I’ve been continuing my experiements on the jiblets.
Leading edge angle
The first issue was the approach angle of the leading edge. The symmetrical aerofoil has a leading edge angle of 90º. This means air will not flow round the leading edge of a sail with this shape.
The larger the angle the more forward thrust we’ll get from the sail but the lower the angle we can go into wind (larger tacking angle). Conversely a smaller leading edge angle will improve the angle we can go into wind but will reduce thrust from the sail. Ideally we’d optimise the angle for Velocity Made Good (VMG) given the performance and drag of the boat, but we just don’t have enough information so a reasonable guess will have to do.
The hard-sheeted jib angle on my hull (Wanderer dinghy) is approx 12º. To allow air to get past the mast, plus allow for the extra drag of a junk rig I’m assuming that the jiblet angle of incidence will be around 15º. This means that the maximum leading edge angle should be around 30º (45º – 15º = 30º).
Playing on airfoiltools.com I came up with this shape that seems to look about right – and something like a sail.
I plugged the aerofoil into my spreadsheet and made a test panel, then cut it around to try the effect of various shape changes. The objective is a jiblet that takes the right shape yet is simple to make.
The conclusions (a bit tentative but will have to do for now):
- The leading edge is constrained by the amount of material between the front edge of the panel and the maximum curve, plus the (stretchy) diagonal of the fabric.
- Extra material (1% of panel width top-to-bottom) must be added to the top & bottom edges of the front 30% of the panel – approximately up to the point of maximum camber. The panel is constrained by the leading edge as well as the top & bottom edges here, so additional material at the top & bottom edges allows the shape to form correctly relieving the need for stretch on the diagonal.
- Adding extra material to the front edge helps the 3D shape form but also tends to produce a step at the leading edge. As long as the extra amount of material is small this is probably an acceptible compromise – I’ll aim for around 5mm in the centre of the front edge.
- Adding broadseam to the leading edge helps (you can see the taped up cuts in the fabric) but probably isn’t worth the hassle. The amount of broadseam would be around 1mm.
I think (tentative but hopeful) that with a slight (5mm) curve to the front edge and extra (1%) material on top and bottom edges forward of maximum camber I’ll end up with a sail that has the correct shape into wind. Once off the wind a bit the force of the wind will stretch the fabric on the diagonals giving a better shape to the luff. At this stage I have to say good enough and see if it works…