What determines the gross weight of a homebuilt aircraft?
I'm building a Zenith Zodiac CH601HDS. The specs from the manufacturer state the maximum gross weight of this aircraft as 1200 lb, but I have seen several references from other builders of this model who have stated their gross weight as reset to 1300 lb. Is it as simple as saying "I won't ever go anywhere near 6 G so I can restate the gross to anything I want?
Answer: Your question is a good one, as there is much misinformation floating around regarding gross weight on homebuilts. From a regulatory standpoint, the builder of a homebuilt is in fact the "manufacturer" of that individual aircraft, and is allowed to set the weight limits, including gross weight, anyplace he/she cares to. There is no restriction on what weight a builder lists as the maximum gross weight, regardless of what the aircraft designer or kit manufacturer recommends. This is why you see many homebuilts with gross weights that differ from what the kit manufacturer’s aircraft calls for.
From the safety standpoint, it's a different story. As you point out, the gross weight of a particular design is set so that the structure will withstand the design loads without failure. If the aircraft is designed to withstand, for example, plus or minus 6 Gs at a particular gross weight, it will not be able to structurally withstand the same limits at higher gross weights. Builders who set gross weight limits higher than what the designer specifies are truly "experimenting", as they are taking the chance that the airframe will withstand the higher loading without causing problems. In addition, remember that during the test phase of the aircraft you must test the aircraft to the max gross weight before you can fly it at that gross weight during normal operations (e.g. before taking anyone up after flying off your flight restrictions).
Another factor to take into consideration is performance, particularly during takeoff and climb. An aircraft with a particular engine installation will obviously not be able to take off in the same distance or climb at the same rate if it is operated at a higher gross weight than specified. Thus, again the builder is truly "experimenting", as the published performance figures will not be realistic.
EAA does not recommend licensing or operating homebuilt aircraft at gross weights higher than that which the designer specifies. If a greater useful load is needed, the builder should select a different design that more closely matches his/her needs.