Bernoulli's Principle

[Say Cheese]

Hi all,

A question that I am hoping you can all help me with please.
I am learning to fly and have been learning about Bernoulli’s principle.
The principle as I understand it is that the air travels faster over the top of the wing therefore creating a low pressure. This low pressure enables the air passing under the wing to effectively be pushing against a lower pressure area and therefore more effective thus helping lift.

My question is why does the air travel faster over the curved surface? I was told that it’s to get to the back of the wing at the same time as the air that goes underneath the wing but what is it that means it has to get to the back at the same time? Why does it have to travel faster? Why can it not travel at the same speed?

All opinions and advice gratefully received.

Regards.

[skwinty]

Airflow across a wing - YouTube

[The500man]

From Wikipedia:

Bernoulli's principle can also be derived directly from Newton's 2nd law. If a small volume of fluid is flowing horizontally from a region of high pressure to a region of low pressure, then there is more pressure behind than in front. This gives a net force on the volume, accelerating it along the streamline.[5] (http://en.wikipedia.org/wiki/Bernoulli%27s_principle#cite_note-Babinsky-4)[6] (http://en.wikipedia.org/wiki/Bernoulli%27s_principle#cite_note-Weltner-5)
Fluid particles are subject only to pressure and their own weight. If a fluid is flowing horizontally and along a section of a streamline, where the speed increases it can only be because the fluid on that section has moved from a region of higher pressure to a region of lower pressure; and if its speed decreases, it can only be because it has moved from a region of lower pressure to a region of higher pressure. Consequently, within a fluid flowing horizontally, the highest speed occurs where the pressure is lowest, and the lowest speed occurs where the pressure is highest.

[Pull what]

The top surface air reaches the trailing edge before the air underneath.

His experiments in the 1730s observed that water flowing through pipes of varying diameter was speeded up as it entered a segment of smaller diameter, i and its pressure decreased.

[TractorBoy]

I was told that it’s to get to the back of the wing at the same time as the air that goes underneath

If an instructor told you that then they should be shot. That's the Equal Transit theory and is just plain wrong.

[Genghis the Engineer]

If an instructor told you that then they should be shot. That's the Equal Transit theory and is just plain wrong.

So what?

I have never seen any real evidence that a detailed knowledge of Bernoulli is of real benefit to either a PPL or their instructor.

G

[foxmoth]

Bernoulli seems to be falling out of favour anyway;-
The Airfoil Misconception in K-6 Textbooks (http://amasci.com/wing/airfoil.html)
Airfoils and Airflow [Ch. 3 of See How It Flies] (http://www.av8n.com/how/htm/airfoils.html)

[mad_jock]

It used to annoy me that some people just couldn't get lift theory, but if you told them it was lift fairys that kept the aircraft up they grasped it instantly.

You can actually do the whole of the PPL with lift fairy theory.

Critical angle of attack = angle which the lift fairys fall over so the aircraft stalls.
etc.

O well didn't make the slightest difference to them flying the aircraft they did get a brief though that on no account were they to tell anyone else about lift fairy theory.

[BackPacker]

Bernoulli and Newton were both 17th century (or so) scientists and were trying to understand why stuff behaves like it does. And they did a good job progressing science. In fact, a lot of their principles have been used to teach science to people.

But after their deaths, science has moved on, and both have been proved wrong, either in generic cases or in corner cases.

As I understand it, the currently considered "good" explanation of air flow around a wing, and the resulting lift and drag, is based around an equilibrium equation involving five different factors (like pressure, temperature and speed) in some sort of integral. But that equation is very hard to visualize if you don't have an extremely good grasp of math, and is too hard to calculate by hand anyway. That's why we have computers and CFD.

So anything from the Lift Fairies, via the Equal Transit theory, to Newton (Impact Lift anyone?) to Bernoulli (high speed = low pressure) is essentially wrong to some degree. And is best used as an approximation of what happens only.

[mad_jock]

Its all to do with these equations.

Navier (http://en.wikipedia.org/wiki/Navier%E2%80%93Stokes_equations)

They give engineering students nightmares until you discover that they just zero out the hard stuff.

And there still isn't a computer thats powerful enough to solve the whole lot.

They are pretty good at doing 2D linear models but as soon as its gets turbulent your into wag land. When your into 3D nonlinear and transient dynamic flows it produces some very pretty pictures and gives you an idea of what might happen but thats about it.

[The500man]

lift fairy theory.

