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Old Nov 26, 2014, 12:28 PM
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Solar Plane Motor

Hey Guys,
I'm a senior in college working on designing a solar electric plane for a research project. I know it's been done before, I know it's very hard to make it work, but I think I'm up for the challenge, and worst case I'll fail and still have a blast working on it. That said, my expertise is more in the power system end of things (although even that I'm not the most experienced in), so I'm looking for some advice in terms of choosing planes and motors. I'm probably going to build and test the power system (solar panels, batteries, etc.) first without the plane, then purchase a glider and modify it to carry the power system.
With that in mind, I've done some basic calculations and decided I'll need about a 3m wingspan plane to make it work, and I'm shooting for about 5.5-6.5 pounds total. With that in mind, I've been trying to choose an engine. I visited a local hobby store and they suggested the E-flite 32 or 46 based on the weight. My issue is that I think this draws much more power than my panels will be able to supply; I'm hoping to use something like a 300 W motor. The calculations I've done say that this should be enough to climb slowly if I use a fairly efficient sailplane design, which would be fine for my purposes. I don't need anything like vertical ascent, I'm shooting for just a bit more than level flight. What does it sound like to all of you? Do I need something much more powerful? Any motor recommendations?
Thanks very much for all the help.
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Old Nov 26, 2014, 02:08 PM
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Hi, cbrooks8593.

Approximately how many volts and amps will your solar cells provide? We will need those numbers to recommend a motor range. My guess is that your best efficiency will come with a geared setup and I really don't know much about them other than the theory.

For an airframe light enough to fly on low power you will probably have to build from a kit or plans or spend a whole lot of money for a pre built. Something like an Oly II would, I think, be ideal. The OlyII is a bit under 3 meters but it has a smaller, lighter, fuselage than the other versions and excels at low speed. It would be relatively simple to extend the wing a bit and you might even be able to shave some weight off by using lighter spars since you won't need to use a winch or Hi-start to launch. This is where i would begin my search.

http://www.skybench.com/

Cheers!
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Old Nov 27, 2014, 04:19 AM
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The weight estimation is too much by any standards, even the Bird of Time "turkey" is under 5 pounds.
"it's been done before", but just as a rechargeable patch for the main lipo, not as main propulsion source.
The solar efficiency is still too weak as a permanent power source for a normal plane. We talk about 1W per square dm effective power delivered to lipo charger.
For example, the central section of Bird of Time can't deliver more than 20W, done that.
A 3m performance glider need at least 35W (3Amps from 3s 11V) to cruise level in calm air, and this means 35 sq dm of flat wing full of cells... is not easy to find so much flat surface on a 50-60 sq dm wing that has curved lines, ailerons, bends, dihedral...

As motor, my take would be one of the modern low kv multirotor motors, about 4-500kv, that can turn a 14-15" prop without gearbox, and draw 2-3 Amps from 3s, for 500 grams thrust, more than enough to fly level with a 2kg glider.
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Old Nov 29, 2014, 08:29 PM
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Thanks for the responses!

To elaborate a bit more on my solar panels, I plan to use SunPower C60s if I can get them. They have a maximum output of roughly 0.58 V and 5.9 A per cell, and a power density of about 218 W/m^2. Assuming a wing area of about 1000 in^2 (on a plane like the OLY II) and that I would be able to cover about 75% of that area in solar cells with some clever engineering, that would yield 102 W using 30 6"x6" cells. If I linked them all in series I could theoretically get up to about 17 V. Is that helpful for motor choices?

I'm also a bit curious about gearing options, I know very little about those. Do you know of any good resources to look into that a bit? Or I can just research on my own.

In terms of my guess on the weight, I was probably a bit high, but using the OLY II as an example, the plane itself if 2.5 lbs, the E-flite Power 25 is 6.7 oz, the solar panels weigh about 14 oz, and a MPPT charger like this one should weigh about 1.5 oz. Leaving about half a pound for batteries (this can get me up to ~4000 mAh which should be more than sufficient) and a little more for wiring, controllers, etc, all that puts me at about 4.5 lbs.

Finally, doing a bit more research I found this tool: http://flbeagle.rchomepage.com/softw..._imperial.html which suggests a motor with about 320 kv and a power of ~150-200 W. Do those numbers seem like they're close? I'm not immediately able to find anything exactly like that, so does this mean I need some gearing to make it work?

Thanks again for all the help!
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Old Nov 29, 2014, 08:47 PM
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I think Peter is right, you are going to need the most efficient motor practical, and I believe that steers you toward inrunners rather than outrunners like the Elite offerings. Make contact with a company like neumotors.com and ask the same question. I think where you will end up is a geared inrunner. Even with the mechanical losses in the gearbox, efficiencies can get very high, and you can run a very efficient large prop. Note that I keep using the word efficient over and over. I will again.

As far as aircraft, the larger the better, for multiple reasons. More room for pv cells, more efficient aerodynamics, easier to see (!), practical wing loadings tend to go down with increase in size, etc. However, cost for all components go up dramatically.

