sail aerodynamics

Discussion in 'Hydrodynamics and Aerodynamics' started by Guest, Mar 21, 2002.

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Remmlingerengineer

Then I try it another way:
Only a viscous fluid will separate smoothly at the trailing edge.
The Kutta-condition is a kinematic description of this phenomenon.
Inviscid potential flow can describe the flow-field around a wing, if the (kinematic) Kutta-condition at the trailing edge is additionally enforced.
Enforcing this Kutta-condition will only be possible if a circulation is created by the wing.
In this sense an inviscid flow-field can explain the lift, if an additional "tiny bit" of viscosity at the trailing edge is added.
Could you agree to that?

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daiquiriEngineering and Design

Or rather - if an observed effect of viscosity at the trailing edge is mathematically modeled and made congruent with the airfoil geometry.

Sorry, it's 6 pm here - 10 minutes of pedantry time for me.

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tspeerSenior Member

I would say there is not a unique solution to the mathematics if you assume inviscid physics. You can add any amount of circulation and still get a valid solution. Since you can get any amount of lift you want to name, the question then becomes how to choose a particular solution. The Kutta condition is a way of choosing a solution that is close to what you'd get if you include viscosity in the physics.

Setting the level of circulation is only one way of getting an inviscid solution that simulates the viscous lift. In Theory of Wing Sections, they describe a method for modifying the lift curve slope, too, so as to fit the inviscid lift coefficient to match viscous test results (attached). The resulting pressure distribution does a better job of matching the measured pressure data.

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Leo LazauskasSenior Member

You're correct on that score, Tom.
In the (infinitely-thin) lifting-surface program I posted several
years ago, the Kutta condition is, essentially, just a constant of
integration (at each spanwise location) that determines a unique
solution of the linearized lifting surface integral equation, or in
2D, the classic airfoil equation.

Tuck found that by assuming that the vorticity drops off at a square
root rate as the trailing edge is approached, he could improve
the accuracy and speed of the method by building that rate of fall-off
into the influence matrix. IIRC, a nice feature of Lan's variation of
the Vortex Lattice Method is that the Kutta condition is satisfied
automatically, at least for wings with straight LE and TE.
With curved LE and/or TE, it's not quite as simple, especially when
very close to the wingtips.

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Remmlingerengineer

I did not want to start an academic discussion on the inviscid simulation of lift. I just thought that if Doug McLean says he feels "Schadenfreude" if he catches a wrong explanation, he himself should be extra careful in his wording.
His statement that lift can be explained without the effects of viscosity might be misleading. Therefore I mentioned that in the real world lift can not be generated without the presence of viscosity. To design a mathematical model for this flow is another story.
Uli

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SukiSoloSenior Member

I've really enjoyed this particular discussion and thank all the contributors. Of special interest is the kind of 'open' thinking which Doug McLean makes us think a little more deeply about. We have mathematical models which work pretty well but it seems we still have areas not fully understood.

It's so refreshing to stay open to new thinking and discoveries. Thanks to all.

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Paul ScottSenior Member

Jib boom

So I installed a Hoyt jib boom on Amati, and it works GREAT, especially downwind, but it puts the foot of the jib, um, kind of high, which seems ok for a catamaran, so I'd like to put a hunk of sailcloth under the boom to seal things off, since the Dashews & Frank Cammas etc etc have had good luck with that sort of thing. The problem is our lifelines get in the way off the wind under the boom. Which begs the question- how much deck dragging is enough? I'll find some pics.

If there's another thread for this, let me know? Here's a pic of the boom to give a general idea

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Paul ScottSenior Member

Here's the rest of the boat. Funny, but the boomed jib is close in performance to the chicken chute

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daiquiriEngineering and Design

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brian eilandSenior Member

Have Wingsails Gone Mainstream

This subject came up over on another subject thread, and I thought it might be more appropriate here. Sorry to have injected it here so suddenly, but if I don't do some of these things most abruptly, I tend to forget them.

http://www.sailmagazine.com/diy/sails/have-wingsails-gone-mainstream/
Check out the diagrams at the bottom of the article, "F = the combined effect of drag and lift vector forces"

There have been several replies to that posting on that subject thread, and they can be read here:
1) by sharpii2, http://www.boatdesign.net/forums/sailboats/maltese-falcon-hit-miss-12459-22.html#post690013

2) by CT249, http://www.boatdesign.net/forums/sailboats/maltese-falcon-hit-miss-12459-22.html#post758272

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Paul ScottSenior Member

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brian eilandSenior Member

Semi-Rigid Wing (SRW) sails

Has anyone seen this?

...the blog http://www.k8sports.com/blog
etc....

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sharpii2Senior Member

Looking at this system, it appears that the boomlets...er, I should say "riblets" have to flip over with each tack, sot the camber of the sail is always to leeward. It also appears that these riblets are attaced to leach spar as well as the mast.

My guess is there is some mechanism in the mast that not only flips these Riblets, but also controls the camber of the sail the degree they are cranked over.

Either that, or they are simply rigid and only have their shape for when the sail cloth is on their side.

With a leech spar, an outhaul, at the end of the Boom, would not be necessary.

From their discriptions, it appears the boom is held down as well as sheeted to a horse which is well inboard of the end of it. Not a bad plan. It's been done before with 12 meters.

Reefing the main might prove interesting, but it does appear that it is designed to be reefed. My guess is that the Leech Spar comes off in sections, in order to facilitate collapsing the sail panel in front of it.

All in all, it is an clever, if not complicated system, which allows a very wing like, high Aspect Ratio main to be set with minimum stress on the sail cloth.

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CT249Senior Member

I can find any race results for the boat, which is odd since there is a Sportsboat class in its area.

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Ben GJunior Member

I'm not convinced it's of much benefit.. I tried a pocket sail on my skiff. While admittedly rough compared to this, it wasn't bad.. but produced no noticeable benefit. The main thing to notice was the pocket filling with water making the boat hard to right.

By far the biggest variable was basic rig dynamics - mast stiffness, leech twist profile, luff round, ability to adjust sail power to match conditions.
Without getting these right, the pocket was nothing.
I've replaced with a normal sail, albeit a nice light one, with carbon battens, and stiffened the mast with a carbon track. feels better already.

I bet the thing they notice most is an extra 20-30 kg of sail flopping about up top.

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