Lift, No technical knowledge necessary Options

[effte]

Lift is generated by shoving air down.

Newton's third law, equal and opposite reaction.

That is how lift is generated.

Then there's the question of just how we shove the air downwards.

And we do know how it works, from a physical point of view.

Bernoulli's theory, in the way it is often used, does not work to explain lift and never did. In fact, it was completely useless for many of the first flying devices thought up by man, as these had a curved, flat plate for wings. No different length along the top and bottom of that airfoil, hence they could not possibly get stuck in that old misconception. It was tacked on later, as a misguided attempt at providing a layman's explanation for a not-so-complex phenomenon.

[Corsair]

Right before graduating with a degree in aerospace engineering, it dawned upon my friends at college and me that we couldn't satisfactorily answer this question either. We joked about marching up to the aerodynamics professor's office, asking him this question, and then sitting down in a circle on his floor to hear his answer.

My personal hunch is that lift is caused by a phenomenon quite similar to the force that holds a water skier up on the water: as the skier tips his skis back, they 'ride up' on the water skim over the surface of the water because the water can't get out of the way fast enough. This is essentially the Newtonian 'action-reaction' explanation that has been given previously.

The necessary 'evil' of this explanation is that in order to plow through the fluid (be it water or air) and rise up, a great amount of force is required to separate and redirect the fluid. This of course is drag. And here's where I stick those troublesome Bernoullis. The air that manages to blow over the top of the wing is against great odds as it tries to rejoin the air below the wing. A significant component ends up just swirling about wildly around the top of the wing creating lots of drag. (The exact amount and where along the wing surface it starts swirling is a function of air speed and the wing's angle of attack). The air that does make it to the end of the wing smoothly will be moving quite fast and will have a lower pressure than the air under the wing, providing 'Bernoulli Lift.' But I wouldn't reckon that this lift is more than 10 or 20% of the wing's total lift. Furthermore, this lower air pressure on the top of the wing isn't truly at the 'top' of the wing, but rather at the rear of the top surface of the wing, causing an actual rearward pull on the wing (more drag) in addition to its upward pull.

....those pesky Bernoullis....

[bernoulli]

....those pesky Bernoullis....

I get that a lot...

[Fast Jet]

The forces for flight are Lift Thrust Weight and Drag. There is profile drag caused by undercarriage aerials, and if you stick you hand out of the window of a light aircraft like a Cessna 152, then that would be profile drag. Another important form of drag is Induced Drag. This is the penalty we pay for the lift caused by the wings. The Lift itself is resisting the airflow over the wings and anything that resists the airflow is drag so, Induced Drag - the penalty of Lift. Thrust is provided by the engine(s) Without thrust you would not move forward to increase speed to cause the airflow to flow over the wings to cause lift. Weight is the force of gravity that affects just about everything on the Earth (and other massive spheres in the Universe) To fly, therefore, we need, to overcome Drag and Weight through Thrust and Lift.
Obviously A glider has no thrust and has to be towed into the air - by a cable attached to a machine on the ground which then releases when the glider is at a certain height or the glider may be towed by a faster aircraft to a certain height when the cable is released by the pilot of the glider. The glider maintains airspeed by gently dipping the nose turning weight to thrust - i.e., going downhill. Sometimes the air is such that thermals prvide lift through the upward flow of air.

[airdave96]

You know I thought I had the idea of down from watching the old AIr Force video's on this subject, but......now after reading all these theories I think I have just crashed and burned!

[Fast Jet]

Yeh! CORSAIR I agree with you. The flummuxing of fluids, I always thought there was more to it than what is taught at flight school. Thing is: if you put it in a wind tunnel (an aerofoil) what would one see? Can it be that that pokey per cent of lift over the wing(s) is sufficient to lift an aeroplane...?

Because, there is often little mantioned of the force of air BELOW the wing. I mean, what about Ground Effect - that initial lift of low wing aeroplanes that happenes on T/O and landing of light aircraft like Warrior and the like?

