Bernouli's Irrelevant?

[shags_j]

Hey all,

 

Was reading an article in this months Australian Pilot magazine entitled 8 common aviation myths.

 

The first myth basically said that Bernouli's principle was irrelevant to flight. While it does in fact exist he goes on to give a lengthy argument on why it doesn't really matter for flight.

 

He says that while it doesn have an effect on a wing, it is only a minor effect.

 

The wings are cambered, according to Austin Collins in order to acheive a greater range of AOA's and still maintain flight.

 

Arguments are:

 

1- Aircraft can fly upside down

 

2- Some aircraft have camber on top and bottom of wing

 

3- Ultralights and gliders with single surface wings (ie. no top or bottom)

 

4- No law of physics that says two air molecules that are seperated must meet again

 

5- Actual "suction" on the wing due to the principle is quite small

 

6- Even a flat peice of board with an appropriate angle of attack will fly.

 

Just wondering what everyones thoughts on this were? I grew up in aircadets and the first principle of flight we learnt was bernouli's principle. Hard to let go that it is irrelevant to flying...

 

 

[motzartmerv]

Yes, its an interesting topic alright.. I think NASA did some extensive studies some years ago which basically disproved what we all get taught.

 

Its mainly the action reaction prinicple (newton) that keeps us all aloft..

 

The camber of the wing does play a part, the coefficient of lift in the lift formula is made up of angle of attack AND camber..

 

However, we still teach it and untill someone changes the syllabus and the test questions (right up to cpl aerodynamics) it will continue to be taught..go figure...

 

cheers

 

 

[Matt]

Interestingly both NASA and the RAF teach and promote this "new theory" and it's one that I was taught and have been aware of for many years. As per Merv's note though, until everyone agrees, particularly theory manuals & exams, there will continue to be people taught and believing a falicy.

 

 

[turboplanner]

I like the explanation in Stick and Rudder the author places the greatest significance on angle of attack IN ALL POSITIONS (turns, pitch etc) then adds the impact of Bernoulli's theorem.

 

So for example they do fly upside down but at a greater angle of attack to overcome the downward low pressure pull.

 

 

[shags_j]

So for the physics nuts out there, are we able to quantify the effect of bernouli's principle?

 

Perhaps compare to the effect of AOA?

 

 

[farri]

Bernoulis Theory.

 

Applies beautifully to the spray gun,not so sure about the aircraft.

 

Frank.

 

 

[turboplanner]

Hope an aeronautical engineer will repond Shags, but I have seen formulae (carburettor designers can set the throat vacuum the same way), and know the lower the power to weight ratio, the more camber is needed. eg an Extra will fly with nearly a flat wing, but the bombers of WW2 carrying tonnes or ordnance had very pronounced curvature on the top surface. Also, if you look at the profile of the Savannah wing vs one of the faster recreational AC there is a very pronounced difference.

 

Could NASA have been talking about some of their re-entry aerodynamis where a lot of speed is involved?

 

This subject got me into hot water once at the time we started using wings in Speedway, just using flat panels with a lip down at the front and up at the rear. We began by putting them over the rear wheels thinking we would get more traction.

 

I made a nuisance of myself around the pits explaining Bernoulli's Theorem and how ineffective our wings would be.

 

My car was powered by a two stroke which fed in a massive power increase about two third of the way down the straight.

 

Round we rolled, the green lights came on, and two thirds of the way down the straight at around 170 km/hr my car rotated under the rear wing with the two front wheels coming up in the air just as the corner loomed up. Training kicked in and I rotated it down with brakes just in time!

 

 

[motzartmerv]

I spose one way to do it would be to work out how much lift is produced at a zero angle of attack. So the lift must be coming from the rest of the fromula, for a constant V and air density and for a given surface area of wing the difference would have to come from the camber..???

 

where is that airfoil simulator??

 

ill find it and check it out

 

 

[Matt]

There's some interesting points on the "How Stuff Works" website including links to fluid dynamic simulators, it also talks about the theory of lift being a complex beast which unfortunately cannot be simplified sufficiently to a "lamen" level. Interesting read.

 

HowStuffWorks "How Airplanes Work"

 

 

[shags_j]

Howstuffworks

 

by NASA. I'm sure this would help us work it out but I'm boned if I know how...

 

 

[djpacro]

NASA has more stuff here: Beginner's Guide to Aeronautics

 

Bernoulli simply related local velocity to local pressure. Trouble is that people have invented simple theories to go with it, just for pilots, on why the velocity is what it is.

 

Personally, I like the one about male and female air particles who decide to meet up at the trailing edge at the same time.

 

Follow the links above to Factors that Affect Lift to see how lift is generated plus that Java Applet.

