Let's go with the Fairies theory. The rest of this seems too much like work.

I curse all of you to a 3 hour presentation by a CFD new grad on his/her wonderful discovery ( a change in the 3 decimal place of a 2nd order coefficient) and how this will make fantastic improvements in the accuracy of their predictions. This meeting will end like all such meetings when the person next to you, who was asleep and drooling on your shoulder, wakes up a points out to the presenter that their changes are still well within the margins of error.

Most of the equations cited in this thread are best used for illustrating the physical phenomena that produces lift. Except for some limited cases they are not very good for predicting it.

Given some good test data I think I could write some equations to predict lift in terms of Fairies.

EEERRRR, wrong !

This one of the other persistant myths that have been disproven time and time again but refuses to die.

Wouter

Please understand that Mr Tilley was indulging in sarcasm, which may well be lost to you as English isn't your first language. Don't take my comment negatively, it's not many in America who can speak their OWN language well, much less two languages.

On the other hand. Tilley's sarcasm gets lost to those of us who have known him for years.

Personally, I think all of y'all are wrong.

There's no such thing as lift. Government sucks, and the drafts thereof drive all things windy. Any perceived deviations thereof are driven by fairies, who are angry because of persecution and discrimination.

sea ya
tami

Dave, you are bringing Donna and da Woo in for our anniversary, huh? We have apparently quite a few folks coming in, so do come visit and sail!

Wout. You are right.
I was just a little scared of dissapearing in a flash of bright white light, though everyone tells me not to worry?... ( the other thread got me shook up)

The most readable, logical, intuitive and understandable article I have read on lift is How Airplanes Fly: A Physical Description of Lift. The credentials of Scott Eberhardt, one of the co-authors, suggest that the concepts it presents should be reasonably accurate.

As its name makes clear, "A Physical Description of Lift" presents a Newtonian explanation for lift. "See How It Flies" on the other hand presents a pressure based explanation. These two approaches may not be incompatible. According to the Wikipedia article and discussion on lift they may be two methods of describing the same thing. This is also addressed on this NASA page.

There is one major disagreement. "A Physical Description of Lift" claims that the Coanda effect has a significant role in producing lift. "See How It Flies" goes to considerable lengths (some of them a little dubious) to claim that the Coanda effect is not involved in producing lift. One of these claims is wrong.

Another article that supports Coanda is How Planes Don't Fly: Debunking a standard explanation of lift.


After contact with the inclined wing, the airstream has changed direction. Therefore, acceleration has occurred:

Personally, I propose we take an initiative from intelligent design proponents and declare that lift is too complex and therefore a supernatural being must be responsible.

Can't we wait till winter for these discussions. Right now I'm on my way to go sailing. It's Summer guys.

Mike Hill

True, although you might have to resort to thermodynamics and entropy to measure "disturbance." However, a symmetric section and a cambered section both at zero angle of attack could presumably have equal disturbances with different lifts. As I mentioned above, some of the reading should be discarded. Also, I mentioned this far too forcefully and stridently. If anyone took offense to this, my apologies.



Partially true. The reason Bernoulli is so easy and useful in a tube is that if you assume no boundary layer, the flow is uniform across the diameter of the tube. Also, a poorly designed venturi with the downstream expansion portion at too steep an angle can have separation. The Coanda effect could be used to keep the flow attached, but I'll get to that below.



Yes and no. Coanda really doesn't have anything to do with it. Please read See How It Flies Section 18.4 It describes the situation in detail.



Uniform flow over a wing is not a jet. The jet of fluid has more energy (from the increased velocity) than the surrounding fluid and can be used to an advantage. Later in that section, he describes how the Coanda effect can be used on a wing to delay flow separation by blowing high velocity air into a boundary layer that is near separation and allow the wing to operate at a higher angle of attack. This Coanda effect air injection could also be used to keep flow attached on a too-steep venturi (if for some reason you were forced to have one) and has been used to remove rotars from helicopters. A key quote.



Generally speaking, the reason the flow stays attached to the wing and turns is pressure. The reason it stops turning and separates when the angle of attack gets too great is a lack of pressure. If you look at a packet of air and three adjacent packets (ahead, behind and above) and qualitatively work out what's happening using Bernoulli, you can work it out. No need for math or exact numbers, just sketch out a few places over the wing and work out what's happening with each term in the equation. If anyone wants me to go through it, I will, but I've probably typed too much already and I'm hoping to go sailing this afternoon.

Also, the connection between all that downward accelerated air that the Coanda proponents point to and the wing is pressure. You can qualitatively work that out with four packets of air and Bernoulli as well, but you'll need some vector force diagrams, too.



That's only in cases where the designer hangs the center of gravity well forward of the wing's center of lift for highly stable stall recovery. Check out Section 6.



And Marchaj was late to the party. If you really want your head to explode, check out http://www.navier-stokes.net/nsfield.htm Navier derived these equations in 1822, so there really hasn't been an excuse for anyone to misunderstand lift for the past 183 years.

