Understanding Wing Technology

Wing Film

The stuff we have been using since 1995 is DuPont Clysar. This is a packaging shrink film that you find around Cd's and Video cassettes.
It is pretty cheap and readily available off the Internet. We have also used 3M storm window kits when in a pinch. These come with their ow supply of double sided tape, but because the pieces are not all that big you have top count on overlaps at the ribs to cover a big wing. But, you should know that the Stars and Stripes 88 wing was covered with this stuff.
We have never been able to determine that one film was any better than another film. So until we know that, I would say any heat shrink will work.

There are different weights, I suggest you try the lightest one.
SHC

 

Understanding Wing Technology--Moth

Comments from Steve on the twist question for the design of a Moth wing:

I think the case for twist needs examination.
Given the short span of the Moth wing and the elevation within the wind gradient. The boom of a foiling Moth is something like 2 meters in the air and the tip has to be at about 7 meters. So it is a relatively narrow band of the gradient.... (not like the 12-13 meter span of the C Class) and it is elevated above the really slow stuff by being up on foils.
It is hard to typify the boundary layer. We have all see the single paraboloid representation, but this is a radical simplification of reality. Water and air temperature, sea state, and myriad other factors influence the rate of change and thus the velocity at different heights. So predicting the delta in wind speeds from the top to the bottom of a Moth wing will lack a great deal of resolution. Take for example a range of 15-50% velocity difference over the span.
I'm not going to do the study of the apparent wind angles at the suitable wind and boat speeds, but I think someone ought to because it can radically simplify the project.
One might consider a wing with a wider range of angles of attack and dispense with all of the twist complication. This will make it easier to build a wing that is strong light and reliable.

More Moth wing discussion here: http://www.moth-sailing.org/imca/faces/forums/show/7.page

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Wing measurement in the C Class is done with: Sail area to be measured in accordance with the "ISAF Measurement & Calculation of Sail Area Instructions" (Last issued May 1985).

 

UWT-Magnus musings

From Magnus Clarke on SA:

Drag: Actually it has way less drag, all the time, Plain and simple square foot for square foot, a decent wing, Which I am sure if Duncan has anything to do with it, it is, has about twice the power and half the drag of a regular sail and mast set up period. Thats what makes them so nice to use. It probably has the same or less drag than a round section of mast of the same thickness and height (Granted that is pretty thick)

At the mooring, weather vaning is indeed the way to go. No doubt there is engineering challenges making it stand up and work pretty all the time, but weather vaing really does work. I would imagine the tail feather on it keeps it from "luffing" or getting into an oscillating feedback loop, so it could be kept settled down very easily in that configuration.

Reefing: You simply take out the camber in the wing, meaning the angle between the front and back element of the main wing section, make her flat and you de-power considerably then just ease the sheet thingy or in this case re-trim their tail feathers I guess as main sheets are passe in this set up.

I cannot tell you how great it is to sail a boat with a wing sail, only 4 lines and you can have all the control over your power etc that any soft sail mast combo needs twice as many controls to achieve. Twist, camber, leach twist and angle of attack and whamo, you have it all. No traveller, no no vang, no halyards, no worrying about rig tension except to keep the boat from racking all the time, its so much cleaner.

Methinks from looking at this rendering that they have foregone the Cogito / Alpha style internal twisting carbon spar in favour of something with less sophisticated twist control, which would make it potentially lighter and easier to build strong, but its tough to say without seeing it in action.

Suffice to say for a few years we too have been thinking about autonomous vehicles with wings and I myself get concerned mostly about hurricane force winds and how to deal with those. Easiest would be to deploy a sea anchor off the bow and let the thing weather vane to leeward of the anchor and ride it out. With a fat *** platform under the wing like they have here that thing is not in a rush to get upside down unlike a C-class balancing act.


Signed: Wingnut

I cut it twice and it's still too short

 

UWT-Magnus musings

More from Magnus on SA:

Well the wing we use on the C-class in not one element, its 2 or three depending on how you look at it.

I just call them front and back, the front one having a flap on it that is 15% of the chord of the front part and deflects up to about 20 degrees.

Normally sailing upwind you have about 20 degrees of "camber" bewteen the front and back sections, and the little flap(s) is not off center line at all.

