rmh

I knew you would come up with that Just waiting for it - and it is an obvious answer - However - part of that drag is induced - part is parasitic.
One is IMPOSSIBLE to eliminate from lift the other -improbable.
Fear of science ?
wanna match patents on new and novel inventions?as long as you are getting pointy fingered .
The original query was " why do people say etc.."
my point is that - it is a result of old -incomplete info -
the top AND the bottom (opposed forces) are required and forour use - call it 50/50.
Why ?
you can't alter one without altering the other .
the picking apart of each situation - not required .

But since you brought it up - the components of lift change with the change of any part of the situation - size/shape/speed / altitude, on n on .
You can get more drag than lift but if you change direction or angle - the ratios change
one replaces the other
(unless 100% is a unscientific sum.)

Ain't this a hoot?

Lnewqban

Hugo:

That definition may be too simple.

The lift to drag ratio would be constant for any airfoil if the lift would be just the ramaining force.

Please see Post #79 above, and also the attached diagrams.

Rocketmagnet

I'm sorry I don't understand why that would be. How does the statement "lift is the remaining force" imply that the lift to drag ratio is constant?

I'm not trying to be difficult, I literally don't know.

Hugo

Lnewqban

Excellent picture, Evan!

I believe those particular air conditions made its temperature be very close to the dew point.
The moving wing reduces the pressure of the air above itself (the mass of air expands), reducing its temperature enough to produce dew or mist.
The moving wing increases the pressure of the air below itself (the mass of air compresses), increasing its temperature farer away from the dew point, so mist is not seen below the wing.

The drag produced by the pressure differential can be seen in the formation of vortex at the wing and tail tips, and more strongly at the gap where the flaps and the ailerons meet at different angles (a rough transition induces more turbulence and friction than at the tips).

This drag is attached to finite wings, for which always high pressure air will find a path to flow to low pressure air.
The slower the flight velocity, the higher the CL and pressure differential most be, the more energy in vortex will be wasted.

…….And yes, a 150 tons jet does not float in the air by just moving 15 pounds of air; as you wrote, it must “move a heck of a lot of air a pretty long way�.

Mass of airplane x Velocity of airplane = Mass of disturbed air x Velocity of disturbed air (by wings and engines) = Work or energy transferred from the engines to the atmosphere to keep that monster afloat for many miles.

Regards!!

da Rock

Guys, posts that are opinions of other posters will be deleted.

And have been.

Mike SVOR

My Feedback: (1)

Thanks Rock for keepin it clean. I didn't get to see what got started, but thanks.
(you're doin a good job)


I'm still baffled about this wing causing more lift on the top side than the bottom.
The idea is to compress the air molecules until they're dense enough to sustain weight. Vacuuming yourself skyward ain't gona happen.
Basically the vacuum theory people are stating this:
Take a knife to a loaf of bread. While you are cutting through it (about half way through), turn the knife sideways and press down. Now that resistance you feel is the bread on the top side causing a low pressure on the top side of the knife and not because you are compressing anything below it.

ArCeeFlyer

My Feedback: (1)

It seems to me that one reason people feel the top of the wing is much more critical in maintaining lift is the ease that airflow can detach from the top. It's basically impossible to detach the flow being pushed into the air with the bottom of the wing because it's a mean stubborn solid wall and it has no choice but to flow against it and around it, but the top of the wing can only beg the air to stay with it using low pressure, vacuum, curving or what ever you want to call it. It's much easier to detach that top flow and stall out the lift. Thus, I would suppose that's the reason for so much design effort of the top of the wing. I think all this could contribute to the feeling that the wing is only or mostly held up by the top. It seems to me that the difference in pressures of top and bottom is quite valid, and I think the top is simply far more critical. Just my point of view. Lot's of great thought provoking info and view points going on in this thread. [sm=thumbs_up.gif]

Rocketmagnet

Sorry everyone. That was me getting over excited.

But you understand that the lift is caused by a pressure difference, right?

You could either increase the pressure on the bottom of the wing, or you could decrease the pressure on the top. Or both.

If, by magic, I could just increase the pressure on the bottom of the wing, and that lifted the plane up, I would say that it was the higher pressure on the bottom which was doing the lifting.

Likewise: if, by magic, I could just decrease the pressure on the top of the wing, and that lifted the plane up (it would, imagine sucking on a straw), I would say that it was the lower pressure on top which was doing the lifting.

Indeed, when one sucks on a straw, one talks about the low pressure causing the drink to move. Even though both the high and low pressures are important, it's the low pressure which is different from the ambient pressure. That's the bit that's really changing, and that's the bit that considered the cause.

