Bernouli's Irrelevant?

[wanabigaplane]

So then...

 

So then a surface moving under stationary air will cause a low pressure close to it and hold the aircraft down on the coveyor belt!

 

Jack. :big_grin::big_grin:

 

 

[ahlocks]

Aww bloody hell!! Onya Jack. thumb_downthumb_down

 

Griffo went to all the trouble of explaining how it all works and you go and throw bloody convey belts into the theory. :raise_eyebrow:

 

 

[Harro]

On Mr. Bernoulli...aerodynamics 101

 

G'day all,

 

Time to weigh in on this, with a slightly theoretical bias, so for what it's worth...

 

Bernoulli's theorem is not really that difficult a concept to grasp. Bernoulli found that the 'total pressure' in an incompressible flow along a given streamline is constant! There, that wasn't so hard! But what does it mean?

 

In technical terms: for a given 'streamline', 'static pressure' + 'dynamic pressure' + a gravitational potential term = 'total pressure', which is a constant.

 

As applied to the 'low speed' aerodynamics of an aircraft (less than 1/2 the speed of sound), compressibility effects of air can be ignored, as can the gravitational term. This simplifies then to: static pressure + dynamic pressure = constant.

 

Knowing this means that we can calculate, for instance, IAS from a pitot static system, which is fortunate.

 

But can we use Bernoulli's theorem to calculate the lift (pressure distribution) on a wing? Short answer, no. However, to say that Bernoulli's theorem doesn't apply to the lift generated on an aerofoil is, imho, a non-sense. It certainly gives us some insight as to why the pressure varies accross an aerofoil. Where the velocity of the airstream changes, then the 'total pressure tango' between dynamic pressure and static pressure occurs, hence the possibility to produce a net lifting force if we are clever enough. Remember, the only force transmitted to the wing from the airflow is by means of 'pressure' acting on an 'area' of that wing. Force = pressure x area, so if we sum up all the respective regions of 'pressure x area', we end up with a resultant force, which in aviation we arbitrarily express as 'lift' (perpendicular to the 'freestream' airflow), and 'drag' (parallel to the 'freestream' airflow).

 

What we need is to be able to calculate these lift and drag forces. The whole concept of engineering is about producing 'models' to enable us to make a reasonable prediction of cause and effect. Bernoulli's theorem is just one such model, within its limitations. Mr. Newton's laws represent another handy and reasonably reliable model, within their limitations.

 

One way to calculate the lift and drag forces would be to map the pressure distribution around the aerofoil, and sum up the component forces to find the resultant. Unfortunately, Bernoulli's equation can't do that for us. Bernoulli's theorem only applies to streamlines absent significant 'viscous effects'. This 'inviscid flow' assumption breaks down in the boundary layer, and in the wake.

 

To get around this, we measure the lift and drag resultants (and pitching moment)directly in a wind tunnel, and establish 'coefficients' (CL, CD and Cm) to be used in calculations. This model works reasonably well because it bypasses the need to understand exactly what is happening in the boundary layer, and models only the measured outputs based on the measured inputs.

 

Another method might be to estimate the change in momentum in the wake (Newton's laws). This will tell us the 'reaction' necessary in the wing to produce the change in momentum of the airflow, but the reaction force on the wing is still the sum of varying pressures (Bernoulli again) acting on their respective areas. The change in momentum is an inescapable consequence of lift production... think of a propellor, or a helicopter rotor downdraft. The momentum method tells us only the total reaction, and nothing about the nitty gritty e.g. centre of pressure.

 

As stated earlier in this thread, lift can be estimated by either model, with the results in close agreement. The models don't change what actually happens, but are just our attempt to get a handle on it, so that we can design aircraft that fly!

 

Also... There has been some discussion about the 'fidelity' of adjacent air molecules i.e. do they meet up again at the trailing edge, or have a fling? I've never liked the 'longer path' explanation for increased velocity, and hence reduced pressure, because it presumes 'fidelity' (to carry on the analogy). Without doubt, in a three dimensional airflow around an aerofoil producing lift, the molecules do not meet up. The opposing spanwise flows on the upper and lower surfaces of the wing act to keep the previously chummy molecules apart.

