I have smoked old servos by switching from a 4 cell to a 5 cell pack.
A 4 cell pack will charge to 6.0V but will drop down to 4.8V in a few minutes under load.
A 5 cell pack will charge up to 7.5V BUT will drop down to 6.0V in a few minutes under load.
An A123 will charge to 7.2V hold 6.6V till it is nearly dead.

The old servos can tolerate a sort duration over voltage but can not take it forever.

Either run a regulator to get down to the lowest rated servo, or use all servos rated for the voltage you are running.
There is no function on a jet that is worth risking a servo failure.