Hello RC India community,
I  am in need of a glider (with an approximate wing area of 0. 4 square meters, preferably composite structural but foam also would do) to conduct some flight tests and test some hypotheses on the viability of solar-electric hybrid gliders. The glider airframe will be used to quantify the improvement in endurance when solar cells are padded on to the wing surface.

Any suggestions for an off-the-shelf model that can be bought or an existing design that can be built is greatly appreciated. Would love to know the RcIndia community's thoughts on the best way to proceed on this project.

Thanks and Regards,
Parithi

Quote from: hobbydrone on September 23, 2019, 05:32:18 PM

1. I am in need of a glider (with an approximate wing area of 0. 4 square meters,
2. preferably composite structural but foam also would do)
3. to conduct some flight tests
4. and test some hypotheses on the viability of solar-electric hybrid gliders.
5. The glider airframe will be used to quantify the improvement in endurance when solar cells are padded on to the wing surface.
6. off-the-shelf model that can be bought
7. or an existing design that can be built

Hi Parithi,
Here are some initial thoughts...

1. The "endurance" of a "glider" depends only on the height of release and its sink rate.
2. If it is a "powered" glider, the height of release is a function of the power available.
3. So if you are talking of "endurance enhancement" with solar power, you are talking about a "solar power assist airplane", not a glider
4. If you are thinking of a lipo powered electric airplane, its increased endurance with solar power is a function of
(a) increased release height
(b) increased duration under "cruise" power.

Assuming that you are talking about endurance under power, and not a glider's endurance unpowered, here are some further thoughts:

1. Any particular reason for 0.4sqm (640sqin)?
2. A glider this size could range from span & chord of 64" by 10" to 90" by 7"
3. 64"x10" easy to build. 90"x7" much harder to build, but potentially much higher performance.
4. If you are only looking at "improvement in endurance with solar cells", and you don't need high performance from the basic airframe, then about 64"x10" is a good approach.
5. Now the cells are typically 5"x5". (And say 0.5v). So about 11" chord is reqd.
6. So about 10/15 * are reqd to charge 2s/3s lipos, or 50"-75"

Hope this provides some food for thought.
Later we can consider readymade vs built...
Regards
Iyer

Edit:
* pairs

Hi parithi,

Currently I'm developing DLG with wingspan about 1.2 m & fuselage length 1.0 m. Its completely carbon fiber composite with foam core.

Mr. Iyer has given you valuable points.

I can help you in this regard. Are you in Chennai? Which part? May I know what for you are testing this solar powered glider.


Regards,
Samayaraj

Hi Mr. Iyer,
Thanks very much for your inputs. As you correctly pointed out, the intent is to develop a "solar power assist airplane" powered by a motor/propeller, and having batteries and solar cells as energy sources, and with a glider airframe configuration. The reason for a glider configuration is for low landing speeds and to reduce damage in case of motor failure or other failure scenarios.
Using our computational and sizing analyzes, we have estimates of how much endurance our best design (endurance of >8 hours) might produce. However, building such an airframe (~wingspan =5m. chord = 0.4m, fuselage (along with tail boom)= 2m, solar cell wing surface area = 1.5 m2) will be quite expensive.
Since these analyses are all still computational, we wanted to derisk some of our assumptions regarding the potential "endurance enhancement" owing to solar cells. Hence, our current plan is to build a proof-of-concept using an existing design or buy off-the-shelf airframes.

1. "Any particular reason for 0.4sqm (640sqin)?"

I wanted to test the solar cells on a rough 1:2 scaled down model. Hence, the wing area reduces by a fourth to ~0.4 sqm. This will provide sufficient wing area to put atleast one row of Sunpower C60 solar cells. Each sunpower C60 cell has dimensions of 12.5mm x 12.5 mm.

2. "A glider this size could range from span & chord of 64" by 10" to 90" by 7". 64"x10" easy to build. 90"x7" much harder to build, but potentially much higher performance.  If you are only looking at "improvement in endurance with solar cells", and you don't need high performance from the basic airframe, then about 64"x10" is a good approach."