This is probably the best theory so far... and is supported by movies such as Tinker bell. ;)

Best not to ask how fairies fly in the first place though.

[custardpsc]

saycheese - one poster has already said the important thing, the explanation you seek refers to the equal transit theory which is not held in high regard because it leads to the very question you ask which is not answerable in those terms. (ie the overall explanation is really wrong/poor).

Google the coanda effect for more info. My understanding is that lift is caused by a combination of the air flowing over the upper surface sticking to it (coanda effect) and thus following the curve downward at the back (and thus being accelerated downward) and also the air striking the lower surface of the airfoil being accelerated downward by virtue of its shape.

The two accererations/deflections produce a reaction (newton) on the airfoil in an upward direction.

Stalls occur when the air on the top unsticks.

I expect someone who is better informed/has the correct instructors explanation can put some more detail to this...

[skwinty]

Here is a link to a paper by Prof Babinsky who made the video I posted earlier.

http://iopscience.iop.org/0031-9120/38/6/001/pdf/pe3_6_001.pdf

[Tarq57]

I have some reason to consider this (http://www.pprune.org/questions/204288-just-after-takeoff.html#post2295083) explanation plausible, at some level.

[deltahotel]

Lift fairies? Pah - it's all about sky hooks.

[mad_jock]

Best not to ask how fairies fly in the first place though

That doesn't seem to bother the people that get it with lift fairy theory.

[John Farley]

Say Cheese

I have never seen any real evidence that a detailed knowledge of Bernoulli is of real benefit to either a PPL or their instructor.

I could not agree more.

As a pilot to be you should not concern yourself one jot with why a wing produces lift but become the world's greatest expert on what factors determine the lift of a wing and which of those are available to the pilot control the lift.

That is what will keep you alive - especially when turning onto finals.

[Pull what]

John,its just that a bit of knowledge of it can help with the CAA exams!

[John R81]

Yet the proof is simple, and you can "experiment" when sitting on the lavatory, if the equipment is to hand.

If there is awall-mounted loo-roll, and the end is hanging down, blow between the paper and the wall, parallel with the wall.

You are adding air and might expect the paper to move away from the wall. It doesn't; it moves towards the wall as the faster air stream lowers the air pressure on on side of the paper and the air pressure in the room acting on the other side of the paper pushes the paper towards the wall.

Flight training should be undertaken in the lavatory!

[Pull what]

I just teach 'draught through a door'-studes get concerned if you go to the lavatory with them

[John Farley]

Pull what

True O King

However it is also true that failing CAA exams will not kill you. Whereas not understanding the piloting aspects of lift as you turn finals could.

So for me it is a matter of priorities and being able to talk about Bernoulli (or any other theory associated with lift) should come after learing how to fly safely.

[Sillert,V.I.]

Any explanation of how a wing produces lift is going to be a gross oversimplification if it is going to be understandable by anyone without a relevant postgraduate qualification. Knowing how a wing behaves is essential if you are to fly safely, but you only need to know enough about why it behaves that way to get through ground school.

[pudoc]

There are a lot of wrong lift theories out there. Some of which are half wrong with half of Bernoulli's mixed in.

I got through my PPL thinking I fully understand how lift is made, it wasn't until I got to my ATPLs that I realised I was wrong. To help me fully understand the correct version, I first had to understand why the wrong theories were indeed wrong.

NASA has put together the incorrect theories and why they are wrong.

Incorrect Lift Theory (http://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html)
Incorrect Lift Theory (http://www.grc.nasa.gov/WWW/k-12/airplane/wrong2.html)
Incorrect Lift Theory (http://www.grc.nasa.gov/WWW/k-12/airplane/wrong3.html)

The first one is the one most people think is correct.

Also I believe the theory of blowing between 2 pieces of paper is also incorrect. Some guy from NASA put a video on Youtube saying that if you try the paper demonstration but hang the paper vertically the paper doesn't move. If you hang it horizontally it does move. This is why the paper thing is wrong. The reason the air moves the paper when it's horizontal is because the papers resistance to being blown on lifts the paper up.

I haven't finished studying POF so I might have misunderstood this video, but I think the general idea was that it doesn't support Bernoulli's.

[mad_jock]

Any explanation of how a wing produces lift is going to be a gross oversimplification if it is going to be understandable by anyone without a relevant postgraduate qualification

That about sums it up nicely.

