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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
well theyre are multirotors that fly for 90 minutes on laptop batteries, but theyre 10% machine, 90% battery... (2kg of battery i fi remember)
the SOFCs i really like, and eventually theyll be the FCs that really take over (i think). The PEMs are just too weak and have precious metal catalysts like paladium and platnium and theyre easy to poison.
and the SOFCs will take any liquid or gaseous hydrocarbon. The limting factor on SOFCs s the time needed to start the pre-heat, and then the ultra high temps that ceramics must survive.
i may have found a company that offers a 500 watt PEM FC at 2kg, thats a scary weight though, im running experiments now with motor an prop test stands to see what power can lift what mass.
the mass of tankage and valves scares me further... i predict the mass of propellent (H2 gas) will be inconsequential compared to all else.
Registered Member #65
Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155
You may want to look into the Watt / Kg rating on PEM based systems... Horizon's 30Watt RC kit (970 g): iirc, these were around $1400 a few years back...
ebay kit: likely the lowest cost option... but a gamble if CO poisoned the PEM.
The disposable 2-week MEDIS systems are awful, and don't last in storage. However, the plastic 2w/4w cell cartridges are around $20, and may offer some design options.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
well zinc-air batteries are energy dense, but theyre not rechargable.
for now, ive made a second 3 cell LiPo, 11.1, 3000mAh, 240 gram battery, ill use two in parallel. Ill remove the 250 grams of ballast brass ive been flying with, which will double the flight power-time, for the same flight wieght.
then, ill remove the safety programming on the ESCs, theyve been limiting the discharge to 3.5 V/cell, and ive been landing at 3.4 to 3.5V per cell. removing the RPM reducing safety feature will allow me another 0.3 to 0.4 V per cell. i dont want to be flying at 3.1V/cell and should already have been in "holy crap, i gotta land!" mode.
ill add a loud piezo and blinking LEDs for a total battery V per cell of 3.3 to 3.2ish volts to prevent battery discharge damage or a lost machine.
( 0.3 V/cell means 30% more flight time, 4.2V to 3.6V is about 4 minutes of flight, for my 1.5kg machine. )
I maybe able to get 8 to 10 minutes of flight for a 480g LiPo.
Registered Member #49
Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362
In 2007 or 2008 the DOE sponsored a competition for a portable power system that could power future soldier's gadgets. I entered planning to do what you describe - simple nitromethane hobby engine with a brushless electric generator and rectifier.
I built several test stands and my conclusion was that the efficiency was quite poor in terms of rated shaft power to electrical. I don't have any of my notes, and at the time I had delusions of grandeur and was being very tight lipped, so I'm afraid I can't elaborate.
I do remember, however, that I had massive problems keeping the engines running without massive flywheels that would blow me out of the mass constraint; it seems these small nitro engines can be greatly impacted by instantaneous loads from the generator.
Also I recall finding it very difficult to match the generator to the RPM range of the engine... I suspect that might have been inexperience, however, rather than an intrinsic problem.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
AndrewM wrote ...
I do remember, however, that I had massive problems keeping the engines running without massive flywheels that would blow me out of the mass constraint; it seems these small nitro engines can be greatly impacted by instantaneous loads from the generator.
Also I recall finding it very difficult to match the generator to the RPM range of the engine... I suspect that might have been inexperience, however, rather than an intrinsic problem.
disturbing..........
well these are exactly the possibilities I feared and anticiapated. I was advised to not use aircraft IC's, and use ground RC engines for better torque survival under sudden magnetic load. But if I'm going to spend money and time I'd rather roll the dice on fuel cells.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
AndrewM wrote ...
In 2007 or 2008 the DOE sponsored a competition for a portable power system that could power future soldier's gadgets. I entered planning to do what you describe - simple nitromethane hobby engine with a brushless electric generator and rectifier.
I built several test stands and my conclusion was that the efficiency was quite poor in terms of rated shaft power to electrical. I don't have any of my notes, and at the time I had delusions of grandeur and was being very tight lipped, so I'm afraid I can't elaborate.
I do remember, however, that I had massive problems keeping the engines running without massive flywheels that would blow me out of the mass constraint; it seems these small nitro engines can be greatly impacted by instantaneous loads from the generator.
Doesn't surprise me much; the wobbly bit has a Q factor, and as you take power out you lower the Q factor, until it stalls. The problem is that it's difficult to get high speeds on a linear bar (it's much easier to spin something fast), so the energy you have circulating in the cycle is fairly low.
So you have relatively low kinetic energy, plus the cycle rate is fairly low. That translates into low power, and even lower extractable power to avoid reducing the Q. So you can end up with low power/weight ratios.
The alternative is turbines; they store energy much better, but turbines don't scale down particularly well either because of viscosity issues.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
Patrick wrote ...
But if I'm going to spend money and time I'd rather roll the dice on fuel cells.
Fuel cells are notorious for low power/weight ratios. Unless you know something that others don't about fuel cells, you're very unlikely to get a win that way. Maybe talk to a university or people that know this shit inside out; you might find there's a high-power, short life fuel cell or something out there which can't be used for cars.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Right now it's looking like horizon fuel cells company, has a 500 watt pem at 2ish kg. That's super scary, since it takes me about 300 watts for 1.7kg of thrust.
I don't know what the cost is though...
I hope Ago and Horizon will assist a college group. The problem is, I've only seen Uav companies make money in two ways... first, weapons of war and research companies---, lockheed, northrop, uai and aeroviroment and similar companies. or second--- hobby and related companies. And a shocking degree of contributions are being made by hobbyists. The demand for better LiPo's for example.
Everyone in between seems to be on the outside looking in... There is a lot of supply, but little demand, every screwball and his uncle is building these things.
I predict the attrition in this business will be severe...
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The Possibility of Nitro-Electric Battery Replacement
