I have a player who is planning a monk aarakocra, and he asked me this: Unarmored Movement just says "speed". Does that mean all speed for each movement type, or just ground speed?
Unarmored movement: "your speed increases by 10 feet while [conditions, also gets faster later]" (PHB p.79, "Unarmored Movement")
So what's increasing? Your speed:
Every character and monster has a speed, which is the distance in feet that the character or monster can walk in one round." (PHB p.182, "Movement," emphasis mine)
So the monk feature increases by 10 the distance in feet that the character can walk in a round. For your Aaracokra that number is 25, increasing to 35.
RAT (Rules As Tweeted), yes.
@JeremyCrawford says "bonuses/penalties to speed apply to your speeds in general, unless the text specifies walking, flying, etc." (Emphasis mine, thanks to @keithcurtis for the pointer.)
I'm usually pretty hesitatant to rely on tweets, even from Jeremy, but this one hits the mark pretty directly: you'd look at all your speeds (25' walking, 50' flying) and add 10' to each for Unarmored Movement.
[Later] and here's one that hits it exactly: Aarakocra Monk Unarmored Movement. I'll spoil the surprise: Jeremy says it's +10' for flying speed, too.