Fan design to improve lift force

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MiracleMax1
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Homework Statement


Can the addition of a second set of fan blades, below the first set and on the same axis, improve the lift force created? What is the theory associated with this phenomena?


Homework Equations


none, practical application in lab setting.


The Attempt at a Solution


Lab trial seemed to support the concept but I have no basis for this.
 
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There have been airplanes as well as helicopters with this motive force. You have to take into consideration that the relative wind on the lead rotor is extremely different for the second.

In the case of a helicopter, the leading edge of the rotor is then subject to the somewhat different original wind as the vehicle's velocity increases.

All such designs are history, probably due to mechanical considerations. More parts means more maintenance and more failures anyway.