on the bottom side, which means that, to me, the majority of the lift is being produced by the angle of the wing to the relative wind, deflecting the airflow down, and thus the wing up. The rudder on a boat surely does its job by deflecting the water directly, not by creating a low pressure side which causes vortices to form? (water is incompressible). And an aileron must act directly on the flow of air? Wind tunnel theory only partly relfects the real world, and could very well be wrongly interpreted.
If someone kicks my butt, I think it will be the toe of his boot I feel, not the air flow from the boot vortice!

Prof2MDA

22nd Apr 2001, 23:03

Nimbus-
You write:
_________________________________________
"Not sure about your statement that the downwash is caused by wingtip vortices…!
I would have thought that the wingtip vortices are an undesirable result of the whole up-/downwash action, not the
cause!"
__________________________________________

While I agree with the statement made previously that you are referencing, I should point out to you that I was QUOTING Dr. HC Smith, Director of undergrad studies in Aerospace Engineering, Penn State! Are you saying he is wrong? If so, then so is John Anderson, Chris Carpenter, et al. Go look at the NASA web site I posted earlier as well. All the PhDs in Aero agree with each other. You don't agree with them. Sorry, but I think I'll beg to differ with you and go along with the folks who write the textbooks on the subject!

As for your statement referencing bricks as "proof" that it's Newton and not Bernoulli, all that tells me is that you truly don't understand what Bernoulli is and how to apply it. You clearly lack the basic concept of what is generating the flow that brings about the differences in pressure. I suggest again, more strongly than ever, that you buy and read Flightwise: Principles of Aircraft Flight, by Chris Carpenter (who is, incidentally, the head of aerodynamics at the Royal Air Force college).

[This message has been edited by Prof2MDA (edited 22 April 2001).]

[This message has been edited by Prof2MDA (edited 22 April 2001).]

bookworm

23rd Apr 2001, 23:48

The theory boys ignore the fact that without wingtip vortices there would be no lift. A wing of infinite span creates no lift (Kermode). So wind tunnel experiments do not explain lift unless the wing is complete (ie has wing tips in the flow).

Well that's one of the more absurd assertions of this thread! Reference from Kermode please, bunyip, so we can see the context?

A flat plate creates lift, even at a small angle of attack, when I would assume no flow separation would occur, so the difference in speed of the particles would not be great.

What do you mean by "flow separation" here? If you mean "some air flows over the plate, some under", I beg to differ. That's exactly what happens. You might want to look at http://www.monmouth.com/~jsd/how/htm/airfoils.html#sec-thin-wings
to see the difference in speed.

chicken6

24th Apr 2001, 09:24

NIMBUS

The only way to get a brick to fly is to provide so much power that the vertical component overrides the weight, and then you might as well call it a helicopter. How exactly were you going to provide this power? Through a jet? A prop? It would only be the upward component of thrust that would get it off the ground unless you had a brick unlike one I've ever seen - one that deflected more air down with the undersurface than it deflected up with the frontal/upper surface.

And parking aircraft outside in horrendously strong winds does create enough lift to flip them over, that's why tiedowns were invented.

Bunyip

The common name for spoilers under the wing is flaps. Different types of flap exist, and can be hinged in different places along the chord, but in general a flap extending from under the aerofoil is called a flap, a flap on top of it is a spoiler, and on the fuselage they are called airbrakes.

Note they are all named after their function, and there is only one called a 'spoiler' because it spoils the lift.

And the boat rudder thing is more to do with the water being 1000kg/m3 whereas air is only about 1.2kg/m3 (at sea level). Being 1000 times more dense, deflecting a certain amount of water is bound to require a much much greater force (then equal and opposite reaction rah rah rah) than the same volume of air. If water was compressible, then we wouldn't even be talking about this because there wouldn't be any dry land on this fair planet.

When I learned all this stuff and could write pages of calculus on the stuff, I seem to remember Bernoulli being pretty well right and Newton being pretty well right, although memories fade with time!

------------------
Confident, cocky, lazy, dead.

Prof2MDA

24th Apr 2001, 15:20

A brick could fly with enough speed if it were inclined, just as a flat plate works, with enough q, of course.

Lift can be explained either with Bernoulli or via the Newton method (regardless of the fluid you are travelling through)

Bernoulli depends on Newton in various ways (such as continuity.

Newton's method depends on airflow being deflected by Bernoulli, so to get the amount of air deflected off the top of the wing you need you must have the pressure differences that are explained only by Bernoulli so the flow is forced around.

