It is my contention that the Hudson Valley 'Triangles' are C5 Galaxy Transport aircraft observed at night by people unfamiliar with the type.
It is the largest military aircraft in operation in the US, has wingtip and centerline/fuselage lights that accentuate/call attention to the 'boomerang' shape of its wing-span, is uniformly painted a dull grey/grey green colour, and seen at a distance can appear much closer than it is. At 274 feet (75.3m) long with 222 foot (67.9m) wingspan it is huge, almost as long as, and half as wide again as, the proverbial 'football field' (American Football, at that).
When turning, or heading directly towards or away from the observer, they can appear to just 'hang' in the sky, again because of its immense size. Also, unless the observer is directly below/behind a C5 under take-off power, they can be surprisingly quiet, and again due to their large size, can appear to be silent and close, when they are really just too far away for the sound to be evident.
There are many, many video's on youtube that show all of these 'features' of the C5, both in daylight and at night. I'll just include a link to the one below, which shows especially well how BIG these things look at a distance, and how they 'hang' in the sky. Also, bonus countermeasures!
You tube link to C5 Galaxy daylight flight over Dover AFB
It's a 10 minute video, but you can see what I mean from 1:00 to 4:00
From 8:30 to 8:45 you can get a look at how the C5's lights might be observed as a triangle at night.
Apologies for exposing anyone who watches to that voice-over/soundtrack combo, though (its an airshhow in the US, you can probably guess how it is).
In the case of the Hudson Valley triangles, right at the northern edge of the 'flap area' is New York Stewart International Airport, just west of Newburgh NY. From 1934 to 1970 the aiport was a military base attached to the nearby West Point Academy (yeah, that West Point, about 18km to the SE, also well within the 'Hudson Valley Flap' area), becoming Stewart Airforce Bace after WWII, then becoming the entirely civilian airport in 1970.
In 1983, however, an Air National Guard Base was opened at the Airport, sharing its facilities, home of the 105th Airlift Wing. From October 1985 the 105th operated a fleet of 12 C5 Galaxy Aircraft.
The most active period of the Hudson Valley UFO Flap is usually quoted as being 1982 through to 1986, mostly pre-dating the stationing of C5's at Stewart ANG Base/International Airport. I suggest that bases don't spring up fully formed on the date of inception, and when you have C5's on hand (in service 1970-present), and the facilities to operate them, you use them to deliver equipment/personnel/other material to their new base.
Now, I don't say that ALL sightings in the Hudson Valley area, or even the Hudson Valley Flap, are down to mis-identified C5 Aircraft, but I certainly do for the 'boomerang' and 'triangle' UFO's. maybe the 'crescent' and 'semi-circle' and 'half-circle' reports, too. Maybe.
I also don't say this accounts for other aspects of the phenomenon - sightings of grounded craft, abduction experiences etc. Those are a whole other barrel/tin/kettle of worms/fish/gray-liens!
I have a couple of questions, too - I've been compiling notes about the Hudson Valley Flap, maybe to start a thread on it -any takers? Also are the Pine Bush UFO Flap and the Hudson Valley Flap essentially the same thing? I can find much more on the latter than the Former, and what I can find on Pine Bush (also very proximate to Stewart Int'l/ANG) mostly overlaps the Hudson Valley stuff - is that the general concensus?
Edit to add:
This post got so long I completely forgot to add that the Duluth/Two Harbours sighting from the quoted post, whilst far outside the Hudson Valley area, sounds exactly like the sort of sighting that might come from seeing a C5 at night. The airbase at Duluth, whilst never seeming to have housed a C5 opperating unit, has a 3km runway more than capable of accomodating one, and a Fighter Wing that will have need of supplies, personnel, and other logistics needs, that could be met by the occassional visit from a C5.
Oh, and here's a picture of a C5 on approach to land at Rammstein AFB.