The goal of this demonstration is to show the "improvement in endurance with solar cells". To get the 0.4m2 (640 sqin) wing area and to put either one or two rows of solar cells on the wings, the span and chord combinations can range anywhere between 54" by 12" (2 rows of solar cells, ~18 of them) to 90" by 7" (1 row of solar cells, ~16 of them).
I understand that the performance for 90" by 7" (because of the higher aspect ratio and hence, lower induced drag) is better than the 53" x 12" option. From your experience, is there a manufacturing cost difference between the two configurations? Can you quantify the endurance for the two configurations for the same available power, say 30,000 mah?
Of course, portability and transportability is an issue that neeeds to be considered for higher aspect ratio wings. Is there something else that I am missing here that needs to be considered? Timeline for development? Flight control charecteristics, etc?

3. "So about 10/15 pairs are reqd to charge 2s/3s lipos, or 50"-75" "

We are planning to use a power control board with MPPTs and a DC-DC converter to modify the energy flow from the solar cells to the battery, and from the battery to the system/components onboard the drone. My understanding is that even if there is insufficient voltage from the solar cells, the DC-DC converter should be able to take care of it.

Regards,
Parithi

Hi Mr. Samayaraj,
Thanks for your interest. In my reply to Mr. Iyer, I have outlined the reasons for solar-electric hybrid project.
I am currently based out of KR Puram in Bangalore. I used to live at Valasaravakkam, Chennai but I do frequently travel there even now. It would be good to hear your inputs and learn from your experience. Perhaps we can setup a call to discuss sometime when you are available.

Thanks and Regards,
Parithi

Hi Parithi,

Sure, Ill support my best. Send me your phone/whatsapp number in private message.

From your experience, is there a manufacturing cost difference between the two configurations?

Yes definitely there will be a definite cost difference in making high aspect wing. In fuselage, there wont be much difference.

1. As the length increases, reinforcement along the span increases.
2. Making slender job (long span wings) needs stringent tolerance.
3. Needs inter connectors for easy transporting
4. Also adding solar panels in high aspect ratio wing needs more reinforcement


Regards,
Samayaraj

@hobbydrone,
Hi Parithi,
Some more thoughts and rough estimates for the first iteration:

'Anything missed?'
Yes, Weight.
A 3s 1000mah lipo weighs about 100gms.
So a 30,000mah 3s will be about 3kg.
Add 5-600gms for motor and servos.
A bare airframe for a 640sqin strong enough to carry 3.5kg will probably weight ~2.5kg.
So AUW of ~6kg (say 210oz) (+wt of solar cells)
Hence wing loading of ~48oz/sqft.
This is 2 to 3 times the wing loading of a typical RC model of 5-6' span.

Assume 300watts needed for cruise (an underestimate).
Or say 25amps draw.
Hence 1 hour endurance.
How much current will the solar cells provide?

Edit:
Since the current draw is small compared to the capacity, ie, only low C rating is needed, Li-ion batteries may be a better choice than Lipo

Edit:
I think that what we've been thinking so far has a fatal flaw - too high wing loading.
A larger, lighter foamie is a better idea. For a 4hour flight using 16,000mah, see this:

http://cyberspaceandtime.com/nZkzJSkzte8.video+related

Hi Mr. Iyer,

We are planning on using Li-ion battery pack, with a 33 Ah capacity. So our AUW (including solar cells and building using traditional means) should be around 5kg, giving a wing loading of ~42 oz/sqft.

  As you suggested, perhaps a lighter foamie will bring down the wing loading. Our AUW (if we have same foamie design from the video) will be 800 gms more, resulting in 3.6 kg. This  allows us to conservatively estimate endurance of a minimum of 3 hours (compared to the video, although we increase the AUW, we have more battery capacity as well). The solar cells (18 cells) can provide a maximum useful current of 4A in a 3S configuration. But this is under best solar conditions providing maximum solar power. Assuming a 75% average solar power for a 3 hour flight (similar to the one from the video you shared), we have 9 Ah of capacity from solar. This should increase the endurance by ~1 hour.
 
Please feel free to point out any inconsitencies in my calculations or if there is anything still to be considered.
 
Are there any off-the-shelf kits or designs that can be built for the required wing area to put 16-18 solar cells?