 

 

[shags_j]

I've seen two references now however to where the particles don't rejoin. They say that there is no law of physics that says particles must rejoin where they left off. I found one reference that even said that the particles on the top surface of an aerofoil generally end up well past the particle it originally started next too (if any of that makes sense).

 

EDIT: I can't remember if it was roald dahl or Terry Pratchett that discussed the theory of lies to children. Where science is so simplified to make it understandable that it actually ends up incorrect in the simplified form.

 

 

[Vorticity]

I like the reply about the male and female particles, I have always pictured them giving a high five and meeting at the other end.

 

It has rightfully been stated that the important part of bernoulli's work is that with increase velocity there is a decrease in surface pressure. The big link regarding a cambered (or flat plate with an AoA) and the creation of lift is why. By studying wind tunnells and tracking particles we can show that the flow does in fact move faster over the top. One interesting theory is that a circulation effect occurs around the wing.

 

If you watch your paddle next time you are canoeing you will see an eddy, or a starting vortex. If you read up in some reputable aerodynamics text they will outline the various things that add to slowing down the bottom flow while maybe contributing to the flow on the upper surface. This starting vortex is key in the process.

 

It's all interesting stuff but as pilots my opinion is that we only need to know the things that increase the lift within our control. Shape, Velocity and AoA. You could look into the movement of pressure regions in various AoA and so on but its not as fun as just getting out and flying.:big_grin:

 

Trust me when I say that if you really get into this stuff, including the maths, it will send you completly insane.

 

 

[turboplanner]

For the masochists

 

Aerodynamics Lab 3 - Direct Measurements of Airfoil Lift and Drag@@AMEPARAM@@/docinfo/337576?access_key=8u2cfyt31oimq@@AMEPARAM@@337576@@AMEPARAM@@8u2cfyt31oimq

 

 

[Tomo]

I've seen two references now however to where the particles don't rejoin. They say that there is no law of physics that says particles must rejoin where they left off. I found one reference that even said that the particles on the top surface of an aerofoil generally end up well past the particle it originally started next too (if any of that makes sense).

Yeah, The book I've got reckons that the air particles never meet again...which makes sense in my thinking, because the top or curve/camber of the wing is longer (from the leading edge to the trailing,) than the fairly flat bottom (leading edge to trailing is shorter)...if you can get what I'm trying to sayi_dunno

 

Also the Camber has a fair bit in helping with the flying I think...get yourself a desert spoon and run water across the back of the spoon, so that the water starts at where the handle attaches (handle sticking up at you) the spoon and travels down to the tip of it...just feel the power of the pull of that, it's quite amazing really, and the faster you have the water running the more it wants to pull in...

 

It's the same as air running over the top of the wings...But before you all start screaming at me, Note Where I said "helps" with the lift, not all the lift comes from that, but I reckon a lot does, and thats why an Extra with flatter wings has got to go faster, because of less curve in the wings, just like a flatter spoon under the tap...

 

Anyway good, bad or ugly, thats my 2 cents:blush:

 

 

[Guest pelorus32]

I spose one way to do it would be to work out how much lift is produced at a zero angle of attack. So the lift must be coming from the rest of the fromula, for a constant V and air density and for a given surface area of wing the difference would have to come from the camber..???where is that airfoil simulator??

ill find it and check it out

Remembering of course the zero AOA is not the same as the zero lift angle!

 

 

[Guest ozzie]

OK my partners grandson is dead sure that an aircraft is sucked off the ground rather than lifted. If he keeps tweaking his theory he'll have me convinced. his main argument is 'that an area of high pressure( underside of wing) moves to an area of low pressure. as the high pressure can't get to the area of low pressure because the wing is in the way then the difference between high and low pressure has to cause more 'suck' than lift. i advised him to keep this to himself otherwise he'll end up like Galileo fella.

 

If the two air molecules are seperated at the leading edge and the lower molecule is 10 inches past the T/E when the upper molecule is at the T/E., Is this then the real measurement of the co efficient of lift? maybe they had to come up with that fancy formula as you can't see a molecule of air to work it out. ;)

 

ozzie

 

 

[jcamp]

As previously mentioned the whole business is pretty complicated see any uni level aerodynamics text for an introduction. One way of looking at it without too much mathematical fluid dynamics is consider two separate (but connected) factors.

 

Lift generation - downwash off the trailing edge of the wing due to wing section including camber, angle of attack, coanda effect etc gives a momentum change which requires a force. Crudely density, span and velocity (rho*span*V) give the mass moved per unit time with chord,lift coefficent and velocity (chord*Cl*V) give the downward velocity; multiplied together we have a momentum change (rho*span*chord*Cl*(V^2))=(rho*A*Cl*(V^2)). The 1/2 in the book formula comes from early drag theory so to make the formula look the same as the drag formula double Cl and divide the whole thing by two.