[Big Pistons Forever]

The equal point theory is scientifically wrong but good enough in a practical sense. The only thing I would add is that I also refer to the the fact that the air is bent downwards when it comes off the back of the wing ( ie downwash). There are practical reasons why understanding this fact is useful.

The whole point of the exercise should be that it becomes the lead in to talking about what really matters and that is understanding the concept of AOA.

Finally I can't believe people are talking about "lift ferries".:yuk: Good god don't you guys know anything :eek:.

EVERYBODY knows that lift is proportional to money. :p

[thing]

Just like women are inversely proportional to money then. The fatter your wallet, the slimmer they get.

[Jim59]

It is commonly stated that as the velocity of a parcel of air increases the pressure decreases - e.g. in a venturi. This is in fact incorrect (if one ignores frictional losses etc.).

If fact the total pressure (dynamic + static) remains constant. As the air speeds up its dynamic pressure increases; for the total pressure to remain constant then its static pressure must decrease.

[HowlingMad Murdock]

Say Cheese - am also reading 'principles of lift'

Mad_Jock -lift fairies - too funny!

Squinty - nice video, thanks

John R81 - fantastic experiment - must try!

[Grob Queen]

Say Cheese,

Me too!! I am also trying to get to grips with Bernoulli and as Physics and I parted company in the third year at School...finding it a real struggle. After trying to discuss it over texts, my instructor says there is a simple explanation which he will discuss on his return from hols.

I await his answer! What sparked it off for me was i'm actually studying Met and asked him what a Venturi was vis a vis Carb ice.....

Good luck with your Principles of FLight study, I have that joy to come. :eek:

[Say Cheese]

Thanks all for your help. I know that there is little benefit in knowing all the ins and outs of the exact elements of lift, however it is just to satisfy my curiosity. I was one of those kids who was always wanting to know 'Why'. I've always had a need to know how things work and was getting frustrated by not knowing all about Bernoulli's principle.

I shall go away and have a look at the different theories linked to on here and will come back if I have any other questions.

[mad_jock]

Say Cheese the problem that you will find is that everything is simplified theory.

First year university doing Engineering

Most of the subjects started with. "Well what you learned in school wasn't quite correct"

Second year was "What we taught you last year wasn't quite correct..."

Then at the end you realise everything is about energy and its conversion and conservation. And the technical bits of the converstions can be ignored and the overall effect on the system is all your interested in.

If you start off understanding the basic physics principles and then build them up to what your looking at its alot easier to visualise when you launch into it. Taking Bernoulli dry without conservation of mass and energy and newtons laws understood is quite abstract.

[peterh337]

This debate comes and goes on pilot forums every so often :)

The wing produces lift by deflecting air downwards. That is the most basic physical principle in play. The lift is the "equal and opposite" reaction to accelerating x kg of air per second downwards at y m/sec.

It's true that the pressure above it is lower than the pressure below it but that is a consequence of deflecting the flow :) You can't have one without the other (in any fluid).

If your objective was instead to just make something that produces a pressure difference across an object, the result would inevitably be a deflection in the airflow...

The wing being an "aerofoil" is of secondary importance. An aerofoil is a more efficient (less draggy) way of deflecting the airflow. Otherwise, if you don't mind the drag, a barn door would do as well.

This is basic Newtonian mechanics. Bernoulli merely worked out some equations which simplify Newton for working with fluids, to an extent that makes it easy to work out flow in pipes etc. But if you applied Newtonian mechanics to each separate molecule in the airflow, you would end up with Bernoulli's equations :) The two are just different ways of looking at the same stuff.

Then, people worked out convenient mathematical treatments for some of this stuff. Like the circulation theory for example. If you integrate the little bits of airflow around a wing you see a net "circulation" around it. But this doesn't mean the air actually circulates :) It does that only at the ends (wingtips) where the vortices come off.

[Cows getting bigger]

I recollect at ATPL groundschool this whole thing came up. Sat in our class was a smart-a*** who happened to be an Airbus engineer from Toulouse. He queried why we weren't being taught about the Reynold's Number (whatever that is). Without breaking his stride, our very knowledgeable instructor who had survived 40 years or so of flying without killing himself came back with the following:

"Look lad, if you want to stay an engineer, keep talking about irrelevant numbers. If you want to be a pilot, just learn that you can only pull the stick back so far before things go wrong." :ouch:

[Weirdfish]

I'm another one who has recently started to try and get my head around lift, a head that is now ready to explode:eek:

Although I'm less concerned with how it stays in the air than I am of what causes it to not want to stay in the air any longer, and more importantly, how to minimise adding student ineptitude to the equation.

