I have seen many times on this and other RC forums that contra rotating propellers, in the case of an x8 or y6, are less efficient, considerably so. however, snooping round the internet a bit more, I have found other sources (including wikipedia...) that says that contra-rotating propellers can be more efficient and the major downside in the mechanical complexity.
The only hurdle I see here is the distance between the props (nessecary for the motors and mountings) becoming a problem. Oh and the complexity of design; I'm sure there is more to it than making sure the pitch on the lowermost propeller is accounting for the apparent direction of flow.
So I guess my question is: With a propperly designed system, would an X8/Y6 be more efficient than a standard oct/hex?
Replies
In the case of blades spaced very far apart (in their relative terms) and considering the second one is experiencing the forward velocity effect by the time the air gets there... and considering we dont use everything at full blast
No it is not more efficient in terms of power, but it can be efficient in terms of space, which is why we do it in the first place!!
to start with, the nature of the things we fly, are such that we usually differenciate the speed of top/bottom propeller.
This alone cannot not improve efficiency, as one propeller is pushing against the other, or is provided with less than optimal flow into itself..
The scenario where multiple radial propellers/fans are more efficient (AFAIK) is when two or more fans work together in a ducted fan design with short distance between fans, when next fan operate in opposite direction, it expoits the angle at which air is travelling after the previous fan.