Originally Posted by
deeper
It does not matter any way as it is incorrect theory, (and it was only a theory after all), before you start typing have a look at NASA's website and type "incorrect theory" in the search engine.
Only 2% of lift is generated by the bernouli theory and the rest is Coanda effect.
Don't argue with me argue with NASA, (you'll lose)
Well, a quick search of the nasa site under incorrect theory doesn't show anything on the ongoing coanda/bernoulli/deflected airstream debate (well at least on the first page of results).
Nevertheless, it amazes me that people continue with the misconception that the three explanations are different. They are all different ways of looking at the same effect. It goes as follows if you decide to start with the momentum change associated with deflecting the airstream. You can, of course, start from coanda or bernoulli and get the same conclusions. It is the same physical effect.
1) In order to produce lift we need to produce a change in momentum (F = dp/dt, where p is momentum). This is achieved by deflecting the airstream passing over the airfoil.
2) In order to deflect an airstream, you need to have a pressure gradient across the airstream. To deflect the airstream downwards the pressure above the airstream needs to be higher than the pressure below.
3) For the air going over the top of the airfoil, the pressure above it is just the free stream pressure, therefore in order for there to be lower pressure below this stream, the pressure at the top of the airfoil needs to be lower than the free stream pressure.
4) Conversely, for the air going underneath the airfoil, the pressure below it is just the free stream pressure, so the pressure just below the airfoil needs to be higher than the free stream pressure.
5) The high pressure below the airfoil and the low pressure above it results in the airfoil being pushed up. This is the mechanism by which the force due to the change in momentum of the airstream is conveyed to the airfoil.
6) Because the pressure above the airfoil is lower, the air velocity needs to be higher to comply with bernoulli's law (P + 1/2*rho*V^2 = constant). Conversely, because the pressure below the airfoil is higher, the velocity needs to be lower.
7) Circulation is defined as the integral of air velocity around a closed path encircling the airfoil. Basically, you can think of the faster upper surface velocity and the slower lower surface velocity as being comprised of an equal velocity above and below superimposed with a weaker circulating flow that goes from the back of airfoil to the front along the lower surface, and then comes back along the top surface. The flow circulating flow along the top adds to the base velocity leading to a higher velocity over the top of the airfoil. The circulating flow is subtracted from the flow underneath the airfoil, resulting in a lower velocity under the airfoil. Note very carefully that the circulation is really just a mathematical tool to describe the difference in speeds above and below the airfoil.
8) The coanda effect basically states that the lift of the airfoil is proportional to the circulation times the free stream velocity. But remember, we only have a circulation because there is a difference in speeds above and below the airfoil, and we only have the difference of speeds because bernoulli requires it to generate the difference in pressures, and we only have the difference in pressures because we are deflecting the airstream.
So, you can't claim that only 2% of the lift is due to the "coanda effect", it is all due to the coanda effect, but then it is also all due to the deflection of the airstream, etc... They are all part of one and the same physical phenomenon.
The theory of circulation (coanda effect) is important mainly from a theoretical point of view, it provides a mechanism for calculating the strengths of downwashes and trailing vorticies. All very useful when Nick Lappos and co want to work out how to make a more efficient, quieter rotor.
Questions?
Daniel