Transonic Aeroelastic Weirdness

Dave

Hi Guys, I've just completed a program designed to calculate the aeroelastic response of an airfoil in transonic flow - I'm having a problem and wondered if someone could offer some advice. My model couples the transonic small disturbance equation (unsteady) with a simple pitch-plunge structural model.

If I have an airfoil in harmonic pitching motion, (for low supercritical Mach number) I notice a phase lag between the airfoil motion and the lift & moment coefficients. When the Mach number is increased (such that the embedded supersonic region occupies almost all of the airfoil surface) the phase lag decreases - the lift and moment coefficients are then roughly in phase with the motion. Both of these behaviours are what I expected.

In order to determine the flutter speed I prescribe a value of velocity, run the code for a steady airfoil (until the solution converges) then perturb the airfoils position - the growth or decay of the resulting oscillations lets me know whether my prescribed velocity is above or below the flutter speed.

My problem is that when I'm dealing with a low supercritical Mach number I am able to crank up the airspeed and induce flutter in my model. When the Mach number is high (high enough that the airfoil motion is in phase with the lift/moment coeff) I cannot seem to get flutter to occur.

The problem seems to be due to the phase difference between the motion and the loads - could anyone offer some advice as to why this is happening?