Ben Lanterman

Ken, a well argued summary.

Not knowing Canola from Coanda is the reason they don't let me out at night!

Tim, When you blow across any of the things in your experiment the action of the air following the surface is Coanda (if I understand it correctly). The flat plat will pick up a little but it is small compared to the nicely curved surface. When you do the reflexed surface you have changed the pitching moment from that of the fully curved airfoil, not just the lift.

The same changes can be seen in regular wind tunnel tests of the airfoils.

I do have an interesting question (well to me) that occurred to me -

Put an airfoil in a wind tunnel at angle of attack. Immediately aft of the airfoil (fraction of an inch) put a flat plate parallel to the tunnel floor and many multiples of the chord long downstream.

It basically stops the Circulation, downwash, etc. and leaves you with Coanda, Bernoulli and impact pressure on the bottom of the wing at work.

Does anyone know what percentage of the original (no flat plate behind the airfoil) lift is left? I don't know so any guess is probably more accurate than mine!

rmh

I looked at Tim Green's examples - a bit puzzled -
I know that the speed of the air changes the Coanda effect.
When you look at fluidics valves - one which switches (I won't get into valve terminology -no need for it), you see that the air ONCE ATTACHED follows an angle or a curve or a straight line.
A small pulse OR a steady flow will "flip " the valve making it follow path "B".
One of the sorta interesting things about Coanda effect.
Which proves -- nothing .
just a comment.

Ben Lanterman

I agree it is rather neat.

People have the same problem. Once convinced to go in a direction some stay there until something better comes along. Some oscillate about like a valve design hitting some kind of resonance. Some go hard over and can't be changed no matter what.

teamselpats

going back to the original question, simply which would have greater lift at the same angle of attack, a symmetrical airfoil, or an airfoil with the topside flat? im assuming it would be the latter considering all formulas and equations aside the air above the flat surface has less interference causing the air to move faster, and therefore creating less airpressure, creating lift. Explain if i am wrong

Ben Lanterman

Assume the same thickness airfoil.
Assume the same angle of attack.

Positive camber gives more lift than symmetrical. This is the typical flat bottom airfoil.
Negative camber gives less lift than symmetrical. This is the flat bottom airfoil inverted.

Don't think of it as interference. The air moving over the curved side is still moving faster but if you look at a CLalpha (lift curve with respect to angle of attack) of a flat bottom airfoil with camber it will have a positive lift at zero angle of attack and at negative angles of attack (say 4 to 8) finally have a negative lift. It is what allows you to fly a flat bottom airfoil inverted which is what your question addresses.

There are airfoils called supercritical airfoils that are used at high subsonic Mach numbers that have a look of being flat on the top and curved on the bottom. It lets the draggy shock formation be delayed until higher Mach numbers than a conventional airfoil. They gain lift back by camber near the rear of the airfoil. Most folks who see them on a airplane wonder if the wing were put on upsidedown. But it is a special airfoil designed for a particular flight condition. You don't see them on airplanes that fly at "normal speeds", those less than Macn = .8 or so.

teamselpats

according to the theory, the longer the top side by way of having a curved top side, the more lift. im assuming that if you made the topside 5 times longer than the bottom by having a very very curved wing, it wouldnt work. this brings me to my question which is what is the practical limit on this theory.

rmh

Without referring to any tech explanations - (I am too stupid to know any),
you can, on some low speed applications have a VERY curvy top surface which at a controlled angle of flight - is extremely effective.
Why?
How well the air can follow a curve is somewhat related to speed .
Caveat-- what is great for one speed is perhaps worthless for another .
When someone offers a "perfect "airfoil - it is perfect for a finite application
I can't draw very well so my model airfoils are usually Florsheim derivitives .
Or just a slab of wood/foam / whatever .

Ben Lanterman

Like all of our simple theory it has limits. I would GUESS that lift will still be developed, just not a significant amount. Remember a round cable or a flat plate can develop lift and drag.

