"What if he bewitched the trunk to make it feather-light, tied it to his broomstick, covered himself in the cloak, and flew to London?"
—Harry imagines using the spell on his trunk in 1993.[src]
The Featherweight Charm (incantation unknown) was a charm used to make heavy objects lighter in terms of weight.
Harry Potter contemplated using the spell to make his trunk lighter in order to tie it to the back of his broom and fly to London in order to get some money from Gringotts due to his belief that he was a fugitive. However, he did not actually use the spell.
↑ 1.01.11.2[[Harry Potter and the Prisoner of Azkaban
↑Though she can carry it with apparent ease, the fact that it extends undetectably may mean that it also becomes lightweight as if there was nothing in it.
↑Underage students cannot perform magic unless in school, though it's possible that an exception can be made; this would also explain why, though in his first year Harry needed both the Weasley twins and himself to lift his trunk into the luggage racks, the other years he had no difficulty with it.