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Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
It ought to be possible to use, as .../Steve pointed out, to use a gas engine, but perhaps in a hybrid.
In a conventional quadcopter, two props spin + and two spin - to cancel out the torque, and get controlled in pairs to apply torque.
If you had one central prop driven by a gas engine providing a large fraction of the dead-lift, surrounded by four small electric props spinning the other way for torque cancellation and control, the electricity consumption could be radically reduced. You would need to rewrite the standard controller for the new configuration. The control authority of the small props would be reduced with respect to total lift ones, which may limit how small you could go.
<edit> No, you'd get cross-over between torque and lift. You'd need two counter rotating gas props, and then two pairs of counter rotating control props <\edit>
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
Ash Small wrote ...
BigBad wrote ...
Hydrogen gas fuel cell? That way the fuel has negative weight.
Not if it is compressed, which would be the only sensible thing to do. Weight would then be comparable (similar) to other fuels of the same calorific value.
Not compressed, you can use a plastic bag type thing as a tank, fitted inside the airframe, at around STP, and get negative weight.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
BigBad wrote ...
For a quadcopter making the blades longer would greatly improve efficiency.
For hovering or going slowly, jet engines suck balls wrt efficiency. For long flights, everything else being equal, you want to throw a large amount of air down slowly (which is what big long wings do), rather than a small amount of air down fast (which is what jet engines do).
It's because energy goes as 1/2 m v^2, whereas the thrust goes as mv, where m is the mass, and v is the downwash speed; so if you work it you want m as big as possible and v as low as possible, so that mv = vehicle weight
Super Dooper important point you make here BigBad, and yes evrything you have said has been my design criteria, long thin "slow flyer" props there called... with low to mid pitch (APC 10x4.7 CW, CWW, paddle props, like on the P47 thunderbolt.) ive maxed out the obivious math and physics so far. Ive even moved away from those suckers who use the quads, im trying to use bi's and Tri's now, power efficiency of magnetic/elecrical circuits increases greatly as volume increases... therefore Bi's and Tri's (properly built) should outlast hexs, octas, and deca copters ... fewer ESCs, Motors and props to break in crashes too.
BigBad wrote ...
Ash Small wrote ...
BigBad wrote ...
Hydrogen gas fuel cell? That way the fuel has negative weight.
Not if it is compressed, which would be the only sensible thing to do. Weight would then be comparable (similar) to other fuels of the same calorific value.
Not compressed, you can use a plastic bag type thing as a tank, fitted inside the airframe, at around STP, and get negative weight.
That's what some versions of Helios do I believe.
People have already suggested using the glo-engines of .049 and .1 size, and using a brusshless motor as a generator, but crap, a governor, and fule handling/carb system, a generator stabilizer/current for magnitizing, the mechanics, the vibration, the possibility of the thing quiting at 100 ft.. poor chemical--mechanical-- electrical conversion effciency......... sh*t this just sounds worse and worse
and a PEM fuel cell, even a crappy one are looking expensive...
your negative boyancy comment means a trival contribution to my efforts, my machine already wheighs 1.2kg and will top out at 1.5kg with cameras and lidar...size of drone must be less than 0.7m in X,Y,Z, lengths.
I may be stuck with lipos' even in left-wing eco california, im still trying to contact the FC EV group in sacremento for academic support... UC Davis and Irvine too, hope thell reply to a mere CSU student/unwashed prolitariat commoner...
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
it looks like half a liter of volume for a DMFC will produce 400-500 watts, on 1M methanol solution at 80-120 C'. i just dont know how much it would wheigh.
Though very few people are working on portable fuel cells, i think its becuase all the governement subsidies are for in plant and off grid solutions or for large vehicals. only Toshiba seems fully committed to mini FC's...
Registered Member #1643
Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039
Would be interesting to see a gas engine quadcopter...Would be a joy to see that heh.
Currently on 3mAh I'm 14 minute flight, 20-30 min charge 5mAh 20-24 minutes with 30-40 minute charge.
Personally I have 4 3000mAh lipos so I have about 1 hour of fly time, which, for most people 15 minutes is a long time, 30 minutes is usually when I quit.
Registered Member #65
Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155
Electric VTOL craft usually have more problems with efficiency losses the longer they operate, and can even burn-off the stator field insulation if airflow is restricted.
Typically for the same airframe weight a VTOL must achieve a minimal 2:1 thrust ratio, but a regular fixed wing craft only needs to achieve a 1:3+ thrust ratio.
A kerosene or gasohol powered motor would not allow accurate rpm adjustment needed to change position in a quad's lateral axis (note the prop pairs are calibrated to counter rotate). This limitation will also hold true for variable pitch rotors.
Simply picking the wrong prop and engine for your operational altitude can also lower fuel efficiency. However, it is also common for people to use the wrong sized battery, and rated continuous discharge rates.
Solid fuel cells are a mature technology, but are quite expensive for private individuals to acquire and integrate.
Perhaps some sort of magnetic coupler to swap packs with a charger in-flight may prove amusing. Assuming average flight time is more than 1/2 change and settling time of 2 spare packs the craft would never have to touch the ground if conserving energy.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Carbon_Rod wrote ...
Perhaps some sort of magnetic coupler to swap packs with a charger in-flight may prove amusing.
That does sound interesting. Have two batteries on board, and jettison one when it is exhausted, then pick up another while flying on the other. The electric equivalent of 'in flight re-fuelling'.
If you didn't want to return to base, just scatter a few fully charged batteries around, and have a choice of re-fuelling sites?
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837
The fuel efficiency of small 2 stroke engines is bad ... 5-10% thermal efficiency bad, throw the 80-90% efficiency of the generator on top and it's hard to come out ahead with energy per weight on a small aircraft.
Haven't been able to find thermal efficiencies for small 4 strokes though, they might be better ... or you could try that one RC Wankel engine in existence.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
im thinking that DMFC is the only type of FC that will work, with 1 molar alcohol (mostly water), but the DF's require much denser platnum catalyst than the PEM type.
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Fuel Cells For UAV Drones