 

As far as I know, aerodynamic theory does not require such a condition that the same molecules meet up at the trailing edge either. A gentleman named Mr. Cutta established a theory (model) known as the 'Cutta condition', which represents the only possible steady state solution for an airflow around an aerofoil. This has to do with the theoretical concept of 'circulation' superimposed into the airflow to account for the observed facts. Initially the rear stagnation point in an aerofoil (set to produce lift in the normal, upright sense) is forward of the trailing edge, on the upper surface. This creates what is called a 'starting vortex' at the trailing edge, forcing a unique 'circulation' value into the airflow around the aerofoil just sufficient to move the stagnation point to the trailing edge i.e. the upper and lower surface streamlines meet happily at the trailing edge. At this point, the transient starting vortex is cast off into the wake never to be seen again, and circulation is maintained resulting in a stable airflow. While there are numerous other constraints, there is no intrinsic requirement for the previously adjacent air molecules to meet up again at the trailing edge.

 

Any questions???

 

Regards,

 

Harro

 

 

[Guest terry]

Ha Harro that's just beautiful

 

 

[Tomo]

Ha Harro that's just beautiful

Couldn't agree more...:thumb_up:

 

 

[Guest High Plains Drifter]

The models don't change what actually happens, but are just our attempt to get a handle on it, so that we can design aircraft that fly!

If that is the case - one could argue then that a light aircraft pilot doesnt need to know these theorys/principles.

 

I tought myself to fly a single seat Thruster and had several hundred hours up before I ever heard of Bernoulli.

 

(Just being :devil:'s advocate)

 

 

[farri]

I tought myself to fly a single seat Thruster and had several hundred hours up before I ever heard of Bernoulli.

(Just being :devil:'s advocate)

High Plains Drifter,

 

I agree with that completely,because,the physics of what keeps the aircraft in the air and the mechanics of actually flying the aircraft are two completely different things.

 

I think this thread and debate is about weather or not the "Bernoulli" theory actually does apply and only physics will give us the answer.

 

Cheers,

 

Frank.

 

"FLYING IS EASY,HITTING THE GROUND IS HARD".

 

 

[Guest High Plains Drifter]

I think this thread and debate is about weather or not the "Bernoulli" theory actually does apply and only physics will give us the answer.

No problemo, I'll butt out ...

 

 

[farri]

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!!

Gosh,Gee Whiz,

 

I wonder if deaf people could become pilots then???

 

Frank.

 

 

[Guest palexxxx]

Gosh,Gee Whiz,I wonder if deaf people could become pilots then???

Frank.

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???

 

 

 

[djpacro]

If that is the case - one could argue then that a light aircraft pilot doesnt need to know these theorys/principles.

I agree, pilot theory goes too far in some cases but I don't get a say in the syllabus nor do I want to waste time arguing with students about what they read in their theory notes. Just had a quick look at the CASA syllabus and that doesn't seem to be a real problem. I wonder whether the current exams require anyone to know about Bernoulli and boundary layers?

I used to like referring to sticking your hand out of a car and varying angle of attack etc but can't do that these days.

 

On the other hand, as an engineer I also need to bite my tongue as I can easily slip into stuff which is way too technical when a student asks a question.

 

 

[Harro]

And just to add support for Drifter's outlook ...

 

We used to be taught that theoretically, a bumble bee can not fly! Lucky they don't know how to read the theory books then, eh! ;)

 

For those (like me) that like/need to understand the workings of a duck's rectum, the information is out there... but can be a bit of a liability sometimes. Horses for courses.

 

I would always like to think the designers of all our aircraft were well versed in both the theory and the practical. In my 'previous life' (decades ago), I've seen designs that frightened the socks off me. And yes, people died as a result.

 

Now I am happy to participate on the piloting side of things, so as long as the duck's healthy...

 

Bee safe!!! :thumb_up:

 

Harro

 

 

[farri]

My thoughts, on wheather the Bernoulli principal has much effect on flight,or not, are realy quite simple.

 

It`s a proven fact that aircraft fly and as pilots of the recreational aircraft that we fly,it`s more important to be able to fly the aircraft,well,than to know,who`s theory principal it is that makes it fly,that is realy for the designers and the manufactures.

 

Theory principals are set as a yard stick, to work to, untill a better one comes along.

 

Cheers,

 

Frank.

 

"FLYING IS EASY,HITTING THE GROUND IS HARD".

 

 

[eightyknots]

This thread has been the most interesting and informative thread I've ever read on this forum. I hope it never stops.

I think this thread has 'stalled' ...as a manner of speaking.

 

Now, which theory would explain that??

 

 

[Deskpilot]

I think this thread has 'stalled' ...as a manner of speaking.Now, which theory would explain that??

