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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  • shyea, and if you charge your laptop with a USB charger plugged into the same laptop, the battery never runs out
  • Hamish, if you take that same piece of buttered toast and attach it butter side up to the back of a cat and then drop the cap, it will levitate. 


    supporting rules: toast always lands on the buttered side and cats always land on their feet.


    Bonus fact: microwaving instant coffee creates a time machine.

  • Moderator

    Carrying beer is a more important task, you can throw the ball


  • Right,

    Every quad would pass "tennis ball test" during manouver in acro mode at least in the absence of significant aerodynamic effects. But when I saw this tread I realized that tennis ball :) would be usefull to verify how my custon stable mode based on the principle of C.O.G. Force Compensation act,this mainly during transaction from flyed to autolevel hovering. Now you may observe that the ball stay stable every time the quad reach and mantain a stable position.

    BTW I designed C.O.G. Force Compensated Stable Mode to have a autolevelled quad targeted to beginner pilot (easy to fly) and Photo/Video (very Smooth douring manouer)

  • Any inertial state capable of taking the ball off the quad would also result in the quad no longer remaining stable and in the air. The quad's stability control works to maintain attitude, your inputs (or altitude control) work to keep it from touching the ground (crashing). Together, these two constraints result in a sum of forces always pointing straight up (and the combined inertia and weight of all masses countering this force in the direction straight normal to the propeller plane)

  • every quad would pass this "test"

    because it does not depend on the vehicel's stability


    it's because the propellers are pushing in the direction of the ball, you can turn off the motors, ball will stay on top

    you can turn them full on, ball will stay on top

    you can tilt it 45°, and do the same, there is no difference (because the quad is moving like tha ball -> no acceleration between both)

  • works the same way a waiter with a tray of champagne glasses,

    This is my JJ-copter (modded Aeroquad) doing "Tennis Ball Test":

    JJ-copter Tennis Ball Test



  • G force is way of comparing forces involved in situations such as the G force meters in racing. These are not G forces they are the forces of both thrust and drag (I know there are more but simplifying) with the centripital force and inertia all combined into readings relating to accelerations of 9.8m/s^2

    The copter is supplying a centripital force to the ball/water to fight against the inertia of the ball/water. This is only in reference to it stabilising the ball/water the movement of the quad in the flight something more but its better to stick to basics.

    Incidentally the monitoring the water level in flight with a 60fps video would provide significant data on how fast and accurately the platform does its calculations for stability.

  • Hi,

    interesting would be to see how water upper level is during the flight.

    It seems it is always level with glass top (copter attitude).

    Would this mean that it is G force ?


  • He can still make the ball fall off though (not that he would want to as it would wreck the props). Spin in place as fast as possible (yaw). The shift in CoG will be rotating at that speed so to counter it the software will have do its best movements but thanks to thrust from the props being relative to the velocity of the body (not the props themselves) it should be much slower to react than his normal flying and fall off.

    Of course it could be that the setup is THAT fast that is just takes it in its stride :D but that is the only way skew the forces involved.

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