RE: Some Questions
How long your battery lasts in flight and how long it last while left idle are two different things.
How long your battery lasts in flight depends on its mAh rating and rate of current draw. For a 50 size airplane, a 2000mAh should last atleast 5 flights after being fully charged. It will do more but its recommended to not try to squeeze out the last drop because you dont know what condition of charge the battery is in.
When left idle, NiCd loses charge with time. Again, how much charge is lost depends of degree of charge, mAh rating and condition of battery.
If you are concerned that your battery will get unusable due to not being used you are probably right. NiCd batteries are subject to a phenomena called "charge memory" in that a fully charged battery when left idle over time will "think" it is fully charged even when it has discharged. In this case it will show full voltage on a multimeter but will die out as soon as or shortly after being loaded. One way to get around this is to cycle your batteries during the off-season- charge to full and then discharge to minimum voltage recommended by the manufacturer. Many chargers can do this automatically for you but you can also do it by connecting the battery to your model and wiggling the sticks on your RX till it stops moving. Some TXs have a servo test feature that also serves to discharge and test your batteries. Most chargers will show a summary giving discharge capacity of a battery which should match the rated mAh otherwise the battery are on the way out. Some old batteries will "return" when cycled. Usually I dont such batteries
With respect to the tank, you should get a hydrolock if your engine is inverted with the tank position you described. If it is upright, the carb will simply overflow
Ameyam