According to Random House, rucksack is a kind of knapsack carried by hikers, bicyclists, etc. If you go back to the origin of the term, the frame is the 'ruk,' and the sak hangs on the ruk. I think of rucksacks as early external-frame backpacks as seen in Bavaria and the Austrian and Italian Tirol. The external frames were mostly made of wood and secured with leather thongs. Picture a Tiroles dressed in lederhosen and carrying a ruksak.
I think the external frame pack w/magnesium or aluminum frame is a bit lighter (i.e., more efficient) than the self-contained 50 or 60-litre backpacks w/internal metal reinforcements. Either way, you should have a protective cover if you're going to check the pack on an airline.
If your internal frame backpack is small enough (30 or 40-litre) it may qualify as a carry-on. That might be something to consider. I, for one, couldn't make it 2-weeks with such a small pack, though.
[ 08-09-2001: Message edited by: Eddie ]