I have DiyDrones Hexacopter with 850kv motors.
Currently running with 3s 5000mAh & 10x4.5 plastic prop.
Hovering at 50% Joystick.
Max flight time is 8 min with 8m/s horizontal speed.
Payload without batt = 400g
50m above sea level.
What should i do to achieve 20min + flight time in 8 m/s horizontal speed.
2x 4cell 6000mah?
I recently had a 3DR Quad with the 880kv motors and APC props fly 24 minutes without any special preparations. All I did was mount 2 4S5000mAH 20C Turnigy batteries to it, wired in parallel obviously. It was pretty easy, and if flew fairly well.
The Hex should be capable of similar. Though with a 400g payload, that will certainly make it that much more difficult. I think you should be able to get to 15 minutes without too much trouble.
I do as well, 20 minutes is a cake walk with the right configuration. My Hexa gets over 35 minutes. Again as bill said, anything is possible with proper setup. But on that note, as a certain weight it goes down hill and fast, I have a huge Octo that suffer the weight problem which gets compounded further when I try to through more battery/mah at it. I think the most important aspect I have found is to get a frame with motors and props that is super light, then have the bulk of the weight come from the batteries - this has yielded the longest flight times for me on my hexas and octos. Of course off the shelf frame have limited me, so I have had to go DIY to get the best results.
2X $300 batteries....
I got 4 of those:( they 5c only, I cant fly my hexa with them, alarm goes off straight away. But flight ok in stabalize mode(if you fly very smooth) get about 16-20 min with nex5 11x4.7