you cant be for real the whole idea of the y lead is so the steering servo gets a full 12 volts and gives its max torque out,so what is the use of putting it on the throttle servo...pass..
OK - I didn't think I'd get away without justifying this.
The failsafe is on the throttle servo, so by putting the battery on the Y cable with the throttle servo, there are less points of failure between the battery and the failsafe - ie the battery doesn't have to go through the receiver and the extra associated plugs/sockets to get to the failsafe. Most cases of complete loss of control (at least where a failsafe is installed), are caused by sudden loss of power. If the power gets through the harness, switch, and Y cable to the failsafe, and you remove everything else, the failsafe should still shut the throttle down.
I would guess that either the rudder servo is either causing enough voltage drop to trigger the failsafe, or it is modulating/spiking the power lines with noise. Another possibility is that it is interfering with the RF signal to the Rx.
With such a big battery, if the voltage is dropping enough to trigger the failsafe, I would say that either the battery may be faulty, or there is a slightly iffy connection (probably either the switch, or one of the plug/socket connections).
With the Y cable on the rudder servo, the servo (power wise) is effectively teed into the connection from the battery to the Tx, so it has a higher chance of inducing interference into the power supply going to the Rx and the failsafe - ie the Rx is not receiving a clean power supply or "ground". Again, this will be made worse if there is a problem with the battery or the switch / harness.
I can see your point, but the loss on the power lines, through the Rx PCB - between channel 1 and channel 2 (or 3), should be very small. The biggest benefit (at least in terms of voltage loss) from a Y cable, is in terms of bypassing any battery eliminator or regulator circuitry etc - which may be associated with the Rx battery socket.
You are right, in that the way you have installed the Y cable will give you slightly more voltage at the rudder servo - not sure whether this would be easily measurable or not, but it compromises the protection given by the failsafe more, and I think it may (only may), be causing the immediate problem - I see it as a relatively easy thing to try out.
Of course, the rudder servo may just be faulty, and draining too much current (or draining too much current due to mechanical obstruction), or causing interference......
Ian