As any DBZ fan knows, the Z heroes use their Ki to fire blasts (a la
Kamehameha...) and to fly and teleport, but I think that since they don't have a whole lot to use
in any amount of time, they have to conserve it, and also in order to do these things, they need time
to concentrate (at lower power levels) or power up.
Like when Raditsu totally took Goku by surprise and zanzoken'd
about a foot, but Goku couldn't move out of the way in time. Raditsu had a high enough power level (1200ish)
or else he was powering up prior to the zanzoken and he could do it pretty quickly. This is why you
don't see everyone just blasting the bejesus out of each other when they fight, they have to save
their Ki. I think of it like mana (if you have ever played Warcraft 2 or Starcraft you would
see the resemblance) in that once they have used it up, they need time to rest and gain it back.
Which brings me to point #2, We all know that Goku ran down snake way the first time, and flew back the
second time, right? Well, while training under Kaiousama, Goku's power level increased considerably,
and this proves my point in a way. He had a higher power level, and therefore he had more Ki at his
disposal. Also, when he first crossed, he flew for a few miles and then you see him pretty burned out
as far as his Ki goes, then he had to run. Apparently, he ran all the way out because he almost fell on
his face, but caught himself just in time. Also, that's why when he is jumping and misses snake way, he
doesn't just fly back up, he had to power up a little (while fighting the hands) because his Ki level was low.
And in HFIL (oh god, there it is again! AAAHHHH!!!) he can't just fly back up! The sky in Hell is really high so
he doesn't want to waste his Ki (besides the fact that he used it all up earlier).
Ok, well what about when he got away Princess Snake? Didn't he fly away and power up for a Kamehameha?
Well you have to remember, he took a nap, had a huge dinner and a warm bath, so he must've been feelin' pretty good by then.
As for Gohan not flying down from the mountain while under Piccolo's training, well there are a few possibilities.
1. He is too young to understand how to fly (4 1/2). Goku can't fly in Dragonball either, he uses Kintoun to get
2. Chichi NEVER let's him train, so he never had time to learn how to control his Ki (Piccolo teaches him this)
That's why he can fly when Goku comes back from Kaio Sama (it would have made a really cute scene if Goku would
have commented on Gohan's improvement).
3. A combination between 1 and 2.
4. He is just afraid of heights...