mihnea,
i was just wondering what you had in mind to make that statement
there are a lot of discussions here and elsewhere about coaxial configurations. some will say they are 10% less efficient, some will say the lower props should be larger or smaller in diameter than the upper props. i don't know if anyone has hard evidence to support any of it. empirically speaking, coaxial heli's seem to be less effected by wind and I've found that by using higher p[itch props on the bottoms I get much more stable descents. i haven't studied any of this in a lab, they're just my casual observations during many flights using all sorts of combinations of things.
some have tried making the lowers turn faster and have said it helps a bit. i believe the moving airflow effectively reduces the real pitch of the lower blades going through the air so I run higher pitch props on the bottoms. like i said, i haven't seen substantial improvements in battery useage but i've found my heli's to be more stable as a result.
there are a lot of discussions about this if you do a search. be prepared to sit for a while and to do a lot of reading!
bart
