Bernouli's Irrelevant?

[Tomo]

The only thing wrong with Bernoulli's theory for a aerodynamics is that, all his experiments were done with fluid dynamics, and that he was not interested in flight and made no references to it in his work. He also made no claims or statements about aerodynamics. He was working with incompressible fluids in a closed system - a tube. We are looking at air, which is compressible, in a system that could hardly be more open - the sky.

 

Anyways I think I'd better stop before someone gets hurt:clown:

 

Ps. The only problem is now that when I come to do my theory test, I just got to remember to right the wrong theory down...:black_eye:

 

 

[farri]

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

WHEN THE VELOCITY OF A FLUID IS INCREASED, THE STATIC PRESSURE IS DECREASED.

 

 

[turboplanner]

Here's a direct quote from NASA - Home, referring to the two camps - Bernoulli and Newton.

 

Australian Pilot magazine could have saved a lot of debate by quoting this.

 

 

 

"

 

 

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."

 

 

[shags_j]

That's what I was wondering. Which is more effective. We know that regardless bernouli's has an effect, question is how much of an effect.

 

 

[markendee]

As a young bloke learning aerodynamics at RMIT I could not get my head around two particular points. (both stated here previously)

 

1. Why is an aircraft able to fly upside down? My thinking was it would be pulled down by the lift.

 

and

 

2. Why did the air particles HAVE to meet up again at the trailing edge?

 

Drove our poor lecturer mad I'm sorry to say.

 

 

[Guest High Plains Drifter]

Whilst interesting, why does a pilot need to know anything about these theorys/principles

 

 

[turboplanner]

Did an early preflight on a Victa early one frosty morning with an Instructor who carefully wiped off the layer of ice on the top surface of the wings and said ALWAYS check this because if the flow over the top of the wing is slowed you'll have a very sluggish take off.

 

It appears he was speaking from a personal experience.

 

That's the Bernoulli part and I guess turning sharply after an EFATO would be the Newton part, but I agree so much of GA training spells out the beginning (If you do this) and the end (You'll crash), but not the middle

 

 

[djpacro]

To follow my earlier post:

 

- Bernoulli doesn't explain the generation of lift, but does enable it to be calculated if you know the velocity distribution around the aerofoil as it simply relates local velocity to local pressure. i.e. Bernoulli applies - it is just that people have misused it.

 

- the form of the Bernoulli equation commonly seen is the incompressible version - the more general one applies to incompressible flow

 

- the stuff that we're talking about is incompressible flow - compressibility effects only become noticeable at higher subsonic mach numbers

 

- boundary layer is not really relevant in the explanation of how lift is generated i.e. you can calculate lift of a wing pretty well using inviscid flow theory. BL becomes relevant as the stall is approached.

 

 

[Guest Howard Hughes]

Where does Bernoulli's say that the particles meet up again?

 

I always thought the basic demonstration of a spoon with a tap, covered that already! ;)

 

 

[Tomo]

I think that's called the "Longer Path Theory"... so because the Particles traveling over the top of the wing have got to go faster to meet it's partner at the TE, and then because it's traveling faster it causes a low pressure area on top of the wing...which seems reasonable, But it could only account for about 2% to 5% of the lift needed for flight... but that theory certainly gets smashed when an acft fly's upside down, 'cause it would want to pull the acft down...:ah_oh:

 

Cheers,

 

 

[vk3auu]

If you take an aircraft such as a Quicksilver GT400 which has a highly cambered wing with a flat bottom, it will fly quite well at about 50 knots at zero angle of attack. The lift is all due to Benouilli, Newton doesn't come into it at all.

 

David

 

 

[djpacro]

....zero angle of attack. The lift is all due to Benouilli, Newton doesn't come into it at all.

Nope, sorry.Bernoulli and Newton are general theories which apply concurrently.

Newton explains how lift is created Lift from Flow Turning and Bernoulli simply relates local velocity to local pressure.

 

 

[turboplanner]

Tomo, to fly an ac upside down (and I've only done it with models) requires a substantially greater angle of attack due to the need to overcome the low pressure area you mention.

 

 

[Tomo]

Couldn't agree more with the greater AOA, possibly dew to not having a camber on top any more, to cause the ( ---------- ) theory to have affect... so you got to compensate with a greater "pushing" down of the air...which is what the greater AOA causes...:big_grin:

 

I left the blank space so you can put what ever theory you go for into...

