Fail of the Week: Two Rotors Are Not Better Than Four

Fair warning: [Paweł Spychalski]’s video is mostly him talking about how bad his “dualcopter” ended up. There are a few sequences of the ill-fated UAV undergoing flight tests, most of which seem to end with it doing a reasonable impression of a post-hole auger. We have to admit that it’s a pretty poor drone. But one can only truly fail if one fails to have some fun doing it, [Paweł] enjoyed considerable success, at least judging by the glee with which he repeatedly cratered the craft.

The overall idea seems to make sense, with coaxial props mounted in the middle of a circular 3D-printed frame. Mounted below the props are crossed vanes controlled by two servos. The vanes sit in the rotor wash and provide pitch and roll control, while yaw and thrust are controlled by varying the speeds of the counter-rotating props. [Paweł] knew going in that this was a sketchy aerodynamic design, and was surprised it performed as well as it did. But with ground effects limiting roll and pitch control close to the ground, the less-than-adequate thrust due to turbulence between the rotors, and the tendency for the center of mass and the center of gravity to get out whack with each other, all made for a joyously unstable and difficult to control aircraft.

Despite the poor performance, [Paweł] has plans for a Mark II dualrotor, a smaller craft with some changes based on what he learned. He’s no slouch at pushing the limits with multirotors, with 3D-printed racing quad frames and using LoRa for control beyond visual range. Still, we’re sure he’d appreciate constructive criticism in the comments, and we wish him luck with the next one.

20 thoughts on “Fail of the Week: Two Rotors Are Not Better Than Four

  1. Interesting post, reminds me of way back when, thanks need more of that :-)
    I recall seeing a Popular Science (or P.Mechanics) article some 30-35 years ago which had a two stroke petrol motor (RC car type etc) around 3/4 Hp or so driving two contra rotating three bladed fans and very close to each other and with dia around a meter or so. IIRC the article touched on stability issues which appeared almost self-evident, sort of.

    When I have more time will look into this at the state reference library as seems a heck of a lot of older printed stuff pre-net didn’t make it to many search engines and AFAIK google and others still behind in terms of indexing/storing vs rate new pages produced ie. Their discimination criteria seems skewed to more newer commercial stuff of wide sorts vs older tech stuff buried in popular mahazines of 30+ years ago…

      1. That’s such a massive increase in mechanical complexity, though. I wonder if you can do it without vectored thrust or vanes with some kind of dynamically moving center of mass, like have two weights on the ends of levers which are moved around the center axis by a servo which can balance each other out for stationary hovering or favor one side to steer.

  2. Pretty sure the quads will have longer flight times even though there are more motors. My Sprite can only stay in the air for 10 minutes if it’s not windy. Coaxials look cool though!

  3. Really cool and fun. Crash, best part ever. Onboard camera FTW!

    It sounded from the video that the motors are not being very efficient. Can’t yaw be controlled with the second motor just counter-spinning a cylindrical mass and let the first one just to the heavy lifting?

    And with regards to the control surfaces, just having longer legs might allow it to have the air flow needed. If you want to be really cool about it, you can make them retract like in Elon musk’s.

    Finally, the minimum thrust allowable should be limited so as not to allow to fast a rate of descent.

  4. Coaxial drones certainly aren’t hopeless, HaD has featured more than one in the past.

    There was project NAVI that used a swash plate on one rotor for steering.
    https://hackaday.io/project/20793-navi-hey-listen

    Then there was the one from Modlab that used a variable torque system to control blade pitch:
    https://hackaday.com/2014/12/16/uav-coaxial-copter-uses-unique-drive-mechanism/

    And there is this bonkers one that uses single bladed rotors to get some interesting thrust vectoring
    https://hackaday.com/2017/07/25/duocopter-does-it-with-two-fewer-propellers/

    I don’t recall any mentions of aerodynamics in the bottom prop for any of these projects, and none of them have tried to use vanes to vector the thrust so this post is novel for those things.

    These coaxial drones are an interesting design that I find interesting to see people iterate on.

  5. …so do tell, what IS the difference between the center of mass and the “center of gravity”…? Spoiler alert: unless you’re a satellite floating in space, sensitive over time to minuscule variations in gravity as applied to parts of your structure in slightly different positions, THERE IS NONE. Unless parts of your object that are closer to your source of gravity are attracted to it _noticeably_ stronger than parts of it that are further away where the gravity is _noticeably_ weaker, there is no difference between the two whatsoever. You need a gravitational _gradient_ to even define a difference, and the gradient of gravity on human scales here on the surface is zero for all practical purposes – the flap of a butterfly’s wings on the other side of the Earth will have a larger effect on you than the amount gravity varies over a few feet of height. Come back when you manage to weigh your model at different heights comparable to its scale and manage to _measure_ consistently different results – after all, that’s how we measure altitude isn’t it (oh wait we aren’t)!

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