Lift, No technical knowledge necessary Options

[talldude]

As pilots we know there can be some controversial topics in aviation such as how and when to lean the mixture, regulations, aerodynamics and so on. So I would like to ask everyone their thoughts about the topic of lift. How does a wing fly, what produces that lift? It’s funny that this simple topic in itself can turn into a heated argument so lets see what happens here. Hint: the heated argument is not the goal.

[Piltdown Man]

The aircraft's natural environment is in the air and all aircraft would be there permanently were it not for the efforts of the infamous duo of "Campaign Against Aviation" and the "Fight Against Aviation". These rather embittered bodies of men consists almost entirely of ex Navigators and Flight Engineers who were all made redundant once the pilot community finally worked out how easy their jobs were. So to get their own back, these people enlisted the support of Bernoullis which are microscopic but immensely dense CAA and FAA supporters. They sit on aircraft and prevent them from flying. Only when an aircraft has built up enough speed do the Bernoullis fall off the back, releasing the aircraft into its natural environment. Helicopters achieve the same effect by spinning the Bernoullis off sideways and some are so ugly that the Bernoullis won't go near them. Also the Earth rejects them. Harriers and other noisy jets are the exception to these rules. They make such a horrible noise that the Bernoullis get scared and jump off in fright. I hope this clears up any misunderstanding. There is a lot of rubbish spoken about aerodynamics.

PM

[27driver]

P.F.M.

And those two embittered bodies also have a great number of people who tried to be pilots, but were unable to be...due to some unforseen occurrance of what could best be termed "CFF" syndrome...Can't Freakin' Fly...

Howz this for heated debate?

[glnflwrs]

Lift is generated when a solid object diverts the flow of a moving stream of fluid. The lift is generated in the opposite direction of which the stream was diverted to. This is most easily demostrated by placing your hand out of a car window in a flat, winglike manner. By tilting your hand to angle up or down, you can feel the force of the indstream trying to move your hand up or down. By diverting the wind your hand is lifted.

The important basic factors in lift generation are the presence of a fluid and the relative motion of that fluid. No fluid or fluid standing still, no lift.

Vershtehen zie?

[Ranger]

I'll swing by the local Victoria's Secret store tomorrow and get back to you on that one. They seem to have that whole lift thing down pretty good.

[WoopWoop]

There is a lot of rubbish spoken about aerodynamics.

PM

Yep..you got that right

[c150student]

And here comes some of that rubbish now.

I did a piece of physics coursework about lift fairly recently. I wont copy it out here (in case someone accuses me of plagiarism) but from the research I did there were 3 main theories, and there is nothing to say that these don't all work together.

The first is the one still taught in flying schools, and which is present in my 'Air pilots manual'. This uses Benoulli's principle (a slight variant on PM's version ) and uses the idea that because the top surface of the wing is more curved than the bottom, that the air must flow over the top of the wing faster to allow it to rejoin the air below it. This higher velocity decreases dynamic pressure, and the difference of pressures below and above the airfoil produce a resultant upwards force, that we call lift. However, research has shown that the air cut by an airfoil does not rejoin with the air it was 'cut from' after the airfoil has passed. I.e. Bernoulli's principle is flawed, though it may still work to an extent (explaining why aircraft with airfoils of high camber can fly at lower speeds without stalling).

The 2nd theory, deflection, is the one Glen mentioned. As Sir Isaac Newton said, 'For every action there is an equal and opposite reaction'. An airfoil at an angle of attack greater than 0 degrees will push air down as it moves past it. To see this for yourself, get a fan, remove it's blades and then replace them with rectangles of cardboard, angled slightly towards you. When you turn the fan on and point it at yourself, you will feel a breeze, as air is moved towards you by the moving blades. This is what happens to an airplane. As the wing pushes the air downwards, an opposite reaction pushes the airfoil upwards. If, say, 200 Newtons of force pushed x amount of air downwards in 1 second along 1 metre of airfoil, then that same section of airfoil will theoretically be pushed upwards by a force of 200 Newtons over 1 second.

