title_body: How much air-intake space do I need in front of my DJI Spark's propellers? I've got a DJI Spark that I sometimes use for close-proximity flying such as inspecting the roof of a house.  The stock propeller guards protect against flying laterally into an obstacle, and the bottom proximity sensor and general vehicle geometry make it unlikely that I'll descend into something, but there's nothing protecting the top.
  
  I'd like to build propeller guards to protect against ascending into tree branches or similar hazards.  How much air-intake area do I need to leave, and does the shape of the gap matter?  (For example, are mosquito netting on a frame and a rigid open grid equivalent if they've got the same cross-sectional area?)
upvoted_answer: Single-rotor model helicopters, flown indoors, will "suck" themselves onto the ceiling when the rotor is within about half a rotor diameter if you're not quick to reduce throttle.  The sucking won't happen until you're that close, though.
  
  That gives you an upper bound for how far away an intake shield should be from a rotor without strongly affecting handling efficiency.  More generally, the farther the shield is from the rotor, the better.  Stiffer is also better because the shield is on the intake side and is thus being pulled toward the rotor.  A grid with thick beams is of course stiffer than a floppy fabric.  So if what you're guarding against is tree branches, not tiny debris, then use the most widely spaced mesh ("roll cage") that you can tolerate.
  
  If you want to experiment with netting over such a grid, just feel how strongly it's being sucked against the grid.  The greater the sucking, the less efficient the rotors.  That may be the most practical way to convince yourself of a particular setup's performance.