I cannot really give advice on this but what I can comment on is , that , I have been working on a simple helicopter flight model , and to tell you the truth I have never been so confused with a physics engine when it comes to the forces masses and scaling , it doesn't really keep itself to specific units which would have helped out a lot .

basicly I learned that setting the mass directly can be a problem especially when the size of an object is to small for the mass setting , the best I found is to model the object according to a realistic scale in units then work according to that units per quant , what I do is I define the units such that I only multiply by a factor for my input values and just leave the physx scale as it was .

I had to come up with a factor for distance and mass to finally get something more or less acceptable for the lift and wing area and air density etc to all work together in some acceptable way ,before my values I displayed on screen were more or less acceptable .

I still don't get how to really get it as it should be ,so I am with you here ,kinda lost and not certain how to approach realistic values ..