Basic lineart practice

My line-art on tablet looks awful, I can't do simple straight line - something is wrong with my Wacom tablet :S.
Anyway here's some example of line-art that I've been working on:

Pzkpfw V Panther (A version)

Mustang MK1 A (Early export version)

 B17 (G version) Flying Fortress in loose formation

Last but not least 

Pzkpfw Jagdpanther (German WW2 tank destroyer)