Hi,
my first post here.
After flying my FC40 for a month I found that flying with 2 batteries gives you more stability and smoother control. Because with 2 batteries it is easier to get a good balance. Plus, the added weight makes the Phantom a bit less jerky. First I tried to attach both 2200 3S batteries to both sides on the bottom of the landing gear. This improved the handling and the video's were smoother especially when panning. But in GPS mode the pendulum effect caused the Phantom to overcompensate so I had to think of something else. And then I found a piece of trunking for electrical wiring laying around in my garage, such as this:
http://www.wickes.co.uk/Mini-Trunking-25x16mmx2m-White-PK1/p/109644.
I cut off two pieces of about 1.5 x the battery length and drilled a few holes in them and attached them on each side with tie wraps to the arms and upper part of the landing gear. This was only meant as a temporary setup, just to test it. The batteries fit tight and slide in the rails/trunks. After that I turned the Phantom upside down, on a scale, so I could find the center of balance and check the weight. I slided the batteries in the rails until I found the perfect balance with Gopro (no gimbal yet) and FC40 cam (mounted to the battery door with velcro). Then I secured both batteries with a velcro strap and I was ready to do a test.
The Phantom rose up completely level. Flight time seemed to have improved as well. I have my Lipo alarm on 2.5 and I got well over 15min flying time with a minute to spare before autolanding. Flying was so incredibly smooth, I just can't believe it. Total weight is around 1250 grams but there was no sign of any problem for the Phantom to lift itself with the stock props. With full throttle it just takes a little longer but then she gets up in air in a straight line and she hovers completely stable. Didn't need to do anything with the gains at all, they are all default.
This way, for just a few bucks you can get the best and most flexible dual (or triple even when you use the battery compartment as well) battery setup you could wish for. If you add weight in the form of extra camera's, gimbals, FPV stuff etc, you can just adjust the position of the batteries by sliding them back or forth. You can also easily attach other stuff to the rails as well.
A little bit nose heaviness turns out to be best for fun flying I found out. But you can adjust so if you want the smoothest possible video you just balance the Phantom perfectly so you have the same tilt on every side. That way, when using course lock, the Phantom will stay level wherever you point the nose (depending on your flying atitude).
I'll see if I can make a few pictures or video of the mod. Don't know how to attach a picture here right now....
Cheers
Jan