Small machines using vectored thrust are going to be noisy, no
matter what, and inefficient (how efficient is a Harrier?). Fly-by-wire
isn't cheap, either (what's an F-16 worth?). These small machines are
no doubt possible, if enough money is spent, but what's the market
going to look like for a two-place model that costs $15 million and
gets 1/2 mile per gallon, giving it a 50-mile range, say? And wakes up
half the city? And don't get me started about electric motors and their
weight and the generators needed and all that.
Thrust is most efficient when it's generated by large-diameter
slow-turning props, rotors or fans. Small units have to spin at high
speed, losing way too much power to drag. A 400-hp helicopter makes way
more thrust than a 400-hp aircraft engine and prop. Airliners use big
fans now instead of the old straight turbojets. All of it proves that
small-diameter air movers are not good and unless there's a powerplant
developed that weighs nearly nothing and turns out huge hp at near 100%
efficiency, these little machines will remain a dream, just like
powered flight was a dream until metallurgy and fuels developed to the
point that the Wrights could build an engine that would actually fly.

Dan