Discussion in 'Boat Design' started by lunatic, Nov 30, 2008.

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### lunaticSenior Member

#### Attached Files:

• ###### DroopingD_yellow[1].pdf
File size:
324.3 KB
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272
1 person likes this.
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### MikeJohnsSenior Member

Thanks for posting the info, but unfortunately it's not very applicable to either the aero or hydro dynamics of sailboats.
The effects of a compressible fluid (air) are the driving factors for high speed aerofoil section design.

cheers

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### TcubedBoat Designer

It is important to understand how compressible and incompressible fluids relate to each other and are defined.

Air has a much lower modulus of elasticity than water. Therefore, water is traditionally called 'incompressible' and air 'compressible'. Technically, all matter is compressible to some degree or other.

However, water compresses very little (about 6 orders of magnitude higher modulus than air) even under even very high compressions so in the context of what we're talking about here, we can safely say it is indeed incompressible.

Air is certainly compressible, but the amount by which it actually compresses in flow speeds of less than mach 0.25 is so small that it can be safely disregarded.

Hence the compressible effects of air can be ignored whenever the mach number is small enough, say less than 150 (even 200) knots..

So slow enough air can be treated just exactly as if it were water, just with a density of 1.2 kg/M^3 instead of 1025 kg/M^3 - and different viscosities as well of course.

So aeronautics is certainly relevant to sailboats, so long as it is not a fast airplane.

In the second link i read Reynolds 2*10^5 which is not very high so that is certainly relevant.

In your pdf attacment i see the flows are rather too high mach to be relevant.

The idea of drooping leading edges is not new. The engineering of a flexible skin for a drooping leading edge is, at least to me. However (in the second link) i see that the camber line is still fairly sharply kinked - you can see the 'hinge line' is just in front of the box spar. Definitely room for further improvements.

The idea is not a stranger to sailboats either.

Ex 1; Wing masts that are over-rotated when the sail is at high angle of attack (say apparent wind at 60 degrees and higher) to keep flow attached.

Ex 2; Loosening outhauls when freeing the sheets - same general effect.

Ex 3; Rudders with skegs (this particular example does not translate quite so directly, though.)

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### lunaticSenior Member

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