Air friction

Air friction is a measure of how long it takes for a character to stop moving from a sideways force when in midair. It is the aerial equivalent to traction, which only applies when grounded. Air friction is measured in negative units/frame². There is also vertical air friction, which applies in some circumstances when a character is falling faster than their maximum fall speed.

Air friction only applies when the player is not holding the Control Stick in a horizontal direction (or the input is being ignored, such as when in hitstun). If the player is attempting to slow down, air acceleration applies instead, and air friction is ignored. This allows characters to be designed with several properties:
 * High acceleration and high friction means a character will change velocity quickly in all situations.
 * High acceleration and low friction means a character will respond quickly to the player's controls, but be very slow to decelerate if the player lets go.
 * Low acceleration and high friction means a character will respond slowly to the player's controls, but be very quick to decelerate if the player lets go. This would lead to an unintuitive situation where a player trying to slow down will take longer than simply allowing friction to take over.
 * Low acceleration and low friction means a character will change velocity slowly in all situations.

Air friction also determines how far a character travels when they are aerial grab released (along with gravity in Super Smash Bros. Melee and Super Smash Bros. Brawl). A character with low air friction will travel a long horizontal distance whereas characters with high air friction will not travel far at all. This is because air friction acts as a force against the momentum granted by the aerial grab release. This results in characters with higher air friction being left in a more vulnerable position after being aerial grab released. This is especially significant in Brawl, due to the long amount of time the grab released opponent cannot act. Characters with high air friction in Brawl are much more vulnerable to aerial grab release followups (unless they have extremely high gravity to compensate), especially if they can be forced into an aerial grab release from the ground. This is the reason why 's aerial grab release in Brawl sends him nowhere horizontally, making him extremely vulnerable to followups.

Vertical air friction applies when a character is falling faster than their maximum fall speed, but is not currently fast falling, or is fast falling, but falling faster than their fast fall speed. This can occur if a status increasing a character's fall speed wears off, or starting in Brawl if a character air dodges while fast falling, which ends the fast falling status, although starting in Super Smash Bros. 4, using an aerial during a fast fall will instantly bring a character back down to their regular fall speed, bypassing this stat. It can also apply due to the downwards velocity applied by stall-then-fall aerials, however most of these suppress a fighter's horizontal and vertical air friction for the duration of the move, and thus they won't begin to slow down until the move ends. Finally, in Super Smash Bros. Ultimate, if a character's fall speed during vertical knockback is higher than their base fall speed, this stat will be applied to slow their descent once hitstun ends.

It is circumstantial as to whether high or low air friction is good or bad and it is also game dependent (as with the aforementioned Brawl aerial grab release example). Low air friction means a character will not slow down as much if the player attempts to use a neutral aerial when moving, and will cause them to travel a bit further during combos. But on the other hand, high air friction helps resist being KO'd to a degree, and it may be beneficial in some cases to slow to a stop quicker without providing directional input.

Super Smash Bros. 4
* Ryu is a special case thanks to his Fighter ability. If Ryu jumps, his air friction is 0, meaning he will not slow down at all unless the player attempts to decelerate. If Ryu becomes airborne through other means, his air friction is a standard 0.015.

Super Smash Bros. Ultimate
* Ryu and Ken are a special case thanks to their Fighter ability. If Ryu/Ken jumps, their air friction is 0, meaning they will not slow down at all unless the player attempts to decelerate. If Ryu/Ken become airborne through other means, their air friction is a standard 0.015.