It is true that, for a given load factor, there will be less bending moment at the wing root when the weight (in the fuselage) is less. However it is not just the wing root that must be considered. Other important bits are also designed to the limit load factor, for example the engine mounting. You should not therefore exceed the limit load factor however lightly the aeroplane is loaded.
At speeds below Va, you will not (in theory) exceed the limit load factor because you will stall first.
For example, suppose that Va at maximum weight is 100 kts and Vs is 51 knots. If you make the aeroplane do an accelerated stall at 100 knots at maximum weight you will pull 3.8 g just before the aircraft stalls.
Now consider that we reduce the weight so that Vs is 45 knots. If you fly at 100 knots and pull hard you could now exceed 3.8 g before reaching the accelerated stall.
In this case Va is now 88 knots.
Last edited by Rivet gun; 15th April 2006 at 10:57.