karl 12
Noble
Don't know if folks agree with Nick Redfern's comments below about flying black triangles not being 'secret aircraft' but there does appear to be quite a lot of FBT sightings which go way back before the 80s/90s and there's also relevant info about National Archives research and British MOD documents declassified under the 'thirty years ruling'.
Given that they exist then suppose the idea that flying triangular objects are man made is a colossal conspiracy within itself but suppose it's also fair to say that after all these years the identity of the agency behind it all still remains a complete mystery and the tech involved still hasn't been displayed in any military arena by any government that we know of today.
Turns out that, like other shaped UFOs, FBT reports are also global in nature and there's been quite a number of remarkable sightings from other countries - also some interesting government documents out there and specific research conducted below from places like Argentina (where apparently they're 'nothing new').
Despite triangular UFO flight behaviour being described as silent, nocturnal and low altitude, witnesses also describe the objects as being capable of extremely rapid acceleration and sharp turns at high speed
They also (are reported to) have some other bizarre characteristics like 'blunt-end-forward' movement, detachable lights/objects and the generation of 'light beams' and electromagnetic effects.
Researcher David Marler has done some sterling work on the FBT subject and after a long time spent rummaging through various newsclipping archives and UFO report databases he presents his findings below - great watch.
Given that they exist then suppose the idea that flying triangular objects are man made is a colossal conspiracy within itself but suppose it's also fair to say that after all these years the identity of the agency behind it all still remains a complete mystery and the tech involved still hasn't been displayed in any military arena by any government that we know of today.

Flying Triangles: Not “Secret Aircraft”
I recently had an interesting debate with a fellow researcher of the UFO phenomenon, on the topic of that one particular type of UFO that has become known as the “Flying Triangle.” When the conversation got going, it didn’t take long before I was assured that (in words broadly like the following): “They’re just military aircraft, and still classified. But one day they will be unveiled for us all to see – just like what happened with the Stealth fighter and the Stealth bomber.” Frankly, I get tired of people assuming – often without doing any deep research – that the FTs are domestic in origin. The reason why they reach that conclusion is actually very simple. There seems to be an assumption on the part of many in Ufology that the Triangles are phenomena that have only been seen since the 1980s onwards. Or, at the very earliest, the late 70s. Now, if that was the case, then yes, of course, an argument could be made that these craft are (a) ours; (b) still-classified; and (c) far in advance of the aforementioned Stealth aircraft. Unfortunately for those who prefer to see the Flying Triangles as vehicles of the military and nothing else, there is a major problem: reports of craft identical to those seen today date back more than sixty years. The issue here is that the “Flying Triangles are ours and ours alone” meme is one that has been fully embraced by lazy souls who simply will not look at the bigger, historical picture. If they did, their views might change – and change significantly, too.
link
Turns out that, like other shaped UFOs, FBT reports are also global in nature and there's been quite a number of remarkable sightings from other countries - also some interesting government documents out there and specific research conducted below from places like Argentina (where apparently they're 'nothing new').
Controversial Flying Triangles.
These flying devices are nothing new in the domestic case histories, and therefore, worldwide ones as well. Their true origin has been, and remains, the cause of major debate in UFO forums. It is an overwhelming phenomenon to someone like Diego, not versed in the subject and caught unawares. The first case involving a flying triangle over Argentina can be dated, according to our exclusive UFO database - which holds information on 5500 cases starting in 1947 - to the year night 1958 in Mendoza, when 'a strange black aircraft' was seen descending in the vicinity of the El Plumerillo Airport. Since then, we have been able to earmark some very interesting characteristics, as follows: - One hundred thirty reports of triangular, boomerang-shaped, flying wing or arrowhead-shaped objects fill our dossier. - They are mostly large in size; - Their color is predominantly black; - 95% of their incursions are nocturnal; - Their movements are variable, as we have description of slow, moderate, swift and even supersonic flight. - 70% of their manifestations occurred in absolute silence. No noise, buzzing or rumbling was heard at all. - The duration of their flight is not extensive. Their transit is short-to-medium in the travel that has been observed. - A slight number of episodes hint at landings or near-landings by these craft. - 95% of the evens suggest that these triangles fly alone. From the 1950s to the 1980s, we only have 17 reports. The massive increase occurs in the 1990s, with 21 reported sightings. During the first decade of the 21st century, the investigated cases totaled 36 and the maximum increase, up to now, was in the 2010s, with the uncanny number of 51 reports. In this newly-commenced decade, we are starting to receive the first episodes of these mysterious devices that fly over our heads with impunity.
Inexplicata-The Journal of Hispanic Ufology
Despite triangular UFO flight behaviour being described as silent, nocturnal and low altitude, witnesses also describe the objects as being capable of extremely rapid acceleration and sharp turns at high speed
They also (are reported to) have some other bizarre characteristics like 'blunt-end-forward' movement, detachable lights/objects and the generation of 'light beams' and electromagnetic effects.
Researcher David Marler has done some sterling work on the FBT subject and after a long time spent rummaging through various newsclipping archives and UFO report databases he presents his findings below - great watch.
Since the early to mid-1980s, there have been an ever-increasing number of UFO reports involving triangular-shaped objects. To many people this seemed like a new development in the field of UFOlogy. Indeed, the majority of UFO sightings that have been documented over the last 50+ years involved a multitude of shapes, sizes, and colors. In comparison to other UFO shapes reported, triangular objects were a relative minority. Marler was amazed to find numerous cases worldwide spanning the last 60+ years that involved triangular-shaped UFOs. He felt it was important to share this material, since many people were dismissing this particular subset of UFO reports as secret military aircraft. These same individuals are erroneously implying that these craft are something relatively new in our skies. He has established that reports of these objects are nothing new and that similar objects have been reported worldwide at least as far back as 1936. Also, that these UFOs have consistently displayed similar physical attributes and flight dynamics that strongly argues against the notion that these are “new” military aircraft.Featured speaker at MUFON Symposium.