| Does it make any sense to use a BEC on a electric sailplane?
Yes. Having one battery is often more convenient and lighter than having two.
| If I use the motor until the BEC shuts down the motor and catch a | decent thermal, it seems like I am likely to run the battery down to | zero and lose control.
Well, most ESCs with BECs shut off power to the motor when the voltage reaches 5 or 6 volts, leaving you with enough power for the receiver and servos for a little while. Exactly how long a little while is depends on your battery and equipment.
| With a gas gauge (ESV) on a seperate rec battery, I will have a good idea of | how much longer I can fly.
Your ESV probably uses more power than your servos and receivers average out to ... so keep that in mind.
If you do leave the ESV hooked up to the receiver, drawing power through the BEC, to test your remaining capacity, check the ESC occasionally to make sure it's not getting really hot. It might, as you're drawing more power than the receiver and servos usually do on their own.
| I am brand new to electrics and have been out of RC for a decade and I am a | bit lost in the new technology but my plan is to build an Astro-Challenger | with an Astro Cobalt geared 050, folding 12 x 8 prop, 7 cell Nicad power | pack, a 250 ma receiver pack, and an ESC with a brake like a Castle | Creations Griffin 40 with the BEC disabled.
That sounds fine. But I'd investigate how much power I had after the ESC shut off the motor. Enable the BEC, remove the 250 mA pack, and go flying. As soon as your motor cuts out, land.
On the ground, start wiggling your servos, and keep wiggling them. Every 10 minutes or so, stop and hook up the ESV to see how much power you've got. Once that drops below 4.4 volts, it's time to stop.
See how much time you've got left. Once your motor cuts out, that's about how much flight time you've got left. If it's enough, then you can ditch the 250 mA pack and enable the BEC.
If your ESC permits it, you may be able to raise the cutoff voltage, giving yourself even more time after cutoff for gliding. (I don't think yours lets you adjust it.)
You can repeat the test with the 250 mA pack if you wish -- it's possible that the BEC may actually give you longer flights after the ESC cuts out than the 250 mA pack gives you.
Personally, I'd use the BEC. At least with that, you have a good idea if you're about out of power -- the motor won't go. With a small pack like the 250 mA pack, it may be that you've still got motor power but your receiver pack is dead, which could get really interesting. If you're worried about running out of power after the ESC cuts off power to the motor, just make it a point to land shortly afterwards (instead of trying to glide for another hour.)
If you do decide to go the no-BEC route, I'd suggest a 700 mA AAA NiMH pack instead. Weight is probably about the same as your 250 mA pack, but duration will be much improved.