Why Do Aircraft fy? Flat Plate Lift Vs Bernoulli?

[VinRouge]

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:

[rubik101]

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!

[False Capture]

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.

[brns2]

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

[Fantome]

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.

[Longhitter]

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.

[galaxy flyer]

For me, it's the MONEY!

GF

[Floppy Link]

...and the Lift Pixies :ok:

[RVF750]

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.:)

[Cyclone733]

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

[barit1]

There was a great (but long) thread almost 3 years ago:
http://www.pprune.org/tech-log/207289-how-aeroplanes-fly-propellers-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.

[deltahotel]

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

[Che Guevara]

VinRouge

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

What is Lift? (http://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html)

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

[Hilife]

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.

[Bradda G]

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] (http://www.av8n.com/how/htm/airfoils.html#sec-consistent)

Enjoy!

[Blip]

it could be argued that the wing is mainly sucked into the (sky) lower pressure air above the wing.

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!

[cwatters]

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? (http://www.mh-aerotools.de/airfoils/hdipolar.htm)

[VinRouge]

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:

[Hilife]

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.

[barit1]

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 70% of its lift on the upper surface and around 30% from the lower surface, 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. :)

[Blip]

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

...thereby allowing the wing to be drawn (sucked/pulled/whatever) into the reduced pressure zone. 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.

[Jake Wheeler]

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

[Avtrician]

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.

[Bjcnz]

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!

[MyNameIsIs]

Coanda helps.

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

[John Farley]

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/v145/johnfarley/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/v145/johnfarley/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!

[Ultralights]

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.

[anotheradam]

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.

[cc2180]

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.

[beachbumflyer]

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.

[Daniel_11000]

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

[indiscipline_girl]

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.

[cc2180]

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.


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.

[Meikleour]

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.

[Pugilistic Animus]

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

[balsa model]

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.

[chornedsnorkack]

how much lift % would we loose if we:
2. "turned off" air density?

That one is trivial. 100 %. No lift in vacuum.
1. "turned off" air viscosity?

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)

[BarbiesBoyfriend]

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!

[balsa model]

how much lift % would we loose if we:
2. "turned off" air density?
That one is trivial. 100 %. No lift in vacuum.

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.

no friction= no flight

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?

[ChristiaanJ]

Personally I think Dash&Thump in post #9 already has hit it on the head. 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....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

[beachbumflyer]

cc2180, you can fly a Bellanca Decathlon at a 90 degree bank angle
with only 180 hp. And what is creating lift here? It has to be the
fusselage, the vertical fin, and the engine.

[balsa model]

Ah.. post #9. Started as witticism but then moved onto some serious stuff. I see, now.
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.

Still sounds a little like a circular argument: viscosity because then we can treat flat plate as if it were another Bernoulli object which requires viscosity.
So:
flat plate + viscosity => asymmetric flow => Bernoulli => lift
I must say that asymmetric flow is not the first think that I would expect from a flat plate but I guess my intuition developed without the help of hours spent in wind tunnel.
Again, a question arises:
flat plate + extremely low viscosity fluid => nearly symmetric flow => nearly no Bernoulli => nearly no lift ?

PS: Thick, I tell you! Thick is my head! :}

[bookworm]

flat plate + extremely low viscosity fluid => nearly symmetric flow => nearly no Bernoulli => nearly no lift ?

Excellent question. The answer is no. Perhaps best explained by means of analogy.

If we balance a pencil vertically on its tip, does the direction in which it will fall "depend on gravity"? How can we answer that?

If we took the same pencil, on the same surface, to the moon (where the gravity is lower) would it fall in the same direction? The answer to this second question is, I hope, obviously "yes", as the magnitude of the vertical force makes no difference to the direction of fall. (Of course the rate of fall may depend on the magnitude of the vertical force, but the direction does not -- it depends on things like the mass distribution within the pencil, and the detailed shape of the tip, and the surface.)

If we took the same pencil, on the same surface, into outer space (where the gravity is zero) would it fall in the same direction? Well, with no gravity at all, the pencil would not fall, so the behaviour is quite different. We could legitimately say "no".

