Well you don't need the top event "on every tick", but that's not the issue there.
If you reproduce your logic there in a brand new capx : like this one
it does work as expected.
So obviously the issue is somewhere else in your code where you probably modify "TurnRate" value after this input.
Without seeing your capx, no way to tell, but this simple test confirms that the logic is working and that you're asking the wrong question/looking in the wrong corner.