"Thanks very much for the detailed answer Ed!

I assume mounting props using this set up where the props have a pretty much uninterrupted air flow between them is better than putting one motor and prop on top of the motor arm mount and the other…"

"Hi Sean, sorry for the ignorance - who do you mean when you say "AC" - did you mean 3DR?"

"Hi Ed, which controller did you use that allows gain adjustment mid flight?"

"Very interesting! Did you see a noticeable improvement in performance and/or efficiency when running in contra-rotating mode?

Did you just feed the shaft of the bottom motor through the middle of the top motor?

BTW did you play much with different…"