Summer schmummer, the wind lightens up here in Summer, so we wait for winter to sail

well, the wind lightens up with notable storm related exceptions...

(scroll down for the answer)




















































42.

This only changes the situation by degree. All properly designed conventional airplanes will have the Cg forward of Cl, just some more than others (some MUCH more!). So in cases where the Cg and Cl are close, the horizontal stab does not need to pull the tail down with much force to change the pitch. But it still pulls down some, and still subtracts some lift. The amount of down force needed could be calculated as the ratio of the distance between the Cg and Cl(wing) and Cg and Cl(stab), with smaller Cg/Cl(wing) and larger Cg/Cl(stab)trending the downforce lower. So if the tail is long or Cg/Cl close the airplane is more efficient.

Jimbo

As it feels like this thread is approaching closure , and grappling for a practical value :
I'm buying a mainsail for my uni-rig F16. All the sails from the various manufacturers represent an invisible hierarchy of "best" to "worst" at hastening the force needed to move the boat forward in the wind conditions in which I intend to race.From the standpoint of pure consumerism and given the abundance of misconception about how sailboats move - how does one distinguish a smart (fast) sail from one cut by a minion of the church of equal transits (buy what's winning ?) ?

I'm pretty new to sailing , has the misinformation about how lift occurs manifest in wide spread purchase of bad sail designs in the past or am I safe assuming most sails offered are "smart"?

Regardless , loved this thread...

PK

scott@aa.washington.edu
Office: 101 Kirsten Wind Tunnel Building

He certainly has a credible location. Means I'm going to have to be reeeeeal certain about the email I'm going to send him about the Coanda effect.

The two approaches, Newtonian and pressure must be compatible. They're both based on physical laws. That's the fallacy that I was getting at with the Coanda/F=ma argument. In all the presentations of that argument that I've seen, there is a lack of connection between all the downward deflected air and the wing itself. Then viscosity and "stickiness" gets brought into it as to how flow stays attached and, personally, I don't think that pans out, either. First, viscosity leads to boundary layers and boundary layers lead to separation. As for "stickiness," as opposed to a solid, a fluid, in and of itself -- the special case of surface tension excepted -- cannot support a load in tension or compression. It's like pushing a rope cut into 10 pieces. But surround that fluid that you are trying to apply a force to with a pressurized fluid of a same or different type and now it can support a load.

Worse, I'm going to go out on a limb, although I believe it's a short, sturdy limb and say that Eberhardt and the How Planes Don't Fly article are wrong in their description of the Coanda effect. If Coanda is viscosity/boundary layer driven, then the flow slows down, the bondary layer thickens and the flow separates and it sure doesn't seem like it would help in keeping flow attached on a nearly stalled wing! I'm going to have to ponder this one, but I suspect that in the water over the glass scenario, the flow leaves the faucet at atmospheric pressure, continues to accelerate down the glass due to gravity, decreasing its pressure to below atmospheric and the surrounding air pressure holds the stream against the glass until the boundary layer slows the flow too much and the stream separates from the glass. The water's surface tension would help keep the water attached to the glass, but surface tension sure doesn't have a role in the rotarless helicopter or the blown boundary layer referenced above. I'm not 100% percent on it, but I'd be willing to put money down on a claim that the water-jet-on-a-glass Coanda effect would not work nearly as well, if at all, on the space station.

Supernatural beings like fairies?

And since I'm disparaging other people's credentials and criticizing credentialed experts, I suppose I should toss my credentials into the mix. I have an MS in mechanical engineering, have been working in hydrodynamics for about 15 years and have designed and analyzed about 15 underwater vehicles of various types and speeds.

I haven't had time to read this whole thread but there are a few things that I have to through in after reading a few of steveh's posts. The reason the Bernoulli equasions don't seem to be accurate for a wing is because most people seem to think that if 2 paricles of air side by side strike the leading edge of a wing and one goes over the top of the wing and the other goes under the wing, the two particles must both meet back up at the exact same time at the trailing edge of the wing. This is an insane assumption. There is no law of physics that says this will happen. The equasions aren't wrong, the assumptions people are making are wrong.
Next, the relation between the Coanda effect and lift. The Coanda effect simply shows how air is accelerated by the wing. I don't mean slowed down or speed up, I mean that the direction of travel is changed. Remember that when you have a wing creating lift, there are only 2 elements interacting, the air and the wing. If the wing is creating lift it must be doing it by interacting with the air in some way. Lift is a force normal to the direction of air flow. The only possible way for a wing to create lift is if it creates an equal but opposite force on the air. That force is accelerating a mass of air downward. Contrary to some of the bizzare circulation theories, air must be deflected downward in order to create a force on the wing in the opposite direction.
Circulation theories, CRACK SMOKING BS. Just a little bit of common sense will show that this idea is totally wrong. If there is a circulation around the wing that produces the lift then I should be able to destroy all lift by destroying the circulation. Since the "circulation" is from the trailing edge toward the leading edge on the bottom of the wing, all I should have to do is greatly accelerate the air under the wing from the leading edge toward the trailing edge to distroy the circulation. What if we bolt a jet engine under the wing to accelerate the air flow. By the reasoning of the circulation theory, every jet that has an engine mounted under the wing should never be able to get off the runway. By use of simple vector math you can show that a circulation plus a flow does not create lift. Some of the assumptions and leaps made by Mr. Gentry are soo wild I can't believe anyone would take him seriously.
I'm sure that many people will disagree with me on the circulation theories and I welcome your arguments but first I ask you to think for your self rather than blindly following the latest craze in lift theories. Does it make sense?

