Thick sail 1

[solidsail]

I am designing a rigid sail system for a sailing boat. The boat is a 30ft. long trimaran with an estimated maximum speed of of 18-20 knots, but normally operating in the 8-10 knot range. The sail will be approximately 30ft span and 5ft chord with a rectangular planform. The aerofoil section I am presently using is an Eppler E395 12.5%.
Is this a suitable aerofoil for this purpose, or would something else be more appropriate? Although I have good boating knowledge, my aeronautical experience is limited to building model aircraft, so may be lost if answers get too technical.
Thanks in anticipation.
Solidsail.

 

[DoubleGeek]

The Eppler E395 was designed for low-speed, human-powered flight, so it probably has good lift characteristics in the speed range you're interested in, however, it is a cambered airfoil, so it is designed to always be lifting in the same direction. If you build your sail with camber, you'll be sacrificing a great deal of performance when the sail is lifting in the wrong direction. You'll find that you always want to sail on the same side, which isn't practical. If you use this airfoil but build it without camber, it might perform worse than other standard airfoils that have symmetric cross-sections (like the NACA 0012).

Also, would you consider building some taper in your sail? Remember, a rectangular planform means the lift is roughly equal along the span, maybe even a little less at the base because of wind interference around the boat. So you'll have to build a stronger mast and shroud lines compared to having a tapered sail.

If you've already thought these through, please enlighten me. I'm just a modeler, and former cat-sailer too (never used the BS-AE)!

 

[solidsail]

Hello DoubleGeek,
Thanks for the reply.
I've designed a reversible version of this aerofoil, so it works both ways. It's a bit of a complicated mechanism at the moment, but I'm working on a simpler version.
I'm intending to mount the sail on the existing mast, which was designed for a much bigger conventional sail. I'm more concerned with getting the maximun power from the new wing than reducing the forces.

 

[DoubleGeek]

If you add camber, you're not really looking for 0012 characteristics. With NACA 4-digit airfoils, the first digit expresses the camber in percent chord, the second digit gives the location of the maximum camber point in tenths of chord, and the last two digits are the thickness in percent chord. So, if your cambered airfoil has a maximum camber of 4% of chord located at 40% chord back from the leading edge and is 12% thick, you need to find the charactistics of a 4412 airfoil.

Once you've determined the airfoil you're looking for, search the NACA reports at

http://naca.larc.nasa.gov/

for lift and drag characteristics, as well as pitching moments. There, you should at least find something close enough to extrapolate from.

 

[mklt]

I can't talk about the Eppler airfoil because I don't have my airfoil info in front of me but I would think a simple sail such as 0012 (12% of chord), 0010 (10% of chord) etc. would give you the ability to tack with the same sail effect on either side of your tack plus they (Percentage foils) are easier to make. Good luck with your boat. She sounds like a lot of fun both experimenting and building!

Best of luck,
Mike

 

[Sparweb]

mklt,

How far along have you come with your design?


STF

 

[JohnFortier]

A tapered foil may give you better performance, due to the reduction of tip vortices. Drag is drag, whether the airfoil is mounted horizontally or vertically. It would also mean that you could use a larger overall sail/foil, since the forces acting on the riging would be concentrated toward the lower part of the foil.

Something like a standard bermuda sail shape, but shorter fore and aft, with the advantages of higher performance than a standard Bermuda rig. You should also be able to jibe quite happily, since the sorter boom won't put as much strain on your rigging or tend to knock your brains out!

Have you considered placing the leading edge ahead of the mast and having the mast at the thickest part of the cord? Just an idea, but it would provide a more streamlined setup overall.

Regards

John

 

[mklt]

Quite a bit. I have a mechanical working model of the conformal wing. It's a single point actuated wing that changes 3 sections of the wing at one point. I not sure if you can post pictures on this but if you can I leave a picture I have of it. I'm going to try a model of it on a friends RC airplane. I think it will greatly increase lift and allow for even slower handling speeds. Maybe even a strong wind will pick it up off the ground. WE'll see.

Thanks for asking.
Mike

 

[JohnFortier]

This sounds fascinating. If you could send me some pictures at jfortier@rochester.rr.com, I'd really appreciate it.

John

 

[Sparweb]

I'm curious, too! sparweb@hotmail.com



STF

 

[Sparweb]

Got the picture. Looks like a lever projecting out of the main body, and hidden inside is a rod or cable mechanism that pitches the LE and TE up and down. Clever double cambering on the TE.

If you get a copy of Abbott & Von Doenhoff's Theory of Wing Sections, you might find some relevant data. An airfoil code like XFOIL might help, too. Another book by Domasch, Sherbey, and Connolly, Airplane Aerodynamics, would make the process of finding coefficients easier. TOWS is available at Amazon.com, and AA can be found on used-book sites, such as Alibris.com.



STF

 

[Felix107]

Does your system allow for twist in the sail from head to foot in order to accomodate the increasing wind speeds as you go up the mast? From the forgoing posts it isn't clear if this is the case.

 

[mklt]

Felix,

I allow for the variable wind speed from surface of the water to the top of the mast through tapering the foil (Leading edge constant). It's relatively easy to do because when the foil is adjusted it adjusts in unison throughout the whole (Bottom to top) foil. So, The boundry layer near the surface through the top of the mast the force on the mast should be uniform thus maximizing lift.

regards,
Mike

 

[Rollerblades]

Do not forget to make the calculation as a thin airfoil.