Forced landings (battery warning) at 51% battery

[sar104]

Follow up: I believe I have discovered the cause of the drone performing auto RTH. After looking at my flight logs and GPS paths, the flights where this came up were very complex and "squiggly" paths. I was flying in woods and around trees a lot. I've come to the conclusion it figured it would be just as complex a route back to the home location and was alerting me at 50%. This was confirmed by 3 flights (one on each battery) at home with basic hovers and nearby fly routes. The RTH didn't trigger at 50% and I returned back on my own around 30%.

Thanks for pointing me to reviewing flight logs. That was key in gaining better understanding of the system!

No - that's not correct. RTH is calculated purely on straight line distance from the home point.

 

[old man mavic]

@sar104 may i ask you a question related to this thread please ,i understand that as i fly around with my MPP, that as the flight progresses the remaining battery level displayed on the moving line will show me an H, which is the point when without any intervention from me, that the drone has calculated i have just enough battery left to return home safely
to the home point
so may i ask this question based on the theoretical scenario i will set out now
i take off and fly one thousand feet from the home point in a straight line,then i hover and wait till the display gets to the H low battery return home point,and returns home
i then put in a new battery, but this time i fly out in an arc finishing up at the same point one thousand feet away, but having flown two thousand ft to get there
would i be right in thinking that my total air time would be the same for both flights even though i had flown a thousand feet further in flight no two
both these flights would theoretically have taken place in identical conditions

 

[sar104]

@sar104 may i ask you a question related to this thread please ,i understand that as i fly around with my MPP, that as the flight progresses the remaining battery level displayed on the moving line will show me an H, which is the point when without any intervention from me, that the drone has calculated i have just enough battery left to return home safely
to the home point
so may i ask this question based on the theoretical scenario i will set out now
i take off and fly one thousand feet from the home point in a straight line,then i hover and wait till the display gets to the H low battery return home point,and returns home
i then put in a new battery, but this time i fly out in an arc finishing up at the same point one thousand feet away, but having flown two thousand ft to get there
would i be right in thinking that my total air time would be the same for both flights even though i had flown a thousand feet further in flight no two
both these flights would theoretically have taken place in identical conditions

All other things being equal, if you fly to that fixed point via a longer path then it will take the aircraft longer to get there, and it may (see below) have used more battery when it does get there. So it would then hover for less time before smart RTH is activated. But that doesn't mean that the total flight time will be the same because those differences will not necessarily cancel.

If you consider the battery to contain a fixed available amount of energy, and use the simple (and not correct, but bear with me) assumption that power (rate of energy use) goes up with tilt (and therefore airspeed) because the motors are working harder, then total flight time available would be maximum if you take off and hover, and then decrease from that with any lateral flight. The more time spent with tilt, the quicker the battery is depleted.

So in your example, more energy is expended getting to the fixed point via the longer route, less time, and therefore less energy, is expended in the hover, and the same energy is expended returning to home. And since the rate of energy use is higher while flying than while hovering, total energy will be the same but total time will be less.

Back to the assumption about rate of energy use - it's wrong because it ignores aerodynamic lift, and so minimum power (rate of energy use) actually occurs at non-zero airspeed. That's the reason for these specifications for the Mavic 2:

Max Flight Time (no wind) 31 minutes (at a consistent 25 kph)​

Max Hovering Time (no wind) 29 minutes​

Max Flight Distance (no wind) 18 km (at a consistent 50 kph)​

Assuming that DJI did their testing correctly, minimum power occurs at an airspeed of 25 km/hr, and yields 2 additional minutes of flight time over hovering.

So in fact it is possible that the longer route, if flown with a Mavic 2 at 25 km/hr, will actually yield a longer total flight time than the shorter route, because more time will be spent in the minimum power state.

 

[old man mavic]

All other things being equal, if you fly to that fixed point via a longer path then it will take the aircraft longer to get there, and it may (see below) have used more battery when it does get there. So it would then hover for less time before smart RTH is activated. But that doesn't mean that the total flight time will be the same because those differences will not necessarily cancel.

If you consider the battery to contain a fixed available amount of energy, and use the simple (and not correct, but bear with me) assumption that power (rate of energy use) goes up with tilt (and therefore airspeed) because the motors are working harder, then total flight time available would be maximum if you take off and hover, and then decrease from that with any lateral flight. The more time spent with tilt, the quicker the battery is depleted.

So in your example, more energy is expended getting to the fixed point via the longer route, less time, and therefore less energy, is expended in the hover, and the same energy is expended returning to home. And since the rate of energy use is higher while flying than while hovering, total energy will be the same but total time will be less.

Back to the assumption about rate of energy use - it's wrong because it ignores aerodynamic lift, and so minimum power (rate of energy use) actually occurs at non-zero airspeed. That's the reason for these specifications for the Mavic 2:

Max Flight Time (no wind) 31 minutes (at a consistent 25 kph)​

Max Hovering Time (no wind) 29 minutes​

Max Flight Distance (no wind) 18 km (at a consistent 50 kph)​

Assuming that DJI did their testing correctly, minimum power occurs at an airspeed of 25 km/hr, and yields 2 additional minutes of flight time over hovering.

So in fact it is possible that the longer route, if flown with a Mavic 2 at 25 km/hr, will actually yield a longer total flight time than the shorter route, because more time will be spent in the minimum power state.

@sar104 thank you for your comprehensive answer ,i always appreciate you technical expertise ,on this forum ,and have learnt a lot from the answers you give to others