Thanks alot for the answer. But I'm not sure if you understood it correctly... anyway thanks. Here, I would like to ask again:
IF I put a cone shape (with two open ends), with big openning after a propeller which direct the flow and speed up the flow to the smaller openning. Will that increase any thrust at all?
Personally, I don't think it will change because there is no change in mumentum right?
I don't know?
thanks
Bonpon