Foiler Design

[sigurd]

Originally Posted by FarmerColin

Sigurd - I think it is quite reasonable to have some loss in performance at low lift coefficients for sections with blunt trailing edges. For the section to produce lift it must establish a bound vortex around the section and to do this it must leave behind a starting vortex. An attempted flow around the sharp trailing edge (which requires a high acceleration in the flow at the trailing edge) promotes the shedding of the starting vortex. Indeed it is one of the boundary conditions in the theoretical treatment of sections that the flow around the trailing edge is zero. Without the sharp trailing edge it is possible to have flow around the trailing edge which will inhibit the establishment of the bound vortex resulting in little or no lift.

Right, and the problem on the proa was at negative incidece of a cambered foil IIRC.

Hm. What do you think about a jet from a channel along the trailing edge as a substitution for the sharp TE. it could most easily be fed from a symmetrically placed intake (which would be the jet nozzle on the other tack) on the leading edge.

Sharp edges, and boundary layer suction to reattach and make it less critical, is another idea.

[sigurd]

"I don't see that angling the pivot axis helps much with regard to interaction between the daggerboard and rudder wakes. Skewing them laterally may help on one tack but may bring the rudder right into the wake of the board on the other tack."

just tack the centerboard if it is an issue.

[Paul Scott]

Originally Posted by Tinman85

On the topic of foil design the majority of current sail boat foil designs are variations of straight wing design. The use of swept wings has been around the aircraft industry for some time now. I know some of the advantages of swept wings in aircraft come at high speeds where air starts to become compressable.
-Do some of the advantages of either forward swept wings or backward sept wings transfer to water bound hydrofoils?
-Has anyone else been looking into swept wing designs for hydrofoils?

http://www.youtube.com/watch?v=1oyWMusaDTI

Swept-ish. But watch the end of the video. Instructive.

It would be interesting to see a windsurfer with a high aspect Moth type main foil. I wonder if it could jump and re-enter as well as the delta foil?

[Tcubed]

I would like to know what was the thinking behind that sweepback, apart from catching less weeds etc.

Sweep like that is not efficient.

I'm referring to the image in post #690 btw.

[bistros]

Originally Posted by Paul Scott

http://www.youtube.com/watch?v=1oyWMusaDTI

Swept-ish. But watch the end of the video. Instructive.

Why do you insist on bringing tawdry facts to a wild conjecture fight? Have you no shame?

Obviously the standard sailboard was in a strong local gust that had not made it to the foiler. If the gust had reached the foiler, it's obvious superiority would have left the "sea-hugger" in the dust. Also, if the incompetent person sailing the foiling board knew what they were doing, they would have been able to jump and re-enter several times, eliminating the remaining nasty hydrodynamic drag and going ever faster yet.

You naysayers will soon be proven wrong soon, when Doug's new foiler comes out of the closet this spring. Jumping the foiler is going to be the next quantum leap in performance and fun. You are all going to have to use Botox to remove the perma-grins from your face when Doug posts the video on Youtube.

[Tcubed]

Some pretty gnarly ones there ..

Hate to think of wiping out and getting into a little underwater 'encounter' with your own board, all bristling wit blades.


Anyone how you's supposed to embed utube vid in here?

Well here's the link since the video doesn't seem to want to embed

http://www.youtube.com/watch?v=g3AV1LJwQus

[sigurd]

Originally Posted by bennn

I'm considering building a twin tip hydroifoil that could be used in kiteboarding. that means it needs to be efficient in both direction (like a snowboard) which is pretty new for a profile....
Any idea on what profile could be appropriate ?
flat bottom ?

Bennn,
Directional foil boards seem to be successful. Nevertheless I would also like to try a twintip foil board.
I'd first try Tom Speer's P30110 (a little camber) or P30010 (twice symmetrical) for the main lifting surface(s), since the more cambered sections may not behave properly at low AoA. Strut(s) will be P30010.

The main constipation is in the pitch stability. I have sketched some alternatives. I currently favor nr 1 and 4.
Nr 1: Likely the front strut will be raked forward a bit, to stop ventilation on its way down to the most loaded foil. Maybe the front foil will also have forward sweep, for the same reason, but if it is not stiff enough this will cause instability since more lift will cause the foil to twist and bend and increase tip AoA. All the foils I can remember have been either unswept or swept back, so for that reason I think I will sweep the front foil back a tad. If I make an inverted T instead of L or inv. Y I can make it oblique instead, which looks cool, but there are twice as many tips to hurt me as an L. Sweeping will delay ventilation and cavitation.
The front foil has a smidgeon more AoA than the aft, to be able to load it more, but not so much as to force me out of the water.

