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Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
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
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Sulaiman wrote ...
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
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.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059
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
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
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)
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
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.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Steve Conner wrote ...
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.
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.
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