What is being missed here is that these are not conflicting theories at all. They are not even complimentary (meaning that you don't get "part of the lift" from one theory and "part" from another). They are just different ways of _quantifying_ the results. Either way you can come up with ALL the force you need. If you want to work harder you could do part and part I suppose, but that would be a bit silly.

bunyip

24th Apr 2001, 22:59

OK, the flat plate would only work until flow separation (ie stall), so it is restricted to a low angle of attack.
The spoiler or speed brake works because it causes drag. It has to be close to 90 degrees to the airflow to do this. It matters not where it is located; on top or bottom of the wing or the fuselage. If it is on top and moves only a little bit it is a spoiler, and works by disrupting the airflow, and causing, again, drag. The 747 spoiler is restricted in range in flight for example. In flight it operates as a spoiler and on the ground as a speed brake. By spoiling the flow it also reduces the lift produced, since a clean flow is needed to set up the whole lift thingy (technical term).
Kermode Chapter 3 has a lot of stuff in it and most is above my head, I admit. But I agree with the circulation theory and it says you need wingtip vortices to have lift. Without them you have just drag. If you have an infinite wing then the CL is reduced because the correction for aspect ratio is to divide the CL by Pi.A (A being aspect ratio). If A is infinite then CL is zero. The discussion is in the book to cover induced drag, but I am assuming the drop in CL would also affect lift. Could be wrong, so don't run out and buy any lottery tickets based on my advice!
However if this is right then there will be insufficient lift produced by a wing to fly, and it is my own theory that the wind tunnel results are not real world. I can see that the force on the bottom of the wing needs to be greater (impact lift to give it a name), and in order for it to give more lift than the theory supposes, I can see that it is affected by the approach of the wing and is diverted downward before the wing actually passes, and so the wing is riding on a rising flow of air, so it planes, much as does a water ski. A well designed wing will be efficient because it reduces drag, as well as producing lift by increasing the downward flow of ambient air. As someone said, the total of all effects gives the lift; nothing is solely responsible.

Prof2MDA

25th Apr 2001, 01:23

<sigh>

yes, could be wrong about your infitite wing not creating lift...

bookworm

25th Apr 2001, 14:36

OK, the flat plate would only work until flow separation (ie stall), so it is restricted to a low angle of attack.

Correct, just like a shaped aerofoil. The flat plate may typically stall at about 10 degrees with a maximum lift coefficient between 0.5 and 1. The shaped aerofoil is better at avoiding separation at higher AOAs, and can manage a lift coefficient of, say, 1.5. That's why it's shaped the way it is! But the principle by which the shaped aerofoil and flat plate produce lift are the same.

Kermode Chapter 3 has a lot of stuff in it and most is above my head, I admit. But I agree with the circulation theory and it says you need wingtip vortices to have lift. Without them you have just drag. If you have an infinite wing then the CL is reduced because the correction for aspect ratio is to divide the CL by Pi.A (A being aspect ratio). If A is infinite then CL is zero. The discussion is in the book to cover induced drag, but I am assuming the drop in CL would also affect lift. Could be wrong, so don't run out and buy any lottery tickets based on my advice!

You misunderstood the left hand side of the formula you repeated. Cl/(Pi.AR) is the "induced angle of attack". That's the correction to the angle from which the flow approaching the wing appears to be coming because of the wingtip vortices.

To estimate the lift of a finite wing you can subtract the induced AOA from the geometric AOA. For example, a finite wing at 10 degrees AOA with an induced AOA of 1 degree will have the lift per unit span that an infinite span aerofoil has at 9 degrees.

As AR approaches infinity, the correction, not the lift, tends to zero!

The induced AOA can also be used to find the induced drag. The lift vector is effectively tilted back by the induced AOA, so the induced drag is the lift multiplied by sin(induced AOA). Again, as AR goes infinite, the induced AOA and thus the induced drag tends to zero.

[This message has been edited by bookworm (edited 25 April 2001).]

bunyip

26th Apr 2001, 01:45

Ah, you caught my deliberate error! But without using the formula for Aspect Ratios (BTW, what is the correction (if any) on lift if the aspect ratio is changed?) the circulation theory says that lift is generated as a result of wing tip vortices. So if you have an infinite A you would not have wingtip vortices and so you would not have any lift. And that's using Kermode again.
I still think there must be more to it, and that the theories are just that. In the real world there could be things happening that do not show up in a wind tunnel.
Imagine that there are some mad scientists who build a round tank of water and put a large water ski on a radial drive and at a positive angle, which moves around the pool. The ski has a weight in the form of a concrete block that spoils any aerodynamic lift and makes the thing sink if it goes too slow. At some speed it will lift off the bottom and as speed increases it will plane on the surface. Now the mad scientists let alcohol or compressed air into the tank to reduce the SG/density. The ski would have to go faster to stay on top, right? And what if the water was so aerated it was close to the density of air? The ski would have to go pretty fast now, with the mad scientists' hampsters pedalling like mad. But it would still plane, right?
If the ski planes, why not a wing?

