Why is it so hard to make large aircrafts?
Boeing stopped after the 747. They haven't made a larger plane ever since.
Airbus A380 took a long time for it to be designed.
In June 1994, Airbus announced its plan to develop its own very large airliner, designated as A3XX. Airbus considered several designs, including an unusual side-by-side combination of two fuselages from its A340, the largest Airbus jet at the time.
From 1994 to 2005, nearly 11 years.
I'm not talking about the market feasibility or the cost here, but more in terms of physics and aerodynamics. We already have so many different models of airplanes. If we have working models of airplanes, how about simply scaling them? Would it work? If not, why not?
It would be great if reliable sources from the Boing 747 or the Airbus A380 development programs could be given.