Hank,

I think we are arguing two separate things ! (as is usually the case)

Sure, a Con di nozzle is a device to accelerate a flow up to Mach =1 at the throat and then supersonically accelerate it further in the divergent section, thus realising all of the available temperature energy and turning it into kinetic energy, after all thats what a nozzle does.

A variable area nozzle is purely a device for matching the plume pressure to the ambient static pressure for maximum efficiency. i.e. fully expanded.
If you underexpand your nozzle you fail to convert all of your energy into momentum.
If you overexpand your nozzle the nozzle walls will experience a pressure force in the drag sense , because the pressure of the jet is below ambient pressure.

I believe that is the thrust ( ho ho ho ) of the original thread. So the real answer is no, to mount this kind of device on a model aircraft would be next to useless.

Nick