Does lift depend on viscosity? Well in the sense that absolutely zero viscosity would completely change the pattern of flow around the wing, yes. But all it takes is a tiny trace of viscosity (very high Reynolds Number) to create the flow pattern, and then the wing lifts as you'd expect. Increasing the viscosity will change the lift a little, but not much, even as the viscosity increases by orders of magnitude, and the lift actually tends to decrease as viscosity increases (Reynolds Number decreases).

[chornedsnorkack]

2. "reduce" air density by a factor of 1000.

This is easily doable. Climb to a height where air density is 1000 times less (about 60 km).

Lift should decrease 1000 times, except that Reynolds number also decreases 1000 times.
1. "reduce" air viscosity by a factor of... (pregnant pause.. + Dr. Evil cunning facial expression..) 1000.
You could achieve same result (increase of Reynolds number) by increasing the size of the airfoil, or by increasing density of air.
So, for a flat plate at an angle of attack, lower pressure on top, same higher pressure on the bottom.
No lift, just drag.


Why should the flat plate experience any forces at an angle to the plate in question? How could the flat plate experience any shear in absence of viscosity?

[VinRouge]

Because if its diverting flow in any direction, there has to be a resultant force as a result of Newtons Laws Surely.

It interests me, as I wonder how CFD software predicts lift ... As someone has mentioned, without viscocity, you cant get assymetric flow, thus without Newton, a flat plate could not produce lift.

It would be interesting to see what effects this would have on a flat plate in superfluid flow (0 viscosity).

[balsa model]

bookworm:
Thanks!
At first you've lost me on the relevance of the pencil example, but eventually I think I get it. Any amount of viscosity changes things.
The question still remains: is it absolutely necessary?
Is it a question that can be demonstrated in a realistic experiment?
Increasing the viscosity will change the lift a little, but not much, even as the viscosity increases by orders of magnitude, and the lift actually tends to decrease as viscosity increases (Reynolds Number decreases).
I'll break it down to a short sentence for thick heads like me:
Increasing viscosity descreases lift. Hm.. Do you meant, beyond certain point, or always? Or, is it that you are mentally changing size to change Reynolds, which you equate with viscosity? Something doesn't tally with the need for viscosity.

chornedsnorkack:
I appreciate your replies, but... my questions aim to find how much the two physical properties of air / fluid contribute to the lift.
Your 1st example didn't separate the two.
Your 2nd example promises to decrease viscosity [effects] by ... increasing the size of the airfoil. Hm.. And divide the results by surface area? Any link with results?

Can we perhaps confine our discussion to one flat plate of fixed dimensions, and not introduce this dreaded Raynolds number. To place us on any graphs that you may have at your disposal, let's say that the flat plate is 10m X 1m (not terribly, but close to C-172).

PS: Merry Christmas.

[chornedsnorkack]

Because if its diverting flow in any direction, there has to be a resultant force as a result of Newtons Laws Surely.

Precisely. And if you have a thin flat plate in the absence of viscosity, it can only exert forces at right angles to its surfaces.

As someone has mentioned, without viscocity, you cant get assymetric flow,

Why not? If you have a thin flat plate at an angle to the flow?

The airflow below the plate is compressed on meeting shock front, and then again on approaching the surface of the plate. The airflow above the plate meets no shock, but when the neighbouring air below is no longer there, it starts to expand against the upper surface of the plate. But the pressure remains below ambient. There is no way for the high-pressure air below the plate to affect the low-pressure area above in any way, because this would take upstream signals, which are forbidden.

[bookworm]

The question still remains: is it absolutely necessary?
Is it a question that can be demonstrated in a realistic experiment?

Apparently yes. All you have to do is find some superfluid helium (http://prola.aps.org/abstract/PR/v108/i5/p1109_1). ;)

Increasing viscosity descreases lift. Hm.. Do you meant, beyond certain point, or always?

Beyond the trace of viscosity (present in every fluid that isn't a superfluid like helium 3) that is necessary for lift, lift generally decreases the more viscous the fluid, with all else equal. I'll see if I can dig up a graph.

