Paper mentioned above: http://www.imavs.org/papers/2013/16_IMAV2013_Proceedings.pdf
Thanks, that makes things a little easier to understand
Blue skies, 75deg, 12mph wind. I decided to try some battery options. I rigged up a parrallel harness and combined 3 750mAh batteries. One battery in the batt holder and the other two zip tie4d on below. The combined voltage was at 4.08v. I hand launched because it would not sit on the pad level. It seemed to fly well but I am not into aggressive moves. Slow and stable is fine with me. I realized that I had forgotten to start the timer but it was too late now so I don’t have any comparrative numbers. I fooled around for a while, taking it up to 170’ in POS_HOLD mode and it held position although it had to pitch hard to windward.
Next up was a pair of 1200mAh batteries combined voltage 4.14v. and this time I was prepared with the timer. Hand launch again because of the belly load but it flew normally, very steady and I had no problem with handling and manueverability. I kept it in a low hover close by as I was going to try to get a photo of the belly mounted batttery and I was most interrested in flight time.
Suddenly it began to descend. I could not stop it with the throttle. It hit the ground at 1min 12sec of flight… I could get it to re-arm but it did not have the will to fly. Battery voltage was 4.02 by the time I got in the shop. I had no more time to work with it so I will get at it again tomorrow. The combined weight of the batteries was within a few grams of the 3 750mAh batts so I expected similar results. I did not expect it to give up after 1min or so.
Jerry, although you had 4.02V resting voltage at the end, you had less voltage than that in the air due to motors running/propellers spinning. Most likely explanation is that with the extra weight your Viper could not deliver to the motors (or the motors could not produce enough mechanical power out, with max electrical power in) the required power (voltage x current) at that voltage to stop descending.
If you have a log it will most likely confirm the story, showing what the voltage in the air was.
Olivier, that is what I expected. I realize that voltage will rebound but I did not expect it to be so great. I have the low voltage limit in the flight parameters set at 3.22v and I get enough warning for the RTL command to bring it home and land. I suspect that there may be another answer which brings up another question.
Today, I was flying with just one battery 1200mAh and the flight time was normal (about 7min) with the first battery up until the end of the flight. It was above 100ft and began to descend without my control. i hit the RTL button and the Skyviper did respond as expected although it did not make the normal LOITER time before settling to the ground.
I made a second flight using the second battery charged to 4.17v (same as the first) and it only lasted about two minutes before going into LAND mode and descending descending rapidly, ignoring all control attempts. It seems to me that I have a bad battery that reports 4+ volts at rest but falls rapidly under load. When paired with the other battery in parrallel, it sunk the combined voltage and if I knew how to interpret a log file it would probably show that, but I don’t.