 

 

[Guest terry]

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

 

 

[Guest High Plains Drifter]

I always thought the basic demonstration of a spoon with a tap, covered that already!

Probably all the average light aircraft pilot needs to know about it ;)

 

If the full understanding of these theorys/principles is required before flying, methinks most pilots have just been grounded

 

 

[Ultralights]

my vampire has a symmetrical aerofoil profile, at 0 deg AOA, i will descend, at about 500 ft/min or more (0deg AOA = 0 Lift), to fly straight and level at 80kts, i will be between 3 and 5 deg nose up pitch. because of the symmetrical aerofoil shape, i need a relatively high AOA on finals with 0 flap, the initial 4 deg to maintain height, and an additional 7 to 10 deg more at 50kts, which means i am almost at stalling AOA as i begin the flare,which has resulted in numerous tail strikes, when i land with full flap, nose up pitch is no where near as bad as with 0 flap, with full flap the wing now has a more curved upper suface, requiring less nose up to cate lift. with zero flap, i have since learned to land at 10 kts more than in the book, to save repairing the bottom of both rudders, again.. the joys of a symmetrical aerofoil profile... most if not all aerobatic aircraft have a symmetrical aerofoil as well. so inverted flight i as easy as normal flight.

 

 

[Tomo]

Probably all the average light aircraft pilot needs to know about it ;)

If the full understanding of these theorys/principles is required before flying, methinks most pilots have just been grounded

Ain't that with everything though...i_dunno

 

I don't reckon many people know what makes there TV function, other than plug it in and hope that it goes... or worse still a computer...!

 

But there's no harm in knowing what makes things tick i reckon...:thumb_up:

 

 

[Yenn]

Tomo. Who cares about how TV works. If it stops working your life is not threatened, whereas in an aircraft it certainly pays to have a fairly good understanding.

 

I work on the assumption that most of the lift is the result of deflecting a mass of air equal to the planes weight downwards, and it doesn't matter if you are upside down or right way up.

 

 

[motzartmerv]

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...;)

 

 

[Guest High Plains Drifter]

Who cares about how TV works. If it stops working your life is not threatened

You dont know the Morning Show audience then - they be jumping out windows if they lost their T.V.s

 

I work on the assumption that most of the lift is the result of deflecting a mass of air equal to the planes weight downwards, and it doesn't matter if you are upside down or right way up.

What about if the aircraft only has one wing ? ...why, on Sky News the other night I seen an aircraft with one wing missing do a successful landing - obviously the hull was flying as well .....................

 

But there's no harm in knowing what makes things tick i reckon

Agreed :thumb_up:

 

 

[skybum]

motzartmerv, then you can demonstrate saving a too low too slow by increasing power and pushing nose down and go UP.

 

I prefer pull back and houses get smaller, pull back some more and houses get bigger again:laugh:

 

Coanda effect- the spoon in water flow trick, And deflection of air mass. When you look at some airline wings there is more curvature under the wing than on top. Adding curvature or shape to the wing gives it more useful range of lift than a flat piece of plate. My understanding of how lift is produced has been stood on its ear. I still believe the formula still works because the coefficient part incorporates all the unknowns of my limited knowledge.

 

Reading an article about an old Loadmaster that the CAF were using for photo ops. They used to fly with the emergency door above the wing open for photo ops. People standing in the door during landing would get themselves blown off their feet as a pressure wave would pass them as the aircraft stalled on landing.

 

The RedBull aircraft have been modified back to a conventional wing shape rather than the symetrical aerobat wing. Gives them more performance in high g high angle of attack positive G racing.

 

At times I still marvel at what I achieve when just sitting at cruise with not much else to do and just thinking about what i am doing.

 

 

[farri]

Has anyone had a think about the early types of Utralights and who`s laws of physics kept them in the air.

 

They had a single surface wing which consisted of a leading edge,a trailing edge and a top surface,no bottom surface to the wing at all,not even ailerons.

 

I flew a few.

 

Cheers,

 

Frank.

 

 

[griffo]

Bernoulli??

 

I dunno!

 

I just push this thingie here.........the noise gets BIGGER......wind gets faster,,,,and when it sounds really 'cool', I pull back on the stick-thing and the ground gets smaller......

 

I dunno why!!..... But I fink its sumphin to do wif the NOISE!!

 

 

[Ultralights]

last time i looked, Birds, had curved upper surfaces on their wings...