The Bernoulli theory is good to a point and a well known member did a huge post on this some time ago - I forgot his name for a minute.... (he is/was an engineer in the USAF)

Anyway, there is a lot of forces coming from underneath the wing, especially as it is inclined slightly upwards into the airflow, (see angle of incidence and angle of attack) - so its all set up ready, for increased accel and increased alpha, up we go...

Maybe that the main force is in fact that from below and what is merely required is the smooth curvy bit on the leading edges of the wing to give the airflow a chance to escape - or it would act like a plank of wood and you would rotate to 90 degrees to the airflow - not good for business! And, that there is indeed the centre of (lower) pressure over the top of the wing.... but that .. that aint the real reason for the lift..? It seems very simple really.

[rjb4000]

The Bernoulli theory is good to a point and a well known member did a huge post on this some time ago - I forgot his name for a minute.... (he is/was an engineer in the USAF)

McchordAFBAirman / Archangel of course! I think he was a mechanic of some sort, couldn't find the original thread though.

[Fast Jet]

Yeh, thats him McChord, cool dude - I wonder where he is and what he is doing now, it gets dangerous on those very large beaches where it is a heck`uv a long way to the sea !

=========================================================================

So about L I F T then, well, I think I was wrong. Because, well, It is true that only about 10 per cent of airflow is on the upper surface, like you said, I think you may find that it is a bit more, but my theory of ``mosrt of the lift is from below the wing`` is really not where its at. The proof is as follows. Based on the fact that GLNFlowers correctly pointed out the Bernoulli theory (the old scientist Bernoulli, not the one who lives in Lafayette, nor the one`s who hang onto the tips of speeding rotor blades and here in the UK try to corrupt a civilised system)

So hear goes---- its sssooooo EASY !!! The lift IS caused by the airflow over the top of the wing - as the airflow speeds up the effect of centre of pressure increases, thereby reducing the pressure progressively over the wing and the faster and most linear the Relative Air Flow the greater the lift.....etc., and, here is the proof. Back to D A N G E R O U S Icing of Hoar Frost - this has nothing to do with a chick giving you the cold shoulder - no siree - but if the deposit of Hoar Frost causes a marked reduction in Lift then lift must be created at the upper surface of the AEROFOIL. Next Proof - If lift were caused by airflow on the underside of the wing then when we approched the stalled condition, the stall would be a serene and gradual and seemless process from the condition of Lift - to the condition of Stall - in fact the Stall may not even happen at all. But, this as you know, is NOT the case, when the aircraft departs from the flying condition at the point of stall, the breakdown of lift to the stalled condition is distinctly rapid (all you student pilots out there waiting to do stalling with your instructor, it aint nothing to worry about and you will hardly notice it, your instructor will give you the full demo)

So, therefore as the departure from Lift to Stalled is a very mild ``snap`` for want of a better word AND as the centre of pressure moves forward and then suddenly moves back (on the upper surface of the wing) then it just proves that the Lift is generated on the surface of the wing. If you wanna say the only 10 per cent of the airflow is going over the upper surface of the wing then go ahead, but the does not mean that the lift is ten per cent uh, uhh !

Why`s that....? Because, the boundary layer over the upper surface of the wing acts like, similar to, is almost like a very, very low pressure area almost, almost a vacuum. The centre of pressure works all the way down to the boundary layer and assuming air has a certain elasticity then the centre of pressure is very low nearer the surface of the wing - bringing back icing re: Hoar Frost into the equation - this don`t work if microscopic mountains of ice are messing up the airflow at, at, the Boundary Layer and that has an affect all the way up and all the way across the wing (spanwise flow and all that). If Lift were not caused by this reduction of pressure (known as the centre of pressure) on the UPPER surface of the wing, then the prescence of a deposit of hoar frost on the upper wing surface would be negligiable - this is not the case. The prescence of this ice (hoar frost) has a distinct effect on lift and the aircraft needs loaddds of extra airspeed/extra runway to stand a faint chance of getting off the ground - which most times does not happen and you are sacked (fired) or burnt in the ensuing fire in the bottom of the ditch at the end of the runway>

The NEAR state of a vacuum near and at the boundary layer) is as powerful as a rubber tip on a sucker dart, same thing - wet hand, wet dart, stick it on yer forehead, cant get it off - gottit? So therefore it takes a basic aerofoil shape to cause consideralble lift at 60kts in a little airplane.

