After watching several ornithopter videos and learning that using adequate servos it is possible to omit the construction of a gearbox I finally decided to once and for all make my own model.
The Delta Sail-Shape
Observing ravens soaring in strong wind, I noticed that the pinion feathers were usually collapsed, stabilizing and protecting the wing (because they wouldn’t need the extra lift from full spread pinions), hence I supposed that a delta shape was adequate for simplifying the building of a wing and probably improving the weight/stability ratio.
Therefore I googled images of several sail shapes:
Finally I came up with this layout: