I recently saw Bob Hoover's famous video in which he is pouring tea while he is rolling the plane. Can you explain the physics that allow the tea to flow "up" when the plane is upside-down? Why doesn't the tea fall out of the pitcher like it would normally if turned upside-down?

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    $\begingroup$ See here for the solution: aviation.stackexchange.com/questions/489/… $\endgroup$ – Peter Kämpf Nov 9 '15 at 14:56
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    $\begingroup$ Hi. Welcome to SE.Aviation! Please post a link to the video. My guess is that he performs a barrel roll in which case, the force you feel is always towards the floor of the aircraft. If you closed your eyes, you would not even know that you were rolling. $\endgroup$ – Simon Nov 9 '15 at 15:33
  • $\begingroup$ Can u please explain how the force is always towards the floor?@simon $\endgroup$ – RocK Nov 9 '15 at 15:52
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    $\begingroup$ You're better off asking on SE.Physics, but what happens when you tie something to the end of a piece of string and whirl it around your head? Centripetal force you seek. $\endgroup$ – Simon Nov 9 '15 at 15:54
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    $\begingroup$ I'm voting to close this question as off-topic because it belongs to physics.se. $\endgroup$ – mins Nov 9 '15 at 21:42

The maneuver Bob Hoover is performing is a specific kind of roll called a barrel roll. If you look at the path traced by this maneuver it is a corkscrew rather than a linear path that you might be imagining. Because the airplane is rotating about a longitudinal axis as it translates forward there is centripetal acceleration toward the axis of rotation, or "up" in the reference frame of the person in the airplane.

To the observer in the reference frame of the airplane (e.g. the pilot or a passenger), the centripetal acceleration is interpreted as the fictitious centrifugal force. This is exactly the same mechanism that you experience when driving around tight curves, though the force is in a different direction. The end result in the airplane is that for a properly executed barrel roll, the centripetal acceleration causes the net force you experience inside the airplane to remain constant throughout the roll (in your reference frame, with the net force pointed "down").

If you were to close your eyes you would not know the plane rolled. Likewise, the iced tea does not "know" it is upside down. For a less dramatic demonstration, next time you are on a commercial airplane pay attention to when the plane is banking and turning and note that a cup of water on the tray doesn't notice the turn. The cup stays in place and the fluid remains level (with reference to the airplane).

You can also demonstrate this with a small bucket and some rope. Put some water in the bucket, tie the rope to the bucket and swing it in a circle vertically (be careful). The water stays in the bucket if you swing it fast enough, and this is the same mechanism you see in Bob Hoover's video except it is the whole airplane being swung around in a circle like the bucket.

  • $\begingroup$ so we can say that the G force applied by aircraft's floor is more than that of earth's $\endgroup$ – RocK Nov 9 '15 at 18:53
  • $\begingroup$ @RocK. Gravity is stil pulling you, in exactly the same direction with exactly the same force. The floor exerts a force opposite to the centripetal force. So gravity is pulling you down with +1g, centrifugal force is applying a force on you to push you away from the centre of the roll. $\endgroup$ – Simon Nov 9 '15 at 19:15
  • $\begingroup$ You must fly different airlines to me... The last plane I was on, the cup certainly didn't stay still! Hopefully it was just the copilot practicing his coordinated turns $\endgroup$ – Jon Story Nov 13 '15 at 20:18
  • $\begingroup$ For the net force to be constant, would not the centripetal acceleration have to be 2g higher at the top of the roll than at entry and exit? I think that would lead to either a noticeably pear-shaped barrel ( narrow end up) or require an airplane capable of accelerating significantly from entry to the top, and correspondingly able to lose all the excess speed on the way down. From the film I have seen, Bob Hoover's 707 roll appears to be somewhat pear-shaped, though it is hard to tell if the camera is panning. $\endgroup$ – sdenham Mar 24 '17 at 22:18

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