Some strutted designs allow the wings to be folded back while  still on the carriage. This can also be helpful when using a  smaller space for storage by folding the wing up without  taking it off the carriage.  [Figure 3-9]  It is also convenient  for sea trikes since the aircraft does not have to be taken out  of the water to fold up the wing.  Modern sails are designed with complex geometry and sewn  to precision to achieve a highly efficient design. Because of the  flexibility of the wing frame and the modern techniques in sail  design, the leading edge can have a curved shape which adds  to the efficiency and stability of the wing.  [Figure 3-11] Strutted wings have a clean upper surface with no holes  required for the king post or wires to go through the top  of the sail. This reduces interference drag on the top of the  wing. Increasing overall efficiency, no holes in the sail also  eliminates any high pressure leakage from underneath the  wing getting sucked up to the lower pressure on top of the  wing.  [Figure 3-10] The sail is a highly refined design that integrates with its  wing frame. Each sail and wing frame are designed for each  other and are not interchangeable with other sails or wings.  Leading Edge Leading Edge Leading Edge Figure 3-9. A strutted wing folded back so it can fit into a trailer for  storage and easy transport (top). Strutted wing with wings folded  back for easy storage (bottom).  Figure 3-10. Clean upper surface of strutted wing.  Figure 3-11. Curved leading edge sail design.   Sail Components 3-6