ORIGINAL: skyhawknut
There are a number of things at work here.
First - when you narrowed the exit you increased the efflux (ehaust speed of the air) which is really everything. Narrowing the opening will increase top speed (at the expense of static thrust) and opening it up will increase static thrust (at the expense of top speed) You may have to experiment to fid the optimum opening to get what you want.
Also - just like a prop - different fans have different pitch. Higher pitch will result in higher top speed but longer takeoffs.
I appreciate your input. That is sorta what I figured. I have plenty of runway to build enough airspeed to take off. So the slow excelleration on the take off roll out isnt really that big of an issue. But the airframes Im flying are rather heavy and have a fair amount of drag to them. Im flying a Mig 70mm Mig 15 and 70mm F 86. So coupled with the airframe drag and the 5000ft elevation that I fly from I need nearly as much available top speed as I can get. But I do agree that If I experiment enough I will find the happy medium of static thrust and top speed thrust. But in short youve more or less answered my question. As the aircraft excellerates the prop (edf unit) unloads and ends up at top operating speeds (rpm)