Why Do Aircraft fy? Flat Plate Lift Vs Bernoulli?
 

Right, a little shamed to admit this, having done a 3 year aerospace engineering degree. I have tried to figure this out for a long while and am no closer to resolving my brain-fart.

At university, we were taught about Bernoulli and lift generated due to difference in pressures. We were taught about circulation, how lift is proportional to the amount of circulation, about starting vortexes. About without viscosity, we couldnt fly. Reynolds Numbers.

we were also taught however in one lecture (I clearly remember it because what he said confused the hell out of me) that lift cannot be completely be resolved purely as a result of faster flow over the top of the wing. It was mentioned that flat plate lift played a major part as well. I think he was talking hoop, although I now have a doubt in my mind.

Flat plate lift cannot explain symmetrical aerofoil sections at 0 degrees AOA. surely, no lift = aircraft falls out of the sky according to flat plate lift? Yet, I can understand that an aircraft MUST generate lift as well as a result of diverted flow at the trailing edge (for every action reaction blah) .

Which hypothesis is correct?

1) Lift purely due to bernoulli?
2) Lift purely to flat plate?
3) Lift a result of both, which both need to be taken into account exclusively?
4) the sneakiest answer (the one I think may be right) that they are the same thing through some form of coupling, in other words, bernoulli can account for downwash at the trailing edge and flat plate lift can go some way to explain I used to hate aerodynamics, as I felt it was the one topic that was the least well understood by the teaching staff.


The only way I can account for a flat plate producing lift (say paper aeroplane) is that at release, for arguments sake released at 0 degrees AOA, the aircraft falls as there is no lift supporting the weight of the aircraft. This downward velocity, in combination with forward velocity, has thus generated an AOA and thus the flat plate wing of the paper aeroplane now generates lift. Or is this paragraph complete guff?

Its only been 7 years since I finished my degree, think I should resolve this one once and for all. Over to you all! :ok:

The Tailplane of an aircraft is an inverted aerofoil yet still produces 'lift' but in a downwards direction. I doubt the aircraft would fly with any semblance of control if it were simply a flat plate!

In their book Aircraft Flight: A Description of the Physical Properties of Aircraft Flight R.H. Barnard and D.R. Philpott dismiss the Bernoulli theory as a simple way of describing lift production which has unforunately stuck.

I think they also state this in the introduction to recent updates of A.C. Kermode's Mechanics of Flight.

I think this thread should be renamed "Why do airfraft fly? Bernoulli or Newton"

Bernoulli theory sure does sound like the most exciting theory to tell someone when they ask why an aircraft flys. Its the one theory that you was drilled into you at your flying school because thats what the instructor read in a book. But i dont believe it entirely.

My paper aeroplanes fly, and they dont have any camber.

High speed aircraft fly and they have minimal/no camber to increase Mcrit and delay the onset of drag

Newtons law explains why a wing will fly. "for every action there is an equal and opposite reaction" Deflect the relative air down and you will go up. Increasing AOA, will Increase the lift produced. Simple

And rubik101 you say:
"The Tailplane of an aircraft is an inverted aerofoil yet still produces 'lift' but in a downwards direction"
Not entirely true. The tailplane produces upward OR downward lift to balance the aircraft, which is dependant on the CofG position

Try Mike Griffith at Oxford Aviation's ground school.

There have been many debates on this subject in Prune over the years , as a search will show. I only wish my late mate Chris Braund had got his flat plate Do28 into the air. Chris also claimed to have invented winglets when he flew a Seafire with the tips turned up, and achieved a 10 knot reduction in Vs.

Look for Oxford at http://ask.oxfordaviation.net/viewforum and make a post there.

Aircraft fly because they all accelerate a portion of the air that flows past them downwards, causing a reaction force upwards (at least that's what I've been taught by my aerodynamics teacher).

