title_body: Do propellers layered on top of each other have twice the thrust of one propeller? If you had two of the same propellers layered on top of each other going at the same RPM, with some space in between, would the thrust be double the amount of a single propeller at the same RPM?
  
  If not, what factors affect this?
  
  The propellers are pushing down on the ground by the way.
upvoted_answer: At least at the small/model scale, the answer is no. There is significant efficiency loss due to the stacked propellers not receiving the same clean air as they would if they were horizontally separated.
  
  This is demonstrated experimentally in a Youtube video by rctestflight which compares the efficiency/thrust production of three different propeller configurations sharing the same propeller type and motor type:
  
  
  Coaxial — What you're talking about where two props are stacked vertically so that they share the same rotational axis.
  Overlapping — Where two props' discs overlap each other by roughly half a prop diameter
  Normal — Where two props are positioned side-by-side without any overlap
  
  
  
  
  To quote Daniel from rctestflight in his video:
  
  
    Any prop overlap is significant in efficiency loss. Traditional side-by-side
    motors are 25% more efficient than coaxial and 8.4% more efficient
    than overlapping props.
    
    To me, losing 25% efficiency is not worth the small form-factor and increased ease of transportability that a coaxial system produces.