A small clip from the high efficiency quad using 29 inch 3 blades props running of U8 135KV motors and 6S batteries.
The setup delivers ~35 min with 16,000 6S lipo and 1.8 Kg useful payload.
More scope for extended endurance beyond 40 minutes with larger batteries.
https://drive.google.com/file/d/0Bz5gyGz_E_bWT3lJTEpZQnNtOVk/view
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To increase 20 min more flightime over the 35 min you will need to add 28A more battery capacity, if your coptere can handle the new weight.