Perhaps you don't QUITE understand what the power needs are for a laptop...
Typically, you plug your laptop into a brick and plug the brick into the wall. ( AC-> brick (AC-DC converter) -> laptop @ ~15-17vDC )
Your car/airplane socket supplies 12vDC, thus you need a different brick: 12vDC (lighter socket) -> brick (DC-DC) -> laptop @ ~15-17vDC.
Now, you could PROBABLY get away with running your laptop on 12vDC as the extra 3-5v are for the charging circuit, but I wouldn't try it.
So, you'll need two rigs for the road if you plan on plugging in on a plane/car. If'n you don't wanna buy/carry two power rigs, perhaps you should look into buying and extra battery.
--JamesT
