RobRiley, "Tunnel data is for a theoretical wing with no ends in a constant steady flow, the air around the sea isnt a lot like that. It could be, but more often than not it isn't."
Sure, but a wing is much better than a sail at recovering and keeping that steady flow. Especially if the wing instantly responds to the variable wind via the elevator.
The whole 'secret' is the elevator keeping the wing perfectly trimmed all the time, especially for the gusts. It is just a sailplane on its side.
RobRiley, "I must say his interpretation of Clmax of 1.8 to be rather 'optimistic', we wouldn't ordinarily expect see figures like that without a flap, or flap and slat."
Yeah, he had confirmation bias all over the place : ) And I don't think he even got his flaps working. I think a reasonable goal though is 60% of the sail area should generate comparable numbers.
Flaps and ailerons add drag faster than they create lift. That is why airplanes don't use them, except when they want to create extra lift and drag. It is why a rudder is necessary to turn to counteract the drag from the aileron.
I am having a devil of a time trying to figure out why the sailors want flaps?
RobRiley, "The other thing was about flaps, moving geometry doesn't come for free. Even a simple split flap adds weight and loads and arent so simple to organise. This is why they are uncommon on ultralights for example where they need to keep structures light. The additional loads generally need another spar in the afterpart of the wing. IE. They become 2 spar wings where there was just one, 3 where there were two."
Flaps add weight, complexity and cost and perhaps most importantly decrease the stall AOA.
I don't think I emphasized why increased drag is horrible, it is parallel to the chord. So yeah flaps increase the lift, but unless the wing is perpendicular (90 to 110˚) to the direction of travel the extra drag will slow the boat down more than the extra lift speeds the boat up. It will absolutely kill trying to point into the wind, which is exactly what a fast boat has to do.
Interestingly the increased lift and drag increases the moment arm (heeling force) dramatically. Say for example the CoE is 20' and a clean wing produces 200 lbs of force and 6 pounds of drag, that is 4000 lbs of moment. Add a flap so that you get 300 lbs of lift and 60 lbs of drag then the heeling force is over 7,000 lbs almost double the heeling force for an extra 40 lbs of lift? What am I missing?
Feathered a clean wing might only produce 120 lbs of force. The same as a single cable supporting a mast.
RobRiley, "Another conundrum I would like to see satisfied is what happens when by some phenomena the wing is hit by a gust or wave square on, and this is what I meant about unforeseen loads. Because now the Cdo is closer to 1, and this would be unnatural for an aircraft wing. It isnt unknown for flat stalling aircraft to break the main spar and lose their wings."
That is what we call 'G' force and it can tear wings right off. Good bearings, a light wing and low moments are the solution.
RobRiley, "I noticed too you seemed unsure of the reynolds number, heres a one line way of working that out:
Reynolds number at sea level is written: Re = 6378 x fps x chord length This because sea level cu ft air weight given as slug (0.00238) and subsequently viscosity, is used for engine performance. Sorry I only know how to work stuff out in imperial figures..."
Reynolds numbers are variable based on temperature and velocity. At 20˚C and 15 mph I get reynolds numbers in the 100,000 range +/- 30,000 depending on chord thickness etc. It is the variability in the low Reynolds numbers that make them tough to calculate, lift and stalling that is.
Apparently at low reynolds numbers the flow is quite forgiving of thick sections but because the energy is low nothing is stable or predictable.....
Talador
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Posted by: taladorwood@yahoo.com.au
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