DEFINE_CG_MOTION for flapping wing UDF
 

Hi everybody!
I'm a graduate student of mechanical engineering working at a Fluent project for my thesys. I have to write a UDF in order to give a motion law to a flapping wing. By using the UDF "DEFINE_CG_MOTION" I need to specify an angular velocity d(theta)/dt so that the oscillation angle of the flapping wing is: theta(t)=25*sin(2*pi*frequency*t+arcsin(3/5))-15
If I use the derivate d(theta)/dt=50*frequency*pi*cos(2*pi*frequency*t+arcsin(3/5)) I lose the constant "-15" so that the oscillation angle is no more from +10 deg to -40 deg. As a matter of fact the positive peak amplitude must be +10 deg and the negative one must be -40 deg.
How can I solve the problem?
Thank you so much!