[indiscipline_girl]

Well, well, well.

I apparently must repeat my previous post to re-concentrate some minds.

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.

If in doubt, ask the birds. ( OK we cannot do that).

[Pugilistic Animus]

Indiscipline_girl

Aw to heck wit'em:}

[Pinkman]

You're all wrong.

The Gods breathe on them and cause them to move.

Happy Christmas

Rob

[Brian Abraham]

The word according to NASA

Lift is the force that holds an aircraft in the air. How is lift generated? There are many explanations for the generation of lift found in encyclopedias, in basic physics textbooks, and on Web sites. Unfortunately, many of the explanations are misleading and incorrect. Theories on the generation of lift have become a source of great controversy and a topic for heated arguments for many years.

The proponents of the arguments usually fall into two camps: (1) those who support the "Bernoulli" position that lift is generated by a pressure difference across the wing, and (2) those who support the "Newton" position that lift is the reaction force on a body caused by deflecting a flow of gas. Notice that we place the names in quotation marks because neither Newton nor Bernoulli ever attempted to explain the aerodynamic lift of an object. The names of these scientists are just labels for two camps.

Looking at the lives of Bernoulli and Newton we find more similarities than differences. On the figure at the top of this page we show portraits of Daniel Bernoulli, on the left, and Sir Isaac Newton, on the right. Newton worked in many areas of mathematics and physics. He developed the theories of gravitation in 1666, when he was only 23 years old. Some twenty years later, in 1686, he presented his three laws of motion in the Principia Mathematica Philosophiae Naturalis . He and Gottfried Leibnitz are also credited with the development of the mathematics of Calculus. Bernoulli also worked in many areas of mathematics and physics and had a degree in medicine. In 1724, at age 24, he had published a mathematical work in which he investigated a problem begun by Newton concerning the flow of water from a container and several other problems involving differential equations. In 1738, his work Hydrodynamica was published. In this work, he applied the conservation of energy to fluid mechanics problems.

Which camp is correct? How is lift generated?

When a gas flows over an object, or when an object moves through a gas, the molecules of the gas are free to move about the object; they are not closely bound to one another as in a solid. Because the molecules move, there is a velocity associated with the gas. Within the gas, the velocity can have very different values at different places near the object. Bernoulli's equation, which was named for Daniel Bernoulli, relates the pressure in a gas to the local velocity; so as the velocity changes around the object, the pressure changes as well. Adding up (integrating) the pressure variation times the area around the entire body determines the aerodynamic force on the body. The lift is the component of the aerodynamic force which is perpendicular to the original flow direction of the gas. The drag is the component of the aerodynamic force which is parallel to the original flow direction of the gas. Now adding up the velocity variation around the object instead of the pressure variation also determines the aerodynamic force. The integrated velocity variation around the object produces a net turning of the gas flow. From Newton's third law of motion, a turning action of the flow will result in a re-action (aerodynamic force) on the object. So both "Bernoulli" and "Newton" are correct. Integrating the effects of either the pressure or the velocity determines the aerodynamic force on an object. We can use equations developed by each of them to determine the magnitude and direction of the aerodynamic force.

What is the argument?

Arguments arise because people mis-apply Bernoulli and Newton's equations and because they over-simplify the description of the problem of aerodynamic lift. The most popular incorrect theory of lift arises from a mis-application of Bernoulli's equation. The theory is known as the "equal transit time" or "longer path" theory which states that wings are designed with the upper surface longer than the lower surface, to generate higher velocities on the upper surface because the molecules of gas on the upper surface have to reach the trailing edge at the same time as the molecules on the lower surface. The theory then invokes Bernoulli's equation to explain lower pressure on the upper surface and higher pressure on the lower surface resulting in a lift force. The error in this theory involves the specification of the velocity on the upper surface. In reality, the velocity on the upper surface of a lifting wing is much higher than the velocity which produces an equal transit time. If we know the correct velocity distribution, we can use Bernoulli's equation to get the pressure, then use the pressure to determine the force. But the equal transit velocity is not the correct velocity. Another incorrect theory uses a Venturi flow to try to determine the velocity. But this also gives the wrong answer since a wing section isn't really half a Venturi nozzle. There is also an incorrect theory which uses Newton's third law applied to the bottom surface of a wing. This theory equates aerodynamic lift to a stone skipping across the water. It neglects the physical reality that both the lower and upper surface of a wing contribute to the turning of a flow of gas.

