I thought I explained. You either measure the current draw with an in line wattmeter or you look things up in a table like the ones I suggested. You can also use calculators such as motocalc but I've never understood those. Here's a simple example. If your setup draws 20 amps at full throttle and you have a 2 amp/hour (2000mah) battery then the flight time will be 1 hour divided by 10 or 6 minutes at full throttlle. That's best case. You can't calclulate it until you know the current draw. You need to know that using one of the methods I outlined. Personally, I don't pay attention to the flight time. I can tell when the battery is beginning to get reduced in voltage enoughthat I need to bring it in just by the waythe planebehaves. It starts losing power from the moment you take off because the voltage decreases as the battery dishcharges. You don't notice it, however, until the voltage gets down enough to make a meaningfuldifference in the way the plane behaves. When it does, then it is time to land it and change batteries.

ESC's have a low voltage cutoff to keep you from discharging the battery too far. What it does is cut power to the motor but leaves power through the BEC to the receiver so you still have control.Since you are running 6S you will be usinga separatebattery for the receiver anyway.However, the low voltage cutoff can occur when the plane is too far away for a dead stick landing so I never let things go that far. When the plane begins tofeel sluggish, I bring her in. At that point the battery usually reads around 3 1/2 volts per cell. I replace it and take off again.

I have a tendency to overbuild the electric systems on my planes. I use motors that are a little more powerful than I need and prop them down a little. I use ESC's one size too big and I use high C rating batteries. This all adds weight and wing loading to the plane but I'm a sport flyer, not a competition pattern or 3D flyer, so it works for me. My planes run very cool and reliablyand produce typically 8-12 minute flight times. I have a 3D foamie that only gets about 6 minutes because I have a smallerbattery in it than I would usually use in order to keep weight down. You can adjust the flight time, as you know, by changing to a battery with a different amp/hour rating.

I generally carry 3 or 4 batteries for every plane I take to the field. That's all the flying I want to do.

I'll give you some parallels to a fuel plane. The battery is the fuel tank. It's size is the amp/hour rating. Higher amp/hour ratings mean larger tank with more fuel. The current is the fuel. The C rating is the fuel line. It needs to be large enough that the motor can draw enough current to do what you want it to do just like you would need a large enough fuel line in your nitro plane to keep from starving the engine. The voltage is like the fuel pressure. In fact voltage is electrical pressure. Watts are the horsepower. By ohm's law power is equal to the current (amps) times the electromotive force (volts.) This formula will handle about anything you need to calculate in the electric RC world.