Wing "Sail/Foil" Question

I do not want to spark arguments about this topic-wings versus soft, battens etc, and not instigate "jib" sail tete-a-tetes...but I do have a question...and I did try to find the topic, but "curved" length versus surface area is difficult to separate...


Wind surf sails are curved along their leading edge-meaning top to bottom-and fast, the light hull etc notwithstanding, but

Why are say C-cat and AC boats not sporting a curved or curve-able wing, leading edge-wize? I generally understand about mast bend (rake) flattening sail and opening leech, etc.


Is a vertically curved leading edge wing too hard to make, monitor underway and maintain, or due to aerodynamic principles?


Keep it simple, I are dim...

 

BTW, obviously, links to topic by Steve Clark, Par and so on are fine.

 

Mast bend (what you describe as a curved leading edge), in sailboards or other sailing rigs, is a necessary function of tensioning the shaped sailcloth panels to adjust the built in "draft" or camber of the sail for different conditions.

A wing sail does not need or use mast bend because the camber of the sail is either fixed by internal structure or adjusted by other means internal to the hollow protion of sail/mast. In a wing mast, the fabric/plastic covering is just to provide the aerodynamic "skin" on the wing and tensioning it has little effect on the foil shape.

 

Gooday bloke. Hey - 'je' - with all due respect - your 2nd paragraph is not correct. A 'wing-mast'/'wing-sail' does need to bend - twist off & in fact - should also be capable of changing section automatically - while sailing - as the wind strength changes. Further more "In a wing mast, the fabric/plastic, etc etc" - - the tensioning of the skin has a whole lot to do with the overall efficiency of wing-masts. IMHO. It would be interesting to know just hsow many wing-mast you have built & raced in how many classes of sailing boats???? Y'all have a great day. Ciao, james

 

I know, I know, but I tried to keep it simple.

As far as the OP question is concerned you don't need to bend the mast fore and aft and you don't need to tension the skin to change camber on a wing sail.

You are correct in that you do need to twist off the AoA and that the skin is tensioned over the ribs to give camber shape, but that is not what the OP was asking about.

 

There are a few basic aerodynamic design objectives for a wing sail:
- create sufficient lift to reach maximum heeling moment (hull flying)
- minimize induced drag for the heeling moment
- minimize profile drag

Downwind, the apparent wind is less, so achieving maximum lift becomes important, especially for boats like the C class cats that are under-canvassed. Reaching maximum lift depends on the section design (slotted flaps, etc.) and having a planform shape that results in each section being evenly loaded.

Minimizing the induced drag for the heeling moment means the sideways velocity imparted to the wake should taper linearly from the foot to the head. This is achieved through planform shape and twist of the flaps to get the right spanwise loading. (Which, BTW, is not elliptical.)

Minimizing the profile drag is done through the same planform shaping that results in the sections being evenly loaded, so there's no extra wetted area - all the wing is pulling its weight. Profile drag is also minimized by the section shape.

Achieving these objectives does not require that the planform be curved. There is little difference in the wake, and thus the induced drag or maximum lift, between a curved planform and a straight tapered planform, especially a double-tapered planform with controllable twist.

It's especially difficult to make a leading edge that is accurately shaped and has a compound curvature. Most of the profile drag is generated at the leading edge because that is where the velocities are highest. Since skin friction scales with the square of the velocity, the high speeds at the leading edge create a disproportionate amount of profile drag. So it's important that the leading edge be smooth and shaped so as to minimize the formation of leading edge pressure peaks. It's much easier to do that shaping when the leading edge is developable. The same thing goes for shaping the core that goes into the sandwich construction there.

The current thinking in sailplane planforms is to have a straight trailing edge and put all the taper into the leading edge. The reason is at the leading edge, the boundary layer is thin and energetic, and can tolerate any spanwise flow due to the taper. But at the trailing edge, it is subject to a destabilizing adverse pressure gradient and more sensitive to spanwise flow that could cause an area of local separation to spread to neighboring areas. So it's better to have a straight trailing edge to minimize any spanwise flow there.

It's not true, however, that existing wingsails don't have curved planforms. USA 17, for example,

definitely had a curved planform. The wing spar was straight so as to be able to take the compression loads with minimum weight. And since the spar was well forward, there wasn't much curvature to the leading edge planform and most of the planform shaping was applied to the trailing edge. Note that the maximum chord was not at the foot, but well up the span - in keeping with making the area distribution follow the load distribution. Note, that the hinge line of the flap is also curved, which is one of the reasons the flap had to be subdivided into 9 segments. So there was plenty of curvature to the planform of that wingsail, even though it wasn't applied very much to the leading edge.