The real details of how an object generates lift are very complex and do not lend themselves to simplification. For a gas, we have to simultaneously conserve the mass, momentum, and energy in the flow. Newton's laws of motion are statements concerning the conservation of momentum. Bernoulli's equation is derived by considering conservation of energy. So both of these equations are satisfied in the generation of lift; both are correct. The conservation of mass introduces a lot of complexity into the analysis and understanding of aerodynamic problems. For example, from the conservation of mass, a change in the velocity of a gas in one direction results in a change in the velocity of the gas in a direction perpendicular to the original change. This is very different from the motion of solids, on which we base most of our experiences in physics. The simultaneous conservation of mass, momentum, and energy of a fluid (while neglecting the effects of air viscosity) are called the Euler Equations after Leonard Euler. Euler was a student of Johann Bernoulli, Daniel's father, and for a time had worked with Daniel Bernoulli in St. Petersburg. If we include the effects of viscosity, we have the Navier-Stokes Equations which are named after two independent researchers in France and in England. To truly understand the details of the generation of lift, one has to have a good working knowledge of the Euler Equations.

[Piper19]

About 75% of the lift is due to upwash/downwash. Look underneath a helicopter. The grass will be blown flat, only possible due to the wind going down underneath the wing/rotor. Action going down is reaction going up of the rotor itself. Same with a paper airplane which has no airfoil, and same with an aircraft flying on its side.
25% rest is Bernouilli.

[Brian Abraham]

About 75% of the lift is due to upwash/downwash
The correct figure is 100%. In NASAs terms, lift is generated by the turning of the airflow (upwash/downwash). As a reading of my previous post you will see "The integrated velocity variation around the object produces a net turning of the gas flow"

[Tarq57]

Slightly OT, here, but Bjcnz, (http://www.pprune.org/tech-log/355523-why-do-aircraft-fy-flat-plate-lift-vs-bernoulli-2.html#post4607435) (post 24), that is pretty blatant plagiarism.
If you're going to do that, at least quote the source, as was quoted here. (http://www.pprune.org/questions/204288-just-after-takeoff.html#post2295083)

[TWT]

Hold your hand palm down out of the window of a moving car.Tilt the angle of your hand up ,your whole hand then raises.Tilt your hand down,your entire hand moves down. :ok:

[cc2180]

A helicopter flies by directing the thrust downwards to overcome its weight. In the same way that a harrier jet does on take-off.

A red arrow flies at 90 degree bank by a slightly positive AoA and raw thrust/weight ratio to literally drag it up through the air to keep it level.

It has nothing to do with the aerodynamics of a fixed wing a/c creating lift.

[MyNameIsIs]

Sorry fellas but how you describe the way choppers fly is incorrect.

They are just so ugly the earth repels them, just like the A380! :E


TWT, thats a good explanation. True or not well, that will be argued about but all i know is that it works!

[barit1]

beachbumflyer's post #41 and cc180's #57 address "knife-edge" flight.

Sorry, but it IS an aerodynamic exercise; the fuselage and fin, rolled to 90 degrees, does form an (albeit imperfect) airfoil. It creates lift (opposite to gravity) because of its AOA, while the wing is now at a zero-lift AOA.

To be fair, since its AOA is 10-15 deg., the prop axis is at this same angle, and thus it's creating some lift too.

So it's not a brute-force "hanging on the prop" condition, but just a poor airfoil with very low aspect ratio making do as a wing.

[south coast]

I agree with one of the paragraphs posted by Brian Abraham below...

Surely Bernouilli's theory is not completely relevant when discussing wings because his theory talks of a venturi.