Now, take a massive wing of a B747 and the same really, really low pressure, almost to a vacuum applies - therefore to lift this monster, is easy - just like crawling up a tall building using those sucker pads - ok?

Not only but also, this is exascerbated (big word to confuse Ranger) by the fact that the air molecules (as GLNFlowers rightly said) split at the leading adge `cos the air over the upper surface has to travel a longer distance that the air below the wing - but physics dictacts that the air molecules HAVE to meet up again at the same position at the trailing edge (of the wing) and therefore this sort of pulls the air behind it - because the air on the upper surace has had to speed up to meet its buddy at the same time even though it has further to travel to get over the curvature of the upper surface of the wing, due to an inherent elasticity of air - and stretches it and basically makes it less than the air below - reduction in pressure, the centre of pressure - all things being equal (and they are not) everything moves from high pressure (under the wing) in the direction of low pressure (above the wing) plus, plus that near state of a virtual vacuum at the boundary layer = this lower pressure on the tope of the wing (centre of pressure) causes the wing to move up toward the lower pressure - because things move from higher pressure to lower pressure - LIFT.

A helluva lot of lift. Yessir, you too can have, scrambled eggs for brains, if you study thae CAA way.

Also, you cannot have lift without weight unless you are in a balloon or in the water.

Don`t clap just throw food!

NOT AT ME !!!! TO ME!!

Modulators Ahoy!! When someone writes ``CAA`` can you insert **I have a dirty mouth**

(excellent!!!)

[effte]

Because, well, It is true that only about 10 per cent of airflow is on the upper surface, like you said,

You'll find that at most altitudes where we normally fly, there's a lot more air going over the wing than below it.

The lift IS caused by the airflow over the top of the wing

Got that right.

If Lift were not caused by this reduction of pressure (known as the centre of pressure)

The center of pressure is important, and something else entirely.

The NEAR state of a vacuum near and at the boundary layer

There's no near vacuum around a wing, but there is a low pressure compared to the ambient pressure.

but physics dictacts that the air molecules HAVE to meet up again at the same position at the trailing edge (of the wing)

No, physics most certainly do not dictate that the air molecules have to meet up. In fact, the molecules usually do not. On a wing generating lift, the molecule going over the top of a wing will typically get to the trailing edge significantly faster than a molecule going below the wing.

Also, you cannot have lift without weight unless you are in a balloon or in the water.

Yes, you can. For example consider the case of a fast jet going vertical in a looping, pulling say 3Gs. That's a lot of lift generated, but no weight opposing it. All the lift goes to centripetal acceleration, pulling the jet around the curved path of the looping. In level flight, you can also generate more lift than weight. In this case, you are accelerating vertically or, as it is usually referred to, pulling up into a climb.



Most of the lift generated by a wing comes from the low pressure over the top surface of the wing. In fact, at the angles of attack where we usually operate, the pressure over most of the lower surface of the wing will be lower than the ambient atmospheric pressure - only less so than over the top surface. This pressure difference creates a net force acting on the wing which is the aerodynamic lift. (1)

Newton's laws dictate that for every action there's an equal and opposite reaction. If the air is pushing the wing up, the wing is pushing the air down. See post #21. I reiterate, how much lift a wing generates depend on how much air it is able to shove downwards. (2)

Combine (1) and (2) and you reach the conclusion that it is in fact the top surface of the wing which is doing most of the shoving. Air passing over the top of the wing is sucked down by the low pressure above the wing and given a downward velocity. It goes down behind the trailing edge, creating downwash.

How is this low pressure generated?

You have the coanda effect. Hold a spoon under a running faucet with the back of the spoon in the stream of water and you will note that the spoon gets sucked in under the faucet. Note the similarity between the back of a spoon and the curved upper surface of a wing.