How this acceleration is achieved is where Bernoulli, Newton and all other theories come into play. I personally think it's a combination of different causes and effects, since one single theory does not cover every aspect.

For me, it's the MONEY!

GF

...and the Lift Pixies :ok:

I remember the BAe ATP flew only because the engines generated more vibration which forced the air molecules apart above the wing, hence the low pressure and up she went...slowly.

On landing, you closed the throttles and the reduction in vibration caused you to fall out of the sky, hence the landings......:(


I would have thought three years of aeronautical degree studies would have got you to the point where you could teach others exactly why.

The answer IS Viscosity. Without that, you have a pointy land vehicle with wings.

A flat plate will fly because the viscosity allows it to generate asymetric airflow, the fundemental requirement to allow Mr Benoulli to do his bit.

The viscosity is the property that allows the air to seperate at the trailing edge and not come back up and join the other stream in the opposite place to the separation point at the front. I think....

How the air gets up and around the front of the flat plate is the bit that I can't figure out......we didn't get that far in Principles of Flight lectures, though Mr Alan Smith did correct a few errors in Barnard & Philpott.

It makes my head hurt, but I got 100% and came out early so it must have been good stuff.:)

Having spent 3 years doing an Aerospace course in our final week we were told that all you really needed was a barn door and enough thrust.

I think it's year 4 where you learn about the lift pixies

There was a great (but long) thread almost 3 years ago:
http://www.pprune.org/tech-log/20728...lers-pull.html

The short answer is: Bernoulli and downwash are intrinsic and inseparable in defining and measuring lift. The downwash is most noticeable downstream of a propeller or helo rotor, but it's there behind a fixedwing bird as well.

Sky hooks - the faster you go, the more you catch. failing that Bernoulli + Flat Plate (Newton)

VinRouge

NASA have an interesting take on your discussion at the following if you have not already read it:

What is Lift?

Naturally there are some that dispute their theories, but wont there always be.

Secondly, have you read Aerodyamics for Naval Aviators? I have always thought that it was one of the definitive books on aerodynamics.
(ISBN-10: 156027140X)

Good luck

Redwine

Q. Which hypothesis is correct? –

Ans. 3) Lift a result of both.

Sticking with Subsonic airflow conditions and not wanting to go into too much detail - which I’ve no doubt you fully understand - a standard high lift camber wing section (Göttingen 387) produces about 70% of its lift on the upper surface and around 30% from the lower surface, so although both sides contribute to total lift, it could be argued that the wing is mainly sucked into the (sky) lower pressure air above the wing.

Upper Surface – Bernoulli’s Principle

Due to the camber - as you point out - the upper air flow has a greater distance to travel and in order for the airstream to meet up again at the trailing edge, the upper airflow must travel faster resulting in a drop in pressure as there are fewer particles of air in any given volume verses the lower air flow - think stretching an elastic band.

Lower Surface – Flat Plate Effect

When mounted onto the airframe, the rigging angle of the wing is set at around 6 degrees relative to the longitudinal axis. The effect of this is that the airflow (relative to the wing angle of attack in straight and level flight) strikes the lower surface and is then deflected downwards - dynamic momentum transfer. Newton's third law results in a force in an upwards direction adding to the vacuum created on the upper surface and more lift is produced.

I am not a AE but I have found Dr. Denker's paper on the physics of flying interesting. Gave me a 'better' understanding of the concept of Lift....

Airfoils and Airflow [Ch. 3 of See How It Flies]

Enjoy!

Yes but what is "sucking"?

Is sucking a force that pulls or pushes?

Does a vacuum cleaner suck air? Is the air being pulled in to the end of the hose, or is it being pushed?

Air pressure on a surface can only PUSH on that surface, it can not pull!

A complete vacuum is simply a complete absence of air molecules and therefore a complete absence of this pushing force. A vacuum is not an existence of any pulling force.

A reduction in pressure can only mean a reduction in this pushing force, BUT IT IS STILL PUSHING ON THE SURFACE. It can not pull on that surface.