The same goes for the wing. The top side tends to feel much more of a drop in pressure than the bottom feels an increase. It is the low pressure area which has changed most significantly from the ambient pressure. (see the pressure diagram a couple of pages back).

Hugo

Rocketmagnet

The other way one could look at it (perhaps this is the way you're looking at it) is:

It's the air molecules bumping into the bottom of the wing which is keeping the plane in the air. Those are the ones doing the actual lifting. The air molecules bumping into the top of the wing aren't helping at all. If anything, they're pushing the plane down!

Is that the way you're thinking of it?

Hugo

cosmo64

Mike,

I think I finally see your view point-now I just need to explain a correction-here it goes.

Your last post stated that the air molecules compress under the wing and cause lift. This is not true as there is no reason for them to bunch up and get "pressurized". However what does happen is as the wing moves through the air a pressure differential happens. The pressure on the bottom of the wing remains the same and the pressure on the top of the wing decreases. So in a sence there is a higher pressure on the bottom of the wing.

Now take a flat board that has no obvious airfoil. You could make it go as fast as you want and as long as the board's angle of attack did not change the board would not fly becasue the pressure is equal on both sides. Now take the board and change the angle of attack (either direction) and the board will magically go the direction the leading edge goes. The reason is that the air molecules have to tavel father on the side that the leading edge raises. this creates the lower pressure on the top of the board and the board flies.

Now in your very early posts you mentioned that you took the covering off the top of your trainer and it flew around just fine. Of course it did! I imagine you left the leading edge on correct? just leaving the leading edge on and the air with a positve angle of attack will need to travel farther on the top and create the pressure difference or greater pressure on the bottom of the wing and you have lift.

This lift could be viewed two ways, One the lower pressure on the top of the wing or the higher pressure (compared to the top) forces the wing in the direction of lower pressure.



This may be a very over simplified version but I think it may help explain what you are looking at.

P.S> to the engineering force of the groupgood job this forum should be kept forever on a lesson in aerodynamics.

Lnewqban

Mike:

You sure have “ruffled some feathers� after your first post; for some reason, this subject always does. [sm=punching.gif]

As I understand it, the lift any airfoil produces is a resultant force from the addition of two main forces:

1) F bottom: The force that is resulting from the product of the area of the wing times the positive air pressure on the bottom of the wing (as you wrote, the air molecules get compressed and heated; or in my words: the wing is pushed up by the inertial reaction of the air molecules).

2) F top: The force that is resulting from the product of the area of the wing times the negative air pressure on the top of the wing (the air molecules get expanded and cooled; the wing is pulled up by the inertial reaction of the air molecules).

By definition, pressure is the number of hits per unit of time of the molecules of a fluid over a surface, ....or something close to that.
The wing, moving through the air at certain AOA is scooping molecules from one half stream to the other.
Hence; the bottom half contains more molecules, that are forced together, against which force they react by pushing the surface up and also accelerating toward any lower density or pressure area (the trailing edge and down).
The top half contains less molecules, that are forced to separate, against which force they react by pulling from the surface up and also accelerating toward any higher density or pressure area (the trailing edge and up).

Now, the molecules at the top are subjected to a higher disturbance, just because the AOA takes the point where the top and bottom streams divide down the leading edge.
So, at least in theory, the molecules on top have a longer distance to cover to react to the disturbance, as explained above, and to find their balance by accelerating faster toward the trailing edge, where both streams meet and continue moving, as forced by the airfoil.
For me, that also means that the lifting top surface is bigger that the bottom one.

The above described effect is more pronounced for cambered airfoils, for which the scooping action is bigger, as well as the difference in top and bottom surfaces (or distance for the air to travel in contact with them).

Tests to measure pressure on both sides have demonstrated that F top > F bottom for positive AOA, and also that such a difference increases with the values of AOA.
This happens until a point is reached, where the molecules at top are so stretched pulling the wing up, that the bond is lost, the molecules detach from the top surface, and a bubble or real vacuum develops between the top surface and stream.
Lift force beyond that point is drastically reduced, since F top = 0.

Then, it is not too wrong to state that:

Lift force = Weight = F top + F bottom = (Negative Top Pressure x Top area) + (Positive Bottom Pressure x Bottom area)

Where F top > F bottom (or there is more pulling on top that pushing on bottom, although both contribute to lift).


Now, Mike, jumping to your example of the rolling fan:

I have inspected my fan, discovering that in effect, is more an air pusher than an air puller.
I believe the reasons are:
1) The AOA of the blades or my fan are around 25~30 degrees, which is a stall angle for most of airfoils. Dust accumulates on the “sucking / top� surface, which accuses air turbulence and detachment there. If the blade is working in stall condition, then F top = 0, as exposed above, then it is not pulling much air.
2) Although cambered, the blade is a flat surface, which makes it a very inefficient airfoil for high blade (airfoil) velocity.

