Building a Solar Plane - Part 2

Thanks to @DIYPerks for his help with this build!

Recently, I talked about this project with the BBC when they visited the Project Air workshop to check out the Guinness World Record for the Worlds Largest RC Aircraft Carrier.

DISCLAIMER: This video is purely for entertainment value. Personal use of video content is at your own risk. Recreations of experiments, activities and projects are the sole legal responsibility of the person(s) involved in replicating them. I can not be liable for any information or misinformation, wrongful use, damage to personal property, death or any circumstances that result from replication of any projects seen. Be safe!

[johntate]

One of the things i have learned about MPPT controllers is that you never attach a load directly to them. Instead, just connect your load to the battery directly. The advantage between a MPPT solar controller and the voltage regulator is that MPPT or maximum power point tracking controller is that it actually operates the solar panel at the maximum power and efficiency whereas the voltage regulator doesn't. This is especially important in the evenings and mornings where the MPPT controller will actually be charging the battery and the voltage regulator will not. Unlike a battery, the solar cell produces the highest voltage (Voc) at open circuit and the highest current (Ia) at dead short and the MPPT controller adjust the voltage of the incoming solar to find the maximum voltage to get the maximum power (Vmmp and Immp). Sorry for the long explanation:( Great project and great engineering!!

[Mad-Jam]

This project NEEDS efficiency to work. To go from maximum power point tracking (MPPT) to a regulator you lose a LOT of efficiency from solar.

[SpacemanFPV]

now we need a cross country waypoint mission, like RC testflight

[MICUEVER]

The fact that the motor mount broke off so easily probably isn’t a bad thing. It saved the airframe, maybe even the motor. It’s a lot easier reprinting the motor mount than rebuilding the plane or replacing the bent motor shaft.

[zwpf]

You could always use carbon fibre rods to stiffen the airframe 👍 Great video!

[xraycharli]

16 years ago I cut my right hand on a Deltawing K10 with a pusher prop. I was badly hurt. 3 weeks sick leave. I was shocked when I saw your friend Emma throwing the plane.
Since that day I use only folding propeller and start the engine when the hand is safe. For your monster that configuration would not work.
Ok ... I am 61 years old :-D which can also be a reason to be slightly shocked 😀
In real life I am an aircraft technician and what I also saw is that the wing cord is very small for that length.
To make it stronger, a long carbon tube with a wooden spar glued into it would also be lighter. OK ... to make that can be a challenge.
However, it is a good decision to go for a delta wing or double glider configuration.
I am sure you will get it happen.
best wishes from Hamburg Germany

[jaketus]

2:40 Well, the simple regulator lacks the capacity to do maximum power point tracking, literally the meaning of MPPT. More efficient would be to have the MPPT in the loop, but connect the load (and maybe the regulator) directly to the battery-terminal of the MPPT along side the battery.

[diegogonzalez]

I’ve done a few solar projects on my own time, definitely not an expert. I would keep the MPPT rather than the voltage regulator though, by connecting the load directly to the battery rather than into the load connection on the MPPT, the other two connections remain the same. In this case the MPPT can charge the battery without worry of disconnecting the load. Keep up the good work Love the projects!

[ДенисСидоренко-фр]

Hi! I really love such solar plane projects and plan to implement it one day. You have made a good progress, although I see some problems in your design. If I were you, I would do the following:
1. Replace a bulky LiPo battery with more lightweight DIY Li-Ion pack. It would be like 30% lighter and you can arrange it into a long narrow pack, which would allow to make a smaller hull, minimising frontal area and drag.
2. Mount the motor inside the hull. It will greatly minimise drag. Mounting it may be a bit more challenging, but solvable with 3d printed bracket. Or you can just change your existing motor mount with a more aerodynamic one, that would be an easy fix.
3. As others have already mentioned, MPPT controller’s purpose is adjusting current which is pulled from solar cells in such a way that generates more output and it works more efficient than a simple converter. I suggest you ignoring load output in your MPPT and pulling power from the battery instead.
4. Improve hull aerodynamics. It looks too big, has no front cone (I don’t see any reasons to keep it open from both sides, except for cooling (which is unreasonable and may be improved by having an ESC outside)). Reducing frontal area and adding a nose cone is quite simple.
5. Issue with motor mount being broken off on landing: use folding props. It will fold on landing and while soaring/gliding (and reduce drag). Be sure that not every folding prop mount suits pusher configuration, as props may fold «inwards» and won’t fold out when spinning.
6. Reinforcing. Aluminium is lightweight, but not as lightweight as carbon fiber rods. They may be harder to find and definitely not as cheap as aluminium rods, but far stiffer. Bonus points for making a special battery pack which may be inserted into a carbon rod (I doubt you have a wing thick enough to implement this idea, as 18650 cell and wiring will be quite large, but that’s just an idea). AFAIK AtlanticSolar did this to distribute load on its wings, thus allowing to have higher aspect ratio and thinner hull (by moving batteries to wings). Also, your L-shape aluminium extrusions may be drilled out in places which don’t bear load (for example a side which goes flat on the wing bottom side) which will allow to shed some weight. I also worried if your aluminium extrusions are connected in the center, if not the wing can snap.
7. Top-sided winglets may cast a shadow on solar cells, reducing overall power output. Mounting them inverted will solve this issue, but will certainly break off on landing. There might be a way to make them detach on touchdown, but it sounds more like the least priority to do.
8. Wing profile. That’s probably the most complicated thing in the whole build. I can’t see it clearly, but assume your bottom side is flat, and the top is also flat. Making top flat is probably the only solution to mount solar panels which can’t be bent, but such geometry (that’s my guess, I’m not even close to being an aerospace engineer) produces lift only when an angle of attack is not zero. This means higher drag by keeping nose up. I’ve seen a solution on rctestflight channel which looks like a ladder: solar panels are put on these «ladders» and form somewhat nice aerodynamic shape.
Btw your idea with joining two gliders is cool, but these gliders are based on real life gliders which have a thick cockpit for a pilot, which adds excess drag. I’ve bought a Voluntex ASW28 for endurance flights and plan on getting rid of stock hull and replacing it with custom narrow carbon fiber one.
Anyway, I’m waiting to see new videos with the solar plane!

