There are 2 styles of transforming to a helicopter using those Vehicles
How to do it:
1st Style: Press Transform Button
2nd Style: Shoot and press the transform button (you will see the transforming is different)

The Glitch only Works! if you have unlocked Optimus Prime (or Finish the game)
It doesn't work on other Helicopter Vehicles like the Chopper,Police Chopper,News Chopper etc.