The 3rd theory is the lesser known Coanda effect /flow attachment. What happens is is that air flows over the curved upper surface of the wing, but actually attaches itself to the airfoil. As the upper surface goes downwards at the trailing edge, the air follows it and is pushed downwards. Again, for every action there is an equal and opposite reaction, so the force of the air flowing down the trailing edge will theoretically push the wing up.

But PM's answer is more in depth, so listen to him (By the way if my above physics is incorrect please tell me so I can put it right, thanks).

[sprint3]

Watch any of the vids on this site where the a/c is taking off or landing when it is cool and humid you can definitely see how the wing produces lift

[AirRabbit]

Bernoulli's principle is flawed, though it may still work to an extent (explaining why aircraft with airfoils of high camber can fly at lower speeds without stalling).

Actually, the reason these aircraft can fly at lower speeds without stalling is that they produce the necessary lift at the slower speeds. If this principle is valid only "to an extent" (meaning that it is only partially correct), you cannot then say this principle is the explanation as to why they they are able do so.

An airfoil at an angle of attack greater than 0 degrees will push air down as it moves past it ... As the wing pushes the air downwards, an opposite reaction pushes the airfoil upwards.

This too is only a "partial" explanation. What is left out of this discussion is a description of what happens to the air above such an airfoil?

The 3rd theory is the lesser known Coanda effect /flow attachment ... air flows over the curved upper surface of the wing, but actually attaches itself to the airfoil. As the upper surface goes downwards at the trailing edge, the air follows it and is pushed downwards ... so the force of the air flowing down the trailing edge will theoretically push the wing up.

This is another "partial" explanation. It's not so much that boundary layer air is a "lesser known" quantity (because the installation of vortex generators have played a large part for a long time in delaying airflow separation to delay the aerodynamic stall) as it leaves unaddressed the aspects of airflow above the wing very similarly to those unaddressed in describing an airfoil at more than a 0-degree AoA ... here, the question is what happens to the air above the boundary layer?

Development of lift is very complex - and the experts will usually tell you (and I certainly DO NOT claim to be one of those guys) that any simplified explanation of just exactly what happens to produce lift, will be just that ... a simplified explanation. As you would probably recognize, anything that is a simplification is not going to be a detailed description (duh!) - thereby NOT allowing a detailed understanding.

Suffice it to say that the airfoil manages to threaten, cajole, convince, or otherwise “make” the airflow turn down behind the wing, thereby taking advantage of the Newtonian requirement that was quoted by c150student, “for every action there is an equal and opposite reaction.”

Watch any of the vids on this site where the a/c is taking off or landing when it is cool and humid you can definitely see how the wing produces lift

Watching the videos on this site will more than likely only confirm that lift is being generated ... I doubt you will see how that lift is produced.

Here are a couple of photos that might get your research juices flowing...
Down Wash One
Down Wash Two

[dash8q300]

What I have been taught is Bernoulli's principal. When air is flowing over an aircraft's wing then the air traveling over the top of the wing will move faster than the air moving across the bottom of the wing. This will cause a low pressure over the top of the wing where the air is moving fastest and on a hot humid day (especially when an aircraft is taking off at high speed) you will be able to see the condensation of water vapour that causes that misty effect over the wing. Underneath the wing is a high pressure and all the air there forces upward to try and get to the low pressure, thus causing lift.

Now I'm sure that the angle of attack of an aircraft will also greatly contribute to how much lift is produced. Does anyone know if, when aircraft such as the 737 is in cruise flight (at around 850km/h), will have a slight upward pitch to sustain flight or is the aircraft moving with 0 pitch in either direction?

Dash,

[glnflwrs]

Yes, any aircraft will have a higher angle of attack as it climbs to higher altitudes. The higher AOA is required because of the lower air density which accounts for less lift produced. To hold an altitude at 25,000 MSL requires more AOA than holding at 4000MSL.

