Imagine a straight, constant-chord wing in a wind tunnel. You are looking at it from the side. The airflow is from left to right.
The wing is mounted on a horizontal pole which is fixed to the walls of the tunnel. The wing is free to rotate clockwise and counter-clockwise on the pole.
There are several holes which run span-wise through the wing, enabling the wing to be mounted, with the pole in several different locations along the chord line.
1: The wing is mounted using the front hole, which is just behind the leading edge. The airflow will exert a very strong counter-clockwise moment and the trailing edge of the wing will tend to rise. As it rises, the angle of attack will reduce and the moment will also reduce. Eventually a stable angle will be reached which will be proportional to the weight of the wing and the speed of the airflow.
2: The wing is mounted using the rear-most hole, which is just forward of the trailing edge. Assuming the front of the wing is supported initially at a small positive angle, the airflow will exert a strong clockwise moment and the nose of the wing will tend to rise. As it rises, the angle of attack will increase and the moment will also increase. Eventually the wing may flip over completely, depending on its weight and the speed of the airflow
3: The wing is mounted using a hole that is located at the aerodynamic centre. (About 25% back from the leading edge). Assuming the wing is initially placed at a small positive angle of attack, the clockwise moments and the counter-clockwise moments will balance and the wing will not rotate around the pole. If the angle of attack is changed manually, there will still be no rotation around the pole. The aerodynamic moments are balanced at all angles of attack.
Last edited by eckhard; 10th Dec 2016 at 07:28.