alt Hacker News> Two is possible if the payload hangs below the props
I think this is incorrect, it's the somewhat unintuitive rocket pendulum fallacy.
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elictronic • today at 1:47 AM
He specifically says if the motors independently tilt. This is active control. The Rocket pendulum fallacy is only valid for systems requiring passive stability.
Tandem rotor helicopters exist.
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Here is a video of one: https://www.youtube.com/watch?v=qO747JB4Nr8
There's a few demonstrated here (26-32 seconds in): https://www.youtube.com/watch?v=vBsyFj8bEJk
And here's one that uses the ailerons instead of being able to tilt the motors: https://www.youtube.com/watch?v=L2fgL97rgK0
I could've sworn Peter Sripol built one while on Flite Test too, but I can't find it atm.
Looks like the rocket pendulum fallacy is about expecting meaningful passive stability from the location of the center of thrust vs center of mass, but even 2-rotors need to be able to tilt independently (or deflect thrust) for active control.
Theoretically this could still work even if the center of mass was above center of thrust, but the tilting/vectoring responsiveness would need to be very high. These RC models at least move so slowly that the air resistance of swinging back and fourth really does help dampen oscillations passively, but they all still have active flight controllers that are trying to keep angular velocity at zero without control input.