I never appreciated the circulation theories, but I do recognize that they can be used as mathematical describtions that will give accurate results. In a way it is not a explanation but a different type of model, one that can be useful in certain circumstances.

For the educated people among us. Complex numbers or imaginairy numbers are just plain BS in the real world but with this math structure we humans are able to solve many complicated problems. I feel circulation theories are comparable to that.

In all other aspects I'm with Steveh

Wouter

Our current summer feels much like winter. Cloudy and plenty of rain with regulary a storm coming through. Another wasted season.

Wouter

To clarify a little about pressure. Total pressure is static pressure plus velocity pressure. Velocity pressure acts in the direction of velocity while static pressure acts in all directions. A pitot tube measures total pressure at the tip (pointed into the air stream) and subtracts static pressure at right angles to velocity (through the little holes around the perimeter of the tube) to give you a reading of velocity pressure which is then allows calculation of velocity. That is assuming you connect both hoses to a manometer. If you increase velocity as in a venturi, there is a reduction in static pressure as it converts to velocity pressure where the velocity is higher (at the restriction). Reducing velocity, the pressure is reconverted to static pressure with some efficiency loss.

In a sail, increasing the velocity on lee side reduces the static pressure on that side compared to the static on the windward. That results in a force or lift which is the difference in the static pressures. I would believe there are other forces involved as well such as a portion of the velocity pressure on the windward side.

Howard

deseely,

Most of this has been covered and to my mind, the Coanda effect as a "how" or cause for the downward deflection is not only DOA, it's a misnomer. Effects can't cause things! Try this.

Bob: How does the air get curved downward going over a wing?
Joe: The Coanda effect.
Bob: How does the Coanda effect curve the air?
Joe: It's like water from a faucet flowing over a glass.
Bob: Ahhhh... But how does the air or water curve?
Joe: (head explodes)

That's pretty much Raskin's argument in a nutshell, but instead of his head exploding, he waves his arms around and says we don't need to know how it works, leaving us

As for the crack smoking circulation theory, as Wouter said, circulation is just a mathematical and visualization tool, not a conspiracy. Check out Section 3.10 of See How It Flies for a very clear description of circulation. Circulation doesn't state that air moves from the trailing edge to the leading edge while an airplane is flying, it states that this kind of flow plus that kind of flow produces the same flowfield as a wing. In fact, circulation assumes that the forward flow under the wing from the TE to the LE is exactly cancelled out by the linear flow passing under the wing from the LE to the TE. You don't need a jet engine to cancel that forward flow, it's already done.

Ironically, a jetstream improperly placed under a wing could decrease lift in that region because of the...



...Coanda effect.

I wish I was kidding.

Wouter,

One could say that the jib works in the mainsail's updraft and the mainsail in the jib's downdraft, but I miss the advantage of the combination.

If I look at the jib, it can be set in a more opened angle due to the main, but if I look at the mainsail, it needs to be moved closer to the centerline with a jib, compared to without it. Doesn't seem to work this way.

Luiz

There is an AWFUL lot of bull sheeeet being thrown about here guys!! The best, truest and nicest answer is that the fairies were the ones what dun it!! And very nice little folk they are too, so DONT GET THEM ANGRY OR THEY WILL TAKE THEIR "LIFT" AWAY - SELECTIVELY, which means that when you are gliding in, quite nicely placed, towards the finish line and you think that you have a top place all wrapped up, it is then that the "lift fairies" will strike!!! And instead of having a favourable wind to cross the finish line with, you will be suddenly "lifted" almost straight up and thrown aback as you watch several other "believers in the powers of fairies" cross in front of you. So take care, you have been warned. (I have seen it happen many times – and that’s the truth -)

There are actual real world physical phenomena described well by complex numbers such as electromagnetic wave theory, where electrical and magnetic sine wave constituents are said to be operating in planes 90* apart, thus creating a 'Z' axis. The X, Y and Z axis relate exactly as the plane of the real numbers intersects the imaginary.

Agree with you on the circulation theory, though. Many people completely misapprehend the intent of this theory thinking that it supposes the fluid is actually flowing backwards on the underside(??!!) instead of simply returning to the same energy state as before negotiating the foil.

Jimbo