The smart fly system kind of sits between the receiver and the ignition for the engine. its a two-piece system, with a fiber optic wire between the two pieces. If the piece on the receiver end senses a loss of battery power, it will kill the fiber signal to the other end, which will cause the ignition to shut off.

This "loss of battery signal" can be caused either by a dead or failing battery (safety feature), or if you set a radio switch to the particular receiver channel that the smart-fly is attached to... you can kill the power going to that switch, causing the smart-fly to cut ignition power... no moving parts or weight like a traditional servo, very limited battery drain, and overall a much more dependable, safe setup. Many RC fields and virtually every competition event requires electronic shut-off systems.

The down side of this type of setup is that you'll probably need at least a 9 channel transmiter and receiver, given all the other channels you want to set up with dedicated controls for each aileron and elevator.

But, it sounds like you had a channel and servo planned for a cut-off servo anyway, so you should be in good shape.

You can get a much better description of the system by going to http://www.smart-fly.com/

There are certainly other systems, but this is probably the most popular.