Is it not wrong as well as inaccurate to assume that all the effects that occur in a venturi will be the same if you were to cut the venturi in half, ie. a wing?

[cc2180]

Barit,

Yes the fuselage will provide some lift, but as it is (hopefully) symmetrical, it will provide as much of an upforce as a downforce. This is why you require a positive AoA to gain that small amount of lift. The fuselage is a source of lift in all scenarios, but is so small that it isnt talked about, except apparently here.

The lift coefficient alone wont be enough to allow most planes to fly straight and level at knife-edge, so certaintly cant be used as a standard example of flight. It requires additional thrust at a positive AoA to overcome the weight as illustrated by a harrier, rocket, helicopter.

Anyway, this thread is about how the force of lift is generated, and the fuselage in your example creates lift the same way that a wing creates lift normally, but due to the extreme AR of (most) a fuselage, lift generated will be insufficient for flight, requiring vectored diversion of thrust upwards to help overcome the weight.

That's all im trying to say :ok:

[Brian Abraham]

Is it not wrong as well as inaccurate to assume that all the effects that occur in a venturi will be the same if you were to cut the venturi in half, ie. a wing?
south coast, a very pertinent and insightful question. Answer once again from NASA.

* An airfoil is not a Venturi nozzle. There is no phantom surface to produce the other half of the nozzle. Velocity gradually decreases as you move away from the airfoil eventually approaching the free stream velocity. This is not the velocity found along the centerline of a nozzle which is typically higher than the velocity along the wall.
* The Venturi analysis cannot predict the lift generated by a flat plate. The leading edge of a flat plate presents no constriction to the flow so there is really no "nozzle" formed. One could argue that a "nozzle" occurs when the angle of the flat plate is negative. But this produces a negative lift. The velocity actually slows down on the upper surface at a negative angle of attack; it does not speed up as expected from the nozzle model.
* This theory deals with only the pressure and velocity along the upper surface of the airfoil. It neglects the shape of the lower surface. If this theory were correct, we could have any shape we want for the lower surface, and the lift would be the same. This obviously is not the way it works - the lower surface does contribute to the lift generated by an airfoil. (In fact, one of the other incorrect theories proposed that only the lower surface produces lift!)
* The part of the theory about Bernoulli's equation and a difference in pressure existing across the airfoil is correct. In fact, this theory is very appealing because there are parts of the theory that are correct. In considering the pressure-area integration to determine the force on a body immersed in a fluid, if we knew the velocity, we could obtain the pressure and determine the force. The problem with the "Venturi" theory is that it attempts to provide us with the velocity based on an incorrect assumption (the constriction of the flow produces the velocity field). We can calculate a velocity based on this assumption, and use Bernoulli's equation to compute the pressure, and perform the pressure-area calculation and the answer we get does not agree with the lift that we measure for a given airfoil.

[bookworm]

I wrote:

Beyond the trace of viscosity (present in every fluid that isn't a superfluid like helium 3) that is necessary for lift, lift generally decreases the more viscous the fluid, with all else equal. I'll see if I can dig up a graph.

Difficult to find anything explicit in the literature. McCormick has some plots of Cl vs AoA at various Reynolds numbers, but they're not superimposed so it's difficult to see the effect clearly. But it shows a flat plate giving a lift coefficient of about 0.75 at Re = 420,000 but only about 0.55 at Re = 42,000. So a 10-fold increase in viscosity causes a small decrease in lift at the same AoA.

Abbot and von Doenhoff also have plots at Re = 3, 6 and 9 million. It's clear that the stall happens at a lower AoA for higher viscosity (lower Re) but it's very difficult to deduce the effect of viscosity on Cl at low AoA.

[Dairyground]

Both the Newton (laws of motion, flat plate) and Bernoulli camps are more or less correct. Lift is created by accelerating mass downwards. In steady flight, the force created by accelerating mass (free air, engine exhasust) downwards is equal to the force on the aircraft due to gravitational attraction (ignoring minor effects such as buoyancy). The downwards deflection of the air is produced by the difference in pressure between the upper and lower surfaces of the wing predicted by Bernoulli's Theorem, although the mathematics describing the flow and its pressure differences is rather complicated.

