Fascinating stuff!! I'm relieved that I'm not alone. Aeronautics graduate and ex-instructor - I always struggled to find a definitive explanation - probably because there ISN'T one!!! As as a previous poster has already pointed out, what's really important as an instructor is to explain when an aerofoil doesn't produce ENOUGH lift and the warning signs!

I've always found venturi and Bernoulli principles to be adequate for pilots although they may not satisfy the geeks. I learned my aerodynamics in the RAF from Kermode (?) and I never broke an aeroplane because of my ignorance. It was also a satisfactory expanation for the many hundreds of glider and jet pilots I have subsequently taught to fly.

Everyone must know that an aerofoil fails to produce enough lift when the angle of attack becomes too high for the bernoullis to hang on and they roll off the trailing edge, often causing a buffeting through the airframe as they roll and their lift arrows thrash the wing surface.

What about the trick of suspending a tablespoon under a running tap? There's no underside here, just water running over the curved surface - and that produces plenty of "lift".

What's that all about then? :uhoh:

Water over spoon theory...
 

When I get a new student for a TIF I ALWAYS make them a cup of tea or coffee first and get them chatting while the kettle boils. I have a big spoon in the kitchen for just this purpose.

The point of it is that the laminar flow accross the back of the spoon causes the water to deflect , the same way as the wing causes downwash at the trailing edge. Newton's Third will have it that every action has an equal and opposite reaction.... and so the spoon deflects the water, and the water pushes the spoon.

Similarly, the wing forces the airflow downwards... and the downwash pushes the wing up.

It is useful for explaining ground effect and also for demonstrating the separation point and loss of lift in the stall. :ok:

Having a brain fart and can't remember for the life of me what its called :ugh:

Do it again - I think you'll find the water pulls the spoon, as indeed the pressure reduction of the airflow travelling over the upper convex surface sucks or pulls the wing upwards.

The other similar demo using flow over the top surface only, is the one where you hold a piece of paper at the near corners so that it droops away from you and blow gently (laminar flow) over its curved upper surface.

Does a carb type venturi need downwash at the trailing edge to produce suction? Of course not - after the waisting of the tube it reverts to same old tube.

Regards,

rts

Would be interesting experiment to suspend a spoon inside a large glass vacuum flask, then play water over back of spoon then see what happens?

Think no one theory explains lift, they all do.

I also noted one day a good example of the venturi effect whilst standing next to a canal near a bridge.
The canal narrows to pass under bridge and leaves floating on the surface speeded up as they passed under the bridge and slowed down on the other side as the canal widened.

Also the QEII had a problem some years back where it travelled fast off the US coast and scrapped its bottom in shallow sea.
Theory was that the bottom got sucked down and hit some rocks that it should have easily sailed past.

You're thinking of the "Coanda effect". How do you use it to explain ground effect?

"Push" and "Pull" are pretty arbitrary when it comes to spoons, aren't they?!

Not in the context of this discussion.

All very interesting but the original question was "If you've been teaching the "Equal Transit Theory" or the "Venturi Effect" theory for explaining "the force of lift" to your students, will you now change after reading this?" and the answer, in my case, is - Certainly not.

There are many ways of explaining why an aeroplane flies and no one 'theory' is superior to any other - they are all no more than 'theories', after all. Why make a simple question difficult (other than to massage a nerd's ego)?

Theories can be proven by mathematical calculations and it has been found mathematically that the equal transit theory for lift does not hold. That's what the NASA website set out to prove; their java lift simulator available at that site indicates that, yes, there is an increase in the velocity of air flow over the top of the aerofoil, and a slowing down of the airflow beneath the aerofoil, and yes, this does result in a lowering of air pressure over the aerofoil compared with beneath .. BUT, the pressure differential is too low to generate the amount of lift that you'd expect from such an aerofoil.

Therefore, Newtons third law of action and reaction is the better theory mathematically and it also better explains how symmetical aerofoils such as the horizontal and vertical stabilizers work as well.

I've just done my first brief on the theory of lift and have presented both theories for the audience to think about. I don't want to be pushing the equal transit theory even if it's partially correct.

Thanks all for your input on the discussion here ... it's all very interesting. :D

Nope, that's not what the NASA site says.

There is an increase in velocity above the aerofoil and the velocity differential between upper and lower surface is related through Bernoulli's principle to the pressure difference. The pressure difference exactly accounts for the lift.

The piece of the "equal transit time" explanation that is incorrect is the association of the velocity differential with the difference in "path lengths" above and below the aerofoil. The velocity differential is much higher than would be anticipated from "equal transit times".

Yes, I stand corrected. Thanks for that bookworm. After doing those java simulation experiements, I found the pressure differential to be around 0.15 psi at 10 degrees AoA and thought it couldn't be enough to produce the 623 pounds of lift. Well, I guess I was wrong. The last paragrah on that page you quoted clears that up.

Cheers!

I wrote a paper on Shallow Water Shiphandling when I was on the staff of the Royal Navy Navigation school in the late 80's - I can't remember all the formulae now but there was a rule of thumb which (I think) suggested squat was a risk when depth < 1.4x ship's draught. Also built some reasonably complex spreadsheets to predict the actual amount of squat vs depth for different classes of ship, based on hull form. Funnily enough, this was only a few months before the QE2 incident.

Interaction between ships due to increased pressures at some points and reduced at others has caused quite a few accidents over the years - we always had to be wary of it conducting Replenishment at Sea operations, when ships approached each other and then steamed alongside.

all i can say is wow....makes good reading though, almost makes me want to pick up that old textbook again..hmm maybe not!!

ok then, going off the basic theory of lift as taught by the PPL sylabus, how can an aircraft possibly fly straight and level whilst inverted? :bored: , just the simple force digram based on that will show its impossible, as lift and weight are acting in the same direction :bored: .... So there must be another reason why an aircraft can fly inverted whilst maintaining hight? :ok:

Am I the last one to realise that this whole thread is a wind-up? Forgive me for thinking for a while that it was a serious discussion.

Does anybody know of any video footage of an aerofoil in a wind tunnel with smoke, on the internet?
Would be nice to show students.

try contacting the museum at farnborough, lots of old research stuff there. Very interesting place to visit too.:ok:

I do love it when "experts" debunk a theory and then don't bother to tell you what applies instead!

If I read it correctly, they seem to say "some Bernouli, some Newton and some other stuff". Sounds very much like what the RAF taught me 20+ years ago. Anyone who teaches combined Bernouli/Newton theory is close enough for the 10% "other stuff" not to matter.

It works - that's good enough. How many pilots design their own aeroplanes?