Although the test flights were successful, the planes all still had problems with the motor mount flexing. The spinning motor put force on the vertical part of the bracket, shown below with arrow, bending it inward and making the plane hard to control. We brainstormed ideas on how to reinforce the bracket at the park, and decided to meet one last time before Leo took the plane to the Maker Faire.
And voilà! The motor spins with no flex on the bracket now: