Oh well, just cos I enjoy an argument - "In straight and level flight at constant speed, ie flight at constant altitude and speed, all the forces cancel out". I would suggest - almost, but not quite!! If you set off in straight and level flight as above (and keep going!) you will end up back where you started having flown a circular path around the earth. There must be a resultant centripetal force acting towards the centre of the earth, ie in the same direction as the weight vector.
Forgive me for being flippant but it is a very quiet Sunday morning.