Originally posted by OUTCAST
You may be correct, but most flutter happens on control surfaces where there is a pressure difference between the two sides, The only time that happens with a rudder is in knife edge. 
There is a pressure difference every time you slightly touch the rudder control. That is how all control surfaces do their work. High pressure on deflection side, low pressure on opposite. The surface is then "sucked" into the lower pressure pocket of air. This is not a knife edge phenomenon
