well, in a season 4 eps, our heroes are close on the ground to a glider taking off and i don't remember seeing any signs of a forceful thrust, like a rocket would make, so the engine must be somthing like anit-gravity or hover in the atmosphere, or maybe if it's in close contact with the ground. now bear in mind i can barely wrap my mind around how my car engine works but...what if a glider uses a type of hover engine when they're in close proximity to the ground, sort of like a harrier engine but not as rough. then when they get higher they can engage a normal thrust type engine, like a rocket.
in the curse, serpents venom we see gliders taking off and there appears not to be any obvious signs of forceful stuff...i know that doesn't make sense but there was no sand flying when it took off in the curse so...it would have to be a hover type of engine...
unless of course i'm just rambling on and making no sense at all.