What is your budget? This is gonna cost ya'!
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Old Nov 29, 2014, 09:16 PM
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Hi again. I have only just begun studying geared motors for small scale applications so this is all a very shaky guess.

There are two main uses for geared motors. They are used in gliders mostly to fit a powerful inrunner in a narrow fuselage to get a very high rate of climb. The other major use is in scale planes where a really small motor can sustain a surprisingly big plane and give longer duration than a direct drive outrunner swinging the same prop. As I (barely) understand it; electric motors gain efficiency at high kv ratings but the gain is lost when they are bogged down without the gearing. One thing is certain. You will need to be able to swing a very large prop.

From what I have been reading; a geared "speed 400" inrunner can achieve nearly twice the endurance as a comparable outrunner swinging the same prop. If my theory is right I think you could wire the cells in parallel to get a useful amount of current and not need the higher voltage required by a direct drive outrunner. Maxx Products has some handy charts that are helping me make sense of the whole thing and I'll include a link.

http://www.maxxprod.com/mpi/mpi-10.html

Cheers!
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Old Nov 29, 2014, 10:12 PM
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For the power levels a bit higher than being discussed in this thread small outrunners (originally intended for helicopters) with a gearbox are a light and relatively economical solution.

For example http://innov8tivedesigns.com/parts/b...-motor-kv-2600 + http://www.reisenauer.de/artikeldetails.php5?aid=233 Would use about 100w to deliverabout 500g static thrust with an 8x5 prop, at 5S input voltage (18.2V)
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Old Nov 30, 2014, 03:08 AM
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You all are right and wrong
The geared efficiency and usage is related to thrust only, and focused on short 30s motor run conditions, specific to competitions. Also, this choice was imposed by smaller diameters available.
For long time, continuous, small thrust conditions, specific to low current cruise requirements, I bet more on a multicopter motor. Just think to the fact that the main battle field for efficiency is now in multicopters area, and guess what... nobody approached the geared solution there professionally.
Conversely, the best motors there are big diameter, pancake motors, with a lot more poles than a standard one.
Hint: increasing number of poles is the electromagnetic equivalent of a mechanical gear, without the mechanical losses
Also, using a ALES propulsion, I mean the high pitch prop, like a 14x10, would be again a mistake, I already debated this fact in another thread. Such prop choice is to have an ascending speed of at least 8m/s, completely different goal then gently cruising level at 40km/h.
And finally, for the above thrust of 500 grams a multicopter motor of 600kv powered from 4s consume less than 60W using a 12-13"x5 prop. Something under 4A from 14.8V. And weight 87 grams
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Old Nov 30, 2014, 03:40 AM
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Comments inline

Quote:
Originally Posted by cbrooks8593 View Post
Thanks for the responses!

To elaborate a bit more on my solar panels, I plan to use SunPower C60s if I can get them. They have a maximum output of roughly 0.58 V and 5.9 A per cell, and a power density of about 218 W/m^2.

This means more than 2W/sq dm^2... please let me doubt, based on my direct experience.
Maybe this could be valid in a hot summer day, with sun rays hitting cells at 90 degrees.
Do you know that only the fact that the light comes from 45 degrees decrease the maximum delivered current with 1.4x? Like from 1.3Amps to 0.9Amps. Why are the solar panels mounted on the ground at an angle ? Or, better on a roof
Will you fly with a 45 degrees banking to counteract this ?


Assuming a wing area of about 1000 in^2 (on a plane like the OLY II) and that I would be able to cover about 75% of that area in solar cells with some clever engineering, that would yield 102 W using 30 6"x6" cells.

Nope, you can't, especially with this immense cell size choice!
I started with 3x3", and ended with 28 3x3 cells on a BoT central panel, 120cm length and 24cm chord.
Because of smaller size their flex on rear part of the airfoil was only 1.5mm at 75mm size. On the trailing part of the airfoil (front of main spar) is impossible to fit anything, too much curvature. So maximum 3/4 area of any wing is available at best, if no flaps/ailerons.
At first landing, very gentle, on grass, two of 28 cells broke, so fragile they are !
Think to something more fragile than an egg shell, which is so hard because is a dome, else if flat would break much easier. Is 0.2mm glass, nothing else !


If I linked them all in series I could theoretically get up to about 17 V. Is that helpful for motor choices?

Voltage delivered by cells has absolutely no importance for motor choice, none ! The motor will be powered from the lipo pack, not from cells, you can't takeoff with the power of cells. Lipo cells are only to charge the lipo pack, and the chargers can accomodate a big range of input voltages, from 8 to 15V.

I'm also a bit curious about gearing options, I know very little about those. Do you know of any good resources to look into that a bit? Or I can just research on my own.

Forget about gearing, read post above. Gears are for instant big thrust for short time only

In terms of my guess on the weight, I was probably a bit high, but using the OLY II as an example, ...etc, all that puts me at about 4.5 lbs.