Actually not, 80K. I hadn't bothered reading it until today but seeing as it's gone on for soooo long, I decided to see what it's all about. Most goes way over my head but I have found one mistake that's been referred to several times.

 

Don't you know that when water is passed at presuure through a pipe, it becomes very highly magnetic. It even attracts plastic.

 

Keep it coming guys, you never know, some-one just might get it right one day.

 

 

[winsor68]

The original post was asking is "Bernouli's Irrelevant"...

 

I guess it is still very relevant... you gotta know it so you can recognize when to not mention that it is heresy. It seems like a waste of breath arguing it. Lift is what it looks like. I reckon the action of a Surfboarder riding a wave gives a pretty good idea of what should be happening when we are flying...

 

 

[completeaerogeek]

On Mr. Bernoulli...aerodynamics 101G'day all,

 

Time to weigh in on this, with a slightly theoretical bias, so for what it's worth...

 

Bernoulli's theorem is not really that difficult a concept to grasp. Bernoulli found that the 'total pressure' in an incompressible flow along a given streamline is constant! There, that wasn't so hard! But what does it mean?

 

In technical terms: for a given 'streamline', 'static pressure' + 'dynamic pressure' + a gravitational potential term = 'total pressure', which is a constant.

 

As applied to the 'low speed' aerodynamics of an aircraft (less than 1/2 the speed of sound), compressibility effects of air can be ignored, as can the gravitational term. This simplifies then to: static pressure + dynamic pressure = constant.

 

Knowing this means that we can calculate, for instance, IAS from a pitot static system, which is fortunate.

 

But can we use Bernoulli's theorem to calculate the lift (pressure distribution) on a wing? Short answer, no. However, to say that Bernoulli's theorem doesn't apply to the lift generated on an aerofoil is, imho, a non-sense. It certainly gives us some insight as to why the pressure varies accross an aerofoil. Where the velocity of the airstream changes, then the 'total pressure tango' between dynamic pressure and static pressure occurs, hence the possibility to produce a net lifting force if we are clever enough. Remember, the only force transmitted to the wing from the airflow is by means of 'pressure' acting on an 'area' of that wing. Force = pressure x area, so if we sum up all the respective regions of 'pressure x area', we end up with a resultant force, which in aviation we arbitrarily express as 'lift' (perpendicular to the 'freestream' airflow), and 'drag' (parallel to the 'freestream' airflow).

 

What we need is to be able to calculate these lift and drag forces. The whole concept of engineering is about producing 'models' to enable us to make a reasonable prediction of cause and effect. Bernoulli's theorem is just one such model, within its limitations. Mr. Newton's laws represent another handy and reasonably reliable model, within their limitations.

 

One way to calculate the lift and drag forces would be to map the pressure distribution around the aerofoil, and sum up the component forces to find the resultant. Unfortunately, Bernoulli's equation can't do that for us. Bernoulli's theorem only applies to streamlines absent significant 'viscous effects'. This 'inviscid flow' assumption breaks down in the boundary layer, and in the wake.

 

To get around this, we measure the lift and drag resultants (and pitching moment)directly in a wind tunnel, and establish 'coefficients' (CL, CD and Cm) to be used in calculations. This model works reasonably well because it bypasses the need to understand exactly what is happening in the boundary layer, and models only the measured outputs based on the measured inputs.

 

Another method might be to estimate the change in momentum in the wake (Newton's laws). This will tell us the 'reaction' necessary in the wing to produce the change in momentum of the airflow, but the reaction force on the wing is still the sum of varying pressures (Bernoulli again) acting on their respective areas. The change in momentum is an inescapable consequence of lift production... think of a propellor, or a helicopter rotor downdraft. The momentum method tells us only the total reaction, and nothing about the nitty gritty e.g. centre of pressure.

 

As stated earlier in this thread, lift can be estimated by either model, with the results in close agreement. The models don't change what actually happens, but are just our attempt to get a handle on it, so that we can design aircraft that fly!

 

Also... There has been some discussion about the 'fidelity' of adjacent air molecules i.e. do they meet up again at the trailing edge, or have a fling? I've never liked the 'longer path' explanation for increased velocity, and hence reduced pressure, because it presumes 'fidelity' (to carry on the analogy). Without doubt, in a three dimensional airflow around an aerofoil producing lift, the molecules do not meet up. The opposing spanwise flows on the upper and lower surfaces of the wing act to keep the previously chummy molecules apart.

