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Mounting flexible solar panel on aluminum aircraft

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12notes

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I am planning on using a flexible 100W solar panel on my Hummelbird to recharge the LIFePO4 battery which will be running the radio, instruments, lights, and possible future transponder. I already have the solar panel, it measures 21.25"x11"x.080" (EDIT:typo originally said 12.25) (540x280x2.5mm) and weighs just under 1 lb.(440g), however, I can trim about 1" from the length and width, and probably will to remove the brass mounting grommets at the corners. This would be mounted to the wing, for the simple reason that it is not flexible enough to conform to the cowling or turtledeck, and is too large for the horizontal stabilizer. I do not want to use a smaller panel, even though 100W is overkill, as conditions would frequently keep the output far from the maximum, and also for more electronics I may implement later. I am not interested in a portable unit to be unrolled and set up when on the ground for three reasons - I would like to recharge during flight, portable systems tend to weigh more, and, most importantly, I am lazy and don't want to have to set it up/break it down.

I've thought of several ways to mount it, but none seem ideal, and was hoping to get a few ideas and hopefully tips from someone who has done it before. My plan is to mount it behind the main spar. This could either be on the center section wing stub (~2' wide, ribs every 6") or outer wing panel (6' wide, ribs every 12"). I'm leaning towards the outer wing panel, as any drag/turbulence/aero weirdness caused wouldn't affect the tail. The panel has a small electronics box on the bottom, I would have to cut a hole in the skin to mount it, so I don't have the luxury of trying different placements.

Ideas I've had:
1. Epoxy the panel on top of the wing skin. This is by far the simplest method, and could make a smooth transition, but would make it difficult to replace, unless there is some chemical solvent dissolveable epoxy that would last until intentionally removed.

2. Screw down .020" aluminum joggle strips over the edges. This would be easiest to remove the panel, but not sure about how aerodynamically ugly. How much does a 11" wide 0.1" bump on one wing matter with a VNE of 150mph, and cruise around 105?
 
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Dana

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Glue (or better,VHB tape) the panel to a piece of aluminium sheet and screw or rivet that to the wing. But glue it while attached to the wing so the curve matches.

Put it inboard, not at the tip... if there is any flow disturbance you don't want it at the tip.
 

Bigshu

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Why not get another panel and put one on each side? That way you would maximize your output from each panel as the sun moves from one side to the other while flying. It would also even out any flow disturbance if outboard of the prop wash. Don't worry about overkill on the wattage, you'll see soon enough that devices eat up more than you can gather under less than ideal conditions.
 

12notes

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Glue (or better,VHB tape) the panel to a piece of aluminium sheet and screw or rivet that to the wing. But glue it while attached to the wing so the curve matches.

Put it inboard, not at the tip... if there is any flow disturbance you don't want it at the tip.
Good idea, I might play with this one.

I wasn't intending to put it at the tip, but near the inboard side of the outer wing panel, roughly 5' from the wingtip and 2' from the fuselage.
 

12notes

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Why not get another panel and put one on each side? That way you would maximize your output from each panel as the sun moves from one side to the other while flying. It would also even out any flow disturbance if outboard of the prop wash. Don't worry about overkill on the wattage, you'll see soon enough that devices eat up more than you can gather under less than ideal conditions.
The fuselage isn't big enough to worry about it shadowing the panel, and I don't want to double the weight if I don't need it.
 

dominiqueghekiere

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just out of curiosity where did you find such a panel and what do you use to regulate the voltage or protect the battery from over charging ?
 

n6233u

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I have thought about putting one on my cowl, I'd like to know where you found it as well.
 

pictsidhe

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Dana's method, with spray glue.
Those connection boxes can sometimes be carefully removed, leaving you some fragile strips to solder wires to. Put it inboard, as someone suggested.
There are flexible panels that will take a tight radius.
You will not get 100W from a foot square-ish panel. More like 10W. Try it out, it may be enough. If not, add another to the other wing. Do you not have engine charging?
 

