I was looking at the movement modes on the pfsrd and couldn't figure out if there are any changes to flight speeds or anything else that might happen while wearing heavy or medium armor. I mean, there's the armor check penalty, but other than that I can't find anything that happens. Would speed be decreased in Medium or Heavy armor? Or am I simply unable to fly in those?
There are no rules in Pathfinder that prevent a creature from flying while wearing medium or heavy armor. However, armor does have some effects on flight:
Fly is a skill affected by Armor Check Penalty – therefore heavier armors (which usually have higher ACP) make flying more difficult
Medium or heavy armor slows the wearer – this affects flight speed just as it does other movement.
- If your speed is not 30 ft., such as the 60 ft. flight you get from fly, use Table: Armor and Encumbrance for Other Base Speeds.
Medium or heavy barding prevents the wearer from flying while carrying a rider – even if the mount is strong enough to carry the weight of rider and barding, there is a special limitation that prevents this.
These rules are, in my opinion, sufficient or even more than sufficient penalties for the heavier armors on flying creatures. Flight is an important part of the game; past a certain point, it becomes mandatory.1 Furthermore, the classes which most depend on heavier armors usually already have the most difficulty getting into the air: Fly is typically not a class skill and they typically do not have class features that allow it. Because flight is so important, this is a major weakness of these classes, and they have to go out of their way (and typically spend a lot of money) to get it. But failing to do so, particularly for those specialized in melee combat, can make them incapable of engaging with their enemies, and even when they can it is a completely asymmetrical situation.
Therefore, I strongly recommend against houseruling this situation to make it harder for heavily-armored characters to get into the air. In fact, I would strongly recommend finding ways to make it easier, at least for those whose classes do not provide innate magic.
1 Unless, by houserule or gentlemen’s agreement, no one is using it and the DM is carefully compensating for that.
I can't see anything in the Encumberance, Fly Skill, Movement (or Fly section of Movement), or Armor sections of the PFSRD which would indicate that wearing Medium or Heavy armor or carrying a Medium or Heavy load would have any effect on a flier other than reducing their flight speed.
I might impose a circumstance penalty on Fly checks if the load were particularly bulky or would otherwise make flying particularly awkward, but I can't find anything in RAW to support that other than the standard "add a circumstance bonus/penalty if appropriate".
You can make the argument that it is not allowable as follows:
Barding, Medium Creature and Large Creature
Barding is a type of armor that covers the head, neck, chest, body, and possibly legs of a horse or other mount. Barding made of medium or heavy armor provides better protection than light barding, but at the expense of speed. Barding can be made of any of the armor types found on Table: Armor and Shields.
Armor for a horse (a Large non-humanoid creature) costs four times as much as human armor (a Medium humanoid creature) and also weighs twice as much (see Table: Armor for Unusual Creatures). If the barding is for a pony or other Medium mount, the cost is only double, and the weight is the same as for Medium armor worn by a humanoid. Medium or heavy barding slows a mount that wears it, as shown on the table below.
Flying mounts can't fly in medium or heavy barding.
Then with encumbrance by weight:
Encumbrance by Weight: If you want to determine whether your character's gear is heavy enough to slow him down more than his armor already does, total the weight of all the character's items, including armor, weapons, and gear. Compare this total to the character's Strength on Table: Carrying Capacity. Depending on the character's carrying capacity, he or she may be carrying a light, medium, or heavy load. Like armor, a character's load affects his maximum Dexterity bonus to AC, carries a check penalty (which works like an armor check penalty), reduces the character's speed, and affects how fast the character can run, as shown on Table: Encumbrance Effects. A medium or heavy load counts as medium or heavy armor for the purpose of abilities or skills that are restricted by armor. Carrying a light load does not encumber a character.
So you can interpret it as a medium or heavy load on a flying creature counts as armor encumbrance, which means a medium or heavy load counts as armor and prohibits flight. However, 3.5 had pretty explicit rules about flying while encumbered, and Pathfinder purposefully left those out, which suggests they didn't want them applied.
TL; DR: No, there are no explicit rules outlining encumbrance, but you could make the case it affects it.