So when you say the pressure above the wing is sucking the wing in to the sky, the forces on the upper surface of the wing is not contributing to the total lift force by pulling the upper surface up, it is simply reducing the force pushing the upper surface down. But the force on the upper wing is still in the downward direction (perpendicular to the surface)! It's just that it is no longer equal to the opposing pressures being exerted on the underside of the wing.

So the way I see it, when a wing is suspended in still air, the air pressure pushing up on the underside of the wing is countered by the air pressure on the upper side pushing down. The forces cancel each other out and the result is zero force (no lift).

When the air is flowing past the wing (with a positive angle of attack) and colliding with hitting it's underside, compressing together slightly and so the pressure exerted by the air molecules on that side of the wing increases slightly.

On the upper side the air molecules are being pulled apart from each other as the the momentum of each molecule does not allow them to keep up with the upper surface of the wing's trailing surface as it slopes downward and away from the air molecules.

As result of this greater distance between molecules, a vacuum is created, ie the downward force on the upper surface of the wing is less than it would be if the wing was stationary. This reduction in downward pressure is not enough to counter the pressure on the underside of the wing. The resultant force is up.

Anyway that is the way I see lift being created by a wing, or a flat plate.

And to answer the question posed above, air is pushed in to the end of a vacuum cleaner hose not pulled!

It's quite interesting to look at graphs (polars) of lift vs Angle of attack. Many produce positive lift even at negative AOA..

For NACA 4412 Cl peaks at about 1.4 but notice that CL is still positive right down to an AOA of MINUS 4 degrees. At an AOA of zero CL is just under 0.5 so Bernoulli is clearly contributing a significant percentage.

AOA = 0
http://www.windmolensite.be/pics/naca4412.jpg

AOA = -4
http://img132.imageshack.us/img132/4979/naca4412lb7.jpg

http://www.mh-aerotools.de/airfoils/images/hdi_pol3.gif

How do I... Read Polar Diagrams?

Looking at that pic though, the flow leaving the top surface (if still attached) has a definite downward vector, thus surely contributing to lift due to Newton?

Thanks for everyones' responses so far, as well as the reminder of that other fantastic thread with fantastic links I couldnt Find. :ok:

Think I am going to stick with the pixie dust explanation. :hmm:

Blip

Oxford English Dictionary

Suck

• verb 1 draw into the mouth by contracting the lip muscles to make a partial vacuum. 2 hold (something) in the mouth and draw at it by contracting the lip and cheek muscles. 3 draw in a specified direction by creating a vacuum. 4 (suck in/into) involve (someone) in something without their choosing. 5 (suck up to) informal attempt to gain advantage by behaving obsequiously towards. 6 N. Amer. informal be very bad or disagreeable.

The above Oxford English Dictionary does not suggest that Sucking is to push in a specific direction, but to draw. If I refer to a thesaurus I could look under synonyms for DRAW and come across Pull and Suck.

Force. Means anything that pushes and pulls.

Isaac Newton did not note that the apple was pushed to the ground, but pulled. Your argument is that there are only pushing forces and not pulling (Sucking/Drawing) forces.

The only reason that ambient air pressure pushes air up a vacuum hose is that the vacuum (produced within the cleaner) allows ambient air to overcome that caused by the reduction in air pressure within the vacuum cleaner.

With or without a camber on the upper surface of a wing, the pushing force on the underside of a wing is not increased as a result of the lowering of static pressure on the upper surface, but the static force acting downwards on the upper surface of the wing is reduced as a result of reducing static pressure, thereby allowing the wing to be drawn (sucked/pulled/whatever) into the reduced pressure zone.

A distinguishing characteristic of fluids is that they cannot carry tensile forces (i.e. pulling, sucking, dragging...) and can rather only produce compressive and shear forces.

