B Matthews told it straight -
to add-- all of these "side thrust /down thrust angles are simply efforts to compensate for a model which is likely going to fly a rather high angles -getting going -or slowing down
Compensation is the key word
IF the model was extremely light (extremely low wing loading) non of this would make any difference.
When the prop (a spinning thing) is forced to slide as it spins-- slide up or down or sideways , it resists this motion --you can test this for yourself with any small electric model held in your hand - or using a toy gyroscope top you will see one of the forces involved
so --IF the plane could always fly at a very low angle of attack - these forces would never amount to a hill of beans
Scale warbirds typically end up with high wing loadings - -and so the plane is often operating on the ragged edge of flying well - -it is at these angles -the prop skidding forces are at maximum.
If flying speed is kept up and th plane is gently maneuvered -th forces are at minimum.