Airfoil selection for perpetual 5m solar eGlider

[WulfKeeper]

Ahh, laminating! Wouldn't that still have a bit of a ridge at the panel edges? It would still be an improvement though. I wonder how much weight it would add. Thanks for the reply, good luck with your build

Actually my comment was perhaps a bit misleading. The 'lamination' is really just on the back of the cells to make a single piece of material you can then apply to the wing. They still then cover it in a transparent (typical RC) heat shrink coating e.g. oralight.

I've got a bit more experimentation to do down the track

 

[WulfKeeper]

According to Clinginpeel the plane photographer, the HK1 made more than 6 sucessful flights, but was not good at generating "feel" on the control yoke, thusly irritating Howard tremendously.

Twisty wings means an assembly failure. I have built 4 meter thermal stuff that was more rigid than 1 meter aerobatic. As long as it isnt heavier per meter, you should be good. Ideally it should be lighter per meter.

Wow. I didn't know it flew that many times! Did it ever get above ground effect height? An amazing build, regardless.

I'm not sure I agree on "assembly failure". That seems to imply that the material selection and design were correct and it's a construction issue. Is that what you mean?

I'd be curious to read more on your 4m construction if you would kindly share Ribs and 'I' beam spar with shrink coating or something more exotic?

Thorougly agree on the lighter/metre the better.... as long as it actually survives flight! That saying "reality bites"... I have firsthand experience with lovely builds (outside of aviation) being destoryed by reality's "big teeth"

 

[Piotrsko]

Yup either not enough glue, poor joints or some other point of failure like sanding too much material off. When cyanoacrylic glue came out, the joke was airplane kit in a bag. Good engineering should cope with the flight stresses.

The 4 meter had X ribs with the load bearing 3rd rib in the middle of the X, fully plywood boxed I beam spars, and 3/32" sheeting to the spar and 2"wide 1/16 rear sheeting top & bottom 1/2oz dacron covering with aeropoxy and chrome monocoat. I think all up weight was ~18 lbs and took a 2 hp winch to launch. It was a thermal flying beast. A representative picture is over on @Jackson T 4meter thread in the beginning somewhere.

Not sure you want or need to cover your solar cells. Wont make an aero difference, but will loose a ton of efficiency.

 

[WulfKeeper]

Yup either not enough glue, poor joints or some other point of failure like sanding too much material off. When cyanoacrylic glue came out, the joke was airplane kit in a bag. Good engineering should cope with the flight stresses.

The 4 meter had X ribs with the load bearing 3rd rib in the middle of the X, fully plywood boxed I beam spars, and 3/32" sheeting to the spar and 2"wide 1/16 rear sheeting top & bottom 1/2oz dacron covering with aeropoxy and chrome monocoat. I think all up weight was ~18 lbs and took a 2 hp winch to launch. It was a thermal flying beast. A representative picture is over on @Jackson T 4meter thread in the beginning somewhere.

Not sure you want or need to cover your solar cells. Wont make an aero difference, but will loose a ton of efficiency.

Thanks for sharing the contruction details Piotrsko. It'll be really interesting to see how the 'super cross laminated' and no cloth-sheath wing compares!

I think the key point is 'good engineering'. A perfectly constructed, poor design is obviously still going to fail. A good design but poorly constructed? Yep, I agree.

 

[WulfKeeper]

G'day all

Yes, this project still breathes... all be it on life support with local COVID lockdown atm. I'm hoping to get the epoxy to laminate the top surface of the wings shortly. In the meantime I've started work on rest of the wing. Namely the front and bottom surface. Being slightly concave I was curious how my 1.5mm ply was going to hold the curve, try rib distance and stringer positions etc. Being 'solid' skin I can get away with a much wider rib distance than for a film covered wing.

From the photos you can see (a rough idea of) the internal structure. The cross laminated surface obviously over the top. There'll be a 20mm CF tube spar running along through the max rib height (maybe a loaded spar - containing the 18650's a la SolarAtlantik!) Still tinkering with how to best integrate the top and bottom onto the leading edge. A couple of good options to try. Still remarkably light and strong. Ultimately the leading edge will have a 50mm CF tape covering.

Oh yeah, I'm seriously contemplating buying a laser cutter for all these ribs (and boom bulkheads)! Lots of people pointing out how slow such-and-such a laser is at cutting... yeah? Compared to an old man and an Exacto knife? As long as the cut is accurate and clean (and doesn't turn the balsa into a BBQ) I'll be happy!

