grumpy_o_g
Well-Known Member
How sails Provide lift and interact with each other to increase efficiency is well understood and has been for a very long time. It has got nothing to do with "jets" coming off the back of the sail. NOTHING!
Only one contributor has mentioned Arvil Gentry. No-one has mentioned circulation theory.
You can't have differences of opinion and expect to be taken seriously. There is one explanation and one only and it has been common knowledge for "ever!"
Just because you don't like the explanation or you don't understand it, doesn't mean that you can make one up and put it out there as a viable alternative.
"Intelligent Creation" might be easier for some people to understand, but it's still a lie!
sam
Given that the true nature of mass isn't understood yet that's a very bold statement - we've a long, long way to go as race before we can say we truly understand anything completely - it may be we never can. At a more practical level I've spent a lot of time talking to and even working with some of the best aircraft designers in the world, people like Gerhard Weibel and his peers, as well as naval architects. None of them had definitive answers so I'll have to disagree with your statement. Circulation Theory is exactly that - no-one has managed to actually demonstrate it yet except in a closed loop system in a lab, though the math's stacks up if you're willing to make some assumptions. Did you know that the most commonly used Kutta-Zhukovsky model is based on measurement rather than theory as it was found to be more accurate? Neither Circulation Theory nor Kutta have any real relevance to a sailor anyway as the airflow is far more complex than can be handled by any simple mathematical model. In practice all you need to know is the basics and then go by trial and error unless you're designing/campaigning an America's Cup boat. To use any of the math's to adjust your sail trim you'd need a supercomputer as well as sensors that could detect both the stagnation point and the vortices or a very good wind speed indicator that worked along the whole of the luff of the sail as well as being able constantly map the sail shape too.
I'd never heard of Arvil Gentry I'm afraid but a very quick look at his web-site (I only scanned two of his papers) suggests that he's simply restated contemporary aerodynamic theory and then used one very specific example to describe the interaction between a foresail and mainsail - the interaction between a non-overlapping blade jib will be very different to that of a huge 150%+ overlapping genoa from the old IOR rules days, and will also be hugely affected by the speed of the airflow and the sail shape and angle of attack. He certainly spends a lot of time telling us what is wrong, but, while I wouldn't argue with any of what he said (except that, like any aerodynamicist, he tends to make the models fit the math's rather than the math's fit the models) he doesn't put forward anything new either. Even if the man in the street didn't know this stuff, Sparkman and Stephens and the like certainly did.
If you want to look to anyone to understand Aerofoils then Zhukosky is the main man (lot's of different spellings as a result of the translation from the Cyrillic alphabet so beware). He was the first person to really study and start to understand the complexity of the aerofoil. There was a model by the Douglas Aircraft Corporation (later McDonnell-Douglas) that was the actual model designers used when I was first into this stuff but it was all based on Zhukosky's work. Effectively it broke a shape into panels (common practice and the basis of 3D Calculus really I guess) but the maths was very heavy going. I never was able to go through it without someone helping me - I thought I could follow it when it was explained but I was probably kidding myself. Later models were refinements of this I believe. There's a thorough but not too complicated overview of the Hess-Smith model here http://www.dept.aoe.vt.edu/~mason/Mason_f/CAtxtChap4.pdf. I have no idea if the Smith that Gentry mentions working under is the same person as the guy who worked with Hess on the DAC model.
FWIW I'll only sort of agree with the statement in another post about momentum as well. Viscosity, density and velocity are the critical factors for the fluid medium. Momentum is in there but it's a derived factor from some of the other parameters. Sorry, but I'm in pedant mode after that sweeping statement by Mr Salter.
