Prop wash. I have flown mine on gens ace 50c 6s 5300mah. Gives about 4 min of mixed flying with 20-25% left in the pack. I have tried nanotech 60-130c with similar results. ( although I will say I killed one of my nanotech packs. could be coincidence as I use these in my trex 700dfc and 800 trekker also) Higher C normally gives better internal resistance numbers over the cycles. Nanotechs seem to puff more than pulse or gensace as well. Moral to story, don't skimp on batteries. Get the best you can afford.