From the book ,"Structures", or why things don't fall down.

     Because of the laws of aerodynamics the C.P.(center of Pressure) of the lifting forces on an aeroplane wing must alway be near to the quarter chord position.To reduce the torsional or twisting stresses in the wing it is therefore necessary to design the structure in such a way that the center of flexsure is well forward in the wing and lies close to the C.P.However,the ailerons (which control the aircraft in roll or to say when banking) apply large up and down forces to the wing tips,and these forces act at points not far from the trailing edge and thus a long way to the rear of the center of flexure. Thus the ailerons inevitably exert large twisting loads on the aircraft every time the pilot banks the aircraft.It will be seen that the direction of the this twist is such as to change the aerodynamic lift of the wing,as a whole,in the opposite sense to the action of the aileron and thus reduce its effect.If the wing is not sufficiently stiff in torsion the effect of the aileron may actualy be reversed,so that the pilot,wanting to roll and bank the aircraft to the right,and appying his controls in that sence,may find the aircraft actualy rolls to the left. Tis effect,which is not only discernting but also very dangerious,is called," aileron reversal" and is not unknown.It is a serious problem in the design of modern fast aircraft. The cure or preventive is to ensure ample torsional stiffness in the wing structure!