I have read some current and older posts questioning why some planes cruise with a nose down or nose up pitch angle. Some have indicated this is due to incorrect physics. Some of these planes may have incorrect physics, but planes do not always fly nose level. I will explain.
A wing generates a certain amount of lift. The amount varies based on speed, altitude and flap configuration. The wing is designed to generate a specific amount of lift for typical aircraft weight, cruise speed and altitude. If a aircraft is traveling outside of the optimal lift envelope changing the pitch of the aircraft will be the only way to maintain level flight at a specific altitude, weight and speed. On some real world aircraft this can be compensated for by proactively loading passengers and cargo to change the center of gravity. Or it can be done during flight by transferring fuel to fore or aft tanks. If your plane in cruising nose up or nose down and you wish to correct it adjust your weight before take off or adjust your speed and/or cruising altitude during flight. Changing the lift characteristics by using flaps can correct a nose up pitch, but in real flying certain flap setting can not be used at certain speeds so it is typically best to ensure you are flying at the optimal altitude and speed to achieve a neutral nose pitch angle.