I am rebuilding a crashed park flyer with my own designs, and would like to understand the 'thrust angle' better.. the WHY of it.
Vertical Thrust angle:
Attach an engine to any object, if the thrust line passes through the C of G, the object is dragged in a direction. But if the line passes over it or under it, the object is also rolled forward or backward in the direction of the pull. So I take it the thrust angle is to get the thrust line to pass the CoG (as viewed from the front or side of the airplane). The engine in this case will point down only if it is located closer to the fuselage than the CoG. If it is higher than the CoG from the fuselage, it should point upward so the thrust line passes through the CoG.
Secondly it regards the AoA of the main wing. There will always be a positive AoA on the main wing (in straight flight). Which means that either the fuselage will point slightly higher and the engine should be mounted 'lower' so in a straight flight it is level. Or the wing itself will be mounted on the fuselage with a slight (sufficient) AoA so the engine can be mounted parallel to the fuse.
Third, is the drag of the airplane, and the thrust line should ideally pass through the middle of the drag face. View a park flyer from the front, and the main wing's thickness is the biggest 'face' of the airplane. Everything else is relatively thin. Which means the center of the drag 'face' is slightly higher. Again mounting the engine higher and yet parallel should be equal to lower and pointed down. This one should be taken with a grain of salt since the drag face changes with speed as the AoA on the whole aircraft changes.
Of these three issues, the CoG is the most significant. Do I have it right?
Horizontal Thrust angle:
I dont get this one. Other threads have said the prop coming down is more efficient than going up, which is why the thrust angle is 2 degrees to the right. But in a level flight if the motor is pointed into the direction of motion, why would the prop going down be any different than when its going up? I'm sure its not gravity. If the angle of the motor is slightly different from the air velocity, shouldnt the motor be adjusted so it is in a theoretically level flight horizontal; so that the prop is efficient and no sideways adjustment is required? I cant come up with any reason why a prop going up or down would be any less efficient.