Suninmyeyes and all those others - you are not only wrong but you are dangerously wrong. People do crash because they do not allow for the effects of turning downwind.
Here are three instances which may bring it home to you:
Model aircraft circling in free flight always rear up and often stall as they turn into wind when near the ground. If they turn out of wind, then they lose height. Their airspeed does not remain constant, due to the effects of inertia. The effect is noticeable near the ground but, oddly, not when they are ten feet or more above it.
If you hang a weight on a piece of string in a car travelling at a constant speed, it will hang straight. If the car suddenly lurches off in a different direction, the weight will move due to its inertia. The effect is there even though the car's speed remains the same.
This happened to me: I was landing a Chipmunk at a farm strip in a crosswind. The circuit direction was such that my groundspeed on base leg was high. When I turned final, I found that the aircraft speed was too high to get into the short strip. This happened twice despite my controlling the base leg airspeed carefully. So I flew the circuit in the other direction, so that my groundspeed on base leg was then quite low. I got in easily from that circuit. Note the large difference in inertia involved.
So much for an aircraft 'always flying in a bubble of air and never changing speed.' Of course it does - and it can be a killer if it is not allowed for.