Bernouli's Irrelevant?

[completeaerogeek]

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...

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

Swings do not need to be cambered they just need AOA. There is no 'suction' on a wing at all. look at www.219sqn.aafc.org.au and click on 'Wings don't suck'. everything y0u need to know is there.

 

 

[completeaerogeek]

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.

 

Coanda is not the reason the air flows a surface. Static pressure and viscosity are. The Coanda Effect (if is exists at all as a distinct phenomenon) is only present in accelerated flows such as upper surface blowing.

 

 

[completeaerogeek]

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.

 

Thomo you are half there and half not. http://www.cam.ac.uk/research/news/how-wings-really-work. There are no vacuums (partial or otherwise) in normal space. also www.219sqn.aafc..org.au and click Wings don't suck! for more. or you could read my article in Australian Flying this month. (JAN/FEB 15)

A wing doesn't 'suck at all' In fact suction is a very misunderstood and somewhat mythical concept. When a piston moves down in a cylinder atmospheric pressure pushes the air into the increasing space . The 'suction' idea is why so many pilots have problems understanding why a MAP gauge goes up and pressure increases when they open the throttle. isn't it 'sucking harder' shouldn't the pressure go down as 'suction increases?

 

A wing (the simplest kind is a flat plate) moves trough the air and as it does so at an effective AOA, the lower surface pushes the 'lower air' air forwards and downwards. Lift Component 1. The Pressure bubble (bow wave_ caused by this disturbance pushes the remaining air up and over the wing where it is held against the upper surface by normal atmospheric pressure. Imagine the wing under water. The physics is identical. Now try to use your explanation. if it doesn't work it is wrong. Simple.

 

The air that us pushed up and over the wing follows the upper surface because it has nearly 15lb/sq in making it do so. As it changes direction a force results (as it must Newton's 2nd and 3rd laws.) This resultant force is Lift Component #2.

 

Lift is caused by the mechanical intervention of a wing bending the air away from its resting position. Pressure variations are a result of lift production not the cause. Flat wing- no AOA no lift. Flat wing, AOA + movement = lift. QED.

 

 

[completeaerogeek]

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.

 

And sorry-no 'Coanda effect.' If the Coanda Effect exists at all (and there is debate abut this) as a separate phenomenon from viscosity effects, it only occurs in accelerated flows such as Upper Surface lowing not in static air.

 

 

[djpacro]

... for more. or you could read my article in Australian Flying this month. (JAN/FEB 15)

good article

[Guest ozzie]

No, the wing don't do the sucking it is the lower air pressure above the wing that does the sucking. The wing is just the device that changes the pressure.

 

Little mate also reckons that gravity is a myth and it is the Earth that sucks.

 

But i new that years ago 'cause i had a skydiving t shirt that said that so it must be true.

 

 

[ayavner]

So here's a question, probably rhetorical. I think the word is starting to get out, and I see alot of articles, forum discussions, and resources such as the ones aerogeek has shared... What is a person to do when it comes time to sit the exam? The books 90% of us have are still outdated apparently, and we know this is what the test writer is looking for. Any move by the industry to incorporate new information as it becomes available? I think it becomes a case of putting what you know will get the tick, and therefore another generation of pilots cruises along with misinformation.

 

 

[Phil Perry]

This thread has been the most interesting and informative thread I've ever read on this forum. I hope it never stops. I wonder how long it will take before I understand it.Thanks guys

Terry. . . . . . Take no notice of these experts. . . . . . .An Airplane flies, . . .Because . . .

 

( 1 ) It beats the air down with it's wings. ( Wolfgang Langweische - 1932 )

 

2) If someone tells you something about an aeroplane that's so complicated you can't understand it,. . . .then it's probably all balls. ( R. J Mitchell - 1938 )

 

So there ! ! ! ! ! Problem solved.

 

Phil

 

• 
   
   
  

 

 

 

 

[Phil Perry]

Lift is increased when you raise the nose and increase power.Lift is decreased when you lower the nose and reduce power.

There...the lift formula demistified...;)

Yes Merv,. . . . . my old Instructor used to say. . . . . pull back on the stick. . the houses get smaller . .pull back harder,. . .they gett bigger again - - - very quickly.

 

Phil

 

 

[Phil Perry]

and furthermore,if he were to crash his plane into a tree in the forest but no-one is there in the forest to hear it, did he really crash???

And. . . . If I say something when I'm deep in the forest and the wife is not around,. . . . .AM I STILL WRONG ? ?

 

 

[kaz3g]

I got lost from the conversation when Thomo introduced gnomes into the equation.

 

Kaz

 

 

[shags_j]

OMG, this thread needs to die. I think everyone is more confused than before...

 

Lets just say magic and leave it at that.

 

 

[Ultralights]

Its a little from Column A and a little from column B. the first 30 secs of this video show it perfectly, both A and B.

 

Bernoulli's Principle in effect by evidence of the water vapour condensing out of the lower air pressure above the wing on take off, especially as AOA increases at rotation, and B, downwash created by the wing... Move a lot of air down, then what ever moves the air down, will go up. equal and opposite reaction. video at 1.20 to 1.40 shows a 757 moving a lot of air down....

 

 

 

[Ultralights]