Semi balanced wing sail pivot axis placement

I am getting conflicting information about the center of pressure location for low aspect(reefed) wing sail. Some sources say that center of pressure moves aft as AR is reduced, but then there is talk of the potential for a laminar separation bubble forming in certain scenarios which can move the CP as far forward as 10% at low reynolds numbers. This leads me to believe that I should place my pivot axis at or ahead of 10% chord to avoid problems such as over-balancing, instability.

Thoughts?

 

Hi dustman, I know nothing of fluid dynamics etc, but a comparison look at Saildrones, (except for the square riggers), shows variable pivot positions from 10% to 30% of chord, from what I have seen in photos of different units. Yes they do have that tail section which would make a difference. They weather storms, and are tough, so maybe have got some things right. The further forward your pivot is, the more pressure is on the rotating gear drive section, and all resisting higher shock loads, so needs to be heavier built than if more balanced. What is the shape of your sail, reefed and not; is it tilted in any orientation?

 

My experience is only with rigid wingsails with a tail and for this particular setup pivots at about 20-24% of chord work well. It's a balance of area ratios and moment arms and a little study of homebuilt aircraft design is helpful. The most significant learning experience in trials was the importance of mass balancing.

 

Dustman,
First of all you must know I am not a CFD engineer, so what I am writing below would required some cross checking.
1-With low AR the wing is likely to stall at higher AoA than hight AR, and as the AoA increases, the Centre of pressure is likely to move aft, it can be a reading of your first remark.
2-Regarding Laminar Separation Bubble (LSB), I am not sure it is an issue, as above 6 knts true wind speed, the boundary layer of the air stream above the water is likely to be turbulent and the wing section will not see any laminar flow. To have a LSB probably you need laminar flow, but if there is only turbulent flow , no LSB.
Not sure it will help much
Regards
Erwan

 

Nominally the Cp is at the 1/4 chord. Our wings had the mast step about 15% of the root chord aft of the nose. The pivot axis was not parallel to the leading edge but intersected at about 2/3 height, where the shrouds attached. The consequence of having the pivot point too far forward is trivial compared to having it too far aft.
SHC

 

The balance point is critically dependant on foil section, hinge location (if the section articulates in a flap) and aspect ratio. Low aspect ratio leads to substantial induced drag where high pressure air escapes up over the top of the wing to the low pressure side. This will indeed move the center of net forces aft, but mostly by sacrificing the efficiancy gain you went with a wing foil for in the first place.

You note other folks are posting numbers between 10% and 24%. That is a massive range. If you went with 24% and the actual center was at 20% the rig would show reverse stability and be uncontrollable. If you went with 10% and the net forces were indeed at 25% it would be very difficult to keep the rig from twisting so that the head was unloaded. Then the head it would contribute drag and weight, but not actually make your boat go faster.

Anyway, it's just a thought.