Regards,
Parithi

Please take a look at Mapper and Batman models from this Allahabad company.
Vortex-rc.com

http://www.vortex-rc.com/product/mapper-v4-1400mm/

http://www.vortex-rc.com/product/batman-plank-rc-plane/

Went back to look at your original post

Quote from: hobbydrone on September 23, 2019, 05:32:18 PM
1. I  am in need of a glider (with an approximate wing area of 0. 4 square meters, preferably composite structural but foam also would do)
2. to conduct some flight tests
3. and test some hypotheses on the viability of solar-electric hybrid gliders.
4. The glider airframe will be used to quantify the improvement in endurance when solar cells are padded on to the wing surface.

Any suggestions for an off-the-shelf model that can be bought or an existing design that can be built is greatly appreciated. Would love to know the RcIndia community's thoughts on the best way to proceed on this project.

Thanks and Regards,
Parithi

Regarding viability of solar electric hybrid, a flight around the world has already been done

Regarding improvement in endurance, an 81 hour flight has already been made by a model plane

Perhaps you may like to reconsider your objectives...
On the other hand, if you manage to do a 4 hour solar assisted flight against a 3 hour battery only flight, it will be quite an achievement.
Well worth pursuing!

Regards

Hi Mr. Iyer,
Thanks for your suggestions. I have been following Impulse's worldwide flight, and also of Atlantik Solar's 81 hour flight on a model plane. In fact, the 81 hour flight has been a motivator/inspiration for me to see if  they solar drone flights made sense for useful missions under Indian sunlight conditions. If you dig a little deeper into the 81 hour flight that was performed, the flights took place in Europe in June, during which, the region experiences almost 16 hours of  sunlight per day. Also, the useful payload that they carried is only about 0.9 kg.

I wanted to see if we could design an UAV that could tradeoff the endurance for a slightly higher payload weight ( say 2.5 kg for a 10 hour endurance). This payload weight and the endurance combination might now make it viable for certain industrial use cases. For a given design, we have some computational estimates of how much endurance is possible for differnt payload combinations.

Your inputs have given us a lot of food for thought, on how to approach this project. In my search for off-the-shelf  airfrafmes, I couldn't find anything that matched our requirments. Do you have any suggestions on what is the best stable, flyer to build  for our initial test flights?

Thanks and Regards,
Parithi

See reply #8 above, maybe you missed it.

Edit
Sometimes I make multiple posts, as my thought process proceeds.
The reader may read only the last post, and miss the earlier ones!
Especially if there are other posts in between.
So when you read one of my posts, do remember to check for earlier posts if any.

I'm going back to basics in my 'thought experiment' , with conservative assumptions.

Solar input 1kw/sqm
Assume avg input 500w/sqm for 8 hours in Indian summer (say 9am to 5pm)
Assume 10% efficiency of solar cells
So 50w/sqm. For simplicity let's say 64w/sqm
Now this is for 1sqm, or 64 of 5"x5" cells
Or 1w per cell, or 2amps per cell

Assume 20v (no load voltage) reqd for charging.
For 20v, we'll need 40 of 5"x5" cells in series. Giving 2 amps.
A 20x2 grid will be 100"x10"
Pretty large, and capable of only 2amps for charging!

I've requested another member, kiran rc, a young nerd, to to join this thread, as he can provide inputs from 'real' experiments, as opposed to my 'thought' experiments...

@hobbydrone
The best material for the glider will be composite/balsa.
Foam will tend to flex a lot and also mounting the solar cells will be a challenging task as the surface temperature of the C60 cells can go pretty high.

Regarding the selection of model/airframe,
An off self balsa kit should be good enough for the initial tests.

Hi Mr. Iyer and Kiran,
Thanks for all your inputs. Appreciate it very much. We will keep you updated on our progress.

Regards,
Parithi

Can you please tell us more about the:
1.Exact objectives that you are trying to achieve
2.The computations that you have run(screenshots would be nice)(is it on MATLAB?)
3.Why are you doing this?Just as a hobbyist or as part of some engineering grade project?
4.Do you have any RC skills?If you are a newbie with no previous experience flying aeroplanes I would suggest that you first practice on a simulator and then go for areliable trainer like the Skysurfer http://rcbazaar.com/product.aspx?productid=1138
5.What is your budget?
6.Do you have any rc equipment already, or will you have to buy those as well?(transmitter,Lipo,Charger,etc.)

If you can answer these question I am sure people will be able to guide you better.All the best.

Hi Karthick,
Thanks very much for your inputs. I have tried answering the questions you had posed.

