Back to basics...

In the early stages of aviation the theory that one particle of air travels faster than it's friend, creating a lower pressure and therefore lift, made lots of sense. It fit current (at the time) fluid dynamics models and explained the phenomenon observed. It was also simple to explain which made it easy to pass on. Unfortunately it's incorrect.

The theory has one simple flaw. How do the air particles know where the other is? They don't care who they are next to. The idea that two air particles try to come together when there's a wing between them doesn't make sense.

The truth lies in a combination of effects. The turbulent effect described above, boundary layer effects and change in momentum all contribute to lift.

Remember don't anthropomorphize air particles. They hate that.

But consider, if the distance method is correct, why are all aerofoils not designed with a straight lower surface - surely that would induce the maximum velocity differential. Most aerofoils are curved on both upper and lower surfaces like this:

To produce less drag. The smoother airflow at the leading edge and the combination of the curves on both the top and the bottom result in a smoother flow of causing less drag thana similarly sized , flat bottomed air foil.

DS.

Originally posted by dead steve

To produce less drag. The smoother airflow at the leading edge and the combination of the curves on both the top and the bottom result in a smoother flow of causing less drag thana similarly sized , flat bottomed air foil.

How would that come about... you have a blunter leading edge (more curvature) - thus the stagnation point would be bigger if anything.

Also, with the curved lower surface there is a longer length for boundary layer growth = more drag.


Admittedly, the slip line at the rear of the aerofoil will be affected, and the result could be positive - but since the upper and lower boundary layers are at their maximum depth, any change would have minimal impact (for two well designed aerofoils).


Note: I am not talking about an aerofoil at an angle of attack, just a postively cambered aerofoil at 0 deg setting angle. [Keeping it simple]

Originally posted by kilcoo316

Originally posted by dead steve

To produce less drag. The smoother airflow at the leading edge and the combination of the curves on both the top and the bottom result in a smoother flow of causing less drag thana similarly sized , flat bottomed air foil.

How would that come about... you have a blunter leading edge (more curvature) - thus the stagnation point would be bigger if anything.

Also, with the curved lower surface there is a longer length for boundary layer growth = more drag.


Admittedly, the slip line at the rear of the aerofoil will be affected, and the result could be positive - but since the upper and lower boundary layers are at their maximum depth, any change would have minimal impact (for two well designed aerofoils).


Note: I am not talking about an aerofoil at an angle of attack, just a postively cambered aerofoil at 0 deg setting angle. [Keeping it simple]

Okay First off it is fair to say you have more education at this than I do.
But I'll kick the the cat and see what noise's we get.

The picture provided is close to the Ideal a-foil section:
the leading edge is nicely rounded into the airflow at 0 Aof A the top and bottom surface's follow smooth curves to the end, which is point slightly downwards.

The stagnation point is a factor of the viscosity of the fluid (air) and will stay about the same size (for us nice slow flying , old school guys anyway) if you change the angle of attack the flow will split reasonably smooth and pass over the wing following the curve to the tail. Those nice curve (ON THIS airfoil) do nicily manage the boundry area vs slip area.

Look at a bird wing, narrow air foil, rounnded edge, smooth curves.

DEAD STEVE
(some one NOT educated in compression effect on wing and air flows)

With all this talk about trailing vortices, wing cross sections, low pressure etc etc you're going to baffle the poor man!

Let’s keep this simple; a heavier than air aircraft keeps itself in the air by displacing air downward with a force greater or equal to its weight.

It's nothing more than good old Newton’s third law, for every action there is an equal and opposite reaction.

A wing is nothing more than a very efficient mechanism for pushing (or pulling, well both really) air downwards with the same force as the weight of the aircraft.

Aerodynamics is used to find the most efficient way to do this. Talking about high pressure under the wing and low pressure above the wing seems to miss the whole point - unless air is displaced downwards your plane isn't going to fly.

I cant believe you are arguing about how a wing works. You are both right. The 'air over the top moves faster' explanation is the standard quick way to explain how lift is formed. Athough not technically correct to and aerodynamisist it serves fine for most people. They do the same in science class. They start off by telling you the atom is the smallest particles as you learn more you get electrons, protons etc and by the time you get you head round that they introduce quarks, muons, bosons etc. doesnt mean the atoms are smallest is good enough for most people.

Originally posted by dead steve
The picture provided is close to the Ideal a-foil section:

Error404... uhh, picture not found

I didn't quite follow your geometry explanation

Anyway, this is close to the ideal aerofoil section:



Although I've seen ones with better lower surfaces, these can give L/Ds of well over 100! (but the AoA capability is extremely limited).

This one has a L/D of near 150 for instance.

[edit on 3-2-2006 by kilcoo316]

Originally posted by Nacnud
Let’s keep this simple; a heavier than air aircraft keeps itself in the air by displacing air downward with a force greater or equal to its weight.

It's nothing more than good old Newton’s third law, for every action there is an equal and opposite reaction.

A wing is nothing more than a very efficient mechanism for pushing (or pulling, well both really) air downwards with the same force as the weight of the aircraft.

Not true, well, kinda true - Newtonian aerofoil theory only applies for very high hypersonics, where only the air pressure on the surfaces with positive incidences to the flow are considered... uhh, that would be the leading surfaces kinda. Hard to explain without a diagram.

If the aerofoil was a flat plate at an angle of attack, only the bottom surface would be considered - thats Newtonian aerofoil theory. It really doesn't apply to anything apart from Mach... 15? maybe, 20+ really high speed and low density stuff.

Look guys, many of you have been taught the wrong things. I was too. This happens all over the world to this day.

All of you debating Bernoulli, disputing pressure, worrying about hyper/super/sub sonics, riddle me this. HOW DOES A PLANE FLY UPSIDE DOWN?

Mr Newton had the answer.

Car windows are useful too.





Edit too add.

I'm only being half serious above.

But if a novice enquires about how planes get lift, the technicalities are not the place too start.

Remember an uncurved, single skin. flat wing can get a plane into the air and stay there.



[edit on 3/2/2006 by Wolfiejohn]

[edit on 3/2/2006 by Wolfiejohn]

Originally posted by Wolfiejohn

All of you debating Bernoulli, disputing pressure, worrying about hyper/super/sub sonics, riddle me this. HOW DOES A PLANE FLY UPSIDE DOWN?





But if a novice enquires about how planes get lift, the technicalities are not the place too start.

Remember an uncurved, single skin. flat wing can get a plane into the air and stay there.

For how an aircraft flies upside down, it does it at an inverted angle of attack (uhh, like pitching nose down [from the pilots perspective]) that the curvature of the wing is generating enough "lift" (it would normally be a downforce) to hold the aircraft in the air.


As for the uncurved flat single skin - yeah, but thats starting to deal with boundary layer growth differences between top and bottom and effective aerofoil shape. That would really start to confuse.

Ok... that kinda explains it... it sure is great to have you here kilcon... I have learned a lot from you...