Thoughts on the wings

Some proagress has been made on the wing design.  Dr. Elkaim’s dissertation cites some problems with the NACA 15% section at low Reynold’s numbers as there is a separation bubble formed just aft of the nose on the leeward side of the airfoil.  

As a result he finally decides on a 21% section with a zig-zag trip strip at 22% of the chord to promote turbulent flow aft of the trip strip.  I have decided to go a little simpler by using an elliptical section at the front of the foil to avoid the small nose radius of the NACA sections.  Otherwise the section is as specified by Dr. Elkaim, except it is fractionally smaller.  It is easy to make an ellipse and if it does not work, it is not terribly hard to change the nose section of the wing.

The wing will be based on standard 4x8 sheets, so will have a chord of 4ft and a span of 16ft.

The wing will be constructed as follows:

1. Build the spar and web as a complete unit using 3mm ply for the web and a 16ft length of spruce (left over from mast building) for the spars.  The width of the web is 10 inches and the spar will be made in two parts glued to either side of the web edges (4 lengths in total).  The web at the base of the wing will be thickened to 6mm by either doubling up on the 3mm ply or using 6mm ply.
2. The wing pivot is to be at the quarter chord position.  As the spar/web assembly is at the 21% chord, the wing pivot will be made from a 3.5 inch aluminium tube glued and fixed to the rear of the spar/web assembly.  This will fit and pivot inside a 4 inch aluminium tube glued and fixed into the two foredecks of the proa.  Thus there will be two wings, one at either end of the main hull.  They will be free to rotate about 360 degrees and their angle of incidence will be controlled by the angle of the tail.  There will be a flap at the trailing edge of the wing.  It will have a chord of 13% of the wing chord.  It will be deployed in the opposite direction of the tail.
3. The nose of the wing will be formed by gluing thin bending ply over plywood formers that have been glued to the front of the spar/web assembly.  The bending ply will extend over the spar and will form the start of the aft part of the wing.
4. The tail section will be built the same way.  This shows the layout of the two and their spacing.
5. The control system will de-couple the wing from the hull so as to allow the wing to maintain the same angle of attack regardless of the orientation of the proa hulls.  Dr. Elkaim found that if the wing angle of attack was reversed too early in a tack it would stop his catamaran and start it going backwards.  This is exactly what we want in a proa as it does neither tack nor gybe.  Thus, reversing the angle of attack of the wings will almost automatically bring about a shunt.  We will see what happens in practice.

Once I am back in Washington I will post some more details of the hulls when I get them off the trailer.

The platform is here

I picked up proaskip's 24ft proa hulls from Kevin O'Neill on our way through Texas to the Port Aransas Wooden Boat Festival.  We are on our way back to Washington.  I have read, marked and learned Gabriel Elkaim's dissertation published in 2001 on his wing sailed catamaran.  Good stuff.  Now I am designing the wing(s) that will go onto the hull of P52 - to be renamed.  Once I get back to Coupeville, I will take some photos and post some more stuff.

Thanks Skip and Kevin for making this possible.