So if an airfoil is regarded as , I read this as: Of the total wing loading (N/sq.meter or psf), 70% of that is reduced atmospheric pressure on the upper surface, and 30% is increased pressure on the lower surface. :)

That agrees with what I am saying so no problems there.

No problems here either. It's just that we seem to be visualising it differently. What you see as a pull, I see as a reduction in push.

OK I think I know how to make my point.

You have a long straw 1 metre long and made of glass. You put the end of the straw in a bowl full of mercury and start sucking on the straw. When you do this you notice the mercury travel up the straw towards you mouth. It seems the harder you suck, the further you are pulling the mercury towards you mouth.

You think to yourself if I suck really really hard I will be able to pull the mercury right up to the top of the straw. But as hard as you try, you can't suck hard enough for it to reach the top and your mouth starts to hurt for the extra effort you made.

If you are determined to get the mercury to the top of the straw, you might decide to let a mechanical pump to take over the sucking. Surely if you had a pump strong enough, you would be able to suck/pull/draw the mercury to the top of the metre-long straw!

So that is what you do. You connect a very powerful pump which is able to create a vacuum so strong, it causes 99.99% of air in the straw to be removed. It's almost a perfect vacuum inside the straw now.

So guess what? The mercury still didn't reach the top of the straw! In fact it only travelled 76 cm above the level of mercury in the bowl. Now does 76 cm mean anything to you? No? How about if you convert it to inches?

Now I ask you? Is the mercury in the straw being sucked/pulled/drawn up the straw, or is it being pushed from below?

What I am asserting is that the sucking of air up and out of the top of the straw, is simply the removal of the opposing downward force on the mercury from that air pressure on the mercury surface on the inside of the straw. The force of air pressure pushing the mercury up the tube from below does so until the weight of mercury in the tube above the level of mercury in the bowl plus the remaining downward pressure in the partial vacuum is equal to the pressure pushing down on the surface of the mercury in the bowl and up the tube.

Same thing with the top surface of the wing. You are reducing the downward force of air pressure on the top surface of the wing. But there is still a downward pressure! It's just that it is not enough to balance the upward force on the underside of the wing.

FM makes airplanes fly. Thanks for all the pretty pictures though. :}

An aircraft cannot fly unless the paperwork equals or exceeds the weight of the craft. Also noise generators are required for propulsion. More noise, the faster/better it flies.

Haven't you guys ever heard of the passenger theory?

Most aeronautical engineers and the general public associate the lift generated by a wing with the differential pressure between the upper and lower surfaces of the wing. Nothing could be further from the truth. In reality, the lift required to make a commercial aircraft airborne is furnished by the passengers. Further, the lift is inversely proportional to both the wing size and the distance to be traveled. Farther further, the distance to be traveled has a non-linear relationship to lift, as will become clear in the following explanation.

1. How passengers provide lift for commercial aircraft:

The lift required for an aircraft to take off is furnished by the passengers pulling up on their seat armrests.

2. How take-off lift is initiated by the pilot:

After the aircraft reaches the end of the runway preparatory to take-off, the captain will advance the throttles on the engines. This action has two purposes: a) to provide horizontal thrust to propel the aircraft down the runway, and b) to increase the Passenger Aggregate Fear Level (PAFL) by raising the noise level in the cabin. The consequent rise in PAFL causes the passengers to strenuously lift up on their seat armrests, thus imparting lift to the aircraft. As we can readily see, the engines have two purposes, to move the aircraft horizontally and to scare the bejabbers out of the passengers.

3. How the duration and degree of lift is modulated by the pilot:

Once cruising altitude is reached, the pilot will throttle the engines back to lower the noise level. The reduction in noise level results in a reduction in PAFL, with a consequent decrease in lift. It is necessary for the pilot to make only minor changes in noise level to maintain straight and level flight. In some instances where the PAFL does not decrease sufficiently to prevent further climbing, the captain may order that free drinks be passed around, thus further relaxing the passengers and lowering the PAFL.