If that is true, and the blades are working in stall condition, it makes sense for a device designed to blow a mass of air, trying to avoid strong force reactions.

May be a better comparison with the sucking effect of a regular airfoil would be the blades of the compressor of a turbofan, which take air from atmospheric conditions to high pressure conditions just before the burner.

Regards!!

rmh

each intrepretation is interesting

Here is a another one:
Lift is simply a momentary disturbance in otherwise equalized air.
The plate or wing or what have you traps air - or hangs onto it . At the same time - the air resists being unequalized.
It will take the easiest path possible to avoid it
The unequal pressure we make, can be useful- or not.
It can be used as lift.
In any case the nature of fluids is that if uncontained - and uninfluenced - an equalization of pressure immediately occurs.
Make a pressure difference and the higher pressure will try to move anything in it's path to re equalize.
High moves to low just as heat moves to cold.
(in space there is no heat and no pressure - thot I would throw that in -
Nobody has mentioned gravity in all this
OK back to sleep

Lnewqban

Please excuse me, Hugo. []

Definitions are tricky, and so are interpretations.

I was just trying to say that the relation between lift and drag changes with the velocity of flight, if everything else remains constant, and also that it is mainly due to the changes that velocity fluctuations do to the several forms of dragging forces.
High drag is associated not only to high velocities and low AOA, but also to high AOA and low velocity.

As you know, for level flight, there must be a balance between weight and lift, and also between thrust and total drag.
Actually, there are only two forces, which we pull apart just for our better understanding.

In the schematic I attached to my previous post, you can see that while having the same lift, an airplane can fly in balance (lift = weight) within a wide range of drag force, by modifying AOA of the wing (and also airfoil for some of them) and velocity of the air.

Thrust must balance total drag; hence, the zone of minimum drag is desirable for more economic flight.
Most of the commercial airplanes have their airfoil selected to operate efficiently in that zone.

Regards!

Rocketmagnet

lnewqban,

I still don't understand how that definition relates to any of what you said. Sure, the relationship between lift and drag changes with all kinds of variables (speed, AOA, etc.). But the definition I gave doesn't affect any of that. The definition of lift and drag don't depend on speed or AOA.


Does anyone dispute that the total force on the plane can be split into lift and drag, and nothing else? (Sure lift and drag can each be broken down further.)
I.E. Force = Lift+Drag

Does anyone dispute that drag is all of that part of the force which opposes the motion?
I.E. if the velocity is in direction V, then drag is in direction -V

If both those statements are true, then the only result is that drag is the part of the force which opposes the motion, and lift is what remains.

I must insist that this is not a valid result. Force=List+Drag does not imply that Lift/Drag = constant.

Does anyone here remember vector mechanics from school?

Hugo

B.L.E.

I used to sail shortboards a lot until a pinched nerve problem made me stop. When a strong puff hits you and you feather the sail, the torque that you experience gives you a first hand feeling for what pitching moment is.
Another windsurfing problem is something called "spinout", the first time it happened to me, I could have sworn that the rear fin just broke off, the letting go was that sudden and extreme. My thinking is that it is due to "cavitation", it's more correctly called aeration but everyone calls it cavitation. What happens is that the suction on the "top" of the fin starts sucking air spanwise down the fin and that destroys the vacuum there and the sudden increase in angle of attack due to the loss of lift just cinches the stall, just like a snap roll.

da Rock

Want to feel lift?

We've all done it as children, haven't we?

Stick you hand out a moving car's window. What do you feel? Hmmmmmmm I think the "mash the air" crowd has an argument with this "experiment".


BTW, I'm fairly certain there has been reputable testing that measured pressures top and bottom.

Mike SVOR

My Feedback: (1)

AWE MAN!!! You must've seen me driving home from work today with my hand out the window at 65 mph! I was gona post this simple experiment tomorrow, but you were too quick.

It's a simple experiment you can FEEL.
There is way more pressure on the palm of your hand (underside of the wing) than on the back of your hand.
All those amazing scientific calculations (really, they are amazing how indepth they are), and nobody did the simplest of experiments.

ArCeeFlyer

My Feedback: (1)

It depends on how you angle your hand to either feel only pure drag (like square on) or lift along with the drag when angled slightly positive. When angled just right to "feel" the lift, I think nerve sensors in our skin are stimulated primarily by pressure. So, we feel the push up much more than the pull up. I used to love doing that as a kid and was fascinated by it. Probably one of the things that got my interest up so much in aerodynamics.

