3D Robotics

Why doesn't the ball fall off the quad?

SSM has a fun physics questions over at RCG: why doesn't a tennis ball fall off his KK quad when he's flying?

Well, I wanted to return a stray tennis ball to some kids playing in the tennis courts at the local park. smile.gif

So I pick up the ball and place it on top of my Quad. The ball is on an upside down plastic sandwich box (to protect the works), with a very slight indent in it, a couple of millimetres deep, so the ball rolls around and can fall off at the slightest bump.

I take off with the ball on board and fly over the court, expecting the ball to roll off at any second, and fall down to the kids. It doesn't. I rock the copter back and forth a bit. No change on the ball. I increase the strength of the rocking. The ball sticks there, like it's glued. I fly the copter around, turning, stopping and starting. No effect. I zoom the Quad up and down. To and fro. Frantically stopping, dipping suddenly, whamming it into turns. The ball doesn't budge. "What gives?", I think. mad.gif

I bring the copter back to earth and place a second ball on top, with the first one. The two balls cannot both sit in the indent, and so they're now really unstable. If I carry the copter in my hand, it's hard to keep the balls on top - they fall off really easily.

So I fly the Quad with the two tennis balls on top to a position back over the court. And I do the rockin' and rollin' and shakin' thing all over again - the works. The balls just laugh at this and refuse to drop down to the kids. frown.gif

In the end, the only way I can get the balls off the top of the copter, inside the tennis court, is to land the Quad. The balls fall off the instant the copter touches down, The boys are amazed.

So what gives? It's something to do with the fact that every time the copter dips in a certain direction, it accelerates in that direction? And this acceleration perfectly balances the inclination, so the ball/s stay centered and don't fall? Even with all that starting, stopping, turning and zooming? confused.gif

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  • I believe the reason for ball(s) stability is thanks to the atual quad. You have added an unstable mass that has (mostly) free movement but is abve the center of gravity. The quad's eletronics, firmware and software are their to stabilise to quads changes in pitch, roll and yaw.

    The balls movement around the top of your container will create moments around the center of gravity which will be sensed by the quad setup and corrected for. Think of it as an inverted pendulum on top of the quad. The reason it keeps one, two or more on top of it is because more mass means larger moments for your sensors which mean they cn react to even finer shifts around the center of gravity.

    As this is a free standing ball and not a pendulumm attached to get it off wait for the ball to be out from the center position and thn spin the airframe (yaw) as fast as possible. Stability of the airframe will be coped with but the ball will press against the lip of your container and will hopefully have sufficient force to get over it. That should be the ony way to get it off (short of flipping the quad) as any others should be completely countered by the quad setup.

  • A very practical application of what's normally causing frustration in near hover accelerometer use :).
  • Go under the basketball backboard and gentlely knock it off.

    Danger is ball will hit prop.



  • To save you time searching from my last comment, it is the Bernoulli Principle:

    "Here’s a question for you: Do you think you could produce enough wind to blow a ping pong ball out of a funnel?

    Did you try? Can’t get the ball out?

    It’s a physics trick. See, the harder you blow, the harder it is for the ball to leave the funnel, thanks to the Bernoulli principle.

    Basically, the faster a fluid flows, the less pressure it creates. In this case the fluid is air. When you blow through the funnel you create a current of fast moving air around the ball. That results in a low-pressure zone below the ball that allows the higher pressure zone above it to push the ball down and keep it from rising any farther.

    It’s the same principle that airplane wings use to create lift."

  • The air flow around the ball may be a factor as well. Look up the ping-pong ball and funnel trick.
  • @David 

    and toast always falls on the buttered side!

  • hmm I was thinking flip the quad upside down..though you probably couldent do that
  • "Why doesn't the ball fall off the quad?"

    Probably because you wanted it too...try wanting to keep the ball on the quad and I am guessing that it would fall immediately :)

    basicly what Mikko said.


    the ball's at the center of gravity of the Quad when so when the Quad rocks the ball moves very little to start with. so what ever momentum the ball has gets canceled out by the way the quad Naturally accelerates and decelerates.


    the only way to drop the ball would be to tilt the frame without moving in the direction or flip the quad over. Both of which you can't do really with a quad. 

  • :)


    Nice video!


    The whole set-up of a quad generates lift upwards perpendicular to the props plane.  In horizontal moves

    the exact same force which makes the quad accelerate in horizontal directions is also directed perpendicular to the frame. In acceleration moments the frame is tilted a bit to have also horizontal component in the force vectors.  Nevertheless the force remains up in the quad frame of reference (and thus in the sandwitch box frame of reference as well)


    The rotating accelerations are all small not to disturb the ball enough to fall off.  Same stuff with the differences in drag forces when the quad is moving through air.


    Very nice video on demonstration in deed!  Thanks for sharing!!!



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