which causes lift?

[javaman1]

i've heard a few different arguments for what causes lift in an airplane. I was thinking that maybe both of the main ones are right or maybe only one is. 1. i've heard that the bernoulli principle (which says that accelerating or fast moving air causes low pressure and slower moving air causes higher pressure) is an explanation for lift. it states that when air is slowed or rather "bunched up" on the lower part of the airfoil, high pressure is created and that low pressure is created when a moving airplane quickly forces air upwards over the top of the airfoil. 2 i've heard that the coanda effect helps create centrifugal pressure gradients when air hits a wing. (centrifugal pressure gradients have high pressure on the outside of the circle and low pressure on the inside, like a tornado or hurricane). the air going under the wing curves downward creating high pressure on the bottom of the airfoil and when air goes over the top of the airfoil, the coanda effects helps the air stick to the wing and the round shape of the air movement causes a centrifugal pressure gradient with low pressure directly above the wing.
im not really sure which to believe. or maybe both ideas are correct and each have something to do with lift. does anyone have any facts or opinions on this topic?

[oimmuk]

Simple answer is both are correct to a digree, but for different reasons which I am not going to go into as I already have a headache...:D. The wind passing over the wings upper surface has to travel a greater distance so it moves at a faster speed and this provides lift, but this applies in wings that are not symetrical<spelling> in shape. In those instances the wind travels the same distance and the same speed providing lift on both sides, this is why a patern craft can fly equally well inverted. Attack angle or Declanation or incidence, choose your poison, also play a roll in lift and a negative angle can inversly effect lift and too much angle can increase drag which will effect lift as well.

But lets get back to your two sources I tend to go with Bernoulli, as if you take a ping pong ball and and air hose you will be able to balance the ball on the air stream but IF you turn it sideways it will STILL support it so there you have a good example of lift.

Now I am either going to get some asprin or a good stiff drink...:D.

[jb48]

NACA, to become NASA later on, conducted a lot of tests and studies on airfoils, back in the good old days. Since the Bernouilli`s Law theories concorded with testing results it became the accepted explanation for lift. NACA found that moving air surrounding a wing creates lift in unequal proportion. On classic airfoils, such as Clark Y and many other convex top wing and almost flat bottom, the lift from a moving wing is created in these (average) proportions: 75% from the moving air over the top (low pressure) and 25% from the bottom. To make it short, that means that the top of the wing is doing the essential «work» for lift, the bottom helping the «process» with a relatively small upward push. This is why despite a badly torn underwing fabric or aluminum covering, an aircraft can usually fly good enough to make a controlled forced landing or even a more or less normal landing. A similar damage to the top of the wing is end of crew and plane.
Before World War I, some airplane even flew without covering at all on the bottom part of the ribs! Accordingly, these were slow, hard to maneuver, dangerous flying machine...but they flew!

[slipstick]

Both Bernoulli and Coanda, together with many other variations are attempts to EXPLAIN lift and to QUANTIFY it.

All lift is caused by the displacement of air downwards. If air is not being displaced downwards there is no lift. Why it is displaced and exactly how much of it is displaced by which mechanism is a bit trickier to work out, hence all the theories. In the real world no one explanation covers all possibilities e.g. if it's pure Bernoulli then flat plates don't create lift.....but they do. Coanda looks very tempting but wind tunnel tests don't actually show the circulation that would be needed for that to be a complete explanation.

If you really need to understand lift, since it is basically about air movement and air is a fluid, a reasonable start would be to learn a fair bit about fluid dynamics and particularly edge effects (what happens at the leading and trailing edge and the wing tips is very important and is simply ignored by the basic theories).

Steve

[BuzzardBait]

OK...

I'm STILL trying to figure out how the (*^&& a symmetrical (sp?) wing creates lift....

is it just the angle of attack of the wing???

[Rabbitcreekok]

OK...