Smurfjet

26th Apr 2001, 06:01

bunyip I don't know what you on about, or what you're confusing in Kermode's book, but please please stop saying ...if you have an infinite A you would not have wingtip vortices and so you would not have any lift. And that's using Kermode again.

My head hurts :) and I'm sorry I can't help you out...

bunyip

26th Apr 2001, 10:32

Why? Afraid you might fall out of the sky?

Evo7

26th Apr 2001, 14:26

A wing of infinite span creates no lift (Kermode)


bunyip, I'm not sure I understand this quote. A wing produces circulation in proportion to its angle of attack, and the
Kutta-Zhukovsky (or Joukowski - not sure of the correct spelling, as both seem to be used) says that lift (per unit span) is proportional to circulation (and, FWIW, density and velocity). Now vortex-lines can't have lose ends, so in for a finite wing they spill off at the wingtips, and mark the boundary of the region of descending air behind the wing that, thanks to Newton's Third Law, keeps us in the air. For an infinite wing, no vortex shedding occurs (obviously, as there are no wingtips), but circulation and hence lift occur unless the wing is at a zero angle of attack. The lift per unit span should therefore be finite, not zero, while the total lift is, of course, infinte. The modifications to make 2-D calculations apply to 3-D are well documented.

Edited for tyops

[This message has been edited by Evo7 (edited 26 April 2001).]

bunyip

27th Apr 2001, 01:19

That sorta makes sense, thanks. I like your explanation in fact. I will put my grey cells to work. I don't doubt it is correct. I was taught aerodynamics a long time ago by some pretty intelligent guys, and the statement that a theoretical wing of infinite span, or in a wind tunnel when the wing extended from the sides of the tunnel, would produce no circulation and therefore, in theory, would produce no lift, stuck. It was explained at the time but I cannot remember how. I might have misunderstood (probably did) and I was hoping someone else would be able to explain it to me here.
But my real agenda is to have someone explain why impact lift is not considered.
When I hit turbulence on final in a Cessna 152 and lose 50 feet real quick, then slam out at the bottom of the air pocket, the little Cessna stops going down in a real hurry. I can feel the wing hitting solid air. No way is it just the circulation starting up again, it is solid air, and feels just the same as a water ski coming down off a wave.
Ignoring all the theory, I cannot see why a wing set at an angle of attack simply does not push the air out of its way and so generate lift. The circulation, Bernoullis theory etc simply explaining why a wing does it so well, and why drag can be reduced with a well-designed wing. A wing at 90 degrees angle of attack is still going to produce lift, yet there is little flow theory working there!

[This message has been edited by bunyip (edited 26 April 2001).]

bookworm

27th Apr 2001, 14:26

But my real agenda is to have someone explain why impact lift is not considered.
...
Ignoring all the theory, I cannot see why a wing set at an angle of attack simply does not push the air out of its way and so generate lift. The circulation, Bernoullis theory etc simply explaining why a wing does it so well, and why drag can be reduced with a well-designed wing. A wing at 90 degrees angle of attack is still going to produce lift, yet there is little flow theory working there!

Newton was the first person to consider what you call 'impact lift'. If you go to very high altitudes (space) where the mean free path of the air molecules is of the same magnitude as the wing chord, then impact lift is what you get. The particles behave as individual packets of momentum bouncing off the underside of the wing.

Let's do some sums.

The dynamic pressure of an airflow is essentially the force applied by bringing all the air molecules in the airflow to a halt and thus changing their momentum.

So if you put a flat plate at 45 degrees to the airflow, the air molecules would change direction by 90 degrees as they bounced off. You'd get a rate of change in momentum downwards of the dynamic pressure times the area of the plate. That would produce a force upwards on the plate, your 'impact lift'.

The lift coefficient for that 45 degree angle of attack is 1 (one). (Of course you'd also have a drag coefficient of 1, but we'll ignore that for now). If you reduce the AOA of the plate, you get less deflection of momentum and therefore less lift. For a 10 degree AOA, you'd get a deflection of 20 degrees, and therefore a lift coefficient of about sin(20 degrees) or 0.3.

In the wind tunnel that bunyip is so fond of, you can do the experiment, to find that the actual lift coefficients are much greater than that. It's about 1.0 per 10 degrees of AOA and is roughly linear. So one reason "why a wing set at an angle of attack simply does not push the air out of its way and so generate lift" is that, from the point of view of reconciling theory with experiment, a perfectly efficient deflector of that sort doesn't generate more than a fraction of the lift actually observed.

