<<So here is my question...  Can you put a butterfly
valve in the intake? >>

I saw the ATP turbine as well and was very impressed.  It uses the core of a
Solar APU and I know an ex Solar engineer - I'm going to ask him how well
they work.  I'm afraid there is no easy answer to the fuel consumption
problem as that has been the Achilles heel of the technology since day one.
Air flow is essentially reduced by reducing fuel flow until the rpm drops,
but since it is a dynamic compressor, it only has to drop a little to have a
big effect.  To make matters worse, the ATP is a single-spool engine so the
prop has to keep spinning fast along with the compressor.  A rule of thumb
is that most turbines burn about the same amount on the ground idling as
they do at high altitude cruise.  It is, however, very tempting to put one
on my ES and if I were building a Legacy the temptation might be too much to
resist.  The cruise fuel consumption might be 50% higher and the idling fuel
consumption much, much higher than a piston engine.  Even by adding another
200 pounds of fuel the endurance might be less than with the piston engine.
It looks like they did a very nice job, though.

Gary Casey
ES project, long ways to go

a piston engine goes "pucketa-pucketa," a rotary engine goes "whir," a
turbine engine goes "whish..."