Why - any thoughts ??
CJ
The usual reason for an overheating motor is either the wrong motor, the wrong prop, the wrong voltage - or perm any two and then add one for fun!
I can't find any current listing for the Robbe PT15 but it looks at least three feet long from your pictures. On a single prop I wouldn't run anything less than a Speed 700 in such a model. An X55 is altogether too much for any brushed motor on the sensible side of a Speed 900BB. I'm not surprised that the poor little Deans motor is getting worked up - it's like putting a 1400cc petrol engine into a quarry truck and then trying to drive it up a wall in fifth gear.
Unless you have a burning desire to go brushless then I would say to remove said Kestral, fit a Speed 700 9.6v motor (#G3308) and run something like a 40mm 3-blade brass prop.
Suit yourself, as someone once said.
DM