So , again, Yes I do believe there is a cover up.
I wonder if we are covering up for ourselves? Meaning a certain branch of govt has a program to make new vehicles. They work, and are occasionally seen by all sorts of people who are not allowed to know. Thus a cover up.
Such a vehicle as a UFO would be of the upmost importance to keep quiet. It could be the silver bullet in a war if needed, but would only be totally useful if it is a complete unknown. I suspect that various agencies through out the years came to some scientific discovery that lends its self well to a new type of propulsion. And if one group can do it, then the other is assumed to be able also. Thus keep your abilities secret till last minute, be first to use your UFO type vehicle.
Also, there have been many weird science experiments in the 50's and 60's regarding nuclear light bulb engine and other open frame, free flowing air cooled nuclear engines. There used to be two in Idaho at the INEEL. Saw them myself. They are now on Wikipedia. They were for airplanes. So USA government openly admits trying to make a nuclear powered plane. Why not make it smaller and better and a drone, and then it becomes the UFO we know today.
If anything is going on , it is covered up. I think the government covers up many things, even things they do not need to. As a procedure of need-to-know and information compartmentalization.