The Avrocar--a more modest test aircraft based on the same aerodynamic principles--was rather a flop:
http://en.wikipedia.org/wiki/Avro_Canada_VZ-9_Avrocar
If it had ever got off the ground, the US Air Force’s 1950s flying saucer would have gone a long way off the ground: all the way to 100,000 feet. A paper recently declassified and made public at America’s National Archives details the project: a few million spent assessing the design feasibility of a flying saucer. The USAF' …
The problem they found with both designs was that as soon as you get into the forward flight mode, a circular "wing" is unfortunately crap.
AVRO had an engineer so addicted to the concept of round wings that he even penned a conventional, fuselaged design sporting a circular wing (like a flying saucer cut in half and glued onto each side). Wind tunnel tests proved that it was still crap.
None of the projects would have got anywhere near as far as they did, save for the US military being all starry-eyed over the idea of OMFG REAL FLYING SAUCERS!!11!!!!
Why not???
Asking why something should be built in the first place is the reason a lot of things stay on the drawing board. Think if this had been built and performed as it suggests at 100,000ft at Mach4 that would of been an impressive piece of kit that would of outperformed the SR71!!
I suspect "why not" is that a design like that would have required electronic control that wasn't available at the time. Its likely to be very unstable when moving at hight speed if left to its own devices even given the gyroscopic effect of a mooted spinning outer section. I suppose it could be built today so I guess as a design it just doesn't trump conventional aircraft when you take everything into account, not just outright performance.
They've only just declassified the fact it was designed in the first place ... five years from now, will we be reading newly declassified stuff about how they built a prototype, found it didn't work but built six more anyway to use up the black projects budget for the year? Or indeed built it, found it didn't work very well so stashed it in a warehouse somewhere to be forgotten.
<-- better way to "use up" budgets.
The Avrocar that people know about and see in footage did not perform well no. However, they made several refinements to the design (as well as producing other models) which improved the performace dramatically. As soon as the thing started working properly, the whole project went "deep black" and nobody knows what developments there have been since. Project Silverbug is an interesting related topic to this. Anyway, I always think that if the USAF were working on stuff this radical in the 1950s, then what the fuck must they have now??
This report on the Avro Canada Y-2 (complete with lots of lovely drawings) was released in 1998.