There are a few vintage motorcycle dynamos or alternators small enough
While this is true, there voltage regulation is often very poor, not a problem for bulbs etc, however no good for switching power supplies. I have had a couple of failed avionics power supplies on Rotax engine as well. Measuring the “regulated” output of the small in-engine generator showed quite some spikes.
BPE-14 14 Volt / 6 Amp wind driven alternator
That looks like nice stuff, but doesn’t really meet the requirement for looking more or less vintage… And am I the only one to find it horribly expensive? For that money one can have two handheld radio’s, rechargeable batteries included. The suggestion for an R/C motor seems much better, and indeed I think the regulator could be easily concocted, perhaps even a bridge rectifier, elco and PB137 would do the job? Add some inductors and ceramic C’s, though, spikes are the devil to avionics as Jesse stated.
Thankfully I think I can get my head round the regulation part of things. I’ve not considered it in detail, but my general feeling is that a linear regulator would be the way to go in order to avoid generating switching noise etc… Possibly in series with a current limiter.
I figure you could size the motor/prop to provide 12V 2A at 40 knots. Throwing away a few watts at speeds above this shouldn’t be too much of a problem – the more energy you have to dissipate the more air cooling you’re going to have available.
Bicycle dynamos would be another option – they use a clever arrangement with 1) high inductance windings so that the faster the motor turns, the higher the impedance and 2) cores that are designed to give large eddy current losses at speed.
Using something brushless should deal with some weather woes but I don’t know which motors would work out best in terms of lubrication.
I’d worry a bit about putting the propeller underneath – the Turbulent really is tiny and it would be quite vulnerable to mud and debris.
And perhaps a series fuse and a big Zener diode before you meet the expensive battery/avionics.
a linear regulator would be the way to go in order to avoid generating switching noise etc… Possibly in series with a current limiter.
The PB137 I suggested offers all that. Even if I seem to remember its current limit is at 1,5 amp, that should be amply sufficient. The chip was conceived for standby applications, I think, such as emergency exit lights. It also avoids draining your battery through the power source when inactive.
Building a voltage regulator for a DC generator is pretty easy. In essence you control the field current to achieve the desired output voltage. The trick is to make it work properly in the presence of ripple and muck coming out of the generator (brushes make for a very dirty DC) and to make it stable as a control loop (that you would just do experimentally).
This is one I built for a Yamaha YCS3E in about 1973:
Today one would do it much more elegantly, and anyway that magneto had some weird field winding arrangement. Amazing, when I think about it now, what one achieved all those years ago (aged 16) because one could not get any girls – because one was such an anorak 
In DC aircraft buses one achieves a smooth DC by having the battery there, acting as a huge capacitor. One doesn’t achieve it by dynamically varying the field current. One could, but nobody bothers because this stuff is straight out of post-WW2 trucks. I believe the GAMI “supplenator” (another device for ever “certified next year”) does this correctly, because that delivers clean DC without a battery.
If you don’t have a battery at all, then you have to either use a huge capacitor or regulate say 2V above the target and then use a linear regulator (a low dropout version of the old “7812” e.g. LM2940CT-12 – that one is made for this job and can take big spikes on the input) to clean up the output.
And yes, have some sort of “just in case” overvoltage protection… could be a big zener diode (on a big heatsink) or a trip circuit which turns off a series MOSFET. This is my ground power unit for the TB20, on which I took no chances, with one protector inside the switcher power supply and another external (the BC337 etc stuff):
I am building another such PSU for an A&P friend of mine, switchable 14/28V and with a +2V overvoltage trip on that too.
Peter, sorry but your first sketch is not really to the point. Simple generators like an “inverted” r/c motor, or the two series coils on the Rotax engines, have no field coil to regulate, they rather rely on permanent magnets. for their field. The only thing to do is to regulate their output. The smallest systems have a parallel regulator, shortcircuiting excess energy before it gets into the battery; common regulators for Rotax 4-strokers like this one use a series bridge rectifier with two thyristors, and doubtlessly some kind of microcontroller to fire them.
And regarding your second sketch, you’ve not yet convinced me about the idea of a series “switch” like your mosfet. Even when duly overdimensioned, it may one day yield up the spirit without you ever realising. Then when something goes really bad, it will be unable to do its job of cutting the supply and there go your precious avionics, all in the slightest puff of smoke. I persist that the one more or less reliable overvoltage protection is a really fast switch, a thyristor most likely, to shortcircuit the supply voltage to earth. Doing it fast is critical for the health of the avionics, a faster fuse or circuit breaker will only serve to limit the one-time power dissipation in the thyristor.
OK, it sounds like a linear regulator would be the best way, if there is no field coil. I would not use a shunt regulator – that’s really crude and wastes engine power 
The MOSFETs I am using are rated at ~600A. They also get tested whenever the output on/off switch is operated.
Kubota alternator
A big capacitor should even out all voltage spikes and weighs nothing.
The alternator and associated regulator look just right for the given job. Good link!
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.
