Here are some inflight images of a medical helicopter run by Superior Ambulance Air Med 1. These helicopter photos were taken from the top floor of a parking garage as the unit lifted off the ground and left the area.
My personal method for photographing helicopters is to use a single center focus point that stays on the subject the entire time that I am looking through the viewfinder. I prefer an f-stop that allows for shallow depth of field to separate the helicopter from the background. Subsequently this also serves to push the shutter speed to a faster setting to ensure that I eliminate motion blur.
This is a Eurocopter EC135 P2+ helicopter managed by Metro Aviation.