The typical curve for a transmitter battery should have it staying in the 9.6-9.8 range for more than 40 minutes. I can get something like 10 flights or more out of mine before it even gets to 9.6. I personally would never fly with the voltage less than 9.4. Suggestions such as "You can safely fly well into the 8 volt range" are dangerous.

Is this a new radio or second hand? If it's new, contact Futaba service and ask them about it. Either the battery is draining too fast or the volt meter is wrong. Either way, something is not right.