What problems can be encountered if an airplane does not undergo wind tunnel testing? SpaceShipOne did not use any wind tunnel testing. According to the Scaled Composites FAQs:


  Q: Did you do wind tunnel testing?
  A: No. All design refinements and performance predictions have been derived from Computational Fluid Dynamic tools.


What problems would such an aircraft face (which is designed completely without wind tunnel testing)?
For decades, there was no wind tunnel testing performed. There will be, of course, more unknowns about performance and flight characteristics, but with modern computer simulation technology those can be greatly reduced.