 

As far as I know, aerodynamic theory does not require such a condition that the same molecules meet up at the trailing edge either. A gentleman named Mr. Cutta established a theory (model) known as the 'Cutta condition', which represents the only possible steady state solution for an airflow around an aerofoil. This has to do with the theoretical concept of 'circulation' superimposed into the airflow to account for the observed facts. Initially the rear stagnation point in an aerofoil (set to produce lift in the normal, upright sense) is forward of the trailing edge, on the upper surface. This creates what is called a 'starting vortex' at the trailing edge, forcing a unique 'circulation' value into the airflow around the aerofoil just sufficient to move the stagnation point to the trailing edge i.e. the upper and lower surface streamlines meet happily at the trailing edge. At this point, the transient starting vortex is cast off into the wake never to be seen again, and circulation is maintained resulting in a stable airflow. While there are numerous other constraints, there is no intrinsic requirement for the previously adjacent air molecules to meet up again at the trailing edge.

 

Any questions???

 

Regards,

 

Harro

G'day all and Harro,

 

Yes you can explain lift very well without knowing anything about Bernoulli. Bernoulli's theorems were base on Newton's Second Law. Lift is caused by turning flow or 'bending the air' if you like.

 

Google NASA Incorrect Lift Theories. The wing does not act like a venturi that to is a myth. The simplest wing is a flat plate and it works very well as up to about 8 degrees AOA. Lift quite simply is caused by AOA and wing camber (if any) which cause the air to be bent downwards invoking Newtons 2nd and 3rd laws. Any pressure differentials are a result of the wing physically moving teh air.

 

The air DOES NOT EVER meet up at the trailing edge if you are talking about the same 'particles' this is a myth called Equal transit time. On YouTube search how wings work Babinsky and watch the streamlines blow up this ridiculous myth.

 

The Kutta condition is a starting vortex flow at the trailing edge that detaches at about 3okts and is never seen again so it not relevant nor is Coanda which is in dispute as to whether it exists at all or is simply viscosity..

 

 

[shags_j]

Jesus when did this get resurrected

 

 

[Guest ozzie]

Look, for the last bloody time. It's called MAGIC.

 

 

 

[metalman]

Besides, Bernoulli isn't irrelevant ,,,his theory on fluid dynamics is quite interesting,,,,,it has f**kall to do with wings !!!but interesting all the same

 

 

[turboplanner]

I'm beginning to wonder how many real people are on this site.

 

 

[metalman]

I'm beginning to wonder how many real people are on this site.

I know I'm real,,,,,,not so sure about the rest of you!

 

 

[flyerme]

Damn you busted me , I'm really a robot ! You can tell by my spelling and grammer mistakes. But please don't tell anyone! I'm slowly being re-programmed to compute the English language to standard lol

 

I always believed lift comes from AOA for the most part and speed and stall from wing camber?

 

A big airfoil gets you slow top speed and slow stall speed. A thin airfoil gets you greater top speed but a higher stall speed. . Having flown single surface wings I come to the conclusion the speed/stall/airfoil has something to do with the air going over the top,

 

And the bottom ( what ever the shape) for lift,

 

so bottom/ underside of wing = lift

 

And top/upside for stall and speed.. Just my theory. And I'm not committed to it!

 

 

[completeaerogeek]

The reason a big or high camber aerofoil gets lots of lift at low speed is that the upper surface camber being so pronounced is equal to the angle through which it bends the air downwards. If you imagine a highly cambered wing's upper surface in isolation, you have a hang glider wing.

 

Even in level flight it is producing a big angular change which means lots of lift. But that is also why it is speed limited. Lots of lift means lots of induced drag.

 

A thin wing with less or no camber needs more airspeed to created the same amount of downwards moving air to support the aircraft's weight.

 

The F-104 had essentially a flat plate wing-hence the high takeoff and landing speed.

 

 

[completeaerogeek]

As to the stall part - stall happens when the upper surface air can no longer 'turn the corner' because the angle of change is too acute and it tumbles into a rotating vortex.. The lower surface does not stall until close to 90 degrees.

 

Single surface wings (hang gliders) use the same principle as sailboats. BOTH surfaces bend the air to a new direction. This bending creates a reaction force that is lift. (Newton's 2nd and 3rd Laws at work) Stick your hand out of a car window and you can feel the effect. It is just less noticeable on the back of your hand but is still there. Wach the Cambridge University video I linked and it will all make sense.

 

http://www.cam.ac.uk/research/news/how-wings-really-work