Planes

One explanation offered for things seen in the sky in Project Blue Book - despite how it did not match the witness descriptions was the 'H-13'.   This helicopter is most often associated with the Korean conflict of the early 1950's and the emerging private and commercial industry use after...or from the television series, "The Whirlybirds"...

The "Flying Wing" was another aircraft offered as an explanation...

1953 saw the US Military, and others, promoting bold new designs for all future aircraft. Unfortunately, all published reports indicate they were failures and the project was dropped...

"Aurora" - supposedly was a code name for a group of super stealthy, fast, and powerful planes - and was used to describe all such super-secret, may-not-really-exist planes of the 1970's through 2000 time span.  The SR-71, Blackbird...who knows what else may have been linked to this umbrella concept. These, some claimed, would describe the mysterious triangles reported across the globe for decades and especially in the 1990's over Belguim and Phoenix.  Like the question of the which came first the chicken or the egg, the reports of delta winged, triangle craft went much further back...Kenneth Arnold's sighting in 1947 was of delta shaped objects that moved like a saucers skipping over water.