 

Adding. This is of course for subsonic a/c. The individual molecules are moving at (loosely) the speed of sound in all directions for short distances (less than 0.1 micron) in a random manner before bouncing off another molecule and going in a different direction. As the arithmetic gets too difficult we have to consider it as a fluid which is nasty enough even though with numerical methods we may look at 1,000,000,000,000,000,000,000 molecules together..

 

Transfer the force to the wing - This is where Bernouli comes in. The downwash accelerates the air on the top of the wing and most (not all) of the force transferred to the wing comes from the Bernouli effect there.

 

 

[Guest Maj Millard]

C'mon guys keep it simple, like flight itself. I'm going for three myself.

 

1. Bernouli's theory: 1/2 venturi = high pressure beating low pressure every time.

 

2. Newtons' action = equal reaction theory.

 

3. Coriallis effect: air passing over a curved surfaces wants to follow the

 

curved surface.

 

Years ago I had a friend who had produced about his fourth aircraft design. He had built up a beautifull scale model. Through a friend of a friend, he gained access to one of the top aerodynamicists who ran the NASA Ames windtunnel at Sunnyvale in California, just South of San Francisco. It has one of the biggest operating wind tunnels in the US, and probabily the biggest on the West Coast. He presented himself and his model to the gentleman one day, and awaited what he thought would be a valuable critique of his new design. The professor turned it around a couple of times studying it and then said.... " hey, you know, if they look good, they usually fly real good".... go figure.

 

 

[Tomo]

Some extra reading on lift...

 

Ok. here is a pretty simple way of putting it...;)

 

The reason that the air sticks to the top of the wing is found in the boundary layer. This is a layer of air, on the surface, less than an inch (2.5 cm) thick. Because the air is slightly sticky (it has viscosity) the part in contact with the wing actually clings to the surface, and doesn't move. You will notice this when your car gathers dust on a dirt road. You come to the tar and put your foot down, hoping to blow it off, but not one grain moves. This is because the air at the surface of your car is completely still, despite the fact that you might be travelling at over 100 km/hr.

 

So a thin sliver of air right next to the wing is stationary. The next layer up moves very slowly, the layer above that moves a bit faster and so on, until at about one inch from the surface the air moves at the normal speed.

 

Imagine a little gnome, only one inch tall, standing on the wing in the boundary layer. He is facing towards the tail of the aircraft. There is no wind blowing on his heels, a strongish wind blowing on his butt and a gale on his head and shoulders. His feet stay put but the rest of him pitches face-down on the surface of the wing.

 

This is exactly what the airflow does — it's pulled down on to the surface of the wing.

 

So now we have this layer of air that clings to the upper surface and is bent downwards. This drags more air down with it. In fact it drags a huge mattress of air down.

 

For those who like figures, a Cherokee or C172 at cruise speed, displaces a 3 metre thick layer of air downwards at a vertical speed of about 9 knots. To supply the 1000 kgs of lift needed for level flight, the wing deflects over 2.5 tonnes of air every second!

 

If you start moving huge volumes of air downwards,

 

you leave a void where it came from. In other words this downward movement of air creates a low-pressure area above the wing.

 

Let's take stock for a moment. We know that the wing gets lift by deflecting air down. But we also know that it gets lift from low pressure above, and high pressure below the wing.

 

Strangely, both these statements are correct. The pressure difference, and the deflected air are both part of the same system.

 

Because many folk battle to come to grips with this, I have developed three different ways of explaining it, in the hope that one makes sense to you.

 

Example 1. The air over the top of the wing is pulled in two different directions. Newton’s first law decrees that it should carry on in a

 

straight line, while Coanda insists on curving it downwards. This conflict causes the air to be lifted slightly above the wing, forming a partial vacuum.

 

Example 2. Imagine speeding your car over a humpback bridge. As it follows the downward curve of the bridge you feel yourself lifting out of your seat. The air curving down over the top of the wing behaves in the same way it tends to lift away from the surface, causing a partial vacuum.

 

Example 3. This example is to clarify the relationship between the partial vacuum and the down force.

 

Imagine your aircraft being picked up by a crane. But instead of ropes round the wings we attach the aircraft to the crane with a whole lot of suction cups.

 

The aircraft's weight pushes down on the ground through the crane's wheels. But it's being supported by suction on the top of the wings. Exactly the same happens in flight — the aircraft's weight pushes down on the air, and it's being supported by suction above the wings. Make sense?

 

For the purist, it's not just a matter of low pressure, or suction, above the wing. Lift comes from the difference in pressure between the partial vacuum above the wing and the increased pressure below it.

 

Finally, some people want to apportion the lift. They think that part of it comes from deflecting air down, and part from suction.

