But a big capacitor will be an elco which does little if anything about spikes. Big means slow, a bit like in “diesel engine”. It can never hurt, though.
Not sure I follow that… a capacitor is a capacitor, with a given ESR which may be too high for the job, but at high cap values one doesn’t get a chance to use a ceramic cap for example.
I think Jan is trying to say that Vic’s big capacitor will be a huge elco, which will stabilize the output, but won’t do much to spikes. Because high rise times of these spikes, it also will mean that these will have a high frequency component. For this high frequencies an elco is useless. As such the elco is good for low frequency stabilizing, not for fast rising spikes, one need smaller caps for those.
Conclusion: You should use high value and low value caps, high value for low frequency stabilization, low value for suppression of high frequency spikes.
Where would the spikes come from? If it is arcing from a commutator, that will have a very wide spectrum, yes. But if you have an alternator with a rotating permanent magnet, there are no brushes.
The main reason I’d go for a brushless motor over an alternator would be the lack of contacts to corrode, foul and otherwise go wrong. In the absence of these, If you were to use a linear regulator with a brushless system you shouldn’t have any high frequency spikes to absorb anyhow. In other words you’re moving the complexity from the mechanical side to the electronics which seems to me the right thing to do.
I suppose the obvious way to do it is to let the motor charge a smallish capacitor through a 3-phase rectifier then use a switching regulator to drop the voltage to something reasonable. If you aim to provide 12V 2A at 40 knots and have a speed range up to 120 knots then you’ll never have more than 36 volts, which isn’t particularly high. My model helicopter drops about 40v to 5 volts 10 amps with a little circuit that weighs a few grams and it has to deal with a load (digital servos, a 2.4gHz receiver) that is far more complex than anything you’d expect to see if you were powering a few radios via a battery. I believe it’s electrically quite noisy though.
I had a colleague whose previous job was in designing switching power supplies and my impression was that they were sufficiently full of ‘gotchas’ to make your life an absolute misery – particularly if you’re not certain in advance what load they’ll be connected to. This was a decade or so ago, so perhaps it’s easier now to buy a coil and a clever chip off the shelf and let them do their thing.
You could also hope to use a dimmer style circuit to regulate the voltage, hopefully without producing as much electrical noise as with a switch-mode supply.
But surely you shouldn’t need such a big heat sink to lose 40 watts at 120 knots, even in hot countries. And as 0.1% or less of your total power flying at that speed, it seems to me that the only reason to avoid it is to keep the electronics cooler.
I worry a little about how the bearings would hold up though – perhaps something to add to the 50 hour chores. I suppose if you oversize the motor and run it slower than the design speed they should do OK.
or maybe four of these at 1 kg :
3,2 V 8 Ah
Thanks for the suggestion: 3.5lb and apparently a generator rather than an alternator. if everything on usergroups is to be believed. Certainly closer to what I’m looking for than most of the larger offerings.
Electronics is only one side of the equation. You also need a propeller capable of giving you the right power at the right rotational speed, at the right IAS. Preferably at a wide IAS as possible, and with as little drag as possible. Although an open propeller with 2 blades (wind turbine style) certainly will be most efficient at one particular design velocity (at any given pitch), a multiblade ducted fan style propeller will be much less draggy and work over a broader velocity range. It will also have a stable characteristics, a propeller has not.
The main reason I’d go for a brushless motor over an alternator
Surely, generating DC is physically impossible with a brushless system. It has to be AC, which can then be rectified.
A “brushless motor” is of course possible, if you want a motor and not a generator, but it needs to be driven with AC, usually 3-phase.
Maybe I have wires crossed 
Well, rectification isn’t a problem (6 cheap diodes individually or packaged together) as you know.
When I say ‘motor’ I suppose I mean ‘generator’ but what I’m getting at is that it’s easy to buy little brushless motors of all different sizes which should work very efficiently as generators. And because they’re produced in the millions for hobbyists they’re much cheaper than a purpose-designed component would be – at least in small numbers.
Although an open propeller with 2 blades (wind turbine style) certainly will be most efficient at one particular design velocity (at any given pitch), a multiblade ducted fan style propeller will be much less draggy and work over a broader velocity range. It will also have a stable characteristics, a propeller has not.
Presumably this is why the commercial offerings are built that way. The thought occurred that I could hide a small ducted fan unit in the mouth of a venturi, though presumably neither venturi or fan would work optimally.
Sorry for this being a bit off-topic but,
In Germany otoh you really want one [transponder], even if only mode C.
What’s the practical use of a mode A/C-transponder in Germany? (Seriously interested as most aircraft in my club don’t have mode S and AFAIK Germany has a requirement for mode S.)