I am curious as to how you had thought would be best, and what anybody else might think.

EDIT: Obviously I will make the foil(s) too long before I cut them down to a manageable AR. And make the tips soft or crushable, somehow (how?).

[sigurd]

I forgot this one with the pivoting tail.

[mark_m]

@doug:I think I've posted this before but:


This was my prototype design based on a Rich Miller concept.

The first impression of sailing with it are on my site, but basically the long foils provided a lot of damping in roll and they took a lot of ankle stress to contol them.

Foilboarding is more about control than efficiency at the moment, as even the inefficient, almost flat plate, initial designs that Neil Pryde were using are way more efficient than a regular board. They just have much less control so you can't use all the available power.

[Tinman85]

Originally Posted by Tcubed

I would like to know what was the thinking behind that sweepback, apart from catching less weeds etc.

Sweep like that is not efficient.

I'm referring to the image in post #690 btw.

Swept foils either forwards or backwards help with reducing drag at higher speeds increasing the top speed. This is the the reason why aircraft have swept wing aerfoils.

A draw back of swept back foils is the wingtip vortexs they produce which is the reason why winglets (little verticle fins on the wing tips) were created for some aircraft. Adding winglets to hydrofoils could increase lift by increasing the effective foil area.

It is forward swept foils that have interested me because they do not have the problem of wing tip vortices if designed correctly as span wise flow along foil is to the centre where the body of aircraft (or in our case the centreboard part of foil) acts like a winglet, but does not incur the additional drag penalties because it is already there.

[Tinman85]

Originally Posted by Doug Lord

Darryl Barrett, the designer of the Aussie Alpha Omega 14 Cat, has. Here is a rudder foil picture and he is using the same sweep on the main foils of a new foiler he is developing:

That is interesting that someone is experimenting with swept foils. Is there any one else playing around with this idea as I am trying to calculate what sweep angle is best but can not find much to help to do this.

I refer to my post 701 about the some ideas around swept foils. I researched and found one of the reasons forward swept wings are uncommon is that they had difficulty making the strong/light enough and when they did the extra structures within the wing reduced internal payload for fuel, etc. Where as swept back foils twisted less and so were easier to make strong enough. However hydrofoils do not have this problem as they do not need to contain internal fuel tanks like there airbourne counterparts.

[Tcubed]

<<<<<<<<Swept foils either forwards or backwards help with reducing drag at higher speeds increasing the top speed.>>>>>>>>

You will find that all aircraft with swept wings have that for mach # reasons which is completely irrelevant in the case of a foil boat.

<<<<<<<<<<<winglets (little verticle fins on the wing tips) were created for some aircraft. Adding winglets to hydrofoils could increase lift by increasing the effective foil area.>>>>>>>

These winglets inhibit tip vortices yes but come at a price. If the area of the winglet were to simply be used to extend and augment the aspect ratio of said lifting foil you get a pretty similar result. Winglets are not a magic solution, otherwise all planes would have them.

<<<<<<<<It is forward swept foils that have interested me because they do not have the problem of wing tip vortices if designed correctly as span wise flow along foil is to the centre where the body of aircraft (or in our case the centreboard part of foil) acts like a winglet, but does not incur the additional drag penalties because it is already there.>>>>>>>

This is incorrect.
You cannot cheat induced drag by simply sweeping the loaded foils forward.
In general, for the kind of flow regimes we're disscusing, any devition from foils at right angles to the flow increases induced drag.

I am talking about incompressible attached non ventilating non cavitating flows at M << 1.

[Chris Ostlind]

Originally Posted by mark_m

... They just have much less control so you can't use all the available power.

So, what's you take up till now, Mark? Are the foils a plus or a minus to date and will you use them in the future what with all the fiddle factor and lack of shallow water launching that is enjoyed by all other sailboard setups?

ChrisO

[Chris Ostlind]

Originally Posted by Tinman85

... A draw back of swept back foils is the wingtip vortexs they produce which is the reason why winglets (little verticle fins on the wing tips) were created for some aircraft. Adding winglets to hydrofoils could increase lift by increasing the effective foil area...

A guy with no less than the credentials of Tom Speer, has already, many times, commented that increasing span is virtually always a better way to get additional lift than is to add drag inducing winglets to any foiled surface.

The commercial jets you see with the winglets are invariably working against an overall wingspan limitation at the airports where they operate.

Yeah, I suppose they look really cool, but that is not the best way to take the problem to a proper solution.