I hate to be the bearer of bad news, but I suspect that the 400 motor will be a dog at 7.4 volts. My experience is that they wake up at about 9.6v and come to life on 3S lipos. Motor life is compromised on the 3S lipos if you are not careful with prop selection, however. If you already have made an investment in the 2S lipos, I would suggest looking at the 350 drive for your plane.