The National Advisory Committee for Aeronautics (NACA) reports are available for free full-text access. Just a note that the Engineering Library collections include a large collection of NACA report in print.
NACA collection - via NASA Technical Reports Server. As of April 27, 2021, indexing in the NASA database does not work well, so collections above are usually better choices.
These are the NACA series:
Aircraft Circulars (AC) - Reprints of articles giving specifications of individual aircraft, often from other countries.
Annual Reports (AR) - Contain NACA’s Annual Report to Congress as well as Technical Reports produced in that year.
Control Reports (CR)
Technical Memorandum (TM) - Reprinted reports from other laboratories, often translations from foreign languages.
Technical Notes (TN) - Report partial results or material of narrower interest. Selected information from these reports was often analyzed and refined for later publication in the more formal Technical Report series.
Reports (TR)- Also called NACA Technical Reports (TR). Usually the final report on a project, summarizing earlier interim reports.
Research Memorandum (RM) - Less formal reports issued to meet the need for rapid dissemination of aeronautical information.
Wartime Reports (WR) - Published after World War II to make more widely available wartime research that had been previously issued in restricted series.