(If we think of the example of a hosepipe playing on a flat plate, the plate slows down the water, so the pressure on the plate increases relative to that of the flowing water - which is the same as that of the surrounding still air, so the plate tends to move in the direction of the jet.)

Fifty years ago I was in the middle of a degree course in Difficult Sums which included, among many other things, a series of lectures on fluid dynamics. The years have eroded much of the detail from my memory, but the basic principles remain.

The partial differential equations that describe the steady three-dimensional flow of a real fluid are complicated, non-linear and do not have neat analytic solutions. When acceleration of surfaces in contact with the fluid is taken into consideration, the situation is even worse.

Without the assistance of modern high-speed computers, the accurate numerical solutions of the full equations for even simple cases are just not possible. The only feasible approach is to make simplifying assumptions, work out the numbers, and then perform experiments (for example with wind or water tunnels) to determine how closely the simplified theory matches reality.

The main sources of intractibility in the full equations are viscosity (or internal friction) and compressibility. So the simplest approach is to eliminate both, introducing the concept of the ideal fluid.

Solution of the simplified equations for two-dimensional flow round a circular object (equivalent to flow in three dimensions round an infinte circular cylinder) is (or was for me in the days of my youth) fairly easy. A simple trick (called conformal mapping) can transform this flow into the flow round an aerofoil shape. The difficult bit is finding a transformation that converts the circle into the aerofoil of interest.

This simplified model works well for some things. It predicts flows (and thus pressures) fairly well, so long as you don't look too close to the trailing edge of the wing. The pressures over most of the upper and lower surfaces tally well with the predictions from Bernoulli's equation. However this simplifies theory has one major drawback - it predicts zero lift (and drag). Introducing "circulation" into the theoretical flow can get out of this problem and produce acceptable predictions for many purposes.

However, the simplified system is not good for predicting flows near the trailing edge of the wing, or where the flow will separate from the wing with the resulting turbulence and increased drag. To model such things as flow separation and the onset of turbulence, the equations have to take account of viscosity, which increases the complexity of the computation dramatically. One of the complications is that there is a strong interaction between viscosity and typical lengths of objects in the flow. In one of the possible simplifications for viscous flow, it turns out that there is a characteristic number, the Reynolds Number, that identifies flows that are very similar though with widely different lengths and viscosities.

By varying viscosity as well as the scale of a model it is possible to set up an experiment in a relatively small space that can provide information about real flows past larger objects in fluids with different viscosity.

Reynolds Number modelling is applicable only where the fluid can be trreated as incompressible. It is initially a little surprising that air can be treated as incompressible for aerodynamic purposes up to speeds fairly close to the speed of sound. Beyond this, into the transsonic range and beyond, a different simplification of the general fluid flow equations is needed.

At subsonic speeds, the air can effectively "hear an aircraft coming" and move out of the way, resulting in relatively simple flow round the aircraft. At supersonic speeds, the aircraft arrives without warning and the result is a shockwave. The flow regime is quite different, and a wing designed for supersonic speeds is likely to be quite thin and symmetric, contrasting to the thicker and cambered low-speed wing.

Computational Fluid Dynamics (CFD) software typically will use very general forms of the fluid dynamics equations, modelling both compressibility and viscosity. Some simplifications may still be used, but fast computers permit much more detailed computations in a given time than are possibly with simpler tools.

So, rougly speaking, Bernuolli explains the cause of lift (the pressure difference), Newton explains the effect of the pressure difference.

[cc2180]

So, rougly speaking, Bernuolli explains the cause of lift (the pressure difference), Newton explains the effect of the pressure difference.
Yes, exactly.

As you alude to, you cannot attempt to model each and every particle going over different sections of an aerofoil. At some point certain generalisations have to be made.

[deHavillandDave]

I wrote a couple of interesting articles for Associated Content that were helpful. The amount of disagreement on this topic is surprising. The shape of the wing and pressure differences are of course important (Bernoulli), but I don't think an aircraft would ever take off without the downward deflection of air via AOA of the aircraft and control surfaces (Newton).