I'm STILL trying to figure out how the (*^&& a symmetrical (sp?) wing creates lift....

is it just the angle of attack of the wing???


Yes. Remember the wing stalls when the angle of attack becomes too great and the airflow over the wing breaks away from the wing and lift goes awry.

[darylm44]

It is magic:D

[Insomniac]

I've heard it has somthing to do with the air on the top of the wing being thrown downwards as it leaves the traling edge.

[p901]

You might find a book, "Theory of Wing Sections" to be very helpful.
Lotsa math but there is also text which explains the graphs and also includes the plots of many airfoils. Of course since the book was printed more airfoils have been developed and specifically for model use,
Selig, Quabeck, etc.

As far as it was explained to me there are different "types" of lift,
impact lift, (when the air hits the bottom of the wing), and induced lift,
(made by the shape of the airfoil and the AoA, Angle of Attack), and others which concern the aerodynamicists.

The induced lift provides the majority of the lift phenomenon, (the magic) which is caused by the acceleration of the fluid, (air) over the top of the wing. The acceleration of a fluid causes it to lose pressure. Less pressure on the top of the wing relative the pressure below, lifts the wing upward. Symmetrical wings must be angled slightly to cause this effect.
This is why for aerobatic planes when they are upside down must keep the nose "up" for that little bit of angle to keep flying.
Hope this helps, P901

[John Boy]

When laminar flow occurs, the equations (partial derivatives) that describe air (or any fluid) flow are dead on with measured results. Lift is created when the pressure on the bottom of the wing is greater than the pressure on the top. In the case of a flat bottom wing with no angle of attack, the pressure on the bottom is equal to the atmospheric pressure. The curved upper surface has a higher pressure near the front and lower pressure past the high point. If there is more surface area behind the high point the wing is experiencing lift. While increasing the angle of attack increases the pressure on the bottom, the curved upper surface always has a pressure less than the atmosphere. So, it is easy to say to a first approximation that the lower pressure on the up side is creating the lift. But the reality is that you have to add up all the pressures every where to get the total.

If wings extended to infinity the laminar flow would be sheets extending to infinity. But wings end. Unless you cap the end with a side a force generator (vertical plate) or a bulb or some other upward curving surface, the air will travel in a vortex (tornado) with it’s base at that wing expanding rapidly behind the wing. This is one of the main reasons that separation is maintained between planes landing. Passenger jets create such strong vortices that they will flip smaller planes a mile away. But this effect is well behind the plane and does not add lift. In fact, the vortices steal lift from the plane that creates them. So, today most passenger jets actually have a vertical plate on the wing to increase lift by killing the vortex.

Aerobatic wings (symmetric) allow the plane to fly upside down just as well as right side up. However the wing only creates lift if there is an angle of attack. (This is the angle of the wing relative to the air it is moving through. It is not the same as pitch or the angle at which the wing is mounted. It is the angle of the wing relative to its forward motion.) In the case of the symmetric wing the pressures are symmetric if there is no angle of attack. The easiest way to see where the lift is coming from is to press the trailing edge down on a table. This region in contact with the table has pressure equal to atmospheric pressure. The under side that sticks up will have higher pressure as will the top front. However, the high point is now forward with a rather large area experiencing low pressure. Thus stable flight for an aerobatic plane requires a positive angle of attack.

I have mentioned laminar flow a lot here. If the flow is not laminar, it is “turbulence” and virtually unsolvable. The control horns create turbulence. Struts create turbulence. Landing gear creates turbulence. Wind through trees is turbulence. The basic rule of thumb is that anything wider the 1/11 it’s length will not conform to laminar flow for air (1/7 for water). Certain shapes such as balls and regular smooth surfaces can be solved, but the experimental results are less likely to match the calculations. So, explaining what is going on becomes much more problematic. As in that butterfly in China just might have some effect. So, to some extent it is also magic, or unexplainable.

[watt_the?!]

fantastic thread ppl!!!!!!!!
we should bottle this.