Re: Need Help : Hydrogen Tank, Molar Quantity.
Sulaiman, Wed Apr 30 2014, 01:00PM

From this reference you get 237 kJ/mol
and since in PV = nRT
n is in moles, I don't think there is a times or divide by 2
so only 96 kJ electrical output

for your battery I get 6.6 x 3600 x 12 = 285 kJ
2.7 times more than your fuel cell
... just under 3 minutes

Re: Need Help : Hydrogen Tank, Molar Quantity.
Patrick, Wed Apr 30 2014, 02:07PM

Yeah, I got less than 3 minutes too. I'm shocked at how poor the PEM fuel cells measure up.
It looks like doubling the battery mass is all I really have left. maybe for 20 minutes of flight.

Re: Need Help : Hydrogen Tank, Molar Quantity.
Ash Small, Wed Apr 30 2014, 03:18PM


That 'is' the way the maths works. For the same payload, increasing battery mass, and if necessary, increasing prop size= longer range. Plot a graph depicting these parameters.

Hint: As well as increased lift, larger props are also more efficient, all other factors being equal.

Re: Need Help : Hydrogen Tank, Molar Quantity.
Sulaiman, Wed Apr 30 2014, 05:18PM

After I wrote my previous post it occured to me that maybe you should be using 297 kJ/mol
because the thermal loss is probably accounted for in the quoted 40% efficiency.
Helps, but not massively.

Re: Need Help : Hydrogen Tank, Molar Quantity.
BigBad, Wed Apr 30 2014, 05:26PM

I'm a bit surprised you're still investigating hydrogen fuel cells; they're practically guaranteed to be a loss with pressurised tanks.

The hydrogen plane stuff mostly wants to use liquid hydrogen. You're really up against it with small vehicles though; the square cube law is not your friend for LH2 tank.

You might get away with insulating with styrofoam though; but cryopumping may be a problem, and the tendency of LOX to form condensing from the air makes it rather 'interesting', but might be OK for short periods.

A lot of hydrogen tanks use thermos flasks for these kinds of reasons.

Re: Need Help : Hydrogen Tank, Molar Quantity.
Patrick, Wed Apr 30 2014, 10:44PM

im a desperate, desperate man, im seeing as Dr. Slack and others are pointing out that a fixed wing multirotor may solve the range, vertical and hover requirements.

some are already doing this.

let me ask, im using 10 and 11 inch diameter props, and relaize effciency goes up as the prop-disc loading goes down. But why is this? im thinking accelerating high mass with a tiny change in velocity, is more effcient than a tiny mass accelerated by a huge delta V. (holding force constant in both cases, due to KE=mv^2) that squared part gets to be problem.

Re: Need Help : Hydrogen Tank, Molar Quantity.
2Spoons, Wed Apr 30 2014, 11:58PM

You are exactly right: the force is due to the momentum change M.dV , while the energy used is M.dV^2. So for the same force a big M and small dV needs less energy.

Have you looked at methanol fuel cells?

Re: Need Help : Hydrogen Tank, Molar Quantity.
Ash Small, Thu May 01 2014, 12:06AM

The way I look at it, the losses with props are all at the circumferance. As you increase the diameter, area increases faster than circumferance, hence more area = less losses. The other factors I mentioned in my earlier post also hold true.

I'm no mathematician, I don't convert everything into 'digital' and then back into 'analogue'. I think in 'graphs', if you like.

I know you need to 'input' some figures at some point, but if you want to increase range, it's not a 'linear function'. Doubling battery weight won't increase the prop size by any 'linear function'. I think this is where your '^2' function comes into play.

As I said, I'm no mathematician.

Re: Need Help : Hydrogen Tank, Molar Quantity.
Patrick, Thu May 01 2014, 02:11AM

What about the pitch? in reguards to M.d and M.d^2 ? do i go larger disc with shallower pitch? i presume id want thrust at lower RPM instead of higher RPM.

also, i cant keep going larger though i clearly need to, so what bout a three blade prop? does that entrain more mass and lower the delta V over the same diamter as a 2-blade prop?


as for the methanol fuel cell, no but are they functionally differnt or better ? butam i trading poor tankage for poor molar concentration?

