RE: Graupner Speed 300
Where to begin?
About the only EDF that is designed for a sp400 motor (I believe the particular motor is a Mabucchi 380) is the HiLine Red Flame Blaster. The RFB's I have use a 4.8v sp400 motor, need 8 cells or a 2S.
At best, these little fans are anemic and leave a lot to be desired. The simply do not produce enough thrust with a 6 or 7.2v sp 400. Perhaps there is a suitable brushless motor that has the same dimensions as the sp400.
Putting two in parallel helps a little but the Wattage, Wemotech and other fans are better. GWS also makes some small EDF's such as their EDF55/300H combo as used on their A10.
Here is my TBJ mkII that uses two RFB's, powered by a 2S2P lipo. The plane flies OK but would shine with a couple of MF480's
I think you would be better off forgetting the sp300 motor and start with a more viable EDF/motor combo.