Even the F-16 loses altitude during a knife edge pass. https://www.youtube.com/watch?v=rHm9ZDPRRQw
Because the F=16 does not have enough rudder to maintain its altitude and the shape of the fuselage is designed for "normal" flight ops, not a knife edge. If you will observe, many of the aircraft which are designed for aerobatics completion has a large rudder and the fuselage is designed to produce some amount of lift. Example, the 4 and 8 point rolls is another maneuver which would require a certain amount of lift from the fuselage to successfully complete those maneuvers during a aerobatic meet. As someone pointed out in these replies, the prop also plays an important part in the knife edge and again, is designed with the attitude that it, the prop, will have all different angles of attack during aerobatics. One aircraft comes to mind, the Zlin250lx I think is the name.
While I am not an engineer, I would think the mission profile for a aircraft design would have a great deal of influence on the prop designers as related to obtaining maximum efficiencies from the prop. Put another way, don't think a prop designed for a bomber, would work very well on a aerobatic aircraft design.
