Okay, let's take off in the Cessna and fly at 50 knots. We manage to get aloft, our flight plan is to fly at 360° for one hour. The the wind picks up; all of a sudden it's blowing 60 knots from 90°. Fortunately we've managed to account for the sudden wind and know our starting point. The speed of our little plane is 50 knots and we chug along. We are flying north at 50 knots and we have no headwind; so our ground speed is 50 knots (air speed - wind speed). After one hour, where are we? Well, we've flown 50 nautical miles north, but we've also been blown 60 nautical miles west. Damn, where are we? From our starting point we're about 78 nautical miles away at a bearing of 320°. So (and please correct me if I'm wrong) we are about 78 nautical miles from our starting point, but west and north of where we want to be. So what's our ground speed? Air speed - wind speed = 50 knots/hr. But we're 78 nautical miles from our point of departure! Damn! Our ground speed is 78 knots/hour at a heading of 320°. Direction matters because we are dealing with velocity, not speed. There is no negative speed.