From the reading recommended in this thread, Here's what I gather:
Bernoulli always works in the absolute sense as it is really just a reiteration of the laws of energy conservation. But the totality of the fluid moved by the foil would have to be considered, which as you know includes fluid in strata quite removed from the surface of the foil, which (probably simplistically) explains why a prediction of lift based solely on Bernoulli disagrees with pressure measurements on the foil surface. Bernoulli does then predict the velocity and pressure of the flow in totality, and at any specific point and strata given knowledge of the complementary parameters for that particular 'blob' of the fluid, but not necessarily how a foil behaves in that total flow. Two 'foils' could perturb the energy state of an airmass equally but produce vastly different amounts of lift by, for example, causing differing amounts of turbulence. So Bernoulli becomes for foils just a meaningless equality like 1 = 1; energy is conserved. We know that; it is always so.
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.
Bernoulli does OTOH, predict well with tubes, where the airflow is constrained. Even in a Venturi with a nice foil shape, Coanda is meaningless as the airstream cannot separate from the surface. Therefore the concept of angle of attack becomes meaningless for Venturis, thus no Coanda.
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.
Coanda effect is the reason the airflow stays attached to a foil's surface as it's pitch (angle of attack) is increased to a point where useful lift begins, but it cannot alone explain how the foil converts kinetic energy of the chordwise flow into lift. If you try to explain that conversion by Coanda alone, you will run into several problems as Steve mentioned, such as the major portion of the acceleration happening in the wrong place and direction.
Basically the flow has to bend as it travels around a foil that is set at a useful AOA. It stays adhered to the foil during this acceleration(change in velocity) because of boundary adhesion and Coanda effect. The change of momentum of it's original velocity causes the pressure drop observed on the lift side. As AOA is increased, more lift force is derived from the flow since the change in the direction of the airflow is greater; a change in direction of motion constitutes an acceleration. The speed of the flow in its original direction of motion remains unchanged, yet that flow has now acquired a new direction as it negotiates the lift side of the foil. Thus its speed increases. A simple vector diagram will illustrate this. The flow actually undergoes continuous change in velocity and speed as it follows the surface of the foil, although in this explanation it sounds as if happens as a singular event (This is where the proponents of the 'longer distance' explanation get sidetracked. It's not the differing distance but the change in velocity! Subtle but real.) More importantly, this creates a conflict between the adhesion of the fluid moving at the surface of the foil and the cohesion of the total fluid mass, with the fluid near the foil becoming rarified as a result. Essentially the opposite is happening on the other side of the foil, though not with equal reaction or lift.
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.
The name Coanda effect is properly applied to any situation where a thin, high-speed jet of fluid meets a solid surface and follows the surface around a curve.
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.
Once again, the Coanda effect cannot explain how the wing works; you have to understand how the wing works before you consider the added complexity of the blower.
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.
In a conventional airplane, the lift of the horizontal stabilizer is subtracted from the total lift, not added as someone stated earlier. The higher the wing's AOA, the greater the subtraction.
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.
Some people are under the misimpression that the tail must fly at a negative angle of attack for the airplane to be stable. That's just not true. The real rule is just that the thing in back needs to fly at a lower angle of attack than the thing in front. If the angle is so much lower that it becomes negative, that is just fine, but it is not required.
The amount of stability you have depends on the angle of attack of the tail relative to the wing, not relative to zero.
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.
