You can access NASA technical reports (and reports from NASA’s predecessor NACA – the National Advisory Committee for Aeronautics) from the NASA Technical Reports Server. This searchable database provides access to
1) NASA technical reports and citations from 1958 to the present,
2) NACA technical reports and citations from 1915 to 1958, and to
3) NASA images and videos via the NIX system.
Many of the reports are available in full text in Adobe PDF format. In addition to citations to technical reports, citations to conference papers and journal articles in aeronautics, engineering, geosciences and materials science are available here. This is a treasure trove of research materials.