I think there are a lot of misconceptions about SFG's in the modeling community. Upon seeing a SFG mounted to a model aircraft for the first time, one might think its purpose was to merely increase surface area during KE flight.

In actuality, there are many aerodynamic forces working at/around the wingtips of our models, changing with each change of speed, pitch, roll and yaw that most of us don't understand or consider when looking at a static model sitting on the flight table at the field.

While I don't claim to be an expert, or to know and understand ALL of the effects that SFG's have on our models, this explanation helps to wrap one's brain around the various aerodynamic principles that are in play and how SFG's may (or may not) affect the flight characteristics of our models.

http://en.wikipedia.org/wiki/Wingtip_device

Regards,

Astrohog