Mike,
What you're not understanding is that kinetic energy is relative.
I'll give you an example-
My cofee cup is sitting beside me. It's on a table. The table is on the floor which is attached to the Earth. The earth is spinning at 600mph and orbitting the sun at about 3000. The sun is orbiting the centre of the galaxy at about 100 000 mph, and the galaxy is receeding from the centre of the universe at a signifigant percentage of the speed of light.
My coffee cup has enough kinetic energy to destroy a fair sized city- better not drop it!!
Of course it's not a problem because the table and everthing around the cup is moving at the same rate, so RELATIVE to it's surroundings, the cup is stationary.
An aeroplane flys due to interaction with the air around it. Imagine a gold fish in a bowl on the back of a truck. Does the fact that the truck is moving at a constant rate make it hard for the goldfish to swim in the direction the truck is moving? No! he can happily swim in circles to his hearts content, not even aware that the truck is moving. In the same way, a turning aeroplane interacts with the "Bowl" of air around it, completely indedendent of the surface of the earth, moon, sun or horse head nebula!!
If the truck brakes, it's a different story- That's windshear, a sudden chane in velocity of the environment the aircraft is in at the time.
As I said- you're sitting in front of a computer- do a little research and find one authorititive confirmation of the "Down wind turn" and I'll eat my shorts!!!