[charliegolf]

JF said:

As a pilot to be you should not concern yourself one jot with why a wing produces lift but become the world's greatest expert on what factors determine the lift of a wing and which of those are available to the pilot control the lift.



and I plan to steal it for myself! Brilliant!

CG

[BillieBob]

Reynold's Number (whatever that is)Bourton-on-the-Water 271, as I recall. Mind you, that was before CFS moved to Yorkshire.

[alland2012]

I am just busy reading Wolfgang Langewiesche book "Stick & Rudder"
And he dismiss's Bernoulli's Theorem.
And if I'm honest, I have a much better understanding of what Wolfgang is saying rather than what I have tried to understand with Bernoulli's explanation that I have read in my Pilot Handbook during my PPL training.

[MPrince]

I've found that if you can understand Bernoulli's theorem, you'll pass the CASA CPL aerodynamics exam (CADA) with ease. Even if it is wrong, if everyone learns it then at least we're all on the same page. So what if it's wrong anyway? There are more important things than why the wing produces lift, like picking a spot to go after an engine failure in the Lance at 400' AGL.

[24Carrot]

I am just busy reading Wolfgang Langewiesche book "Stick & Rudder"
And he dismiss's Bernoulli's Theorem.

An excellent book, but he does not "dismiss" Bernoulli! He is making a different point. He actually says (my bold):
... Bernoulli's Theorem doesn't help you the least bit in flying. While it is no doubt true, it usually merely serves to obscure..."

He then goes on to say that Lift is produced when the wing pushes the air down, which is fair enough for piloting purposes, just don't expect to calculate much that way, at least not on on the back of an envelope! He also talks a lot about angle of attack, which is obviously very important for pilot training, and also lets you do quite decent calculations.

The key issue is what the student wants from "Lift Theory".
Some example motivations:


Understanding how lift is even possible, and why it seems to need airspeed and wings.

Regurgitating what is needed to pass one particular exam.

Understanding how to keep a particular aircraft type flying safely.

Wanting to "design" an aircraft, for a Flight Simulator, perhaps.

Wanting to design an aircraft that somebody could actually fly!


Each requires a hugely different level, and even area, of understanding.

In my view these "competing" theories e.g. "Bernoulli", "Newton", "Downwash", "Circulation Theory", are not competing at all, they all describe the effect in different ways, and their main difference is their ability to produce numerical results, with or without massive computational effort.

As a simple example of the cross-linkage, (and sticking resolutely to motivation #1), you could note that Lift does seem to require wings, which produce an upwards force, but are essentially horizontal plates in contact with just the air. You could further surmise that the pressure above the wing must be lower than the pressure below, there is no other force available.

There is no flow through the wing, but the above-atmospheric air below the wing will induce a flow downwards toward the atmospheric air below it. Similarly the sub-atmospheric air above the wing, will induce a downwards flow from the atmospheric air above it. Bingo! We have "Newton", conservation of momentum and "Downwash". There will also be some leakage around the wingtips, so you should expect trailing vortices. Unless you are near the ground...

Thinking about that pressure difference above and below the wing, you could also say that Bernoulli predicts that the airspeed above the wing must be higher than the airspeed below it. This means that from a ground observer's view point, (if he could somehow see the air), the air is mostly still, but as the aircraft passes, the air above the wing has a backwards ground speed, and the air below the wing has a forward ground speed. Viewed from the ground, the air appears to "circulate" around the wing as the wing moves forward. And that circulation joins up very neatly with those trailing vortices... Bingo! We arrive at the "Circulation Theory" of Lanchester and Prandtl, about one century old now. I say "arrive", because there is a lot more to it, but I will stop there because I'm only trying to show how this stuff all links up, and also it gets quite hard quite quickly.:)

[peterh337]

you could also say that Bernoulli predicts that the airspeed above the wing must be higher than the airspeed below it

Yes, but that is just another part of deflecting the airflow. The deflection causes a pressure difference, and a lower pressure means a higher velocity.

As you say it's all part of the same thing.

There will also be some leakage around the wingtips, so you should expect trailing vortices. Unless you are near the ground...

Or the wing is very (very :) ) long.

[Glamdring]

To quote a well known NATS college instructer....

Bernoulli? ha ha ha, I think not.

How does Mig fly upside down?