sail aerodynamics

Clever Leo, thank you. Anytime we can draw parallels from nature, there's pretty solid backing to the design.

 

It's not so simple.
First of all, raking the tip on a swept wing while keeping the spanwise projected loading the same increases the magnitude of the root bending moment. The reason is that this moves the raked portion farther from the wing root. Note: To calculate the structural moments at any location (e.g. the wing root) due to a load at some distance, what counts is the load's distance projected onto the spanwise spar axis at the bending-moment location. This spar axis is swept on a swept wing, and includes a streamwise component. The lateral distance as seen from the front is only part of it.

But what really counts on a transport airplane is the bending moments at the critical max-G load case, which is set by regulations. The rake adds static washout twist into the outer wing under excess G's. This washout in turn moves the load distribution inward and thus momentarily reduces bending moments over the inner wing where most of the structural mass is. the passive washout effect allows the wing to be made lighter.

Clearly, using a raked tip on a wingsail because "the 787 has it so it must be good" is a bit naive. Any benefits are way more complicated than that.
The closest wingsail analogue might be a tip which passively unloads the upper sail when it's hit by a gust. In strong gusty conditions this would allow you to load up the sail more for a given capsize risk.

PS.
On tuna tails and swallow wings, the crescent tip provides some twist over the span during a power stroke, which is necessary to keep the local Cl's from getting too excessive near the tip. A similar effect happens on raptor wings, but here the individual tip feathers twist.

 

On sailboats, this has long been achieved by bendy topmasts on a fractional rig.

 

Thanks for the clarification, Mark.
I had a cursory look into the aerodynamics a long time ago when van Dam and his colleagues made some rather strong claims concerning (rigid) planar lunate wings. IIRC, the claims weren't supported by experiments and their claim of "optimality" was never very convincing. I'm not sure if Ilan Kroo made any progress with them after that.

Do you know if there are any animals that have endplate-like structures (i.e. ones extending above and below) the ends of the wings or other lifting surfaces?

All the best,
Leo.

 

wingtips-birds and others

This paper is interesting: http://www-scf.usc.edu/~tchklovs/Proposal.htm

 

There are also several references to curved planforms and wing tip studies in the biblio at:
http://www.cyberiad.net/wingbib.htm

 

Chris White's MastFoil Rig

What do you suppose Chris White has in mind with the aerodynamics of this new rig??

http://www.chriswhitedesigns.com/atlantic_cats/mastfoil/index.shtml

 

It's too early to tell, for me. a roller furl jib and a roller furl mizzen staysail, but there's not enough detail to understand the subtleties.

 

Looks like a staysail ketch with a balanced aft staysail and a mizzen on the aft mast.

I suppose he's looking for lower Center of Area (CA), over a more traditional cutter or mast aft rig.

The balanced aft staysail is interesting. It doesn't need quite as much back stay tension to stand well (I know. I had one on my first boat as the jib) It also has its luff slightly to the weather of the jib.

Another potential advantage is the mast for the large forward jib can have a better back stay angle than a mast aft rig and possibly have a tighter fore stay and stand better.

This rig eliminates one of the worst problems of the mast aft rig: there is no place to hang a sail aft the mast and the mast has to be somewhat forward of the transom to get any kind of effective back stay angle.

Oops. Looks like the aft stay sail is not balanced but just club footed.

 

Added my comments in between the quotes above.
Brian

 

Articulated Foils on MastFoil

Back to Chris White's new rig, the subject of this lastest posting. Did all of you take notice that his 'mastfoils' were two element foils themselves??
"Why does the foil have an articulating trailing edge flap?
Because a flap adds lots of power to the foil with very little additional weight and complication. Reaching and running the flap is set at a significant angle (approx 40 degrees) to the main foil which increases the overall camber of the foil and can nearly double its power. Sailing upwind only a small amount of flap angle is used but it helps create additional lift with very little drag"

This is an important and distinguishing feature of his design. This imparts a much greater contribution to their effectiveness, both alone, and in support of that headsail located in front of each MastFoil.

This unique design deserves more analysis before being tossed to the trash pile by the 'traditionalist'.

 

The mastfoils not having spreaders, even on a beamy cat, limit the upper mastfoil area. It'd be nice to see the details up there. And the mastfoil doesn't rotate fully, so I'm puzzled how it can feather, if for example, boat is tied dockside and wind is off a stern quarter or fully astern. And I purely can't fathom that Chris White penned the optimistic lines that would have the cat sailing sideways off the dock with an on-dock breeze, by setting each mastfoil to lift to windward.

 

In my last comment, I was trying to say that the mast on a mast aft rig must be some distance from the very aft end of the boat for the back stays to have enough leverage to resist the head stay tension, even if the back stay only goes halfway up the mast.

And, if it does go only halfway up the mast, an upper back stay is required, along with a spreader, to hold the top of the mast back.

Once you have all that rigging aft the mast, you can not hang any kind of a sail there. So, to get adequate sail area you have to go higher.

I am not knocking the aft mast rig here. It is one of Phil Bolger's favorite rigs, especially for ocean going boats. All I'm saying is Mr. White's rig allows a mizzen sail hung from the mizzen mast, where the mast aft rig doesn't.

I might add here that stay sail schooners have been around for a long time. This is the first stay sail ketch I have ever heard of. Having the shorter mast aft allows better back stay geometry for both masts.

As for the wing masts, I have no idea what he is trying to do. Perhaps he is trying to cut down mast drag. I think they are a bad idea on a boat that can flip. Just my opinion.

Anything that is intricate and complex is bound to fail in the worst conditions. If it fails to feather in a storm, the boat and crew are in great jeopardy. I would go with a round mast section and a tear drop section mast sock. It's not as good as a real airfoil section, but it's a lot better than a plain round or oval mast section. And it can be pulled down, if it misbehaves.

 

Can the masts become assymetrical foils that might develop lift at 0 or negative aoa?

http://www.worldofkrauss.com/foils/784

I'll see if I can find a more specific example, but generally...

Paul

 

Some foils do provide lift at negative AOA.

This is not really anything special. Each profile has an angle of zero lift. More AOA gives positive lift. Less AOA gives negative lift.

Your example was an apparent attempt to use a highly asymmetric foil to approximate a symmetrical foil designed to operate with a flap extended well off of centerline.

This points out a fundamental gap where the most available technology on wings does not fit our desired use.

Most airplanes use flaps only to generate extra lift at takeoff / landing where they can tolerate much higher drag. They then reposition the flap to "restore" the design profile - a lower lift but high efficiency profile for cruising.

For boating, wing flaps are being used to address additional needs. One major need is a just the power and efficiency of a cambered wing that can be reversed during tacking. The other is adjustable camber to provide for the different needs of upwind vs downwind sailing. For upwind we want efficiency (max lift vs drag). For downwind, it is more of a need for max lift (similar to an aircraft at take off). The other major consideration is twist. This is where we are dealing with different apparent wind angles from near the surface up to the top.

Given that the vast majority of wing studies and wing publications are for airplane applications, it is not surprising that sailors do not find what we really need.

For the Chris White boat, his web page is a bit confusing but it would seem he is using of a inner (probably round) structural element (mast / spar) with an exterior that provides a wing with flap. The round spar as a main structural element has been used quite a bit and allows for easy construction of a very strong element.

I like the basic concept where you want to skip the traditional main and use large roller furling Jib/Genoa as your soft sail surfaces and wing sheathed mast(s) to provide small but efficient rear driving surface(s). However, I am not sure I like his choices for and/or placement of the components. Then again it is easy to sit back and dream stuff up. He is to be commended for pushing to actually get the concept on the water.