Down hill you have a maximum of 40 degrees of camber between front and back and the number 2 flap kicks in about 15-20 degrees held there by the idlers, which are little fingers that hold the trailing edge of the front section.

So uphill, on the leeward side what you see is a nice fair curve with a little slot half way along the wing. Thats the important bit is that the leeward side is nice and fair with a little slot in it.

downhill, it looks the same, just deeper fatter more powerful and draggy section.

So essentially when you tack or gybe the thing flips inside out and goes the other way.

The point here being that by combining two symetric sections, you get a more powerful assymetric section with the option to power up or de-power. so as for donwhill, you have a big fat deep section that is way more powered up than uphill, so thats what makes it work down hill.

As for apparent wind, it should still be forward of the beam to be useful, any further aft and you're sailing like a retard with the wing, she just does not work well when too deep unless its 4 knots of breeze and then you just drift straight down hill with the barn door up there nudging her along (Not as fast as the wind for those on that other thread)

 

This is a really informative thread!

I must admit that when I read of 2 or 3 element wings and camber angles I recall the undercambered wings (i.e., with concave undersurfaces) used on the sailplanes of my model aircraft days many years ago. I did a web check and not much seems to have changed with those, apart from material advancements. The undercamber (concavity) provides greatly increased lift at low speeds, with less drag than merely increasing alpha.

That might be of interest when designing a wingsail for a boat, especially for sailing in light airs. Which leads me to a question.

Is the 2 or 3-element wing a best attempt to emulate an undercambered wing so that it can be configured for different tacks? Or is the slot essential?

The slot may be primarily to reduce separation as stated, or it may be to minimise the imperfect emulation of the cambered wing. I have read in landyacht websites that the lee surface must be kept smooth at all costs.

As an example of this might lead, if the ideal wing turns out to be a "one-tack" undercambered wing with no slot and discontinuuity on the lee or weather surfaces as in a model sailplane wing, for an out-and-out speed attempt a one-piece wing would be the way to go.

 

UWT-A Class wing

Real good info here from Steve Clark, David Smith and Tom Speer:

http://www.boatdesign.net/forums/multihulls/wing-sail-class-cat-5771.html

 

Doug do you have any links to sidtes with foil profile data? I found a good one years ago but cant find it anymore

 

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This should help: http://www.ae.illinois.edu/m-selig/ads/coord_database.html

 

Thanks Doug: interestingly the figure attached to DSmith's post http://www.boatdesign.net/forums/multihulls/wing-sail-class-cat-5771.html#post34031 shows that, for upwind with the slot closed the wing does indeed emulate an undercambered wing [for reduced drag] but for downwind the slot opens [to prevent separation] as the wing is operating at a higher alpha to deliver higher lift in the relatively lower apparent wind. My understanding of the reasons in square brackets.

 

I think you have pretty good grasp of the concepts. What matters is the camber of the chord line. You can have camber and a flat or convex lower surface. It when thickness/chord is low (thin sections) that the wing has "under camber"

If you like to play here is a fun link

You can play with speed, area, aspect ratio, camber, thickness, and angle of attack.

For high speed the goal is high L/D.
For sailing applications you need high CL.

The difference between the zero lift angle of attack and the stall is the maximum apparent wind angle the "sail" can use efficiently.

 

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Thanks for the link, Randy!

 

Hey Doug,
Where abouts on SA are you linking to?


Cheers
Mojo

 

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I'm scavenging wing related posts from all over-mostly-so far- in "multihulls" on SA.
Best thread-"Fred is in so......"

 

Note: The default in the simulator is to model the stall, but the AoA of stall is set at 10 degrees. It took me a while to figure that out. You can turn off the stall model and see ideal flow, but that can be misleading too.

For those interested in sailing fast, it is fun to see what the Maximum AoA can be at different multiples of wind speed. At 3-4 times wind speed the Max AoA is very low and within the limits of this application.

R

 

Thank you. That is an interesting tool: preliminary experiments suggest that undercamber may increase lowspeed lift but it degrades lift/drag ratio; I expected the first but not the second. however, there are too many variables to arrive at any useful conclusions without a lot of experimentation but it is handy for checking out a design. Learning will take time: sigh! It only seems to provide one profile which is a severe limitation.