I was just wondering about the 'two-propeller stack' in the
attachment...I understand how more blades help absorb
HP...but would the configuration shown in the attachment
be:  efficient?  noisy?  Otherwise desirable?  Thanks.

Gene Kahn

I don't have a clue :) I do know the fewer the blades the
more the efficiency down to one.

The other thing I know is efficiency of the tip is a strong
function of the thickness of tip. That is one reason metal
props are better than wood props.

Paul Lamar