[Saltman66]

Yes, any aircraft will have a higher angle of attack as it climbs to higher altitudes. The higher AOA is required because of the lower air density which accounts for less lift produced. To hold an altitude at 25,000 MSL requires more AOA than holding at 4000MSL.

Recently, it occurred to me that whenever people ask for an explanation of lift, you usually get mixed answers involving a few different theories to explain it. What baffles me is that there seems to be no cut and dry straight answer. Why is this? It seems to me that this equation should be easily explained. We have all the variables at our disposal. Forces and airflow about airfoils can all be measured with CFD or wind tunnels. What is missing? Obviously, there are things I don't know. Anyone care to take a stab?

[dash8q300]

Recently, it occurred to me that whenever people ask for an explanation of lift, you usually get mixed answers involving a few different theories to explain it. What baffles me is that there seems to be no cut and dry straight answer. Why is this? It seems to me that this equation should be easily explained. We have all the variables at our disposal. Forces and airflow about airfoils can all be measured with CFD or wind tunnels. What is missing? Obviously, there are things I don't know. Anyone care to take a stab?

I don't think that there is a straight answer to this. Perhaps lift is a combination of things such as the AOA and Bernoulli's principal and possibly something else. After all, lift is only a theory and we can't really see it being produced.

I just wonder how aircraft manufactures are able to generate wings for their aircraft that are the perfect shape and size to allow the aircraft to sustain flight. How are the wings tested? How do the designers know how much camber will be needed for sufficient lift?

A lot of this is completely beyond me. We don't exactly know how lift is created yet can still build aircraft that are perfectly capable of flying.

Dash,

[27driver]

A lot of this is completely beyond me. We don't exactly know how lift is created yet can still build aircraft that are perfectly capable of flying.

I already said the answer: P.F.M.

There...fully explained...now, who wants ice cream?

[Aspiring Boeing ...]

I have also learned Bernoullies Principle in Y9 physics at school, but this can't be fully true because there are turbulent vortexes at the trailing edge of the airplane, this is mentioned somewhere above (to do with the "water sheets" that occur above the wing). If it was to obey the principle fully then there would be less vortices created by the wings in low speed flight.

You certainly know your stuff C150, well to an extent anyway.

Regards,
Chris

[dash8q300]

I already said the answer: P.F.M.

There...fully explained...now, who wants ice cream?

Sorry, but I have never heard of P.F.M. could someone please explain?

Thanks.

Dash,

[glnflwrs]

Aerodynamics, like any other theory such as electrical or nuclear, are just that, theories. A theory is not just an educated guess or someone's opinion. Theories are scientifically generated, experimentally validated explanations of how a process works. It is a theory because, unless we can follow the individual electrons in a wire or every molecule of over a wing, we can't prove as fact the explanation a theory presents. When you can show through experimentation every time that your explanation works, you have a theory.

Bernoulli's theory worked, in everyway, to explain flight to the extent that we even designed some of our most sophisticated aircraft using his theory. It worked, and still does, for the most part. It wasn't until NASA and its Glenn and Langley Flight Research centers got to be so advanced, that Bernoulli's theory started coming up short in its capabilities.

We may never prove any theory of flight as a fact of physics. But, we can sure stumble along with what we have fairly well. Doncha' think?

[Ranger]

Sorry, but I have never heard of P.F.M. could someone please explain?

Thanks.

Dash,

Pure Freakin' Magic.

Or something close to that.

[27driver]

Pure Freakin' Magic.

Or something close to that.

Something close to that for sure...this IS a family site, after all.

I've tried to sufficiently and completely explain lift to so many different people so many different times that I simply don't care enough to try to do it anymore. Now we do the "Cliff Notes" version:

Suffice it to say that as the airfoil moves through the air, the pressure under it is higher than above it and the thing goes up. Keep the air moving at a sufficient velocity and eveyone gets to live...yay. Let the air drop below that suffucient velocity and everyone dies...booo.

Any questions...? No...? Good. Now, about that ice cream...

[Ranger]

You're buying. Three scoops. Haagen Dazs. Coffee chip. And where'd you say that bar is.......?