The coanda effect dictates that a fluid flowing across a curved surface will attempt to follow the surface. As the air flows over the top surface of the wing, it follows a curved path turning downwards. Consider the air as being on a carousel. It gets thrown outwards, which in this case is upwards. What happens if you "pull" on a mass of air? It expands, creating a low pressure. The curved path and the centrifugal force (insert disclaimer about centrifugal force not being real, lack of centripetal force, yadda yadda, here) pulls on the air near the top surface of the wing creating a low pressure.

Good observation regarding the hoar frost and the disastrous effect on the lift generating abilities of an airfoil, and the conclusion to be drawn about the distribution of lift generation between the top and bottom surface of the airfoil. That's one I'll make sure to remember as a good example. You can do a lot to the lower surface of the wing without reducing its lift generating capability significantly, but start messing about with the top surface and things go pear shaped very fast indeed. For the very same reason we like to keep flap track fairings and all the other junk hanging off wings below the wing rather than protruding up from the top.

Cheers,
/Fred

[Fast Jet]

re-edited due to disillusionment. To effte . . Hey Fitty! You said that the air molecules on the upper surface of the wing / aerofoil reaches the trailing edge first becasue it is faster - that is not so. The molocule may be faster but it has got further to travel, see: curvature of the upper surface of the wing. If you relate these two buddy molecules (one under the wing, one over the wing) to surface tension of water you may get the idea that if you split them up - they try to re-join together again.

Also, you said that the Centre of Pressure was - well, you inferred that it was a seperate entity and was apart from lift. Well, I reckon that the Centre of Pressure is everything to do with Lift, in fact it is the resultant factor of all that we are discussing - this resultant factor that causes LIFT (There are other things that happen to the airflow which are also resultant factors but have f--- all to do with LIFT)

Not only but also, if, IF you had the upper air molecules arriving at the trailing edge first, then your vortices would converge inwards and upwards and they don`t the diverge outwards and downwards.

So prove me wrong and I will accept defeat - but I am not a happy shopper.

[Fast Jet]

You know I thought I had the idea of down from watching the old AIr Force video's on this subject, but......now after reading all these theories I think I have just crashed and burned!

No Way have you crashed and burned. The current books of the PPL, CPL and ATPL nature will have all this in them. you can see what books you need by asking your local flying school club, flight school or anywhere you are doing your training. you can also get excellent, excellent videos from KING training videos and all of the aforementioned also give you stuff relevant to your ground exams - so don`t pull the yellow and black handle yet - ok Dude. It seems daunting but we squash years of study into one post and it makes us seeeem like einstein - when in fact we are just suffering from verbal diorreah - you will get all the concepts of lift in one afternoon of studying or a solid month of studying if you want to wear five bars on your shoulder, but I digress.. . . .as does the airflow as it hits the leading edge of the wing otherwise known as an aerofoil - read on. . . ASA test prep books do fantastic books for your exams and they also recommend other theory books that you can study for mega in depth knowledge. Pilots in general, sweat for an exam and then study for real in their spare time. I have not met any senior captains yet who have not studied their a-- --f. Now where is efffety. .

[Fast Jet]

)





For the very same reason we like to keep flap track fairings and all the other junk hanging off wings below the wing rather than protruding up from the top.

Cheers,
/Fred
[/quote]


That, above, is your writing. This is cool. Are you an engineer or something - or do you build planes. . ? Your`e the CEO of Boeing are you not. . .? I can feel it in my feathers. . . I still don`t like what you said about the molecules of air over the upper surface of the wing not meeting up - if you wanna go to the level of M.I.T. then I will give you my theory but you will probably have me locked up.

[effte]

I still don`t like what you said about the molecules of air over the upper surface of the wing not meeting up

It is just like gravity. You can like it or dislike it, but it's still there. You just have to accept it.

Someone put together a nice collection of images:

http://www.eskimo.com/~billb/wing/airgif2.html


BTW, do you drink coffee?

[Fast Jet]

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.

Wow! Kewel !!!