[ThomasWinget]

"so I hopped on my Mac"
"and yes, this is all running on a Mac"

(shows B-roll of it running on Windows)

Very interesting video(s) and project, but I couldn't help but laugh at this part.

[simeonhunter]

As stated previously here, keep the MPPT controller. Run your flight controls off the battery, not off the MPPT. If you’re going to reinforce with an aluminum spar, use a thinner box extrusion as it will be stiffer in torque than angle for the same weight. Run as many prop motors as you like; the amount of motors does not dictate your power consumption in level (break even) flight. In fact, running more props at lower RPM with more propelled surface area will increase your efficiency dramatically, and also give you the ability to climb when you need it. Your penalty will be weight of course - but not power. I flew in a two-seater solar plane with 5kw on the wings, and a 15kw motor.

[Gosuminer]

Congrats for this partial success. You really stepped up your video production game. There is a reason people use MPPT-Chargers instead of DC regulators. The reason is that we do not really care about voltage but power and the power from solar cells gets really low if the current is too high. This happens when the battery voltage drops, e.g. when there is voltage sag when more motor power is needed. It may be worth testing your charging setup with the actual solar cells and changing lighting conditions and motor power. Also, with a plane that valuable it is always a good idea to provide a small extra battery for the receiver and servos so the plane can be landed as a glider if push comes to shove.

[willdrunkenstein]

Use carbon and glass fiber laminates to form the wings. No additional reinforcements are needed, probably gives you the lightest fuselage.
Composite materials also make it easier to produce more aerodynamic joints. For your solar powered plane, efficiency matters

[stephenjohns]

Great effort! Please dont let Emma keep hand launching like that. She is going to get hurt by the prop.

[nickldominator]

Seems like a very backwards step to eliminate the MPPT charge controller, due to it having a load safely cut-off. Use the MPPT still, but only for charging. Connect the load directly to the battery. A DC-DC converter wont be nearly as efficient in varying sun conditions.

Another recommendation I would say is to introduce a BMS in front of the battery. RC batteries don't feature one for weight and cost reasons, but in an application where you are continuously charging and discharging a lithium pack, you are very likely to introduce gradual cell imbalance, which your charge controller cannot fix. At some point, the cells *will* go out of balance enough to cause damage, if not a fire. At the very least, add an active cell balancer to ensure imbalances in the cells are handled quickly. They are fairly inexpensive & light as well. They won't have the OCP, OVP, UVP and so on of a proper BMS, but that could even be an advantage as there is no chance of powering being shut off abruptly.

Great video!

[homermorisson]

Does Emma actually work with you on design and/or construction?
If so, we need more Emma, if she's comfortable with that... always great to see a (historically) stereotypical "male profession/hobby" performed by stereotype-busters. =)

[infinitytec]

There's a company called PowerFilm that makes super thin and flexible solar panels. I don't know if they are as efficient as the ones you are using, but they could possibly be placed over a frame or even just wrapped around foam or something.

[ErickC]

Your design, as I see it, has several things working against it:

1. That aluminum spar is probably more than the wing actually needs in terms of stiffening. You'd probably have better luck with a less-substantial aluminum or carbon-fiber reinforcement, or you could build a torsion box out of balsa. The challenge isn't making the wing strong enough, it's making it just barely strong enough. If it didn't break in the first test flight, chances are good that a very minute amount of reinforcement would be enough to stand up to most turbulence. Your wing probably only really needed any real reinforcement towards the root.
2. Swept wings inherently have poor lift properties at low speeds. Your wing is actually swept well into jet airliner wing territory - unless you're in the mood to start adding high lift devices, you're kinda working against yourself.
3. There's no reason why the chord of the wing inboard of the elevons shouldn't be extended to match the chord at the elevons. Sure, the overall aspect ratio of the wing would decrease, but I doubt it'd create enough drag or weight to offset the lift benefit, especially since I'd wager increasing the chord at the tips is adding to induced drag...
4. You're going to have to optimize the powerplant for efficiency with respect to thrust... a ducted fan of the same diameter as your propeller is probably the best approach. While the duct imposes a drag penalty at high speeds, you're low and slow.

I don't think a delta wing is going to solve your problem - in fact I think it might make it worse. Delta wings are low aspect ratio by nature, so you're going to be sacrificing a lot of efficiency to no real benefit since your airplane isn't meant to fly at supersonic speeds. You're designing a low-speed airplane. High aspect ratio straight wings are the way to go.

[akujiwar]

The simple solution would be connecting battery and ESC to the same terminals on the MPPT. Skip the "Load" terminals as they have a cutoff.

Ideally you can find an MPPT controller for 4s lipo. The one in the video has a slightly lower voltage limit, so wont start charging until voltage drops a bit, but should work just fine after that.
That voltage regulator has no MPPT, this reduces panel efficiency, likely by a significant amount. you could use MPPT + regulator, but this adds weight and extra losses.

[ReneSchickbauer]

You could try to mount the solar cells inside the wings with a transparent cover. This should reduce air drag and help (somewhat) to protect the cells in case of a crash. (Yes, this will slightly reduce the efficiency of the cells).