One may observe that on most aircraft the first-class passengers are automatically anaesthetized by the use of free booze. Clearly first-class passengers are a source of surplus lift and must be dealt with accordingly.

While the airline industry will never admit it, passenger seating assignment is governed by national characteristics. For instance, Italian males are hardly ever upgraded to first class since they are easily excitable, respond very quickly to outside stimuli and provide almost immediate changes in lift. Clearly one would not want to get the Italians drunk. One difficulty associated with using Italians in this manner is their clannish nature; getting them evenly distributed (left and right, front and back) within the cabin can sometimes be difficult. Stewardesses will often resort to eyelash batting and hip wiggling to move the Italians about the aircraft.

While at first blush it may seem that the French would also be a good source of lift, their uncooperative nature makes lift modulation difficult. One should never fly on an aircraft containing more than 45 percent (by volume) Frenchmen.

The reader will note that Lufthansa, SAS and KLM fly only very large aircraft. Raising the PAFL for the stolid Germans, Swedes and Dutch is notoriously difficult, requiring as many people as possible in each aircraft. The British never fly.

The high takeoff-accident rate for Aeroflot can be attributed to the fact that Russians are generally drunk before they get on the aircraft and are not a reliable source of PAFL-induced lift.

Descent and landing are accomplished using a combination of fatigue and passenger discomfort. It is a happy coincidence that travel over greater distances takes a correspondingly longer time. Even the most casual observer will note that after the aircraft reaches cruising altitude the plane will begin a slow and gradual descent for the balance of the trip. This descent is due to passenger fatigue and discomfort. A detailed explanation of the fatigue factor is unnecessary; suffice to say that with time one's arms get tired and the upward pull on the armrests is reduced. By reducing leg and hip room, passenger discomfort is increased with time, and this distraction is also sufficient to reduce the Passenger Induced Lift, or PIL. The common airline practice of showing only the most boring of in-flight movies is also a lift-modulation technique.

Nota bene: The decrease in the amount and quality of airline food has not been found to be an effective method of PAFL modulation; biogas production offsets any decrease in lift. (See Hindenburg disaster, reference no. 75.)

Several recent instances of sudden aircraft descent have been attributed to air pockets. The air pocket explanation is clearly a feeble attempt on the part of the aircraft crews to avoid blame. In reality the crew neglected to closely monitor passenger fatigue, discomfort or degree of inebriation. Luckily sudden decreases in altitude are self-correcting due to the consequent rise in panic levels and increase in PAFL-induced lift.

Most passengers and the general public believe that the oft experienced practice of circling the airport many times prior to landing is caused by the weather. This is not wholly the case. During bad weather the PAFL increases as the aircraft reaches its destination. This undesirable increase in PAFL and consequent increase in lift must be dissipated by prolonging the flight and further tiring the passengers.

4. Historical basis for this theory and the role of PAFL in aircraft design:

As your may recall from early aeronautical history, the Wright Brothers' aircraft had four wings with a very large surface area. The large surface area of the wings inspired great confidence in Wilbur and Orville, decreasing their PAFL and, as a consequence, decreasing the altitude and flight duration capabilities of the Wright Flyer. As aircraft design advanced, it was found that smaller wing surfaces inspired greater PAFL, with a resultant increase in aircraft performance. Indeed it was not until the advent of the multipassenger aircraft (with a higher PAFL factor) that increases in range and altitude were possible. The only reason wings (albeit very small ones) are still included on aircraft is that they look nice.

It is a little-known historical fact that the general unpopularity and eventual demise of the supersonic passenger aircraft were brought about by the fact that as soon as the aircraft reached supersonic speeds, the passengers could no longer hear the engines. No noise, no PAFL - and no PIL. The aircraft would drop like a rock, causing the PAFL to spike drastically, and the aircraft would then climb precipitously to a supersonic altitude, with a consequent loss of engine noise. The process would then repeat. The resultant sinusoidal altitude and speed changes have rendered supersonic travel impractical.

