It is known that winglets can reduce induced drag by reducing the tip vortex. They can also increase aileron efficiency, giving better overall handling characteristics. However an incorrectly designed winglet can absolutely ruin the performance, especially at higher cruise speeds.
Johan Bosman, JS Chief Aerodynamicist, explains: Designing winglets is probably the second biggest challenge faced by sailplane aerodynamicists. The basic design variables are:
- Aspect ratio
- Toe-out angle
- Cant angle
The winglets for the JS1 Revelation were specifically tailored for the wing shape and tip airfoil. KKAERO was used to obtain the optimum induced efficiency factor for all configurations by adjusting toe out angles and sweep angles. Care was taken not to overload the winglet and create load peaks at the tip. For all operational angles of attack, the load on the tip region was kept within limits of the maximum airfoil lift coefficient, for safe handling characteristics.
Due to the polyhedral wing configuration of the JS1 Revelation, the last wing panel is already at an angle of 24 degrees which increase the angle between the winglet and wing. This reduces the detrimental 3D flow effects at the junction corner due to super positioning of adverse pressure gradients which causes separation.
Oil flow tests have also revealed no separation problems in the winglet junction area. At low speeds, the winglets give around 5% performance increase compared to a wing without winglets. The cross-over speed, which is the speed where the presence of winglets becomes detrimental to overall performance, is above 220km/h.