britbrat makes a good point.

What I said in my earlier post MUST be applied within given restrictions. Props slip and some blade shapes are more efficient than others. The prop must be correctly sized to the engine AND plane. For example, A 16-8 will fit the Zenoah G-23 on my 14#, 1/4 scale Fokker Dr.1, but will it fly the plane? Probably not, or not very well. It just doesn't move enough air to overcome the drag of the airframe. A 12-4 will fit the .40 on a pylon racer, but will it fly? Probably, but it will be slow and mushy. Phrases to be aware of are "engine loading", "engine over-revving", and "cavitation". All three are very important. The prop recommendations made by the manufacturer are not written in stone.

The "truth", as you put it, is to find the prop that flys the plane the way you want it to.

Dr.1