While further research by really annoying and pedantic people may bring my theory into disrepute, one must keep firmly in mind that even with all of the efforts to reduce personal space aboard commercial airliners, they have yet to remove the armrests. Think about it.

Hope that helps!

Coanda helps.

Bernoulli? Hmmm, i have my doubts about that. However, thats the CASA syllabus so we 'have to' believe that theory.

I have thought five times before joining in on this thread as the whole Bernoulli/Newton thing has been examined by better minds than mine for many years.

However as a young lad I was fortunate to be educated in a practical world of wind tunnels and flight test where one’s day job was primarily to measure and observe things.

Because of this I would like to extend my condolences to VinRouge regarding his cry from the heart and to explain why the Bernoulli ‘notion’ works as well with a flat plate as it does with a traditional curved aerofoil. (Something actually not surprising given so many aircraft use flat plate tailplanes and fly very nicely).

Understanding why this is so becomes easy if you examine the practical evidence concerning two terms not often bandied about on PPRuNe ‘stagnation streamline’ (SS) and ‘stagnation point’ (SP).

http://img.photobucket.com/albums/v1...ley/CH10F6.jpg

I trust the above diag shows how the SS is just an imaginary streamline that defines the plane of separation above which all air goes over the wing and below which it all goes underneath and the SP is the point where the SS meets the aerofoil.

Now consider a flat plate at 90 AOA as in the diag below where the SS and SP are as shown:

http://img.photobucket.com/albums/v1...ley/CH10F7.jpg

If you now reduce the AOA the SP moves forward but remains behind the leading edge until the AOA is exactly zero meaning that at say 10 AOA the air above the SS has further to travel that that below. QED for the folk who follow Mr B

Sorry! I did not intend the diags to be as large and offensive as that!

go down to the biology lab at a university, and have a look at a birds wing cross section.. it is curved.. and im sure if you look at a discovery channel doco about insects, you will see slo motion video of a fly, flying, its wings are flat, yet at such a high AOA, that they create a vortex above their wings to create lift.

How about this. A 747 is at the end of a runway about to take off. Now ask the question "what makes it fly?" For me, the answer has to be "brute force", the engines, and that has to be Newton. If we have enough Newtons, we can send it into the air like a rocket. Bernoulli is really there to help reduce the amount of force needed to get it airborn and to help control it.

Lift is created by reducing the cross-sectional area of a stream flow over an aerofoil. This convergent streamtube accelerates a mass of air, and due to bernouilli theorem, causes its pressure to decrease and therefore sets up a pressure differential between top and bottom surface. It is this pressure differential that causes the upwards force (by newtons).

The bottom surface similarly reduces the pressure of the air, which is why a symmetrical aerofoil at 0 AoA has a zero lift coefficient.

We've all been said that airplanes fly because of the shape of the airfoil- forget the engines, airplanes can glide-, the upper camber being longer
than the lower one, thus, creating a low pressure area above the
wing and a high pressure area below, which makes the airplane fly.
I was told once by an aeronautical engineering professor that the shape
of the wing created a suction force on the upper surface. But, I've been
asking myself for a long time that if this is so, why airplanes can fly
inverted or at a 90 degree bank angle.

..... and water-ski... no camber, no sucking... but they fly (on the water), just flat-panel theory...

KISS. Newton's third law. To every action is an equal and opposite reaction.

That is the answer in a nutshell.

Aircraft fly because the upward force opposite to their weight is equal.

The way aircraft produce such opposing force is by deflecting air downwards. Now whether it is by aerofoils , bernoulli, coanda, flat plates or pure chance, it makes no odds. It is simply the downward deflection of the air that causes an opposite force upward on the aircraft.

