I was making a drone animation and i wanted to know how i can make the three propellers i made to rotate like a helicopter does at the same time in an animation.
Similarly to my answer here I believe drivers are a great way to animate mechanical objects.
For a very simple case the rotation axis, in this case z, can be "spun" by setting the rotation to the frame, by typing
frame (known to drivers, as the current frame) into the field.
Adding other variables as custom properties on the parent empty, eg on / off / speed, as well as on the individual rotor empties themselves, could change the speed of each.
Create a cube.
Name it something like Rotator.
Move it to some other layer.
Now in Properties windows, select your propellers & add "Copy Rotation" constraint.
IN "Copy Rotation" -> target , select the Cube Rotator.
Animate your Rotator and it will in turn, rotate your propellers.