I am going to hold a lecture on aerodynamics for a group of high school students, after which a competition on constructing the best glider is held. I think the planes will be made of sheets of balsa wood, and they are therefore likely to have flat wings. To help them succeed, I plan on telling them about stability in the lecture. According to what I have read, and a little reasoning, I think it is not possible to have a plane plane with symmetric wings and tail plane fly with the same setting angle on both surfaces. Then I came to think about a paper plane which has no tail surface, and which obviously can fly, I just do not understand how My argumentation is as follows: For a symmetric profile the moment around the center of gravity will be zero, and the lift will always be located at the center of gravity. Three conditions must be full filled for a plane to fly stably: 1) the moment around the center of gravity must be zero, in the flying attitude 2) the aerodynamic center must be located behind the center of gravity 3) the lift must be(approximately) equal to the weight of the plane As I see it all three conditions cannot be fulfilled at the same time. If the aerodynamic center is behind the center of gravity to fulfill condition 2, in order to fulfill condition 1 the lift must be zero, and condition 3 is not fulfilled. Condition 1 and 3 can be fulfilled at the same time, if the center of gravity is at the same point as the aerodynamic center, but then condition 2 is not fulfilled. ...when theory and practice does not agree it is seldom practice that is wrong. Practice clearly shows that a paper plane can fly, so there must be something wrong with my theory but what?