The Quapaw transport helicopter now works! You can lift anything up to the largest jet.
Use a bit of caution when taking off with a slung load, if you shoot up into the air too fast, you may wear your load. Durability is increased to aid survivability incase of any collisions.
Credits: Big thanks to Noiz on the Just Cause 2 Mods site. His modded civilian helicopter really helped in understanding how some of the values worked.