[Fast Jet]

It is just like gravity. You can like it or dislike it, but it's still there. You just have to accept it.

Someone put together a nice collection of images:

http://www.eskimo.com/~billb/wing/airgif2.html
BTW, do you drink coffee?

Yes, I drink coffee but I do like to fly at 200ft AGL doing 800kts playing with nearby SAMs/RPGs/Manpads and sometimes Stingers, just to keep awake, just to keep the peace in the world so you have the freedon to ask me if I drink coffee - leary.
My hobbies are Dodging US fighters over the Grand Canyon and Interstellar Sex -

BTW: Do you sniff Cocaine?

[Fast Jet]

It is just like gravity. You can like it or dislike it, but it's still there. You just have to accept it.

Someone put together a nice collection of images:

http://www.eskimo.com/~billb/wing/airgif2.html
BTW, do you drink coffee?

Guys be reasonable, for once. The molocules of air split up at the leading edge of the wing. some go over the wing, some go under the wing. The molocules that go over the wing have further to travel than the molocules that go under the wing, and yet, the molocules all arive, and meet up, at the same time, at the trailing edge of the wing. The reason the molocules on the upper surface make it for the meeting at the trailing edge with the molocules under the wing is because, they, travel, faster. Why do you think they travel faster?

[Fast Jet]

. What happens if you "pull" on a mass of air? It expands, creating a low pressure. The curved path and the centrifugal force (insert disclaimer about centrifugal force not being real, lack of centripetal force, yadda yadda, here) pulls on the air near the top surface of the wing creating a low pressure.


Cheers,
/Fred
[/quote]

================================
See, what you say here is brilliant. There`s your lift. I still cannot accept that the molecules over the wing do not meet up with their mates (the molecules under the wing) at the trailing edge (or where-ever due to washout spanwise flow etc).

This ....speed of travel is what pulls the air (the air thats flowing over the wing) this also causes the reduction in pressure, this is great! This is what causes the lift. . . . . . but, the Bernoulli Theory states that the molocules all meet up at the trailing edge and you say that the upper wing molecules arrive first because they are faster but although they are faster - they do have further to travel which is why they all meet up at the same time - da daaah!

I am not trying to prove a point here - because y`all know the G------n answers anyway, but my gripe is why do they teach us that - the point I am trying to make about the molecules - and Bernoulli did all this work all these years ago and Aviation in general adopted it and teaches it (all over the world) and you . . . guys are not only disputing it but failing to l o g i c a l l y prove it otherwise. . . ?

Next Week: Preparing your garden for Spring.

[27driver]

Yes, I drink coffee but I do like to fly at 200ft AGL doing 800kts playing with nearby SAMs/RPGs/Manpads and sometimes Stingers, just to keep awake, just to keep the peace in the world so you have the freedon to ask me if I drink coffee - leary.
My hobbies are Dodging US fighters over the Grand Canyon and Interstellar Sex -

You dodge RPG's, huh? Just to keep awake...? Over the Grand Canyon, huh? Cool. Wish I could do that. Seriously.

I'm just wondering...I'm not quite as skilled at playing with them as you, I imagine. They don't let us do such things. Keep it up...and don't hit any of the donkeys in the Grand Canyon...they get most annoyed when you do, I imagine.

[effte]

Yes, I drink coffee

Try drinking less.

but I do like to fly at 200ft AGL doing 800kts playing with nearby SAMs/RPGs/Manpads and sometimes Stingers, just to keep awake, just to keep the peace in the world so you have the freedon to ask me if I drink coffee - leary.

Are you trying to quote Timothy Leary, or is it a bull**I have a dirty mouth** act and an insult to those who actually do wear a uniform? Sorry, can't be polite anymore.

The molocules that go over the wing have further to travel than the molocules that go under the wing, and yet, the molocules all arive, and meet up, at the same time, at the trailing edge of the wing.

They. Do. Not. And you've seen the wind tunnel and CFD imagery to prove it.

The reason the molocules on the upper surface make it for the meeting at the trailing edge with the molocules under the wing is because, they, travel, faster. Why do you think they travel faster?

