Ultracapacitors for electric RC airplanes?

One of the big issues with a capacitor as a storage device is that the voltage drops as you drain off charge. Drain off half the charge, and the voltage drops by half. You would have to design a power system that operates efficiently over a very wide voltage range.

As for energy density, let's do the math:

The caps you cited have a Vmax of 2.5 volts.

Assume we put 4 in series so we can charge the pack to 10 volts. [now that pack weighs in at 8+ ounces, and has a capacitance of 87.5F].

Assume we want to provide 40W of power. Initially, we draw 4A at 10V, but the voltage starts to drop exponentially, and the current has to increase exponentially to maintain a constant power level.

At T=80s, The voltage has dropped from 10V to 5V (and current has to increase to 8A). At T=100s, the voltage has dropped to

3V, and at T=110s it's 1V.

It's probably impractical to desing a motor controller and motor that operate well over much more than a 3:1 voltage range, so that limits flights to 100 seconds.

You've got a "battery" that weighs 8oz and can put out 40W for

100s.

That's equivalent to an 8 cell 120mAH NiCd pack that weighs 8oz.

Don't know what sort of planes you build but 8oz isn't "almost nothing". It's about twice the weight budget I'm allowing for batteries for my next model.

They have been proved to be almost completely useless.

At best they allow a otoir ro rev up a bit quicker, but at the sort of currents in use, they are effectively flat in less than half a second.

They are marginally useful for car racing where they may hold charge long enough to get a bit more grunt coming out of a slow corner.

A half second?

If you charge one of those 350F caps to it's max rating of

2.5V, you've got a charge of 875 coulombs stored up. To drain that in a half second, you'd have to draw 1750 Amps for the entire half second. Even allowing for "flat" to be defined as 50% discharged, you'd still have to draw 875 Amps for half a second.

The cap: (1/2)*(2.5V)^2*(350F) / (2*oz) = 547 Joules/ounce.

A 500mAh from SR: (1V)*(500mA) / (0.66oz) = 2730 Joules/ounce.

So my little speed-400 job with the 7 cell pack (weight = 4.6 oz) would have to go to a 12 capacitor array (weight = 24 oz), for an overall aircraft weight of 36 oz -- and that's before I build the switching motor controller that'll deliver the power to the motor?

Well, I suppose it'll crash less often, seeing as how it wouldn't get airborne.

I agree, it doesnt sound practical but a good thought.

I built what I call a zero length electric high start for a glider. It supplied enough power to the motor to get it up to altitude and then stopped. It powered a speed 400 and had a 750 mah battery plus a solid state circuit that timed out. To launch again push reset and start. let er go. I never tried it on the airplane....but bench tested it. Impressive. It may no longer be practical with current tech.

If you read Flying Models magazine you'll find that the electric freeflight folk do just this, only with tiny little models that are essentially electrified rubber-powered models. Flying Models, by the way, is a great magazine if you can't decide what to fly when you grow up -- they still manage to publish at least two construction articles a month of not three, and it's still a pretty even mix of RC, CL and freeflight.

For some reason I find Flying Models to be my favorite magazine, although I'm strictly RC. Always something to pique my interest. Now if they'd only make it bigger....

PCPhill

I fly RC and CL when I get the chance. I fly RC because I could never get a FF trimmed to my satisfaction, and because I always end up living in the hills with lots of trees.