Beechbum
 

Symmetrical aerofoil produces as much force downwards as it does upwards. A cambered wing (which produces more up force than down at 0 AoA) can still fly inverted, but must choose a high AoA to balance the forces.

Understand, the cambered planes you fly... the bottom surface also produces "lift" towards the ground, its just that it's overcome by a greater uplift of the upper surface.

E.g Acrobatic/Military planes use a more symmetrical wing to help with this as one advantage.

Planes can fly at extreme attitudes only when they have powerful engines where pure thrust is used to overcome the lack of vertical lift from the wing (because its at 90 degress).

A red bull racer can fly straight and level at 90 degrees bank. The symmetrical wing of such an acrobatic plane means it hardly changes heading when banked (achieved with elevator input), whilst the huge thrust of the engine compensates for the weight.

VinRouge

Sorry that I can not quote the exact link but I did come accross a NASA link once on this very topic which suggested that about 20% of total lift came from Bernoulli and the rest from reaction ( ie Newton) and the flat plate effect. I think the Bernoulli bit has a lot to do delaying the flow separation which is very bad on a flat plate.

Indiscipline_girl

Exactly :D

wing/rotor/prop/turbine etc pushes air down / back/ forward

air pushes wing up/forward /back:ouch:

any further requires mathematical obscenity:8:mad:

also, most students who start basic aerodynamics class wearing Gucci sunglasses and begin their introduction by ignorantly 'spouting utter tosh' :} about Coanda effect blade element theory etc. receive a F:suspect:

Lester:E

viscosity vs. density
 

I keep hearing that viscosity is essential. Yet, point a firehose at an inclined flat plate and it will move. It moves because of fluid's inertia, action, reaction, etc. No viscosity needed.
It sounds like viscosity is something needed for Bernoulli effect. An added bonus.

Perhaps cambered airfoil and related Bernoulli effect figure so prominently in the aerospace curricula because when the science was being created, the starting point was what we saw on birds. Also, early on, power-to-weight of the available powerplants was so low, that, indeed, the airplanes would not fly without the benefit of camber.
Off course, we should keep the camber; it improves performance and saves money, but it is still a rather complex starting point for students, and the resulting confusion abounds.

So, I would like to pose a corollary question:
On Cessna ... ok, ok.. this is a "professional" forum... on Boeing 737-xxx, how much lift % would we loose if we:
1. "turned off" air viscosity?
2. "turned off" air density?
I'm guessing that the above distribution will be a function of speed & weight but any data will help me (and others?) getting sense of order of things.

That one is trivial. 100 %. No lift in vacuum.
This one is more interesting.

Making viscosity equal to zero makes Reynolds number infinite.

What will happen to the coefficient of lift of a fixed airfoil shape when the Reynolds number is increased without limit? (This can be done by the unphysical method of "turning off" air viscosity, or by merely impractical method of scaling up the airfoil to absurd sizes)

According to a book wot I got called 'Flight of Fancy' (tries to explain theory of flight in laymans terms)

On page 47 it says "Flying thus depends on gaseous friction' The point being made is- no friction= no flight also

no friction= no thrust, therefore no flight.

Ask Sophie!

I don't know whether it will change your (or your model's?) answer but I was thinking more about medium's physical property.
Perhaps I could rephraze the questions to highlight what I'm after:
1. "reduce" air viscosity by a factor of... (pregnant pause.. + Dr. Evil cunning facial expression..) 1000.
2. "reduce" air density by a factor of 1000.

Somehow, I don't think so. If a plate diverts the non-zero density flow direction, force is used, and force will be felt back. This doesn't seem to depend on any friction or viscosity.
Am I creating something unrealistic with my simplifications?

Personally I think Dash&Thump in post #9 already has hit it on the head. No viscosity, no trailing edge separation.
So, for a flat plate at an angle of attack, lower pressure on top, same higher pressure on the bottom.
No lift, just drag.

He's probably right, otherwise why would we fuss so much about Reynolds numbers?

CJ