The problem is our lack of understanding of the processes and math that will describe the process. Theories are generally developed to fit observed processes. Of course the good thing is that physical processes do work based on a whole lot of physical laws that we have a chance of knowing based on our observations and math. We may not know all of the really little details involved and might not even care. We are sometimes lucky to get a good hold on some rather narrow parts of all the possibilities - those would include the useful range of airfoil shapes.

Within the range of useful curves and flight conditions we do know a fair amount, some of us know more than others :-). I personally find the math frightening in that I am unable to look at the math and understand it's implications and conversion into the real physical thing going on. The guys with Dr. in front of their names have that gift - I am really jealous. To look at a set of differential equations or other "stuff" and know the physical reality they describe is a real talent.

Your question fits in an area that was looked at probably once way back when, when folks were investigating how thick airfoils could be and still be useful - when no practical use for the super thick airfoil could be found it was put in the "interesting but doesn't make money" bin (it is a rather large bin).

If we knew the full math describing all of the complexities involved in the analysis of very very thick airfoils like the case you spoke of then there would be no practical limit to our theory. However, in keeping life simple, the basics (without complexities) which work on a "normal wing" won't work too well on your example. It isn't a fault of anything but our need for simplifications to make things easy. However our knowledge and theories within the limits we normally set is pretty solid (in spite of the arguments here). Aerodynamics is a fairly exacting science.

The nice thing is that disciplines like airfoil design don't have sudden "brick walls" where things don't work. It lets the practical engineers like Dick (and me too) get away with a lot. So my little front yard RC F-4 Phantom works fine with a flat wing.

LouW

Very well said, Ben. I've been away for a little while, but let me run this by the group.

I have been criticized by some in this forum for using such everyday standard english terms as, air being “pulled” down, “negative pressure”, “vacuum”, “sucking” etc. They are right of course. No matter that these terms may be common, intuitive, and highly descriptive, they are none the less imprecise. I am reminded that one defining characteristic of a fluid, such as air, is that it has zero tensile strength. Air cannot “pull” or “suck” anything.

On the other hand, air has some compressive strength so it can certainly “push” something. I believe we all agree that due to Mr. Bernoulli, Mr. Coanda, and other great men of old, along the upper surface of a lifting wing there is a pattern of reduced pressure. This reduced pressure cannot by any means "hold" the wing up. Instead, the lifting force arises from the difference between the pressure above and below. The relatively higher pressure below tries to move into the relatively lower pressure above however, being blocked by the physical presence of the wing, it simply pushes upward on the lower surface.

This being the case, why would not a more precise explanation of lift be,

“As a wing moves through the air it rides on a “cushion” of air that supports it by pressing on it from below.”

Uhmmmm ?????

Shoe

And after the wing passes, the relatively high pressure air below the wing rushes up to fill the area of lower pressure above causing a torrent of upward momentum?

Shoe

I don't think "theory" really suggests that a longer top side generates more lift. I think this is a misconception that comes from the "molecules separated at the leading edge must meet at the trailing edge" urban legend. Consider the 2 airfoils shown below. Thin airfoil theory suggests that wing sections A and B will generate the same amount of lift per degree of angle of attack, even though the top of section B is much longer. I don't think it's an issue of theory reaching a practical limit. I don't think "theory" suggests a direct connection between a longer airfoil top side and more lift in the first place.

The flow at the "peaks" of airfoil B will be flowing faster (less pressure) and the flow in the "troughs" will be flowing slower (more pressure) than at the corresponding points on airfoil A. There's no conflict with the Bernoulli equation here, but the molecules split at the leading edge will not reach the trailing edge at the same time (for wing section A or wing section B).

LouW

After the wing has passed, the low pressure area has passed with it so there is no motivation for continued upward momentum. There is not even any upward momentum to continue since the air was prevented from moving upward by the physical presence of the wing. I am hoping for some other comments and I will expand on them all.

Shoe

How is it that the physical presence of the wing prevents upward momentum but allows downward momentum?