Ducted propeller/fan theory 101

Here are a pair of well produced crash course videos on ducted propeller and fan theory by "RC Model Reviews" on YouTube. He explains a few ways that ducted fans are more efficient.

In this second video, he explains how ducted fans in hover are self stabilizing when travelling at forward speed. I believe this phenomenon, which prevents mono-rotor hovering ducted fans from travelling too fast a forward speed, is reduced for multi-rotor ducted fan because of the thrust moment arm from the ducted fans being further away from the CG. This is because the moments caused by the Coanda effect get swamped by the moments caused by the fan thrust. The thrust moments are effectively zero for single central fans.

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