dunno, play with it and see. I would guess that one component of the array is north-south speed, one east west speed and the third vertical speed. If your chopper was going roughly northwest (say) and you wanted a speed of 5m/s you would have something like
chopper1 setvelocity [3,-4,0]
As I say, I'm only guessing since I've never used it. But have a play and see how you get on. Once you've figured it out post a comment in the online
Command Reference to tell people how to use it. It may be km/h rather than m/s.
That line of script should go in the on activation field of a trigger or waypoint, or in a script file.