Re: Need Help : Hydrogen Tank, Molar Quantity.
Sulaiman, Thu May 01 2014, 05:54AM

If you look at elecric fans designed to move high volumes of air efficiently
they use as much blade area as possible

Aircraft use thin blades for a couple of reasons;
. engines are more efficient at higher rpm
. gearboxes loose power
. high speed airflow is required for high speed flight

the vertical airspeed of a 'copter is almost zero

I still think that you should try fan blades like those used in domestic free standing fans
with ducting/cowling.

This would however require a gearbox to use high efficiency (power/weight) motors.

Side note;
if the 'copter is meant to be used in hazardous areas it should itself not be a hazard,
a tri-copter or even quad-copter will probably crash if one fan fails for any reason;
a hex-copter can compensate for the loss of one fan.

So, I think that you need six or more large low speed ducted fans for this purpose.
or two really large fans, somewhat like a heavy lift helicopter

Re: Need Help : Hydrogen Tank, Molar Quantity.
Patrick, Thu May 01 2014, 06:09AM

some of the ultra light or small quads use a reduction gear from motor to prop. I think for the reasons you site.

All the props, including those for supposed multirotors, seem to be mostly fast velocity fixed wing propellers slightly modified.

I wonder if the under-cambered props of small helos, would entrain more air at a lower velocity. Adding a wide blade chord, as c Sulaiman suggests, would all drive this new prop towards the ideal for hovering purposes.

Re: Need Help : Hydrogen Tank, Molar Quantity.
Conundrum, Thu May 01 2014, 07:28AM

Yup, MFCs would probably work if the efficiencies were a bit higher.
Also why stop at methanol, simple methane would probably work at least somewhat and is easy enough to store as a liquid.
-A

Re: Need Help : Hydrogen Tank, Molar Quantity.
Ash Small, Thu May 01 2014, 09:26AM

The questions regarding higher RPM and greater pitch are connected. Greater diameter, lower RPM and lower pitch is more efficient than smaller diameter, higher RPM and higher pitch.

We haver discussed this before, all the losses are at the interface of the fast moving, accelerated air and the 'static' air surrounding the craft.

EDIT: Also, one large prop is more efficient than six small ones. It's a 'surface area' divided by 'circumferance' thing.

Also, I don't think 'slip' is really a factor here, as it doesn't really contribute to efficiency. I can ellaborate if required. (Having said that, greater pitch is more 'lossy' than finer pitch, all other factors being equal)

Re: Need Help : Hydrogen Tank, Molar Quantity.
Steve Conner, Thu May 01 2014, 10:14AM

Don't underestimate the size and weight of the support equipment needed to make a fuel cell work. Pumps and the like.

Hydrogen has a poor energy density by volume, and compressed hydrogen has a poor energy density by weight when the weight of the pressure vessel is taken into account. The metal hydride storage tank is a recent invention that might be relevant: BMW briefly experimented with it as a fuel tank for hydrogen-powered cars.

Re: Need Help : Hydrogen Tank, Molar Quantity.
Patrick, Thu May 01 2014, 03:16PM

I looked at the hydride 3.2Mpa storage of H. It came out to 90g hydride for 15g of H.

As you said, tankage, valves and such add up. I wasn't surprised by re high mass low power of thy 1kw unit, but 18,000 dollars for a PEM fuel cell, was shocking. Even with platinum present, that's freaking high.

This is a 200w version note the graph, and note-the note "not for VTOL use" :

But all these companies and Darryl Hannah say it's so easy, just plug them in and it'll all work.

Once the solid oxide FCs get going in 15+ years, I think these PEM FC's will be doomed, for fixed locations.

Re: Need Help : Hydrogen Tank, Molar Quantity.
Patrick, Thu May 08 2014, 04:03AM

A mouse whispers in my ear : Aqueous sodium borohydride, for STP storage of hydrogen. And a small reactor for the evolution of H2.

1kw with all the bells and whistles, for a mere 20k$ for a single evaluation unit. And 12k$/unit for a production run.