Something is wrong with your basic assumption that air is incompressible at speeds below 230mph. That statement is false, air is compressible at any speed. So that original statement needs to be run down and understood. I think it is causing all of your confusion.
Agree with this statement, although it's been a long time since I got my ME degree. To the OP, please identify the source of your info? Seems to me there may be some confusion there since air absolutely is compressible, but in calculating line losses in fluid flow it was acceptable to treat air as "incompressible" within a small range. If the pressure drop (hence the change in density/volume) was too great you would you would have to break the system into smaller segments to do the calculations.
Of course, that was back in the day when YOU actually did the calculations, and not just feed info into a computer that spits out a number.
