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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?)

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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.

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The more open the air-intake area you have, the more efficient the drone will be. For example, something as fine as mosquito netting will likely cause the drone to be less efficient and less agile while something with larger holes and more open areas won't impact it as much. If you were flying under a roof, a few plastic bars would protect the drone but if there are small branches above the Spark, you would need something a bit finer.

Assuming that you are building this to protect against small things like branches I would recommend something that is very light, has small but not tiny holes, and something that doesn't interfere with the drone's operation.

There are a few pre-made full guards designed for the DJI Spark like this one: enter image description here

https://www.banggood.com/Propeller-Props-Guard-Full-Protection-Cover-Cage-for-DJI-SPARK-RC-Drone-Quadcopter-p-1452878.html?cur_warehouse=CN

Or if this one won't work for what you want, using some netting with slightly larger holes than mosquito netting should work fine about an inch above the props.

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