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I have a mini quadcopter controlled by remote controller. I have set the motors (e.g 50% thrust) it will remain at altitude at 10 cm~ but once I increase to 80% and then follow by decrease to 50% I should remain at a certain altitude (e.g 2 meters) but it will then drop back down to 10cm~. My question is, why does it drop back? If 50% of the motors can lift the quadcoter, can't It do this at any altitude (I do understand air gets thinner above certain altitude but we talking only a few meters off ground)

From what I know, motor output is Normal force which is pushing it up which is 'constant' and gravity is force pulling it down which is also constant.

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Great question, I've seen this happen before but never thought about why before.

The reason is Ground effect.

50% thrust isn't enough to hover in free space, but when it gets close to the ground then there is increased lift which acts as to stabilize to altitude.

The pressure is slightly higher under the wing (rotor) when its near the ground which mean extra lift.

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