sail aerodynamics

[peterraymond]

I have only gotten to page 19 of this thread, but since wings are a current topic I thought I'd jump in. Apologies if I'm repeating recent content.

In message 104 of this thread Tom Speer has three charts dealing with the optimum plan form for a sail. You can always reduce induced drag by making the sail taller, but at some point there is too much heeling moment. Also, the classic elliptical plan form assumes no gap at the base of the sail. Using lifting line theory and with some other assumptions, Tom showed the plan forms that will give you the lowest induced drag for fixed heeling moment and he did this for different gaps at the bottomand different relative luff lengths. Even for the case of no gap, the elliptical plan form is beaten by a taller shape that's more tapered.

If everyone remembers post 104, this is nothing new. But, when I think about how you would design a C-Class wing, I'm not sure if I understand all of the assumptions, or the format isn't quite what seems easiest to me.

A C-Class obviously has fixed sail area and the righting moment you can generate is based on boat and crew weight and that's fixed too. In the design process what can you control? You can change the gap at the foot and you can change the hoist.

If you increase the gap there is more flow under the boom and therefore more induced drag. Also, you lose power at the foot, so you need to sheet in to get back to the same heeling moment.

As you make the sail taller, induced drag goes down while the center of effort goes up. For both reasons you need to sheet out to get back to the same heeling moment.

Tom's graph looks at performance at different gaps at the foot and different luff lengths. What you read off the chart is the induced drag and the height of the center of effort for each optimized solution. I think the relative induced drag on the horizontal axis is exactly what you want, but on the vertical axis, I think it would be nice to see the lift you generate for constant righting moment. It seems like this gets you one step closer to having the numbers you need to figure out boat speed.

However, this is really just the start of the process. In the real world you have restrictions on the wing shapes you can generate. If you twist the wing to minimize induced drag, how do you then twist it to account for different apparent wind angles with height? You can also design this lovely plan form, but if you have a three element wing like Cogito, don't you want the trailing edge of the second element (the flap) and the leading edge of the third element to be straight?

I see how you could have a whole team just designing and modeling the wing.

[Schoonertack]

I don't think Marchaj's results are wrong, I think what we see with the Crab claw sail in particular is the effects of scale.for example why a Dragonfly flies. the air is relatively thicker and more viscous for small scale items than large. So things don't really scale up. Like sail area wetted surface ratios, good when comparing the same rig less so for comparing different rigs. I would like to say thanks to everyone for their inputs. Wind abhors a vacuum.

[peterraymond]

I'm actually caught up now, but it's hard to go through the posts without looking up some of the terms, and as a result I came across some interesting sites.

The first is a paper on a model for sail performance. It's by Krebber-Hochkirch and extends the work of Hazen and Jackson. It looks like it could be used as the sail model in a VP program. Here it is: http://www.futureship.net/downloads/...irchHPYD06.pdf

Also, I wandered to a site out of AU that has a ton of information on foil and hull performance. Here is that: http://www.cyberiad.net/science.htm

I will admit that I haven't spent a ton of time at either site, but they both look good.

[yipster]

coming back on the crabclaw aka deltawing i dont think Marchaj's measurements were wrong eighter Schoonertack and you got a point worth checking. perhaps the research was not complete enough in translating and sure the delta has a low lift aspect and drag in attack angle, but many scientific papers confirm the extra vortex lift measured was about rite, even in so called low speed deltas. its intriquing and frustrating for many as i now read up Bernard Slotboom who investigated the crab claw as low aspect camber sail and belives Marchaj made an error somewere witch can be also possible. Martin Hielkema who raced crab claws mentions in his blog somewhere Marchaj's tunnel tests were at 12knt (i think and will check again) witch i missed in Marchaj's book where not much on speeds was said, have to reread again but there too may be a scaling or so. many way's to solve the mystery but looking at deltawing planes they come in with a high angle of attack still faster as regular wings, i imagine that also the delta sail probably only works well under speed and slow sailing it powers only the low aspect, without enough vortexis developping. guess it also needs a very fast, maybe planing platform than to maybe perform like a "flying" proa. proof it, hm, allready took my sleeping pil and probably forget mentioning more points but wanted to share my thoughts, let me hear yours

edit: not beeing a real mathwizzard i did learn to second check my calculator to avoid making obvious mistakes, now only checking windspeeds i see Martin Hielkema's 12knt but on page 153 of Marchaj's sail airodynamics 15.7knt or 29.3ft/sec compared to 8m/sec hmm.. am i still sleeping or are we all? still beliving in vortex lift i'm checking for example a delta wing kite drag/lift at the i-net here http://www.grc.nasa.gov/WWW/K-12/airplane/kiteprog.html and reading up some more

[Pericles]

Here is a sail that deserves wider recognition. The Vela Latina Canaria. It's tacked like a dipping lug.

http://www.youtube.com/watch?v=8TtcZjAuF4A

[BobBill]

Originally Posted by Pericles

Here is a sail that deserves wider recognition. The Vela Latina Canaria. It's tacked like a dipping lug.

http://www.youtube.com/watch?v=8TtcZjAuF4A

Crazy fun, looks simple. Also on page 12...this thread.