1.Exact objectives that you are trying to achieve
We would like to quantify the endurance increase owing to placing solar cells on the wing surface. The intent is to test the airframe endurance under 2 scenarios: 1) solar cells + battery 2) only battery. This has already been demonstrated in Europe and the US. As Mr. Iyer pointed out, the Atlantik Solar project could fly for 81 hours, but these flights tests took place in European summer with 16 hour days. We want to eventually ascertain and see if this round-the-clock endurance is possible in Indian sunlight conditions. But first off, see if the endurance can be extended to 10+ hours with a 2kg payload.


2.The computations that you have run(screenshots would be nice)(is it on MATLAB?)
Yes, its on Matlab (but can be recreated on Python or any other high level language). We have created a computational sizing tool and a design optimizaiton framework, that predicts to a reasonable accuracy what is the endurance possible for different aircraft and environmental parameters. Not sure if I can share screenshots of the tool, as some of the work is proprietary IP, which we are hoping that we can leverage commercially in the future.

3.Why are you doing this?Just as a hobbyist or as part of some engineering grade project?
To understand the technological challenges and then potentially commercialize if there is a viable usecase.

4.Do you have any RC skills?If you are a newbie with no previous experience flying aeroplanes I would suggest that you first practice on a simulator and then go for areliable trainer like the Skysurfer http://rcbazaar.com/product.aspx?productid=1138

It's been a while I flew fixed wing aircraft, although I have experience flying quadcopters in the recent past.

What simulator would you recommend for practice?

5.What is your budget?
Another 1-1.5 lakh, besides the 1 lakh we have already put in. 

6.Do you have any rc equipment already, or will you have to buy those as well?(transmitter,Lipo,Charger,etc.)
We have all the RC equipment (transmitter, receiver, flight controller, battteries, charger, etc.). We are currently procuring 
the MPPT charge controllers and

We don't have a lot of experience building and flying fixed wing aircraft with wingspans greater than 1m. So, we are trying to derisk some of our assumptions by getting an off-the-shelf stable flyer, if available.

Are there any places in India where we can get carbon fiber ribs? For a prototype, would you recommend ribs made of out aeroply or basla, for wingspans of 2.3 meters or above?

Thanks and Regards,
Parithi

Very happy to hear your response.
I will answer whatever questions I can and leave the rest to real experts with proper experience (unlike the armchair rc pilot/theorist with no flying experience that I am )

For a simulator,you can go for RealFlight software( a bit pricey though)(I have used it and can vouch for it)
I am fairly unaware of other options.

I completely respect your IP rights. With respect to computation and modelling,this might be a nice watch:


In fact,you can even try creating the above model for starters!You can make it solar by just adding 1 or 2 cells!

I can't answer your question about gliders.I have no knowledge about the topic.

P.S:These threads might be useful to you:

https://www.rcgroups.com/forums/showthread.php?3007750-BNF-SolarBear-pure-solar-powered-32-wing-Free-plans/page6#post42181217
https://www.rcgroups.com/forums/showthread.php?2830795-F5E-RC-Solar-Powered-Aeroplanes/page22#post41890113
(if you haven't seen this already)

Please do consider a flying wing style glider if possible as they are more efficient than conventional gliders(Proof:Ho-229,B-2 Bomber, MQ-25 Stingray)

For personal and educational reasons,I might not be able to reply to your posts/queries in the future.

All the best for your endeavours!
Really nice to see that somebody is trying to build a business model out of this,that too in India!

So what did you guys settle for? Any updates? We are eagerly awaiting some and we do hope that you are able to take the project all the way to completion.What you are doing is super difficult and I wish you all the best.

Thanks for the wishes, Karthick. We decided to with our own design with a scaled down model of ~3m wingspan and chord of 350mm. We have bought most of the components and building the airframe now.. Hopefully, should be done in the next couple of weeks. Currently doing some benchtop testing of the solar cells to get some data on its performance.

Eagerly awaiting the data 

And a few photos/ build log please.

Checkout this paper too:https://www.ijitee.org/wp-content/uploads/papers/v8i5s/ES3484018319.pdf

Ha!As always,yet another Indian attempt at making a solar UAV in hibernation!(including me :-))

Quote from: Karthick Ashwath on January 09, 2020, 10:42:08 PM
Ha!As always,yet another Indian attempt at making a solar UAV in hibernation!(including me :-))

Are you doing this?