Auto takeoff is set by the mission control scripting only. The takeoff mission specifies a takeoff pitch and a target altitude. During takeoff Plane will use the maximum throttle set by the THR_MAX parameter. The takeoff mission item is considered complete when the plane has reached the target altitude specified in the mission.

Before takeoff it is important that the plane be pointing into the wind, and be aligned with the runway (if a wheeled takeoff is used). The plane will try to hold its heading during takeoff, with the initial heading set by the direction the plane is facing when the takeoff starts. It is highly recommended that a compass be enabled and properly configured for auto takeoff, as takeoff with a GPS heading can lead to poor heading control.

If you are using a wheeled aircraft then you should look at the WHEELSTEER_* PID settings for controlling ground steering. If you are hand launching or using a catapult you should look at the TKOFF_THR_MINACC and TKOFF_THR_MINSPD parameters.