Lnewqban

Quote:

I used to sail shortboards a lot until a pinched nerve problem made me stop. When a strong puff hits you and you feather the sail, the torque that you experience gives you a first hand feeling for what pitching moment is.
Another windsurfing problem is something called "spinout", the first time it happened to me, I could have sworn that the rear fin just broke off, the letting go was that sudden and extreme. My thinking is that it is due to "cavitation", it's more correctly called aeration but everyone calls it cavitation. What happens is that the suction on the "top" of the fin starts sucking air spanwise down the fin and that destroys the vacuum there and the sudden increase in angle of attack due to the loss of lift just cinches the stall, just like a snap roll.

Very interesting, indeed!
That stall in water must be stronger than in air, I believe.

Some good animations about pressure profiles can be watched at:

http://www.mh-aerotools.de/airfoils/...tributions.htm

Notice that the low pressure on the bottom surface for negative AOA is not as intense as on the top surface of the cambered airfoil.

“Flying may not be all plain sailing, but the fun of it is worth the price.� Amelia Earhart (1897-1937)

banktoturn

Mike,

Forget molecules; it's about pressure. The lift is simply the result of the pressure difference between the top & bottom surfaces. You continue to insist that the lift generated by a wing must be a result of high pressure on the bottom, rather than low pressure on the top. First of all, it's the difference that matters, so getting fixated on the pressure on one of the surfaces is rather meaningless. If someone really had a desire to give one of the surfaces more credit for generating lift, he could compare the pressure on the top & bottom surfaces to the ambient pressure (the pressure far from the wing). If the pressure on the top surface were to differ from ambient pressure by more than the pressure on the bottom, he could say that the top surface is generating most of the lift.

Take a look at the included image, which is an actual plot of the coefficient of pressure (cp) for a real airfoil. Cp measure of how much the local pressure differs from the ambient pressure. When it is negative, the local pressure is lower than ambient, and vice versa. The plot clearly shows that the cp on the top surface is more negative than the cp on the bottom is positive. In other words, the top surface is "generating most of the lift".

It's clear from several of your posts that you find this counterintuitive. I must tell you that the fan-on-the-skateboard and knife-in-the-bread experiments do not constitute counterexamples; they simply are not relevant. Nor is your insistence that "vacuuming yourself skyward ain't gonna happen".

You started this post by asking the question, and it's been answered several times. Even though it's counterintuitive, it's the right answer.

banktoturn

Rocketmagnet

Yup. That's what everyone's saying too.

Well done Mike. You will revolutionise aerodynamics with your simple experiment which nobody in the course of airfoil history has ever thought to try. Not NASA, not British Aerospace, not Skunkworks. All these stupid aerospace engineers making their fancy-dancy real working aircraft. What do they know?

Hugo

Warnberg

My Feedback: (1)

http://www.grc.nasa.gov/WWW/K-12/airplane/short.html

Read this.... there is a page here about INCORRECT theories of lift.... http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html

They go through three incorrect theory's
longer path or equal transit
skipping stone theory
venturi theory

All based on Bernoulli and Newton

Rocketmagnet

Wicked little animation!

da Rock

Honest, it is going to be a violation of RCU Policies if you personally address another poster and demean him or his ideas.

Here again is the rule you agree to with each and every post:

Please resist the urge to curse, flame, degrade, insult or embarrass someone in your post. We encourage the free flow of your ideas, but believe that they can be communicated (and received) much more effectively if you keep things civil. If you have to vent, take it offline. We carefully monitor posts and will ban individuals who engage in offensive conduct within the forums. Thanks. (RCU Policies)

rmh

Lift is simply - pressure differential I always thought it was obvious
So why is there an attempt made to determine which side contributes he most to the phenomonen called lift?
In the end - higher pressure will nudge any obstruction - or go around it - which ever is easiest.
The vacuum ONLY idea can't work -of course - if there were only low pressures on one side of a plate - moving the plate toward the low pressure zone would create low pressure on the other side the plate and you would accomplish nothing. (pressure would equalize.)
Ditto, if only high pressure were present and there was no low pressure on the other side.
If the contribution from EACH side is not maximized- efficiency is not maximized
did I miss anything there?
Looks like a 50/50 contribution is ideal.
My statement that lift and drag are the same - seems to insult some - frustrate others .
But - if you examine any lift situation - the porportions of each (lift n drag) can be changed -easily - one becomes the other .
It is -I believe- an interesting exercise in logic.
I don't care to parrot formulas - It is too much like reciting some religeous phrases
Just words