RE: Webra Speed 91 (old)
Mr Hughes,
The 14x4 will be too small a load on that high torque Webra.
I believe Webra recommended a 14x6 as a minimum, for this engine at that time.
A 15x4 is the bare minimum.
And... you might encounter some overheating, if hovering is your favorite mode of "flight"...
I have seen it happen with some other .91s.
This engine was designed for flying at speed, when planes were actually being flown; not hovered...
Hovering is for helicopters, which have engines with a special cooling head to keep them at proper operating temperature, with the reduced airflow.
Real planes; including those that can 'hover'; like the Edge 540, Extra, Cap... all have prop pitches that are at minimum ~65% of the diameter...
That would translate to a 13x8 prop on your Webra, which will actually fly the plane nicely too...