Before you say "but it's a part of it" and go all additive on me, bear the following in mind. In reality, in the density of air that we fly in, the airflow pattern doesn't look anything like the simple "deflector" picture outlined above. Because the air molecules interact with each other over short distance scales, the cross-section of air disturbed is much greater than the cross-sectional area of the wing. The easiest way to calculate the lift produced is to use the laws of physics that were designed to work with interacting molecules of fluid, namely Bernoulli's theorem and, more generally the momentum theorem. If you do that, with the airflow patterns predicted by circulation theory, you get a pretty good and complete picture of the lift coefficient and the way it varies with geometry (as well as avoiding the ludicrously high drag coefficients that the 'impact lift' theory also predicts).

bunyip

29th Apr 2001, 11:54

Thankyou, Mr Worm, good information. I am sure you are right, and will not argue with you. I don't think that it is necessary for the angle of attack to be so high as to make a 90 degree change of airflow; a small diversion is enough, which would not have a high drag penalty. But I guess any lift so generated would also be small.

Prof2MDA

29th Apr 2001, 19:39

Impact lift does have an application in hypersonic flight vehicles at the edge of the atmosphere, actually, where that is the source of most of the lift.

At lower altitudes its contribution is very small, as has been stated here.

Speedbird252

10th May 2001, 22:35

...so how do aircraft fly upside down??

Evo7

11th May 2001, 10:52

Very well, thanks.... :)

http://www.monmouth.com/~jsd/how/htm/airfoils.html#sec-inverted-camber

explains it better than I can...

pulse1

11th May 2001, 14:39

A fascinating discussion and I have one question. If the major component of lift is due to Newton, why are differential spoilers such a useful alternative to ailerons? They seem to be effective on a wide range (speed) of aircraft from microlights (Snowbird) through the ATR to some fast jets, where a very small application of spoiler seems to have a rapid effect on roll rate.

------------------
"If you keep doing what you've always done, you will keep getting what you've always got"

Prof2MDA

11th May 2001, 20:04

It is incorrect to state "that a major portion of the lift is due to Newton". Newton and Bernoulli are inseparable, it is not a case of "part from one, part from another" at all. Rather, you need Newton to get Bernoulli and vice versa!

Speedbird252

12th May 2001, 01:48

Hey, evo, its me!!!!! The guy that got you flying at Goodwood!! Are you flying upside down already? ah...thats that Mike bloke. He flys 757`s in the week, hes just showing off dont you know.

Still going to have that beer!

john_tullamarine

13th May 2001, 19:04

Several things are of importance.

(a) any discussion must address the reality of what is observed in flight, windtunnels, or wherever

(b) if one looks at windtunnel smoketrail traces, there is a clear momentum (mass flow if you prefer) change associated with the flow downwash. The force balance (Newton if you like) provides the vertical entity we term lift. If you like the idea of throwing air down to get lift, then that is fine.

(c) however, if one considers how this momentum change is going to transfer a force to the aircraft, one has to consider how forces are transferred in any fluid flow - and that is by the effect of pressure differences acting on surfaces of the aircraft. Hence we measure the pressure patterns at the aircraft surface by means of pressure tappings and do some sums to figure out how much force and moment gets involved. In this activity we look at the Euler or Bernoulli relationships to predict numbers.

Trailing vortex flow is conveniently visualised using the ideas of spanwise tip flows.

Flow separation is conveniently described in terms of pressure gradient problems.

For the needs of pilots, the pressure pattern approach is most useful.

If you like pretty pictures of pressure tapping plots to explain lift and drag forces, then fine.

(d) Vortex flow exists. We can see starting and trailing vortices. If a whirling cylinder is trialled in the windtunnel, then a very significant lift force is observed. These observations lead to the ideas of circulation theory which are of use to the aerodynamicist's calculations if not the pilot's immediate needs.

(e) These approaches are just different ways of looking at what is happening - use whichever is most convenient and/or appropriate for your needs at the time.

(f) impact considerations are fine for waterskis on the TOP of the fluid (water) under consideration. However an aircraft is immersed in the fluid - not planing on top of it. In addition, smoketrace observations clearly show that the real world fluid flow is not similar at all to a planing waterski - and, in any case, at the low densities of air, the forces predicted by particle impact calculations are far too small to account for observed forces.

(g) a lot of people get confused by talk of pressures. The various plots you might see are talking in terms of gauge pressures, not absolute, so that when one speaks of a negative pressure or vacuum, one means a slight reduction from ambient. Similarly, a positive pressure means a slight increase above ambient. We are talking about quite small pressure differences overall.

(h) windtunnels are not mystical entities from the world of JRR Tolkien. There are some things which have to be taken into account, such as wall errors and similitude effects, but the flow in the wind tunnel pretty much matches the flow in the real world.

Have I confused the issue ?