I did those detailed engineering drawings BY HAND.  Computer skills are, unfortunately one area where I am a bit behind.  However, the primary reason I made a working model instead of a computer model is that a computer does not prove that something works.  I have seen a LOT of impossible animations, demonstrations, and simulations.  A physical model show that either it works or it doesn't. 
  
That being said, a good computer model would be a convincing and fantastic complement to a physical model.  But time and money....