Hi we are the Non Practical Planners. During the past two weeks, we have discovered more problems because we are trying to make the design practical. For instance, how we are going to restrict the plane to only rotate in a specific direction. After days of thinking and careful consideration, we got an inspiration from the mechanism of a ball pen (retractable pen). After applying this mechanism to our design, when the plane is trying to rotate clockwise due to gravity, since there will be 4 retractable cylinders, which prevent the gear in the middle to rotate clockwise. The plane wouldn’t be able to rotate clockwise either because the gear is connected to the plane directly. Oppositely, when the plane is trying to rotate anticlockwise, the 4 cylinders wouldn’t prevent the gear to rotate. As there is a spring inside each cylinder, they will retract according to the curve of the gear, and when the gear has rotated 90 degree, the 4 cylinders will extend back to their original length, thus the plane can rotate anticlockwise as much as it wants.

　　The reason why we want the plane to only rotate anticlockwise is the fact that there wouldn’t be anything supporting the plane once it starts moving downward, and it will topple causing all the rubbish that it was carrying to be poured out. However, we don’t want to restrict the plane to rotate in both direction because we need it to topple when it arrives at the very bottom of the building, so we try to find solution which satisfy both our requirements (1. Do not rotate clockwise 2. Can rotate anticlockwise), and at last we came up with the design described above.