How an Airplane Wing REALLY Generates Lift - Associated Content (http://www.associatedcontent.com/article/726335/how_an_airplane_wing_really_generates.html?cat=16)

How an Aircraft Wing Generates Lift - Associated Content (http://www.associatedcontent.com/article/709305/how_an_aircraft_wing_generates_lift.html?page=1&cat=15)

Bernoulli's Principle - Associated Content (http://www.associatedcontent.com/article/747795/bernoullis_principle.html?cat=58)

[modelflyer]

rubik101 - My model aeroplanes fly quite well with flat tailplanes and fins (ususally made out of 1/82 or 1/4" balsa wood). A few of my small 'planes have "flat" wings too and they fly OK. Having tailplanes and fins with "sections" doesn't seem to make much difference so far as models are concerned.

Maybe everything changes on "full-size".

[Checkboard]

Hold your hand palm down out of the window of a moving car.Tilt the angle of your hand up ,your whole hand then raises.Tilt your hand down,your entire hand moves down.

Hold your hand palm down, hand flat, out of the window of a moving car, fingers pointing forward. Tilt your hand up, your hand is pushed back quite a lot, and up a little bit. Now curve your hand a bit, and tilt it up. Your hand is pushed back a little bit, and up quite a lot. Nice demonstration of the effect of a curved aerofoil, that :ok:

[Pugilistic Animus]

for understanding of this subject

I recommend that one has familiarity with two dimensional flows, Navier Stokes solutions, spherical harmonics, Curl, Gradients, Flux, Divergence, volume elements, vector math the Laplacian the Legendrian and slope fields, as well as {Oiler's}:} theorem

here one method to arrive at your answer from Abbott and Van Doenhoff

xe =1/Ux ^8.210* INT[U^8.210] dx


Sheesh:\

Happy New Year Pprune:}

PA

[Bjcnz]

Heya "Tarq57" (post 24) well not really, because I had said "Haven't you ever heard of the passenger theory..." plus if I was going to quote my source, that would mean me providing my email address and password and isp website, and well, I'd kinda rather not do that. So pretty much you're saying, when I repeat a joke to anyone, I should be stating my source otherwise its plagiarism? I dont think many people mind to much.

"What did the police man say to his tummy?.....You're under a vest!" - Christmas Cracker from 2008 Christmas lunch (sorry not sure the date of manufacture or factory this Christmas Cracker came from)

[nedleyoldpal]

An aeroplane flies because of velocity and angle of attack of the aerofoil,
cause and effect are in play here purely down to Newtons theory, Bernolli has only a small part if any to play here.

Think of this, if an aeroplane really was sucked into the air then all those really big aircraft will have to have very strong rivited sheeting to hold hundreds of tons up aloft, also how could an aeroplane fly upside down and not be sucked into the ground (and don't talk about centre of pressure), also the comments above are correct there is no such thing as pulling or sucking it's another matter of cause and effect a common mis-conception, I will try to explain another time.

[PETTIFOGGER]

I propose a simple explanation. Lift is caused by differential velocity between a flat plate solid and a Newtonian fluid. The amount of lift (and drag) is caused by the pressure ratio which may be varied by changing the angle of incidence between fluid and fp solid, and/or the shape of the solid.

[FakePilot]

In trying to figure this out I have a couple of facts I cling to in order to understand:

1. On fixed wing aircraft, the engines provide a fraction (like 1/2, 1/3 whatever) of the force needed to lift the aircraft. I realize a few aircraft actually produce more thrust than weight.
Question is, where is the extra force coming from? It would seem a conservation principle is violated here.

2. Helicopter engines produce enough HP to lift their max weight at 1 g if they were on a cable. I.e. their "thrust" is equal or greater than their weight. I believe this allows hovering.

3. In level flight of a fixed wing, the weight of the aircraft is equal to the mass of the air being accelarated downward. (Newton)

Am I close? Or should I just go back to playing MSFS? :)

Therefore, I guess that the column of air collapsing into the lower pressure area behind the aircraft adds the missing force in the case of fixed wing aircraft with less thrust than weight.