12notes

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just out of curiosity where did you find such a panel and what do you use to regulate the voltage or protect the battery from over charging ?
I have not decided on which charge regulator to buy or if I'm going to build one myself. I haven't found one that lets me route excess power once the battery is charged, but haven't done a deep dive into options yet.

Panel info in next post.
 

pictsidhe

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The cheap solar regulators on eBay would work. A lot of them will disconnect the battery if the voltage gets too low. I suggest not using that feature, or just using it for less important stuff, like the transponder and radio... Some of them have digital readouts, so may double as a battery gauge. But, they aren't compact. Get one with at least 2x safety factor on your charging current. A 10A cheapo is going to let some magic smoke out around 11-12A. Many of then have adjustable charge voltage.
 

12notes

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Dana's method, with spray glue.
Those connection boxes can sometimes be carefully removed, leaving you some fragile strips to solder wires to. Put it inboard, as someone suggested.
There are flexible panels that will take a tight radius.
You will not get 100W from a foot square-ish panel. More like 10W. Try it out, it may be enough. If not, add another to the other wing. Do you not have engine charging?
First, I had an important typo in my original post - the length is 21.25", not 12.25", so this is a bit larger than you think, approximately 0.135 m^2 of solar cells. And there's a simple trick to getting 100W out of this size solar panel - you just lie about the output. Stupid me for not doing the calculations. Also, this panel is no longer available where I bought it, so I think someone else did the math as well.

Depending on solar cell efficiency (19-23% is typical), this is a 25-31W panel. I seriously doubt these are at the high end of that scale, so 25W is probably closer to what this panel is. Since my total current draw is likely to be less than 1.5A, this is sufficient to keep the battery charged, and more than enough to recharge while on the ground. I might get an actual 50W panel from Sunpower or another reputable name instead, but that would require me to relocate the boxes. Not a big deal, but I thought the box on the rear looked cleaner, and sealed better. However, the Sunpower is a bit more than twice as heavy, so finding another 25W actual panel may save a tiny bit of weight.

I haven't seen a flexible panel that will bend more than 30 degrees, the radius of my fuselage is approximately 12", and there's not enough length to fit it at the nose, the turtledeck is bent even tighter, so the wing it is.

I do not have an engine driven electrical system, so I don't have to have ADS-B to go under a Class B. ADS-B might be added if my finances get a bit better, but I would still like to avoid an alternator for other reasons.
 

12notes

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The cheap solar regulators on eBay would work. A lot of them will disconnect the battery if the voltage gets too low. I suggest not using that feature, or just using it for less important stuff, like the transponder and radio... Some of them have digital readouts, so may double as a battery gauge. But, they aren't compact. Get one with at least 2x safety factor on your charging current. A 10A cheapo is going to let some magic smoke out around 11-12A. Many of then have adjustable charge voltage.
All the electronics will run exclusively off the battery, solar would only be used to recharge. I'd like to find a controller that will allow me to use the excess power to run a fan in the cockpit on the ground once the battery is fully charged. Canopies get quite toasty in the sun.
 

pictsidhe

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Solar panels are rated under bright sun. Noon, cloudless sky, not too far north. They also produce max output a bit higher than battery voltage, so you lose a bit there. It won't be point directly at the sun very often, so there goes more output. Figure you'll mostly see under half rated power.
There are a lot of 'Max power point' regulators on eBay. Most of them ar lying, though. If you find yourself a little short, they could be an option for another 20% or so
 

pictsidhe

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The fan could be piggy backed from a cheapo regulator. When the pass transistor is off, switch on the fan via another transistor.
 

dino

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Mount it near trailing edge. Airfiol flatter, boundary layer thicker and maybe able to run wires through flap hinge gap.
 

12notes

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Mount it near trailing edge. Airfiol flatter, boundary layer thicker and maybe able to run wires through flap hinge gap.
On the Hummelbird, the distance from the main spar to the trailing edge is roughly 24", so "behind the main spar" I mentioned would have the back edge of the panel near the trailing edge. And there's no flaps, but I haven't riveted on the top skin of the left wing yet, so no worries on routing the wires.
 
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