I have a spirit pro and a normal one (red). And I have Goblin 570 and just building the Kraken.

Which spirit should go to which heli optimally?

My first thought was higger heli - spirit pro. But then I thought smaller heli might be more work for the FBL. Or doesn't matter?

For smaller helicopters (470 blades or a shorter) we recommend smaller Spirit units.
For bigger helicopters Spirit Pro is better because of more ports.

But you can use smaller Spirit units also with 800 class helicopters if everything is OK.

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Spirit System developer