cant find the link
Basicall if i remember correctly all these engines lose efficency at higher speeds. As the limit of the air speed flowing out of the back of the setup is reached you lose the thrust. For a prop its 200mph, fan 250 mph, turbine 600mph. This is for size of engines we use not the big ones and are just approx. values. I am not an aeronautical eng. This is what i recall from this forum. Hopefully someone will explain it to us. Im just answering because nobody else is. I will look for the link tomorrow.