I had bad problems doing 8 to 9 hrs a day sling loading in helos, i can speak for helos, its a combination of constant feet position on the two pedals and the two shoulder postions for the collective and cyclic, all this together is back pain. I started using a roll cushion its about 12 inches long and about 7 inches in diameter, you sit it at your low spine above your butt and youd be surprised what a difference it makes. A guy i flew with loved my cushion so much it became a part of the aircraft, it worked for me and I even use it in my car, give it a go, see what happens