It's not hard with lightning spells. Lightning has no travel delay and it also has a degree of auto aim. Dragons fly too erratically for bow shots. At the very least it's extremely difficult.

couldnt have put it better myself

I know what you mean about unhittable dragons. I have a few dragons that show up now and again that don't trigger my follower's aggro, even though they swoop low enough to make the ground shake. I haven't been able to hit those dragons. They eventually fly off.

One thing about aiming at flying dragons - aim at the center of the chest between their wings. It's the only place that you'll get a hit, unless you're just plain lucky.

Skyrim Bear Unit (SBU) - It's how far he can run from a bear before being winded.

Sorry, man, but it did happen. Only time I've ever seen it, but it happened.

I just wanted to add for all the PC users out there, try toggling collision and going up in the air after the dragon.
I've done it a few times for fun, and the dragon acts strangely (no wing flapping at all - looks almost frozen). Relevant to the OP, when fighting dragons this way, you cannot hit them on the head, neck, wings, or underbody. You can only hit them on the top half of their body, from the base of the neck to near the end of the tail. I am wondering if this is true when shooting from the ground.

The hit box for a hawk is alot smaller than the hit box for a Dragon, that's for damn sure. Don't even get me started on the seemingly impossible shots i've made in Oblivion and Darkfall.

You can shoot them in the belly/chest area, I even see bloodsplats when the arrow hits.

Only by spamming lightning attacks since they land instantly, I dont even bother trying with the bow unless its hovering around.

Yup, can do it - just have to have a good aim.

For some weird reason I have never (afaik) hit a Dragon mid-air either. And my aim don't svck that badly either. I've only managed to nail em while hovering.

Lightning spells are your best friend in this instance. Just have to make sure their within range. Love it when the killing blow is landed in air, and the beast crash lands.

Has killed a couple mostly then they stop in the air to do the breath attack. They are hard to hit while flying, probably as they are father away than you believe, I always underestimate their size.

i am a crappy shot with a bow, but i almost killed one with a hunting bow and iron arrows (AKA crappy bow and crappy arrows). a soldier killed it, but i did most of the damage. i'm a mage-type, but i find the bow better than "fire and forget" spells (i use flames/sparks) for dragon attacks and ranged combat

You know what I would really like to do? To be able to hit them when they're flying really low (like when they glide low above your head) with a sword (or axe, mace...you get the point, melee). Just like in the trailer.
I never managed to get my timing right, so I have no idea if it's possible...do they have different hit boxes when they're flying/hovering/on ground?...

Not sure about the hitboxes, but I have hit a swooping dragon with my sword enough times to know it is possible.

Stormcall and dragonrend hit a dragon while flying.

I quite like Stormcall in secluded places because sometimes a dead dragon crashes to the ground face first and leaves a furrow where it skidded to a stop. In busy places like towns etc, Stormcall can aggro the guards and get you into real trouble.

Good to know! Thanks! At least now I know that if I don't manage to hit them it's because I'm terrible at it, not that it's not possible.
Will try to improve!