Bernouli or Newton??

[Speedy G]

I'm sure this is a stupid question to be asking but I'm fed up of not getting a clear answer.

As a physics student I was taught initially that lift of a wing was caused by Bernouli's theory that the wing caused an area of low pressure above the wing and an area of high pressure below the wing, causing it to move from high to low and thus lift.

However I was later told that it was due to newtons third law that for every action there is an equal and opposite reaction, so the wing pushing down on the moving air causes the air to push back on the wing untill the forces become unequal and the wing lifts.

Both seem sensible - so which is it? Microsoft F.S 2000 explains it due to Bernouli with a cessna as the example.

Encarta and other text books explain it using Newton, but give large plains and an example.

Which one is commonly accepted as being correct - or are they both correct as they apply to different types of wings?

I am most confused, can anyone in Flight Training tell me what they have been told as it is most bemusing!

Cheers Speedy.

[Token Bird]

Trevor Thom PPL books say 'Bernoulli', though maybe it'll be like when we got to A-levels and found out that we'd been told a whole bunch of lies at GCSE. Perhaps when we get to CPL level they'll say 'Newton',

Token Bird

[Willy Jazz the cans 2]

Bernouli :)

If you haven't done already, why don't you just post this in the Tech Log Forum?
I'm sure you'll all the anwers in there........not sure if I'll be able to understand them though!
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I'm on my way!

[This message has been edited by Willy Jazz the cans 2 (edited 25 January 2001).]

[foghorn]

Both are right! They complement each other.

Bernoulli describes lift in terms of pressure and flow velocity.
Newton can be used to describe it in terms of force and momentum.

But wait a minute, I hear you ask, two different theories for one effect at the same time? Well, they are directly linked. Bernoulli in essence says that the pressure is lower on top of the wing and slightly higher underneath. The force applied on the wing by this pressure difference is lift.

So, we have an action, and we can apply Newton's Third law, every action has an equal and opposite reaction. What is the reaction? Well, it just so happens that if you look at the flow patterns over a wing, the air moving over the wing drops after the wing has passed: this is called the downwash. That is the reaction to our lift force.

Personally, I find Bernoulli a better way to visualise lift, since it is a more complete description of what is going on: it's a pretty self-evident model once you apply it to the wing situation. With Newton you have to analyse things a little deeper: you have to know about the existence of the downwash for Newton to fit into the picture correctly.

(speedy G I've simplfied this for any lurkers who are not so physics-minded as you)

[This message has been edited by foghorn (edited 25 January 2001).]

[Perfect PFL]

For flying purposes I would stick to using Bernoulli.
I agree that both are correct, but I think that the effect of lift is generally described using Bernoulli's principal in aviation circles.
I seem to remember using Bernoulli rather than Newton in fluid mechanics at uni as well, but I can't really remember many of my uni lectures, I did finish a year ago. :)

[BN2A]

ATPL Principles of Flight lessons say Bernoulli, and the venturi effect.
If the JAA say that's what it is, then that's what it is.
Disappointing to know that they're right though :)

[johnv]

If Bernoulli is to blame then how can aerobatic planes fly upside down. Newton's law is the reason the plane flys but Bernoulli makes it more efficient. I read somewhere that if Bernoulli was the only thing at work a Cessna 172 would lift off at over 300mph.

[Speedy G]

Thanks guys - it seems I will have to accept that they are both correct in their own little way.

It seems that both are effects in their own right, the only debate is which one is more important, or creates more "lift".

Thanks

[Dan Dare]

(from the dim distant past at uni in the '80s)

At hypersonic speeds Newtons theorys work quite well (sine sqared law?), but at slower speeds the random molecular motion makes these laws inaccurate. At these speeds I think that both (modified) Newtonian equations and Bernoulli's equations predict the same results.

Again from faded memories, CL max is about 55 degrees in hypersonic flight.

Bernoulli's equation allows inverted flight (see recent thread on the subject). A symetric wing doesn't know or care which way up it is, a cambered wing will just produce lift less effectively inverted.

Could it be time to get the books out again?

[This message has been edited by Dan Dare (edited 25 January 2001).]

[herniair]

At low speeds, Barnoffi's theory is a better way of imagining what happens.
Not proper physics but tastes good and stops you stalling.

[mrt]

Bernoulli is simply an extension of Newtonion physics. At low speed and high angle of attack, there is a significant amount of lift, and drag, because of the impact of the relative wind. Think of the downwash behind the wing. Induced drag is high at low speeds, and parasite drag is high at high speeds. Have fun.

[Cuban_8]

All,

Just to put the facts right here guys, here's what the theories say - I was lectured at Uni by a guy who was an International authority on the development of aerodynamic theory!!

Newtons approach was one of this first directed towards the phenomenon of lift. Newton assumed that lift was created by the momentum exchange between the flow and wing, thus leading to a reaction force. HOWEVER, Newtons law was flawed and predicted very low lift levels for a given wing size - so much so, that it was though that it would be impossible to fly!

Bernoulli's approach was developed much later, when the study of fluid mechanics had advanced considerably. Bernoullis effect explains lift by means of the Bernoulli pressure-velocity relation and the pressure difference generated over an aerofoil. This is the theory that is used today (low speed flight anyway!).

Hope this helps - thought I would correct things since everyone seems a bit confused!

Cheers.

Cuban_8