Sin Episodes Level Creation/Making a blimp fly
Introduction
You need to know basic hammer editor mapping and how to make a 3d skybox to attempt this tutorial
Notice the advertisment blimps that float around the in sky in Emergence, this tutorial will show you how to make one and how to make it fly using a func_tracktrain and a few path_tracks.
Making the Blimp
Add a prop_dynamic into your 3d Skybox, then access the object properties setup. While making sure smartedit is activated, select world model and add "models/highrise/dirigeable_01.mdl" to the value box then click apply and exit the object properties setup. You'll now see the blimp model shown infront of you in the 3d viewport. Make sure that it is facing east in top view (Otherwise it will fly reversing)
Now for the tricky part. Make a brush about the same size as the blimp and move this brush so it covers the blimp like a shell. Texture the entire brush with tools/toolsinvisblenonsolid and tie the brush to an entity. Make this new brush entity a func_tracktrain and name it blimptrain. Now go back to the prop_dynamic's (our blimp) properties and enter the value blimptrain into the keyvalue parent (this will make the blimp move with our func_tracktrain).
Okay, now add a path_track entity into the map, you'll want to put this where you want the blimp to start moving from. Name it Blimppath and press shift (this will cause it to make a duplicate) and drag it away to another point. You can see the two path_tracks are now connected by an orange line (This because the next stop target has been assigned to the 2nd path_track, which Hammer should name blimppath1, this is the first part of the track, now grab the second and shift-drag it to another location ahead of it, this is the first turn in the track. Make a looping track by continuing to shift-drag the path_track around then when done goto the properties of the last path_track in the path and change next stop target's value to blimppath (this will make the path loop).
Lastly goto the func_tracktrain's properties and enter this value for First stop target: Blimppath and Change Angles to Ease In/Out. You may want to set the speed (I recommend around 25 units per second) and other variables in the properties list.
Final Note
You can also use this method to make a boat move around a distant bay, by using the boat model instead of a blimp.