Originally Posted by Basingstoke Roy
Cant help thinking that an contra with a gearbox is more complicated than needed.
There was a gentleman that used a fluid driven contra-prop on his f2b plane about 20 years ago. The second props sole purpose was to straighten the airflow over the airframe. The idea worked but, apparently, the extra effort in the form of a special machined hub, free spinning, left-hand prop wasn't worth the trouble. Other things that must be considered are the extra weight and power it extracted from the driven prop/drive train.
Driving the rear prop with thrust from the forward prop is a much simpler system, but it fell short. Kimmo's solution may seem
more complicated than necessary but his approach, execution and operation are brilliant.