What happens to a ducted fan jet in la mens terms is it looses thrust with speed, if your fan is moving air out the tail cone at 170 mph and the jet is going 120 mph your thrust to speed is equliing out causing loss of thrust. I am not sure of the spec but the volosity is around 500 mph exiting a micro turbine, so you have to be going well over the speed we fly at to start to equlizing thrust to speed ratio. I would like to learn myself the math or formulas of what the true specs are on this subject.