[PETTIFOGGER]

Fakepilot,
1.Please see my short post above. The lift is caused by the forward motion of the wing through the air (the differential velocity). The forward motion is caused by the thrust of the engine, which does not need to equal or exceed the weight/mass of the a/c.
2. Helicopters have rotary wings. The lift is produced by the differential velocity of the blades (or wings) moving through the air. Directional thrust and speed is produced by tilting the blades.
3. Yes, but what about drag. Do you think that should be added to the weight? Concerning the second part of this question, I think that the answer is no, as there is no 'missing' force.
I hope that this helps.

[red button]

Yes, its true. My hand actually creates lift. I suck it outta the car window, and the harder i pressed the accelerator pedal, the higher my hand went.

Thats all i really need to know about that! :ok:

[barit1]

fakepilot -

The relationship between thrust and lift is identical to the relationship between drag and lift. We call this the lift/drag ratio, or L/D. The number can range between 5 and 50, depending on the cleanliness of the design. Sailplanes might exceed 50 - and the Harrier etc. might be much lower than 5.

The L/D can also be called the glide ratio (http://en.wikipedia.org/wiki/Glide_ratio). If a plane can glide, power off, for 20 miles while losing 1 mile in altitude, then its L/D is 20.

barit1
(aeronerd) :)

[Pugilistic Animus]

The technical answer you are looking for is found in The Kutta Condition



just a method of laying siege to certain second order differential equations-provided certain conditions are met or assumed:hmm:

in airfoil design--- certain equations have been found to approximately model the lift or drag characteristics of a wing section but nevertheless the WIND TUNNEL remains the reall proving grounds as aerodynamics is an empirical science that lends itself to a more mathmatical --as opposed-- to verbal discussion--
Let's not even attempt a discussion of high lift devices:\


I propose a simple explanation. Lift is caused by differential velocity between a flat plate solid and a Newtonian fluid. The amount of lift (and drag) is caused by the pressure ratio which may be varied by changing the angle of incidence between fluid and fp solid, and/or the shape of the solid



absolutively:}


PA

[Lasiorhinus]

Well Mr Bernoulli -

What rule of physics, mathematics, or horticulture states that two air molecules which seperate at the leading edge of a wing, ever meet up again in the future?

[PETTIFOGGER]

Well Mr Bernoulli -

What rule of physics, mathematics, or horticulture states that two air molecules which seperate at the leading edge of a wing, ever meet up again in the future?

I am no Mr Bernoulli, but the "equal transit time" principle has been shown to be incorrect in most cases. See the third link in deHavillandDave's post of 30 December.
rgds pf

[b377]

A Physical Description of Lift (http://home.comcast.net/~clipper-108/lift.htm)

Try this link article by D Anderson.You will find that Newtons second law prevails when considering the mechanism of lift from a physical standpoint. The classical Bernoulli explanation is quite misleading if not downright wrong. (The Bernoulli diff pressure effect is present alright but fails to account for bulk amount of lift generated and is therefore secondary) It is the Coanda effect which is responsible for the bending of the flow around an aerifoil resulting in the downwash which by reaction momentum exchange produces lift. Lift is not a free lunch, the power required can be nicely calculated by this Newtonian approach.

D Anderson's book is basic but very good written by a blend of a physicist and pilot.

[barit1]

D Anderson's book is basic but very good written by a blend of a physicist and pilot.

One of the better introductory treatments of lift, IMHO. :ok:

[bookworm]

D Anderson's book is basic but very good written by a blend of a physicist and pilot.

Neither role, even in its pure form, is insurance against some fundamental errors in the physics of flight.

"Lift requires power" leads to some unphysical conclusions. "Ground effect" gets a sign wrong.

If you want rather more solid attempt written by an Anderson, try John Anderson's "Introduction to Flight".

[PETTIFOGGER]

I think that this is a good quick reference guide:
The Aviation History OnLine Museum Theory of Flight Index (http://www.aviation-history.com/theory/index-theory.html)