Maybe，the 6DOF model of simulink is wrong?

If the body 's force and torque of 3 direction is zero(figure 1)，the initial velocity of x direcion is 100, initial body rotation of y is 100(figure 2),I think this body will have an uniform linear motion，but，why it has motion in z axis(figure 3),I think the equations of motion is wrong.

Answer 1

[Sergio Tamayo]

This occurs because the tangent acceleration term (Omega X Vb), generates an acceleration on the Z axis. At t=0:

cross([0 100 0],[100 0 0])
ans  = 
   0    0   -10000

Since no other accelerations are present (due to forces and moments), the acceleration along the Z axis is integrated, causing the displacement along the Z axis.

[Justin Beri]

Hi Sergio, I think your answer is not entirely correct or needs further clarification.

I reproduced Songzhou's question.

As you can see there is indeed a displacement on the z axis (in the inertial frame). However, let's also tweak the numerical integration settings. I have reduced the absolute and relative tolerances.

Now we see that displacement is extremely small. So this leads us to think it is numerical error influencing the displacement.