Reverse engineered the Diminishing Return in Robocraft (for hovers).  Bare in mind measurement error and me rounding things to whole number and fitting the numbers on a trend line.
1st-8th blade adds 100% lift each
9th blade adds 90% lift 
10th blade adds 75% lift 
11th blade adds 60% lift 
12 blade adds 45% lift 
13 blade adds 30% lift 
14 blade adds 15% lift 
Not much a surprise, I kinna knew intuitively that adding blade beyond 10 is a waste of cpu.