pic: 6CIM WCD

[T^2]

What hasn't been mentioned yet is that belly pans are almost critical for resisting shear -- never mind torsion. My bosses at work call shear the forgotten force, and for good reason; it's more difficult to simulate than simple point/line loads or torsional loads. Something as thin as .025" sheet aluminum, even with additional lightening holes, will make your whole frame much stiffer when it experiences shear forces.