@winterro They diffuse by default but when you go to aircraft designs and edit the XML file, you can add
diffuseInertiaTensors="false" under Aircraft. you can find why some would want that and an explanation here

@scratch how to diffuse inertiatensors?

I think you should disable aircraft collisions of the wheels if you haven't already and, turn off diffuse inertia tensors, and add more mass to the frame