Foiler Design

See the posts from Mark Drela and Alan Smith(#154 and #157) here:
http://www.boatdesign.net/forums/sailboats/new-high-performance-monofoilers-25366-11.html

 

Front foils must be loaded more than rear foils whether it is yaw or pitch.

However this can be circumvented by other methods of stability.

In the case of pitch height adjusting sensors these will overwhelm natural pitch instability up to a point. I have not researched this enough to give you an answer as to where the limit is. For yaw there is a limit as well and that i can not answer precisely either.

In the case of yaw i made an experiment with a windvane directly attached to a bow rudder, with no linkages whatsoever, and this worked very well. This kept the 'balanced' bow rudder pointing at the given angle to the wind and the rest of the boat 'trailed' behind.

There is though a difference between the systems namely angle fixed reference (and this is not at all fixed because it apparent wind), and displacement fixed reference (which is not so fixed either due to waves).

 

tc if you look at DL's link you will find one answer. about 2:1 cl back /front IIRC. Not a limit ofcourse but practical. There is no limit, you can sit behind the rear foil and have the front foil pull down.

 

Where exactly do you see that 2:1 , are you really understanding M.D.'s graph or are you seeing something i'm not?

As you say you could have negative loading on front foil with a surface sensor but in a practical, dynamic situation you would eventually find a limit where it becomes uncontrollable due to inertial/pitch heave coupling oscillations.

 

ok 3:1.
"It seems that at high speeds it is acceptable (stability wise) to move the CofG that far aft that you are sailing with a cL on the rear foil 3 times that of the front foil. However best performance will usually be achieved when the cL's of both foils are equal."
^alans

 

Tom where is that tutorial? I looked on his website but didn't find mention of optimizing two foils in relation to each other.

 

It used to be on the IHS site. However, I don't see it there any more, and he doesn't have the same discussion on the tutorial on his own website. I believe the formulas are the same, however.

His symbology has always been confusing. For example, he says his Froude number is based on chord length, but his drag equation #6 appears to have two different Froude numbers in it. I suspect Frb is the Froude number based on chord and Frz is the Froude number based on the distance between the foils.

The Froude number for wavelengths equal to the distance between the foils is 1/sqrt(2*pi). When Frz in equation 6 equals this value, the wave drag in his formula goes to zero. That would make sense if the following foil was recovering all the wave drag from the first foil. Hence my suspicion that Frz is based on the separation between the foils instead of the chord.

However, I would expect that to happen when the wavelength was twice the distance between the foils, instead of equal to the distance between the foils. The reason is at one wavelength from the first foil, the flow in the wave should be moving down at approximately the same speed as the downwash off the foil. But if the following foil is located at half the wavelength from the first foil, it will be in an area of upwash. Induced drag comes from the lift vector being tilted backwards by the downwash, and if the upwash from the wave tilts the lift vector of the following foil forward, the two drags can cancel out. In practice, it won't work out quite that well. But I think that illustrates the principle.

Note also in many of his formulas that you see the quantity (h_bar/lambda). His lambda is the aspect ratio and h_bar is the depth divided by the chord. If aspect ratio is span/mean chord, that would make (h_bar/lambda)=(depth/span). So while at first it looks like chord is the most important dimension, it turns out span is really what matters.

 

Slightly off topic- in the latest Windsurfer Magazine (April '09), pgs 74-75 (warning; I think there is a possibility that this article might be an April Fools prank) there is pic and article of a foiler by Rush Randle. (Hydro)Foils are all attached on the bottom of the skeg, main wing forward, small tail aft, vertical tail aft of tail foil, pointing down. He has attached an aero wing on the nose of the board, 8'4" span, 24" at the root, 14" at tip, 2.5" thickness as root, 1.5" at tip. Wing In Ground effect. Says it works well, although he claims (twice!) he can go straight upwind and straight downwind, and can use a smaller sail with it.

The article says that there's a vid of it on the mag's website, but I can't find it yet, so you might have to buy the mag.

A bow WIG might be wand replacement device?

Paul

 

Not on his site yet but who knows...
http://rushrandle.com/hydrowindsurfing.htm

 

paul, sounds weird. if you buy it, any chance you can scan it and put here?
Maybe he can go straight upwind by jumping up and down like the trampofoil?

 

I did buy it, but it's copywrited stuff. I'm going to leave a message at Windsurfing Mag, and at the photog's site, and see if they'll release a pic to the net.

Paul

 

Surface Ventilated Foils-(no wand)

I'd sure like to hear from Tom,Mark and Alan about these foils-advantages and disadvantages: heres a link and also see the Tomahawk thread:
http://www.tomahawkfoils.com/
Thanks,guys!
This is what Jonathan Howes the inventor says on the site regarding its application on a windsurfer:



"The foil’s inventor, Jonathan Howes*(dl) explains the concept: “It’s a totally new type of hydrofoil that solves many of the problems inherent in conventional foil designs. The ventilated design has features that give a wide operating speed range without moving parts and exceptional tolerance to rough water, making the windsurfer simpler, more versatile and easier to sail”

The concept uses subtle upper-surface shaping to provide progressive changes in lift force with increasing speed in a similar way to an aircraft’s wing flaps but without any moving parts. This means that the windsurfer lifts on to the foils at a very low speed but automatically corrects for higher speeds, giving smooth performance throughout the speed range.

Ventilated surface running allows the foil to move in and out of waves with minimal impact on foiling performance and gives speed potential without sudden changes in handling due to uncontrolled ventilation or cavitation. Not only does it fully exploit the concept’s forgiving characteristics, but a windsurfer fitted with the hydrofoil still behaves like a normal windsurfer. "
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I'm not positive but this may be the Jon Howes who is behind the Monofoil concept. I've written to ask him...
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Some info on a ventilated foil by Jon Howes here, post105: http://www.boatdesign.net/forums/sailboats/oh-lordy-doug-14706-7.html

 

It is

Simon

 

Tomahawk foil photos

Photos from todays Dinghy Show posted at:
http://www.fishwick.demon.co.uk/howesfoil1.jpg ... howesfoil4.jpg
all (c) S Fishwick
picture 1 is a general view of the boat
2 general view of the foil
3 view of top surface against the light to show the shape
4 section of foil at end.

Cheers
Simon