Throwing physics out the window.

[doctor_ocks]

This would all depend on what the atmosphere is like on your planet.  Perhaps high altitude doesn't thin the oxygen as much (or the entire oxygen level used by creatures/beings is higher/lower than Earth).  Perhaps weather is far warmer, so altitude doesn't frost over as much.

Regarding speed - real simple.  Put your hand out the window of your car while driving.  You'll find 10-20 miles and hour is okay.  More than that, namely 60-70 mph would be very tough/nigh impossible to work on an open deck (unless you want guys using carabiners and special equipment to work under such conditions).  Still aerial travel at any speed faster than walking would be a worthwhile venture.  Particularly if able to carry large amounts of cargo.

If you're using magic rifts etc., you could consider maybe the ships traveling on wind currents driven by these magical rifts/tears.  With a dangerous ground-level you can  get cool fortresses built high on mountain tops, re-fitted with food and equipment by airship.  Maybe some risky underground tunnels/cart systems for those who risk it.  Perhaps dragons and other flying beasties who present a serious threat even while airborne?

Right on thanks. Some great ideas there. I was thinking of having mechanically propelled air travel be about 20-30 mph but then having low level "jet streams" move at about twice that speed. This streams would be a by product of the rifts. While they're faster, the "streams" would also be monitored by [enter generic tyrannical empire name here] Navy meaning that smugglers and the like would probably avoid these or use other not so well known streams.

[doctor_ocks]

the problem is not the oxygen, but altitude adaptation. As long as You don't go over 2000m, it's basically ok. if You rise or fall too quickly, it can be a problem

Okay thanks.  I think I'll stick with around 500-1000 feet.

[doctor_ocks]

I have a possibly off-topic question about your world. When did the monsters attack out of the rifts? How long ago did the cities build the walls and fortifications, and how long ago did airships become a fact of life? I ask because if the events happened long ago, many of your questions can be answered with "that's how it's always been". 

I was thinking around 100 years ago. Long enough for a generation of humans to be around who do not know the world to be any other way, but not so long ago that some of the older races would have forgotten.

My favorite stories always involve good people with rebellious streaks and lovable brigands. I'm also a bit of a libertarian and one of my favorite topics is what freedoms are you willing to give up for safety - possibly get eaten by a giant monster-demon-thing or live your entire life like cattle behind a giant wall. I imagine most of these walled metropolises being rather Tyrannical in how they treat their citizens in order to keep the peace. This would leave some fun doors open for the more free spirited citizens of the world something to rebel against.

[Supercollider]

The idea of it being a lost human colony and the floating ships/islands idea is reminiscent of an old game called the Skyrealms of Jorune.  Maybe it's worth reading a bit about it, see if there's any inspiration there?

http://grognardia.blogspot.com/2010/03/retrospective-skyrealms-of-jorune.html

As the article says; I remember it having a rish and interesting background, but I don't think we ever played it more than once!

[Elbows]

I was thinking around 100 years ago. Long enough for a generation of humans to be around who do not know the world to be any other way, but not so long ago that some of the older races would have forgotten.

My favorite stories always involve good people with rebellious streaks and lovable brigands. I'm also a bit of a libertarian and one of my favorite topics is what freedoms are you willing to give up for safety - possibly get eaten by a giant monster-demon-thing or live your entire life like cattle behind a giant wall. I imagine most of these walled metropolises being rather Tyrannical in how they treat their citizens in order to keep the peace. This would leave some fun doors open for the more free spirited citizens of the world something to rebel against.

I like this idea.  So while the world is indeed very dangerous (warp beasties or dragons flying around etc.) the over-powerful kingdom/empire plays it up much bigger than it is to keep the people subdued and dependent...so your airship pilots are brave rogues, which are not necessarily criminals but viewed as a bit crazy/brave by average people.  Due to the fact that they travel wide and far the airship crews/pilots would pose a bit of a threat to the empire because they might spread information around when in dock etc...or they deliver goods which are rumoured to be extinct or unavailable.  Empire soldiers/customs guys might be really strict on airships looking for smuggling of goods/people etc.  Trying to keep information/rumours/free thought limited amongst the populace.

Perhaps the empire cities even have dragon-riders (police etc.) yada yada.  Lots of cool potential there for an abrasive relationship with the powers that be. 

[FramFramson]

Just wanted to mention that another possible reference source for ideas is the old 70's-era sci-fi Callisto series by Lin Carter.

[doctor_ocks]

The idea of it being a lost human colony and the floating ships/islands idea is reminiscent of an old game called the Skyrealms of Jorune.  Maybe it's worth reading a bit about it, see if there's any inspiration there?

http://grognardia.blogspot.com/2010/03/retrospective-skyrealms-of-jorune.html

As the article says; I remember it having a rish and interesting background, but I don't think we ever played it more than once!

Cool thanks. I've added it to my reading list!

[doctor_ocks]

I like this idea.  So while the world is indeed very dangerous (warp beasties or dragons flying around etc.) the over-powerful kingdom/empire plays it up much bigger than it is to keep the people subdued and dependent...so your airship pilots are brave rogues, which are not necessarily criminals but viewed as a bit crazy/brave by average people.  Due to the fact that they travel wide and far the airship crews/pilots would pose a bit of a threat to the empire because they might spread information around when in dock etc...or they deliver goods which are rumoured to be extinct or unavailable.  Empire soldiers/customs guys might be really strict on airships looking for smuggling of goods/people etc.  Trying to keep information/rumours/free thought limited amongst the populace.

Perhaps the empire cities even have dragon-riders (police etc.) yada yada.  Lots of cool potential there for an abrasive relationship with the powers that be. 

Yes. This. Exactly my thoughts.

[doctor_ocks]

Just wanted to mention that another possible reference source for ideas is the old 70's-era sci-fi Callisto series by Lin Carter.

Oh I like the look of those ships!  Thanks for that.