Hi every one,
I have worked for some weeks with FAST. I model a small 3-bladed downwind turbine and I would like to investigate more on the stress analysis of mechanical components such as main shaft and blade roof.
When I apply a constant yaw speed of 45 degree/s to the nacelle and rotor turns at 34 RPM, the total vertical bending moment on shaft (LSShftMys) is caused by rotor weight, wind shear and gyroscopic load which is close to my hand calculations. But FAST results show also that a bending moment exists in the horizontal plane (LSShftMzs) which is about half of LSShftMys.
The same phenomenon also occur when I reduce wind speed to a very low value or I apply a negligible air density ; LSShftMzs is always greater than zero.
Is someone can explain me why LSShftMzs is not equal to zero even is yaw speed is constant ?