They travel faster and arrive at the TE before their below wing counterparts due to the low pressure above the wing accelerating them. There's no low pressure in a fluid without acceleration, and there's no acceleration without a pressure drop. To realise this, consider the pressure forces acting on any given parcel of air.

but my gripe is why do they teach us that - the point I am trying to make about the molecules - and Bernoulli did all this work all these years ago and Aviation in general adopted it and teaches it (all over the world) and you . . .

Bernoulli had it all figured out. He'd do about 2300 rpm in his grave if he knew how his theorem is abused though. The world is full of people who cannot understand what Bernoulli really said, or who can understand it but write text books and do not believe their readers will be able to. Dumbed down text books may teach venturi effect or equal transit time, for either reason. If you ever find those explanations in real aerodynamics literature, it will be under the section for humorous anecdotes.

Do you believe NASA has lift figured out by now?

http://www.lerc.nasa.gov/WWW/K-12/airplane/wrong1.html

[Fast Jet]

Er, no. I was not insulting those in uniform, for whom I have unbounded respect, admiration and loyalty. If I inadvertantly did this, due to negligent writing, then I apologise unreservedly. I will look at your NASA link. Thank you for the link. For me to be on ex over the Grand Canyon or on ops over someplace else then 800 kts at 200 ft suits me just fine, whatever they are chucking at me. Also, my reference to leary was mispelt leery as in you were being leery asking me if I drank coffee. I don`t know who Timothy Leary is, but I was referring to your asking me about the coffee and its implications and inferences, as in innuendo - and by innuendo I do not mean an Italian suppository, nah uh, I meant that the question thereby discredits what I write by mere implication. I thought we were having a discussion/debate on the theory of Lift not trying to discredit as in a courtroom. It `aint THAT important. So long as the Fokker flies thats all I need.

==============================

I will look at the link again. Thank you very much. It is a great link which I did not know existed.

I am still hit by my causing the previous mis-understanding. (not the lift) My head is straight.
Back to coffee and cigarettes.

The real reason for Lift, of course, is the will of the pilot. Aircraft cannot fly, they are just tons of metal. What makes them fly is the pilot`s determination to get the damn thing to get airborne. Its Buddah or other spiritual stuff. The pilot produces so much will power at the start of takeoff, that Karma and all the rest of it is so agitated by the vibrations from the pilot`s will, which resound throughout the universe, this alarms the Gods, Who, in Their quest for peace and b y tranquility let the aircraft get airborne. It is this inner power that all pilot`s have, which makes them super human that makes the aircraft, heavier than air, fly.
Same as landing on a dark, rainy and windy night. Forget ILS MLS VOR NDB, VDF, RNAV and all the rest of it, this is commercial clap trap just to sell some kit to the unwary - in actual fact, the entire landing takes place and ends up with a precision touchdown due to the will of the pilot. The pilot has a choice, either he lands this. . . big heavy. . . thing, or, he stays up there all night. Pilot`s don`t like this last idea, it means missing their date with Juicy Lucy, or Juicy Luca for the girls, or . . anyway, and in addition the thing close to the inner spiritual core of a lot of pliots is a vision. The vision is invarably of a nice big plate of steak, chips, or potatoes, veggies and, for the gifted, gravy. Now imagine the steam coming of that lot in a dream. . ? See what I mean? I can see you all, right now, just reaching for the mustard! So it is the pilots will that causes all aircraft to fly and land. In the olden days, when a 9 to 5 meant going out and grabbing the odd wild cow with your club and avoiding being eaten by a prowling dinosaur (As in diners Club), mankind had clubs and the rest of his crew to complete their day to day jobs out in the work place. As clubs would have little or no affect on getting 250,000KG of Jumbus Aeronauticas airborne then, obviously, there was a need to develop inner spatial spiritual stuff and with one last OMMM (which is usually used for the Approach) the pilot wills the aircraft to get airborne dammit - and it does. but don`t tell anyone. (re-edited 6-7/3/09)