I've been looking into the issue of brownouts in a bit more detail lately since I've been experiencing them when I was bench-testing the Electra I'm building. The issue I've had is more to do with the linear BEC in the ESC overheating than with the main pack being too low. Most of the ESCs that have linear BECs (rather than switching) are ridiculously overrated. As an example, consider the one I'm using for the Electra:
Mystery 30A BEC Brushless Speed Controller (Blue Series)
http://www.hobbyking.com/hobbyking/stor ... oduct=9483
Spec.
Amp rating: 30A
Burst Rate (10sec): 40A
BEC Output : 5.0V / 3A
Voltage: 7.4-11.1V (2-3 cell Lipo/5-10 NiCd)
Dimensions: 43x23x6mm
Weight: 25g
Now, if you run this on 3S, like I am, than in order to produce a 5V output from the BEC then the voltage regulators on the ESC are going to need to drop somewhere around 7V (assuming about 12V from the pack under load when charged). At 3A out, this would be 21W dissipated in the regulators - there are 3, and looking at the datasheet they are rated for an absolute max of 1A each. There is absolutely no way that those parts can handle that much heat, especially with the heatsink being on the other side of the ESC, and there being no air movement (on the bench). In practice, I find that the BEC portion starts browning out at around 0.6-0.7A continuous. This equates to about 4-5W being dissipated in that area of the ESC, which I would find much more realistic (and is closer to what I might expect a design like this to do). During the brownouts, I can measure the voltage gradually dropping until it goes into the 3.5-4V range, where the AR6200 I was using began having issues and losing its lock. I'm starting to suspect that this is the cause of an awful lot of the 'radio troubles' that tend to bring down planes - I can easily see getting to these currents even with smaller servos if the linkages were binding at all.
Unfortunately, if this is the cause of your brownout, then the capacitor linked to earlier will have virtually no effect. Under the loads I was looking at (2 mini/micro size servos moving continuously under no mechanical load) the 3300 uF would give you somewhere around 10 ms at best before your receiver would be at 3V and shutting off. In practice, genuine Spektrum receivers seem to shut off at about 3.5V from my testing. The real reason Spektrum sells these capacitors is to help reduce brownouts induced by static for RC cars - the static is a higher frequency effect, so the capacitor does genuinely help to smooth out the voltage in that case.
The only thing this really helps with in airplanes is if you have any switching noise from the BEC, if you are using a switching type. Out of curiosity, I did try putting 6800uF of capacitance on the linear BEC output to see if I observed any difference at all. There might have been a VERY slight improvement in the frequency of the brownouts, but I certainly wouldn't trust a plane to it, since I could see it still browning out, just about 2/3 as often as before.
My conclusions are that I would derate most of the linear BEC outputs by a factor of 3-5 at 3S from what they claim, and anything with servos bigger than the 9g ones is going to get a switching BEC, either as part of the ESC or standalone. The linear BECs are probably fine at 2S, and in the smaller planes there is typically only one voltage regulator on the ESC, so they would be able to run closer to their rating (maybe 1/2). There are some 3 Amp Turnigy switching BECs for $4-5 that look good to me (although they claim 5A max: that's false, since the part they're using is only good to 3A, but they are genuinely capable of 3A). I've got a few on the way.
http://www.hobbyking.com/hobbyking/stor ... oduct=4319
If you wanted to put in a capacitor capable of doing anything (like helping with the additional load of retracts) Hobbyking has a couple of supercapacitors that would be able to source current for a couple of seconds, rather than a few ms. They can be found at the links below. In most cases, though, I think a bigger switching BEC is probably a better bet for the weight. I did order the first one for my RC sailboat to help ride through the sail being pulled in, but in that case it's just running off a Nimh receiver pack, so I just wanted a little better instantaneous current capability.
Turnigy Voltage Protector 550000uf (1.4sec)
http://www.hobbyking.com/hobbyking/stor ... duct=17098
Turnigy Voltage Protector 783333uf (3sec)
http://www.hobbyking.com/hobbyking/stor ... duct=17100
Of course, I also think the Scorpion product Bill started the thread with looks like a good idea too. But I figured I'd post this before everyone ran out and bought the capacitors thinking it would make their planes safe.
