W. Utomo, this is a laymans explanation, but one that I think is valid.

Down thrust. Most planes requiring a large amount of dow thrust are those who's thrust line, the axis of the crank shaft, is below the source of drag on the airframe. The wings are usualy that drag source. The photo is an example of extreeme drag and no down thrust. This is a Radio controlParafoil designed and built by a local college aerospace class. It is in level flight at the time the photo was taken. As you can see, the thrust line was way below the maximum drag. The results is for the thrust to arc upwards with the drag as it pivot. More down thrust on the engine would have put more stress on the lift of the wing, but would also have given more forward velocity, resulting in more lift. In this case, drag won over lift, photo 2

A later flight in which the "balance" of the engine cage was changed so the engine had more down thrust was a success, other than stearing issues. I look forward to recording the next flight.

I'll leave someone else the pleasure of describing how right thrust keeps the plane going straing on the ground.

Don