Some of the material regarding wings I have read, mostly on another site,reminds me of chili, pizza and meatloaf recipes ... every one can be a chef ... not to denigrate those efforts and improvements.

However, I think there is definitely going to be a sail plan that will either replace wings or partially replace them and allow boat to be moored etc. With the more durable materials, seems near certainty. Alas, am too old to dabble.

That said, I may mess around with the "humpback" bumps on my flexible spar to see if my catboat pointing will benefit...

Good stuff, good enough.

[BobBill]

Originally Posted by nibbit15

The slot effect is a myth. Don't get confused by all the aerodynamic models people try explain to you - most of them are degree level or beyond. If you want to sail fast then get a fast sailor to show you how to trim sails and if you really want to learn about aerodynamics or CFD then talk to students/graduates/lecturers from a university that does aeronautical engineering. Lastly if you want great advice on sail set up, talk to a competitive sail loft like North or Hyde.

Huw

I think "lee bow" stuff is mostly conjecture, if not myth on a practical level.

The informed and thoughtful comments above notwithstanding, it may have been mentioned or implied, but when I learned to sail - when boats were on buoys in Montrose Harbor in Chicago - I learned one rule I still use today for sails, trim and all around generally:


"When it looks good, it is good; when it looks ugly, make it look good."

[tspeer]

Originally Posted by peterraymond

...Tom's graph looks at performance at different gaps at the foot and different luff lengths. What you read off the chart is the induced drag and the height of the center of effort for each optimized solution. I think the relative induced drag on the horizontal axis is exactly what you want, but on the vertical axis, I think it would be nice to see the lift you generate for constant righting moment. ....

That is basically what the vertical axis is. The height of the center of effort is the heeling moment divided by the lift. The lower it is, the more lift for the same heeling moment. Plotting the lift for a given heeling moment would go to infinity as the height of the center of effort went to zero. It's cleaner to avoid plotting something that has a singularity.

[BobBill]

The info above on Humpback fin bumps was intriguing. I did some minor research on the matter. Basically, what I found was not much and that the bumps assist the Humpback's manoeuvrings and not speed. The whale's flukes are smooth as they are on most fish tails and other sea mammal flukes.

Now, seems the idea might have some application to rudder foils and rotating masts...including aircraft.

Or, am I simply out of my league, as usual?

[daiquiri]

Originally Posted by BobBill

The info above on Humpback fin bumps was intriguing. I did some minor research on the matter. Basically, what I found was not much and that the bumps assist the Humpback's manoeuvrings and not speed.
...
Now, seems the idea might have some application to rudder foils and rotating masts...including aircraft.

Following your post I did a research on "Humpback fin bumps", imagining that some kind of revolution might have been going on in aerodynamics in these years. What I got was tons of pages with bombastic articles like this one: http://www.gizmag.com/bumpy-whale-fi...dynamics/9020/

Now I am really starting to believe that industries might have a serious problem with communication.

Leading-edge vortex generators have been used on airplane wings for decades, with the same purpose and results as those claimed in the said articles. And are much cheaper than complex shapes like humpback fin bumps.

[BobBill]

Daiquiri, Great stuff. I missed that one. Can see all sorts of possibilities. Kind of stuff, besides sex and sailing fast cats, that makes one want to be younger...

In fact, I am making a new dinghy rudder foil (blade) and may add "turbercles" to see if they will increase efficiency...

May see these things showing up on wings and making soft sails nearly as efficient at wings, without having to remove for mooring. Whoa, Steve Clark...

Thanks.

[Pericles]

Great minds think alike.

http://diy-solar-and-wind-energy.co....-cutting-edge/

[Guillermo]

Humpbacks bumps have been discussed already quite thoroughly in these forums:

http://www.boatdesign.net/forums/sea...archid=2080327

Cheers.

[BobBill]

Is dead, Guillermo.

[Guillermo]

Ooops!
Well, just search the forums for "humpback bumps"
Cheers.