Much better than 6 lbs

Finally, doing a bit more research I found this tool: http://flbeagle.rchomepage.com/softw..._imperial.html which suggests a motor with about 320 kv and a power of ~150-200 W. Do those numbers seem like they're close? I'm not immediately able to find anything exactly like that, so does this mean I need some gearing to make it work?

Me too I am a big fan of webocalc, but depends what was your input choice, 150W sounds a bit high to me to cruise level a 2kg bird, maybe for takeoff only.
Most FPV setups using flying wings of this weight shows power consumption much smaller than 100W.
A very efficient designed platform, able to fly 40km and back, for example, consume less than 100mAh per km, 8000 of 10.000 mAh for 80 km flight, from 4s pack. For a 90 minutes flight, this means 60W power consumption.


Thanks again for all the help!
My webocalc take on this propulsion attached
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Old Dec 01, 2014, 09:39 PM
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Comments re solar

Back in 2007 I flew a 2 energy source e-motor glider for Cal State Los Angeles and U fo Oklahoma Stillwater on an out and return flight. It exceeded the WR at the time. 78 miles in 3 hours. It used a geared Hacker Inrunner with a large prop, maybe 18 to 20 ". I think it would climb at 20 watts. The airplane was about 14' span and weighed 5 Kilos.

Contact Cal State LA energy lab, (they did the power plant, fuel cell and LiPolys) and U of O Stillwater, who did the airframe. I suspect a 12" Esprit, converted to solar, will do the trick.

Don't hesitate to try dual energy sources.

Best ot luck

JDK
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Old Dec 02, 2014, 09:29 PM
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Alright so summing things up, I need something with low KV (WebOCalc says about 200 KV), but low power (~100W ). It seems like if I go with an outrunner brushless and direct drive I would end up with a monster motor capable of something like 2kW and weighing 300-500g which is definitely not ideal.

With that in mind I would think an inrunner with a gearbox has to be the way to go. Would something like this with about a 4:1 gearbox to step down work?

I've also heard that if you want high torque and low power brushed motors could be a good option, any thoughts on that?
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Old Dec 03, 2014, 01:48 AM
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Not sure how you searched for the motor, seems you didn't read my comments... this is sad, being the only coming from direct experience with the subject in this thread with current technologies, not 2007.
Once again, a multirotor motor is what you want. Something like this:

http://montorc.com/Pulso-U39M-300Kv.aspx

or, if budget is tight, the chinese "equivalents":

http://rctimer.com/product-575.html
http://www.hobbyking.com/hobbyking/s...nce_Motor.html

I own the motor from your link, tried to use it on a glider, and is is ludicrous inefficient... and heavy... not all inrunners are the same

Good luck, I think I finished posting all the relevant info here...
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Old Dec 03, 2014, 02:50 PM
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Hi renatoa,
Thanks for the posts, it's very much appreciated. I definitely didn't mean to blow off your first post but as you can see I've gotten a number of conflicting suggestions. Never having worked much with these motors, can it create problems trying to run a plane on a multicopter motor? All of these motors specifically say that they're meant to be used with multicopters, which was why I was leaning towards one intended for a plane since I haven't worked much with either kind before. That said, you're right that those are exactly the right type of low-KV, low-power, high efficiency motor I would want. Here are the main questions I have about it:

1) Just looking at them, it looks like these multicopter motors could run with any prop a traditional plane motor could. Is that correct? I'd be hoping to run something like a 20+inch prop if I can, and these say they're optimized for significantly smaller props, but hopefully something along those lines could still work?

2) These motors seem to require LiPos with a larger number of cells (4s-8s). I've been able to find some good 4s batteries (like this one?) which I think I could charge with this MPPT charger. Would that combination work well with one of those motors?

3) The only other concern I have is mounting it to a plane/aerodynamics, but it's actually not a lot wider than the motor I linked, so I'm sure I could work something out there.

Thanks again for the help!
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Old Dec 03, 2014, 03:35 PM
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They are not conflicting at all the suggestions, just coming from people with different experience.

Yes, they can run any prop... a prop is a prop, thrust is thrust.
Not 20, but 17" are usual for the above motors, and voltage going as low even as 3s.
And the charger is ok for 4s too.
4-5cm diameter is not so wide... is like most glider fuselages
Here is the flight data of a multicopter using such motor and 17" prop: 1.658kg Quad, so 414g thrust per motor, drawing 2.8A per motor, 31.7 W per motor.
Total energy used 9562 mAh, so under 2400mAh per motor for 51 min flight time
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Old Dec 03, 2014, 03:45 PM
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Hi again. The 540L is definitely the wrong motor. It actually has a low kv as inrunners go and wouldn't be amenable to driving a geared prop at relatively low voltages. To adress the other points...

1 and 2. While these motors use a relatively small prop with a high voltage; you could certainly use a bigger prop at lower voltages. I don't think your solar power plant will produce enough current to fry a low kv outrunner running a larger prop with less juice. The danger is that it simply won't spin the prop fast enough for flight.

3. At the speeds you will be flying a larger diameter motor won't be much of a drag problem.

Cheers!
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