When I raise a units Z axis, do I have to do it to all of the .kf and the FX nifs in addition to the .nif?

Also, can an airplane function as a normal moving unit if the XML is set up for that without modding the animation?

You use a program called nifscope. You can probably google it and download it. I think there's a tutorial that may have a link. Once you have that find every nif file for the unit that has a picture when viewed in nifscope and type in a new value at the location as highlighted in this picture. As for the airplane why don't you try it out and tell us. Science is sharing.

Thanks, so all I need to do is raise the z axis on the nifs and kfs where I can see the model?

Yes, I think so. Some of them affect each other so you will find that it is already done on some once you do the main ones, but you have to look at them all and make sure. There are often bunch of models for different actions and positions like freeze1, freeze2, death1 etc... and if you don't change them it will be wrong during that action. The main one with the name of the unit should be first and the fx last because I believe it takes its sheet music from all the others.