Any tips on practicing u-air chains? Which characters are easiest to start with? How fast should the u-airs be? Does turning around between uairs add anything? Can giant DK do uair chains on yoshi's story? etc.
When I learned up air chains I just went to DL in training mode and practiced short hop up air > shield drop up airs as fast I could.
It's easier on characters that fall slow because you don't have to up air as quickly. In my experience, it's probably easiest to actually juggle Luigi or Samus, but they don't have much hit stun so often times it's not a true combo.
The heavier a character is and the faster he/she falls, the more likely it is that they are truly in hit stun. If you can up air chain Pika, Ness or Kirby, it's still fairly easy and most of the time it's a true combo.
Link, Falcon, and fox are the hardest IMO. You have to go super fast for them.
So basically, the up airs should be as fast as you need them to be. It's all character dependent. For fast fallers, the up airs need to be pretty fast. For characters that fall slow, not so much.
Regardless of which way DK is facing, the up air will always send the opponent on the opposite side of him. So if DK is facing left, and the opponent is on the right side of him (behind him), up airing the opponent with send them in front of DK. If DK is facing left, and the opponent is on the left side of him (in front of him), up airing the opponent will send them behind DK.
So in terms of positioning the opponent while you're comboing them, no, it doesn't matter which way you face when you up air them.
The timing, however, does matter. The hit box comes out in front of DK first and then it ends behind him. So if you're about to drop the combo because the opponent is about to hit the platform, make sure you're facing towards them so that the hit box come out faster. If you're about to drop the combo because the opponent his too high, then face away from them so that it gives them time to fall into the end of the attack.
IDK anything about giant DK on Yoshi's.
I hope this helped lol.