As I understand it (and I am more than willing to be corrected), the USAF carried out three partially public-facing and (perhaps) largely-since-declassified studies of the UFO phenomenon:
1. Project Sign (1948)
2. Project Grudge (1948 to 1952)
3. Project Blue Book (1952 to 1969)
In addition, there were also several panels and collatory investigations during those years such as Robertson, Condon, Battelle Institute, etc.
There are, however, apparently conspicuous omissions in the record ... Read even more
