Johan Bosman, JS Chief Aerodynamicist, explains:
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.
Designing winglets is probably the second biggest
challenge faced by sailplane aerodynamicists. The
basic design variables are:
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
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.