Why do the leading edges of aerofoils need to be rounded?

[godlameroso]

Here is the Cambridge video:
http://www.youtube.com/watch?v=UqBmdZ-BNig

It clearly shows that the air does flow faster on the upper side.
What puzzles me is what happens afterwards?
If the leading edge of the upper & lower flows never got back into sync then there would be an accumulation of air along the upper path - because there is more air leaving the upper trailing edge each second.
Obviously that is nonsense since precisely the same quantity of air is entering each path of the system on the left so the same quantity must be leaving each path of the system on the right.
OK the video is not showing the air leaving the system on the right - that is still further to the right & it is in that unshown area that the mass balance must be restored.
What happens? Just a lot of turbulance as it sorts itself out?
But that contradicts what Kiril says in the post above this.
Going back to the statement I made that there is more air leaving the upper trailing edge each second. That does not make sense because where did the extra air come from?
The air has expanded to fill a bigger volume?
So if it has expanded then it is not incompressible flow?

Overall it's the same amount of air, but displacement changes pressure, and to a small extent pressure also changes density.

[godlameroso]

Bernoulli’s principle in my frame of reference says that speeding up a fluid flow will lower its pressure/density. When air molecules speed up they essentially become spaced further apart thus lowering the pressure. This works for wings and a carburetor venturi for instance. The equations may differ for compressible and incompressible fluids but the principle is sound for both.

Bernoulli's equation says that when speed increase pressure decreases while density remains constant. Air behaves like water at slow speeds - density stays constant.
The exmple of the car you made is a bit misleading. 1d-traffic equations are hyperbolic - i.e. have the same mathematical character of navier-stokes equations when air flow is supersonic, that is so fast that air behaves as compressible.

At slow speeds air is treated as incompressible - it is a mathematical model that cuts out the energy equation (which is a modelling semplification) but it works very well.

As far as Newton and Bernoulli, besides advising everybody to try and find and read "How airplanes really fly - Stop abusing Bernoulli" what I can say is that bernoulli's equation is nothing else that the newtonian momentum conservation equation rewritten under some hypotesis (irrotational motion,no viscous effects etc) - but I agree with you: better not stir it up (otherwise we would brake the non viscous, irrotational hypoteses!)

Overall, key word being overall, density stays constant, however in actual practice only physicists can get away with constants.

a bulbous shape is good for initial flow attachment, read low air speeds. And a thin shape would probably yield similar downforce at a certain speed. However thin shapes have difficulty creating that initial flow attachment so the effects of a thin leading edge become more abrupt. You don't want abrupt lift or lack thereof at low speeds, especially in a plane full of people. In an F1 car you want the aero to start working with some sort of consistency at any speed over 100kph, otherwise you have a car that is very twitchy to drive.

[Tommy Cookers]

since the 1940s some aircraft (DH Mosquito, Comet, BA Hawk? and many others) have used aerofoil (sections) that (at large AoA) develop a flow seperation near the leading edge that reattaches downstream, this is known as 'leading edge bubble'
there is a degradation of the L:D ratio

such aerofoils are relatively sharp near the leading edge
they are efficient overall at the Re Nos for which they were designed
but the 'bubble' range of AoA becomes dangerously large at lower Res
one section widely used in 'proper' aircraft shows this bad behaviour in a well-known homebuilt ultralight aircraft

car wing Res are low, in the range where blunt LE aerofoils are the safe bet at large AoA
(gliders are optimised for small AoA)

'jet' airliners have 'droop' flaps at the leading edge to get good lift in the lower speed range and thus avoid using large AoA
some prop airliners are potential 'bubblers', especially in poor weather conditions

[olefud]

Bernoulli’s principle in my frame of reference says that speeding up a fluid flow will lower its pressure/density. When air molecules speed up they essentially become spaced further apart thus lowering the pressure. This works for wings and a carburetor venturi for instance. The equations may differ for compressible and incompressible fluids but the principle is sound for both.

Bernoulli's equation says that when speed increase pressure decreases while density remains constant. Air behaves like water at slow speeds - density stays constant.
The exmple of the car you made is a bit misleading. 1d-traffic equations are hyperbolic - i.e. have the same mathematical character of navier-stokes equations when air flow is supersonic, that is so fast that air behaves as compressible.

At slow speeds air is treated as incompressible - it is a mathematical model that cuts out the energy equation (which is a modelling semplification) but it works very well.

As far as Newton and Bernoulli, besides advising everybody to try and find and read "How airplanes really fly - Stop abusing Bernoulli" what I can say is that bernoulli's equation is nothing else that the newtonian momentum conservation equation rewritten under some hypotesis (irrotational motion,no viscous effects etc) - but I agree with you: better not stir it up (otherwise we would brake the non viscous, irrotational hypoteses!)

Overall, key word being overall, density stays constant, however in actual practice only physicists can get away with constants.

a bulbous shape is good for initial flow attachment, read low air speeds. And a thin shape would probably yield similar downforce at a certain speed. However thin shapes have difficulty creating that initial flow attachment so the effects of a thin leading edge become more abrupt. You don't want abrupt lift or lack thereof at low speeds, especially in a plane full of people. In an F1 car you want the aero to start working with some sort of consistency at any speed over 100kph, otherwise you have a car that is very twitchy to drive.

The differing views concern simplifying assumptions, i.e. air being incompressible and inviscid outside of the boundary layer, that are common in aero working theory. Up to about Mach 0.3 incompressibility is workable. While F-1 vehicle speeds are within these bounds, the slipstream speeds may push the limit.

As you observe, theory and practice sometimes differ.

[riff_raff]

Air speed is definitely a concern. If you look at a propeller, helicopter rotor blade or turbine engine fan blade, they always have sharp leading edges and thin airfoil section. The same is true with wing airfoils of high-speed aircraft.