 

You can't do this — they are both components of the same system, which accounts for 100% of the lift. Apportioning the lift is like saying that a car is partly supported by the air pressure in its tyres, and partly by the road. Each is a component of a system that supports the entire weight of the car.

 

So that's it.

 

• Lift comes from deflecting air downwards.

 

• The suction above a wing is caused by the air leaving a void as it's bent downwards by Coanda Effect.

 

 

 

[Guest ROM]

Newton's third law; For every action there is an opposite and equal reaction.

 

For level flight or for flight of any sort, a parcel of air with a mass of 1.1 kgs per cubic metre at standard pressure and temperature has to be accelerated downwards with sufficient force so as to exactly balance the mass of the aircraft multiplied by the free fall acceleration of 9.8 metres per second squared arising from the earth's gravitational field.

 

This is the fundamental basis of flight which there is no getting around.

 

Wing loadings, operating speed ranges, types of operation and aerodynamic losses and profile drag dictate the type of profile and wing configuration that will encompass the operating regime that the aircraft designer envisages for their aircraft design.

 

The Bernouli principle in all of this is a subsidiary one as it merely explains the manner in which the airflow behaves around a wing profile and by this behaviour, in an airfoil's case the acceleration of the airflow over the wing camber and therefore a reduction in pressure, enables the downwards acceleration of the air mass which as above enables the ability for flight.

 

Tomo is of course correct in that all these factors are part of the same requirement that an air mass has to be accelerated downwards to achieve constant flight.

 

 

[bilby54]

If you hang around flying discussions for long enough then you will hear some stalwart letting this little cherub out of the cradle! I am sure that it is done by magazine editors to increase circulation... ;)

 

I must appologise as this is a lengthy post but is trying to present the argument in a bit cleare fashion.

 

My alter ego earns his living by measuring and controlling gas flows in heavy industry so I try to give visual pictures to those learning from me. To make it clear, Bernouli's theorum generally states that the sum of energies in a streamline are equal. To simplify that a little, if the fluid speeds up then the pressure decreases and vicky verky.

 

To say that Bernouli's Theorum does not apply is like saying we don't need the elevators to fly an aircraft just the other bits. Lift theory can be as simple or as complicated as you like to make it but most (not all) flying instructors do not understand much about this subject other than what is in the training manual, so it is no wonder that a lot of students have trouble.

 

Visualise a wing with equal camber on the upper and lower surfaces that has air flowing directly over it at a speed capable of producing lift. The air separates equally at the leading edge and flows symetrically around both the upper and lower surfaces. The amount of energy displaced on the upper surface is exactly the same as the lower surface so the energy sum is zero - no lift is produced no matter how fast the air flows over it. This is the only time that the molecules have a chance of regaining their freindship at the trailing edge!

 

If the same symetrical airfoil is tilted to give an Angle of Attack then the scenario changes somewhat. If you can visualist the leading edge where the air separates to get around this wing that got in the way then the air flowing under the wing essentially slows down due to the surface area that it is confronted with. Essentially, it bangs into the wing and the air surrounding the lower edge of the wing so the pressure increases - that sounds like Bernouli...

 

The air flowing over the top of the wing is presented with a different set of circumstances as it belts over the top and completely misses the wing a bit like Wiley Coyote going over a cliff and this is explained by one of Newtons theories about bodies staying in motion until some other force changes its mind. The air over the top flows away from the upper surface and the pressure drops. The drop in pressure is trying to drag the airflow back towards the wing surface and without any resistance to flow such as wing stuck in the way, it speeds up to get back to the wing as fast as it can before the other molecules notice! :big_grin:

 

The sum of the energies at this point are unequal as there is a wing in between that separates the two pressure areas. The only way for the energies to equalise is for the wing to move toward the lower pressure area so some lift is produced. This is also the reason for wing tip vortices as they cannot occur at zero lift, But that is not all as was stated earlier.

 

Visualising the trailing edge gives a whole new part of the lift thingy. The faster moving air over the top surface is trying to get back to the wing but most of it misses and this collides with the slower moving air from the underside that is trying to get out of the way of the wing. The result is downwash and this contributes a bunch more to the lift produced and this part can be attributed to Newton.

 

There has to be a pressure change to effect a flow change so it is not reasonable to discount Bernouli or Newton or Cant or all of the others. It can always be helpful to draw the situation or work it out in your mind - no maths or formlae involved.

 

Cheers,

 

 

[motzartmerv]

Thanx Bill, thats cleared it up for me..:thumb_up:

 

 

[farri]

Come on guys,we all know that it`s the pilot that makes the aircraft fly.:clown:

 

 

[Guest High Plains Drifter]

Whats this "Bernouli's theory" thingy ??? did I miss something