General

The ArduCopter flight modes differ from that of DJI aircraft. The behavior of these modes should be understood before flight. The flight mode switches on the drone are configure for altitude hold and loiter mode.

The aircraft has four flight modes accessible to the pilot.

1. Alt. Hold

2. Loiter

3. RTL

4. Auto

Changing flight modes can be accomplished from a pilot controlled switch, from the GCS, or by internal processes like failsafes. The Loiter and RTL modes require a GPS lock. Acro and Stabilize do not require a GPS lock. If the flight mode is commanded to change from any source (R/C, GCS, failsafe) the mode will take effect immediately as long as it can be engaged (requirements like GPS lock are met). The most recent command for flight mode will always be attempted. If the copter enters an undesired flight mode, simply commanding another flight mode will allow control to be retaken.

Altitude Hold

In this mode the drone will hold altitude. The left stick will control yaw and altitude. Moving the stick up will command the drone to ascend. Moving the stick down will command it to descend. The right stick controls the drone's attitude (pitch and roll angles). With the stick centered the drone will hold a level attitude. Moving the right stick up will pitch the drone forward, Moving the right stick right will roll the drone right. The drone will hold an angle relative the stick motion. If the stick is relaxed to center, the drone will return to level.

Loiter

In this mode the drone is holding position using GPS and other sensors. The left stick will control yaw and altitude. Moving the stick up will command the drone to ascend. Moving the stick down will command it to descend. A centered throttle will command the drone to hold the current altitude.

The right stick is used to tell the drone a direction and velocity to move. Moving the right stick up will command the drone to move forward. Moving the stick right will command the drone to move right. The drone will move with a velocity proportional to the stick deflection. When the sticks are relaxed the copter will reduce the velocity targets to zero. This will cause it to coast to a stop and then hold position, heading, and altitude.

This mode is great for use on a well functioning aircraft. However, the additional targets of altitude and velocity make it a poor choice to recover from unusual attitudes or for use when the aircraft is damaged and/or unusually loaded. 

Altitude Hold vs. Loiter

The difference is all in the response to the right stick. In altitude hold the pilot has direct control of the vehicle's attitude. In loiter the pilot is guiding the copter to where it should be. The copter will adjust it's attitude accordingly. As an example, in a strong cross wind a copter in altitude hold will drift with the wind. A copter in loiter will fight the cross wind to hold position. If the right stick is deflected up the copter in altitude hold will pitch and fly forward but drift downwind. If the right stick is deflected up in loiter mode the copter will move directly forward and fight the wind to maintain that course.

RTL

This mode will engage automatic guidance to return the copter to a predetermined home point. The home location is set to the location of last takeoff. However this location can be overridden  and set from the GCS to allow the copter to takeoff from one location and return to another when the mode is engaged.

There is an adjustable minimum altitude. If the copter is below that altitude when RTL is engaged, it will rise to that altitude before beginning to move laterally towards the home location. If it is above the set altitude, the copter will move laterally to the rtl location without changing altitude. Once above the RTL location the copter will descend to a safe landing.

RTL is dependent on a GPS lock. If there is no GPS lock then the copter will resort to triggering an immediate landing. The copter will slowly descend to the ground and disarm once a landing is detected. Because the precise position is not known without GPS the copter will drift with the wind during this fallback landing procedure.

Auto

This mode will follow a pre-programmed sequence of waypoints and actions. Missions can be programmed and uploaded using a GCS. Once engaged the copter will fly the preprogrammed course. The mission may end with an action such as “Land”. If the mission has no specified ending sequence then the aircraft will hover in place at the end of the mission awaiting a change in mode.

Additional Info on Flight Modes

https://ardupilot.org/copter/docs/flight-modes.html