I have been tinkering with solar powered RC planes this summer. The photos above shows the latest incarnation.


This model uses the Parkzone Vapor's radio gear and motor ( http://www.parkzone.com/Products/Default.aspx?ProdID=PKZ3380 ), plus five Powerfilm solar cells ( http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001... ). The motor/prop are quite well matched to the cells, delivering about 2.5 watts in direct sunlight. Here are some test data:



All up weight is 38g, 18g in solar cells. It has a tiny 20mAh battery (1g) in parallel with the solar cells that acts to smooth out the voltage. Before I flew, I ran the motor using battery power until the voltage dropped too low to run the motor, then I plugged in the solar cells. That way, when I flew, I knew I was really running on solar power alone. I flew it for about 5 minutes or so, but it was a pain to fly because the wings were too torsionally floppy. I am planning the next revision that should address some of this design's shortcomings.


Related Work

There have been many successful solar airplanes. I found the following efforts particularly interesting because they are not military or NASA scale projects, and they are well documented.


Andre Noth's (ETH Zurich) 3.2m span Sky-Sailor, which flew 27 hrs in 2008

www.asl.ethz.ch/research/asl/skysailor


Carl Engel and Adam Woodworth's (MIT) 3.1m Aphelion, which has flown over 7 hrs

http://www.rcgroups.com/forums/showthread.php?t=572000&pp=15



Solar Cell Selection

In many applications, solar cell performance is measured by efficiency. In my experiments with the Powerfilm cells, I got about 70 watts per square meter. Using a fairly standard irradiance value of 1000 watts per square meter puts the cell efficiency at something like 7%. This is not particularly good compared to the state of the art (see e.g. http://en.wikipedia.org/wiki/File:PVeff%28rev100414%29.png).


The Aphelion uses an array of 18 A-300 Sunpower Solar Cells. The total encapsulated array weighs about 350 grams and can deliver 55 watts of power. These cells are reported to be upwards of 20% efficient. BTW, if you dig deep enough into the Aphelion thread referenced above, Carl reveals his secret supplier of these cells:

<quote>

Also, here is the contact info for our solar array supplier, now that he has given permission for it to be posted. One note that he wants to be made clear: he is very busy, and it's a small operation, so there can be a serveral month lead-time on ordering an array. With that, here is his info:


SunCat Solar is the name of the comany, and our contact in Alain Chuzel(pronounced "Allen"). His email address is
ahmchuzel<at>aol<dot>com. (replace the <at> with @and <dot> with . )

He can fabricate pretty much any size or confguration of array, given a good drawing (and enough money )

</quote>


The weight of solar cells accounted for 47% of the total weight of my airplane. Therefore, I would argue that for a solar powered airplanes, an important performance measure of performance is watts per gram. I measured about 0.27 watts per gram for the Powerfilm cells. This compares favorably to the Sunpower cells at 0.16 watts per gram.


Some other attractive things about the Powerfilm cells is that they are reasonably priced, and readily available in small quantities. I have not contacted Suncat Solar to determine the cost/availability of the Sunpower cells.


Scaling Arguments

I think it would be very cool to build a solar powered drone that could stay up all day, and possibly all night. My little airplane has essentially no payload capacity. Presumably, a larger airplane would be required to carry up a GPS, IMU, two way radio, etc.


As the airplane gets bigger, we have more area for solar cells, but the weight gets larger too. So how does the power budget work out with a bigger airplane? Here are some scaling arguments to help answer that question objectively.


Definitions:

Preq: required power

Pavail: available power

Rp: the ratio of available power (Pavail) to required power (Preq)

D: drag

V: flight speed

W: weight

g: acceleration due to gravity

rho: air density

S: wing area

CL: lift coefficient

Kcell: power output of solar cell per area

LOD: lift/drag (or glide ratio)

WOS: wing loading, weight per area



Relationships:

(1) Pavail = K * S (assuming the entire wing is covered with cells)

(2) Preq = D * V

(3) V = sqrt( 2 * W * g/ (rho*S*CL) )

(4) D = W * g / LOD

Combining (2), (3), and (4) gives

(4) Preq = W * g/ LOD * sqrt( 2*W *g/ (rho*S*CL) )

Combining (1) and (4) gives

(5) Rp = K * S ^ (3/2) * LOD * sqrt(rho * CL) / ( (W*g) ^ 3/2 * sqrt(2*g) )


Finally, simplifying (5) gives

(6) Rp = K * (1/WOS) ^ (3/2) * LOD * ( sqrt(rho*CL/ 2) * g^(-3/2) )


There are four terms in (6). The first term depends only on the solar cell. The second term depends only on the wing loading. The third term depends on the aerodynamic efficiency of the airplane, and the fourth term we can treat as a constant. Thus, starting from my little airplane, as we increase the size of the airplane, if we use the same solar cells, maintain the same wing loading, and maintain the same LOD, then we should still have enough power to fly.


The bad news is that as airplanes are scaled up, their weight usually goes up faster than their area, thus their wing loading goes up. Stated another way, bigger airplanes usually have higher wing loading than smaller airplanes given similar structural technologies.


The good news is that as we increase the size of the airplane, we can expect the LOD to improve. The LOD of my airplane is around 7. Doubling or even tripling this value should be possible ( see e.g. http://www.rc-soar.com/tech/perfanal.htm ). The wing loading of my airplane is 540g/m^2. If we double the LOD, we can increase the wing loading (WOS) to 850g/m^2 (i.e a factor of 1.58) and maintain the same power ratio.


Also, as we make the motor and propeller larger, we might expect an improvement in propulsive efficiency, meaning that we fly with a reduced power ratio.


Considering these factors, building a bigger solar airplane is probably harder than building a small solar airplane. Based on the successes referenced above, I must conclude that it is possible, but not easy (or cheap).










Views: 19832

Comment by tshado on September 14, 2010 at 11:14am
Hello
Very interesting project.
Congratulations for your achievement !!

Jean-Claude
Comment by AVS on September 14, 2010 at 11:27am
Wow. This is exactly what I have been looking for. Well done and thanks
Comment by Theodor Chervyakov on September 14, 2010 at 11:32am
I built plane like this but with not radio and battery

Moderator
Comment by Gary Mortimer on September 14, 2010 at 11:32am
Nice one
Comment by Alex Arevalo on September 14, 2010 at 11:37am
Shades of Paul MacCready. I wish you success on this interesting project.
Comment by arashi on September 14, 2010 at 2:21pm
Nice! What factor most determined which solar cells to use?
Comment by Mathew krawczun on September 14, 2010 at 4:41pm
great work on this and whats better is there are some new types of solar cells on the horizon which should make this easier in the coming years.
Comment by Chris on September 14, 2010 at 6:02pm
no video of the flight come on this is a wicked idea, more images/designs/video would be spectacular.
Comment by David on September 14, 2010 at 9:03pm
Very impressive, please keep us all updated on this project's progress!
Comment by Chris on September 16, 2010 at 4:12pm
suggestion up the size, with things like this especially solar, the more area you get exponential increase in performance, as you increase wing lift, as well as area for power input

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