Production of Lift

[ryan_vd]

Hey,

My name is Ryan, I'm currently working towards a Commercial License. I'm a little bit confused about the production of lift.

Just recently, I read an article about this very point, and the conclusion was Bernoulli's Principle only contributes to 2% of Lift produced while most Lift produced is based on Newtons Third Law, for every action there is an equal and opposite reaction. After doing some research, looking at NASA webpage and RAF webpage. I came to the conclusion that they also dispute Bernoulli's Principle, and agree that Newtons Third Law is the fundermentals for production of lift - like it to water(fluid) running over a spoon.

Every text book you read, describes lift production around Bernoulli's Principle - High's and Low Pressures etc. I'm just wondering what peoples ideas are and their conclusions, and if other fundermentals, like drag, and especially production of thrust is effected.

I hope you sort of understand my query,

Ryan

[CBLong]

Hi Ryan,

The exact mechanism for the production of lift is quite controversial, but as you say, most authorities seem to agree that relying solely on Bernouilli is wrong. This (http://www.av8n.com/jsd/) site will give you a lot of background - try the link in the fourth bullet point labelled "critique".

For what it's worth (and I'm just a moderately maths / physics literate computer programmer / PPL), I think that whenever you get this sort of "holy war" over an issue, the most likely situation is that all the different points of view have some merit.

It's certainly true that the only way that any aircraft can generate any forces on itself (including lift, but also drag, thrust etc) is by applying a force on the surrounding air - it then feels the equal and opposite force as per Newton. Of course, applying a force to the air causes the air to accelerate, which leads to pressure diferentials, which lead to more accelerations, and so on - making it very difficult to analyse. Bernouilli's Theorem relates changes in velocity of a fluid to changes in its pressure, but it can't (easily) predict how much "change in velocity" will result from a given aerofoil moving at a given airspeed at a given angle of attack, etc.

I'm fairly sure that the Coanda Effect (the "water flowing round a spoon" thing) isn't relevant to aerofoils - the two situations are quite different... or so I've read...

Hope this helps... now that I've stuck my head above the parapet, let's see if my post starts a mini-"holy war" here!

Cheers,

cbl.
:)

[Genghis the Engineer]

There are numerous versions of how lift is generated - Bernoulli, Newton and Glauert to name just three.

Good news is that they're all right.

Bad news is that they're all wrong.

Bottom line is that they all are specific explanations to serve particular functions. In the right context, each is perfectly correct - but the people who go about saying that Bernoulli is wrong are technically correct - albeit also usually irritatingly unhelpful.


Bernoulli is I think universally used in pilot training, and there's little about it that will lead you astray from a pilots viewpoint. If however you want a more detailed version and to learn about some of the other methods, I'd suggest either Kermode's "Mechanics of Flight" or Houghton's "Aerodynamics for Engineering Students". Any good University should carry both in the bookshop.

G


(Genghis' definition of a good University - one which offers a dedicated aeronautics degree. That excludes Oxford and Cambridge.)

[bookworm]

Bottom line is that they all are specific explanations to serve particular functions. In the right context, each is perfectly correct - but the people who go about saying that Bernoulli is wrong are technically correct - albeit also usually irritatingly unhelpful.

Only if you persist in mislabelling the equal-transit-time argument as "Bernoulli". If you calculate the velocities at the wing surface and use Bernoulli's theorem to deduce the static pressures, you get the right answer.

The issue is not whether the stream of air over the upper surface travels faster, but why. None of the popular explanations of lift do a very good job of that.

[Genghis the Engineer]

Which re-inforces my point really, Bernoulli adequately explains lift - if you assume that the unequal flow is true. Problem is, that the explanation then given routinely assumes no viscosity, which when analysed makes no sense at-all.

But in context, the explanation works as you say.

G

[fruitloop]

Now I'm really confused !!!!!

[Jetstream Rider]

Don't worry about it - do you know exactly how the speedo in your car works (if you don't it might surprise you). What you really need to know is the effect of your car's speed, or how to change it.

As a pilot, you do not need to know exactly how lift is produced, but you do need to know what is hot and what is not. For instance, the shape and texture of the wing is important, so snow or ice or even sometimes a light coating of water is bad. You also need to know the effect of flaps, angle of attack, speed etc, the exact mechanics of lift are more the interesting thing for Engineers.

If you really want to know, read the books Genghis suggests, if not, then see below:

Lift = MAGIC

I don't mean to sound patronising with that - I went to one of the Universities Genghis mentions above and learnt all about it, but still, to me, lift is magic (although I bear in mind all the important things you need to know).

The key is, that lift is very compleax and that different people have different mathematical models that work in certain regimes. But they are just that - models, or in other words inexact. Bernoulli works well at low speed in 'incompressible' and 'inviscid' flow (say up to Mach 0.3) for certain shapes for instance. It all falls apart as you get faster though, but still plays a little part in a lot of the maths. When you start talking turbulent, compressible, sonic, supersonic, hypersonic, extremely hot/cold, viscous or any combination of those, it gets really complex and no one on earth understands it (literally), although good aerodynamicists understand a lot more than me.

[Genghis the Engineer]

There is an alternative theory.

(1) Lift is generated by expenditure of money (otherwise termed reverse reaction to lightening of the wallet.)
(2) Lift required is not actually proportional to weight, it's proportional to beauty.

Hence a glider, tends to need very little money to generate lift - and the more beautiful it is the more lift it generates for less money.

Your average light aircraft needs a moderate amount of money to generate lift.

An airliner, which only it's mother could love, needs large amounts of money to make it fly.

And a helicopter, which is so ugly it's mother hasn't made her mind up yet, and probably never will, needs astounding amounts of money to make it generate lift.

Unfortunately like all other models it breaks down in places. Firstly it ignores microlights - which are ugly but generate lift very cheaply, and harriers which are very beautiful but need lots of money to fly. It also ignores purchase cost, which is almost completely unrelated to flying cost.



So why am I spouting this line of airbollox. My point is that virtually all scientific / engineering theories are just models. They are there to allow you to predict what happens under certain conditions - even the one I've just given. The really clever aerodynamicists actually probably don't understand lift better than a PPL who has passed his technical exams - what they do know is (a) a lot more different models, and (b) the limitations of each model far better than anybody else. That allows them to know which model or set of equations to use, to predict what they want under any given circumstance. The really clever ones can take a combination of models and generate their own to predict an aircraft's behaviour under particular circumstances.

G

[fruitloop]

Thanks Genghis,I enjoyed your hypo....(too much red wine). When we throw Bernoulli,Charles,Boyd's,Newtons and half a dozen other theory's into the pot it doesn't really make a lot of sense except in a perfect world !!!
Cheers:}