How does an airplane fly?

[newtron]

How does an airplane fly? The math works out, but is the common conceptual model that most people learn in school correct? There is loads of stuff about this controversy on line. I just wanted to get some opinions from fellow ABers. Thanks in advance.

[Clanad]

I'll get back to you when I have a little time, but if you could clarify what you are actually asking it would help. I fyou mean how does a wing provide lift, that's simple enough, bt if you mean something else, pleaseinclude that...

[Douglasmkv]

when lift overcomes weight, and thrust overcomes drag, flight is obtained

[newtron]

Hi Clanad, I was hoping you would take part in this thread. Well the standard or �conventional� model of how a wing produces lift relies on the shape of the wing to produce a low pressure zone on the top of the wing. This low pressure zone is created because the air flowing over the top of wing is flowing faster than that flowing below the wing because the air above the wing has to travel farther in the same amount of time. The development of this pressure difference can be described by Bernoulli�s Principle. This is what everyone�s been taught in grade school. Well, if this is the primary reason a plane flies, then theoretically, a plane should not be able to fly upside down, because if the wing is inverted, the low pressure zone will be below the wing. First of all, there is an assumption that adjacent air particles that diverge at the leading edge of the wing meet up at the trailing edge of the wing. This is just plain wrong. Actually, the air particles above the wing are traveling much faster than the air particles below the wing, and air particles that were adjacent at the leading edge do not meet up at the trailing edge.

[newtron]

Continued...

The �physical� model says that lift is created by diverting air in a downward direction, and according to Newton�s 3rd law, the air pushes the airplane upward with an equal force. With this model, the shape of the wing is not as important as the angle of attack. This can then explain inverted flight. Also this is exactly how sail boats, paddles, and the ferrying of a raft work. You can demonstrate this by sticking your hand out the window of a car and varying the angle of attack. There still is a low pressure zone above the wing, but it might be argued that the low pressure zone causes the air above the plane to travel faster instead of the other way around. Anyway, this is how I understand it. You can read more about the physical model by googling �why can a plane fly upside down� or something similar. So how do you understand it?

[Peter Pedant]

Yeah I asked a pilot and he said when you fly upside down, you sort of do it a different way than when you do it the right way up.

The big thing is taking off upside down, but I think there is a video of a crazy doing that as well.

upside down, the angle of the wing to the direction oftravel has to be around 45' - I think or something

[Clanad]

Good questions, newtron (just got back from 3.2 hours of defying the law of gravity successfully again, by the way).
What's happened, over the years, is that a simple, somewhat valid explanation for your question was granted legacy standing, that being the oft quoted Bernoulli Principle. Problem is, as you have deduced, it doesn't withstand in-depth scrutiny. Firstly, it's been known for years that when two molecules of air are separated by the leading edge of a wing, the chances that they will reunite at the trailing edge is less that nil. It is part of the KISS explanation of lift and one, unfortunately, promulgated through repetitive generations of flight instructors. I was taught the old canard and used it when teaching students until curiosity got the better of me.

Firstly, Newton's 3rd law, as you've discovered, is the controlling force along with the phenomena decribed as the Coanda Effect. Although developed for fluid dynamics it holds true for air in motion as well. It describes the tendency of a fluid which comes into contact with a curved surface trying to follow that surface. The reason it does is termed viscosity. Air has a certain stickiness causing the flow pattern. At the wings surface, the relative velocity between that surface and the nearest air molecules is exactly zero. This area is termed the boundary layer. The farther away from the boundary level the faster the air moves...because the fluid near the surface has a change in velocity, the fluid flow is bent towards the surface and will follow the surface unless the bend is to small of a circumference. The "tighter" the bend the more force is applied to the air. So much so that the greatest part of lift is produced at the most forward part of the wing... somewhere in the neighborhood of half the total lift...

Contd.

[Clanad]

Contd.

So, OK, cut to the chase... Newton's 3rd and the Coanda Effect work so that when the air is bent around the top of the wing, it pulls on the air above it accelerating that air downward. This pulling causes the pressure to become lower above the wing. It is the acceleration of the air above the wing in the downward direction that gives lift. All of this can be quantified and given mathmatical values to help the engineer determine the best size, shape and curvature of wing for a given application as well as determine the required power to make it all work (unless it's a glider). For the most part Peter Pedant's offering describes the requirment for inverted flight... it's another story, but similar in it's explanation. Thanks for the interesting question... let me know if more clarification is required. (Some verbiage assistance from a text in my library from The Dept. of Aeronautics and Astronautics University of Washington

[newtron]

Thanks Clanad. Yes, you describe the "physical model", which makes sense to me. I just think it is crazy that everyone and their dog is taught the wrong explanation. From the reading I've done on the web, it also appears that there are many people that will not accept any other model.

[ugly_bob]

and there was me thinking they were all held up by tring on sky hooks.

[parkesquay]

It is called the 'Venturi Effect' and is as clanad says. However to simplify things and to prove it yourself take 2 pieces of say A4 paper and hold one piece between your thumb and 1st fingers of both hands and the othere piece between the 1st and 2nd fingers of both hands. Just let them hang naturally and gently blow down between the 2 sheets of paper. You would of course expect them to tend to move apart but you will see that they actually move into each other.
What you have done is to reduce the pressure between the sheets relative to the pressure outside them. You will have done this because the velocity of the air inside is greater than outside hence you get the pressure reaction.
When you see the centrally inwardly curved cooling towers at say power stations the curves are reducng pressure inside and forcing waste air out to aid cooling.
Hope this is easy to follow.

[Skids]

80% of what keeps an aircraft in the air is suction. That is to say, 20% of lift is higher pressure air pushing up on the wings and 80% is lower pressure air 'sucking' up the wings from above.

[mibn2cweus]

newtron, Thanks for pointing out this facinating controversy!
Lots of interesting stuff here

[newtron]

Thanks mibn2cweus. That was an interesting read.

[sally-anne]

Thanks everybody. It's a bit scientific but I get the drift.