Uair is better than Dair for momentum cancelling because it ends slightly faster. In fact, Dair is worse than airdodging.
Quick rundown on momentum cancelling.
There are 2 basic actions.
Action 1 (A1) can either be
a) Air Dodge
b) Aerial Attack
Action 2 (A2) is
a) Double Jump
b) Fast Fall
c) Special Attack
The basic idea is A1 gets you out of hitsun, A2 is what actually reduces your momentum. This means that the faster A1 ends, the more quickly you can momentum cancel.
The preferred method, most of the time, is to Uair (A1) -> Fast Fall (A2). This is because you can fast fall while inputting the aerial, and Fox doesn't have any fancy buckets and skull bashes and **** to let him do it faster.
Here's some in depth-ish stuff you should read so you can understand all of this.
Yes, Uair is the only move to end shorter than the target time. Coincidentally, you don't have to worry about getting launched sideways most of the time and trying to adjust A2 - it's still Uair. (Some characters are better off airdodging).
If you get launched upward/diagonally upward: Uair -> Fast Fall
(Situational) If you get launched sideways: Uair -> Double Jump
The reason why you want to double jump in that case is because you're not afraid of the vertical trajectory killing you, but the horizontal one, and you want to recover above stage, etc. Use your judgment.
But Zhamy I can't DI and momentum cancel at the same time like that it's hard waaaaah
...
Screw you; I'll explain common DI situations some other time when I'm angry and it's way too early in the morning for my own good.
Quick rundown on momentum cancelling.
There are 2 basic actions.
Action 1 (A1) can either be
a) Air Dodge
b) Aerial Attack
Action 2 (A2) is
a) Double Jump
b) Fast Fall
c) Special Attack
The basic idea is A1 gets you out of hitsun, A2 is what actually reduces your momentum. This means that the faster A1 ends, the more quickly you can momentum cancel.
The preferred method, most of the time, is to Uair (A1) -> Fast Fall (A2). This is because you can fast fall while inputting the aerial, and Fox doesn't have any fancy buckets and skull bashes and **** to let him do it faster.
Here's some in depth-ish stuff you should read so you can understand all of this.
This is the reason why MK, despite being so light, is able to survive so long - Uair finishes in 13F, comes out on 2F. Fox is kind of like MK in that Uair is almost always the best option. Here's some more numbers to compare (time it takes for each move to finish):The # of frames before a character can do either of A1 (This is universal)
a) Air Dodge = 14F
b) Aerial Attack = 26F
Fox's Air Dodge lasts for 49F, so 49F+14F = 63F
That's over 1 second. However, the number by itself doesn't tell us much. We have to compare it to the time it takes for an aerial attack to finish.
So 63F - 26F = 37F
If Fox has an aerial attack that is less than 37F, he is better off using an aerial than air dodging. (GUESS WHAT? HE DOES!) If he had an aerial that was exactly 37F, he'd still be better off using the aerial because it gives him the option of fastfalling.
Code:
Target = <37F
Fair = 49F
Dair = 49F
Bair = 48F
Nair = 41F
Uair = 35F
Fair Total Time = 75F
Dair Total Time = 75F
Bair Total Time = 74F
Nair Total Time = 67F
Air Dodge Total Time = 63F
Uair Total Time = 61F
If you get launched upward/diagonally upward: Uair -> Fast Fall
(Situational) If you get launched sideways: Uair -> Double Jump
The reason why you want to double jump in that case is because you're not afraid of the vertical trajectory killing you, but the horizontal one, and you want to recover above stage, etc. Use your judgment.
But Zhamy I can't DI and momentum cancel at the same time like that it's hard waaaaah
...
Screw you; I'll explain common DI situations some other time when I'm angry and it's way too early in the morning for my own good.