Originally Posted by Dietr777
Exactly, now the key is to break the circle.. I am gonna try and run two 8000mah 4s packs to begin with at roughly 1.8lbs a piece. If I get good results then I am definetly going to consider going to two 4s 11000mah packs at roughly 2lbs a piece. I am going to start with a single data logger on one motor just so I can get a feel for what the motors are doing then I hope to eventually run a logger on each motor to see how different flight regimes draw different amps..
Your max flight time, for a given frame, as it relates to batteries, is given when the weight of your battery or batteries is one-half the ready to fly weight of the quad. Less then that, and you are below max capacity, more then that, and you have to lift more weight then the energy you gain. For the proof, look at this thread: http://www.rcgroups.com/forums/showthread.php?t=1733300
Go down to post #8 for the mathematical formula showing this. Post nine shows a graph of a quad showing flight time vs mAh, and the peak of the curve is just about where the battery matches the weight of the rest of the quad (14,000 mAh, in this case).
I found another thread about efficiency in general, 89 posts long, and I mainly just skimmed it and bookmarked it for later. It may be of use, it talks about other things then the batteries, because although good quality batteries and matching the weight to the weight of the rest of the quad is important, there are other important topics. Motors are more efficient as a certain speed/loading. Propeller size matching and other factors. ESC efficiency. Motor speed, pack voltage (3s, 4s, 5s, ..), and so forth. http://www.rcgroups.com/forums/showthread.php?t=1719368
Those are the only things I had saved in regards to long flight times, which are of interest to me as well. However, I myself cannot yet implement them, as I have not yet built a quad. I have an order place, and it got to New York on the 3rd or so, and last I knew it was still sitting there (USPS tracking is down right now). Hopefully my research was able to help you.
Now I'm curious. If the max battery size for longest flight time always equals one-half the weight of the quad, for a given frame, then what happens with a heavier or lighter frame? You raise or lower the weight of the battery to match, but there is more or less weight, respectively. Hmm...