 

[WulfKeeper]

I took to the mockup section in the previous post with the 'if you don't need it structurally - get rid of it' attitude. Weird resulting shape based on that criteria but 20% lighter! Was always destined for 50mm diagonal CF omnidirectional tape. Also 25mm sheet either side of the ribs. The PV cells won't be structural but will help retain the true foil profile. I received the 18mm ID/ 20mm OD CF tube in the mail for the 'loaded spar' very stiff! Maybe too stiff if I want some flex along the whole 5m span. Hmmm.

 

[WulfKeeper]

This project is still bumping along Managed to get the epoxy and associated parafinalia to laminate the wing after lockdown lifted but need it to dry out a bit now! In the mean time I've been FEA'ing a few ideas on the anit-twist straps on the wings. That's looking promising. Actual build has turned to less glamorous but essential aspects such as the tail boom. 2mm ply bulkheads with 45deg balsa sections on the sides to remove lateral flex and provide increased bonding area to the 12mm CF tubes. Photo is a rough knock up with ye olde blue painters tape. Flight version will be CF tape and epoxy. Suprised at how light and stiff it is. Means it's probably too strong = too heavy

There will be a 1mm ply frame/platform for the PV cells to attach on across that top flat side. Clear laminate skin for the rest. I'll 'space' the bulkheads too - as much for cable routing as weight saving

Grand plan is to build the entire airframe minus the (expensive) PV cells and associated electronics/electricals. Just fly it as a regular e-glider. Make sure it 'works' as a plane. I'll even add mock-up 1mm ply 'PV cells' to test the contruction techniquies as much as I can. Maybe even appropriately distributed ballast to make sure the load and stresses are as close to full perpetual solar spec as practicable.

 

[Piotrsko]

Dont sweat the flex, everything man designed and built flexes to some extent. Ever watch a B52 take off? Wings need tip wheels to stop dragging, but flying it has obvious diehederal. As long as the flex doesn't change incidence, you're good

 

[L Edge]

Only way possible to make this system work is you need potential energy(thermals) to help you. So the FC needs to sense updrafts and use that to circle and gain altitude till it falls apart(30,000 ft or more?). Then glide back(high L/D) down without using the stored energy. Saving that for the night. Then it might be possible to accomplish your task.

Buy the way, are you aware of the winds speeds from 1000 foot up? Look at some charts. Weather and wind are a big factor.

Good tinkering project to get it to fly for one day.

Here's is your design:
Just went to 76,100 ft and spent 18 days taking data. Called Zephyr and produced by Airbus.

https://www.airbus.com/en/products-services/defence/uas/uas-solutions/zephyr

 

[WulfKeeper]

Only way possible to make this system work is you need potential energy(thermals) to help you. So the FC needs to sense updrafts and use that to circle and gain altitude till it falls apart(30,000 ft or more?). Then glide back(high L/D) down without using the stored energy. Saving that for the night. Then it might be possible to accomplish your task.

Buy the way, are you aware of the winds speeds from 1000 foot up? Look at some charts. Weather and wind are a big factor.

Good tinkering project to get it to fly for one day.

Here's is your design:
Just went to 76,100 ft and spent 18 days taking data. Called Zephyr and produced by Airbus.

https://www.airbus.com/en/products-services/defence/uas/uas-solutions/zephyr

G'day L Edge

Yep, Zephyr is ceratinly a good one. I think their record is almost 26 days aloft? Ravin' mad. Wing span is 5x what I've got planned ...and even then I've got domestic pressure on build and storage requirements

There's a few seriously good papers written on the SolarAtlantik project too (more my scale).

Totally with you on the weather and wind. Obviously a huge impact on performance! Been the limit on many a solar flight.

Flight profile is an interesting topic all in itself. Gain max height during the day then glide down at night seems common. Automagic thernalling would sure be nice... Just have to wait and see

Thanks for the tips and suggestions.

Cheers
W

 

[WulfKeeper]

Dont sweat the flex, everything man designed and built flexes to some extent. Ever watch a B52 take off? Wings need tip wheels to stop dragging, but flying it has obvious diehederal. As long as the flex doesn't change incidence, you're good

Yep, dihedral flex is good. Tail boom flex... not so much.

I think this build is going to be... umm.. iternative. I always expected as much.

 

[quorneng]

WulfKeeper
As weight is going to be so important to what you intend it will pay to explore and test what design of structures give the best weight to strength ratio for the size you intend.
In most cases modellers have 'adequate' power so plane structures tend to be of the 'plenty' strong enough category rather than the lightest possible for the required duty.
In principle you have to ask for each part is is over strength for its duty and would a different material (not necessarily stronger) give a weight benefit.
The bigger the span the more important material selection, weight analysis and testing becomes.

 

[WulfKeeper]

WulfKeeper
As weight is going to be so important to what you intend it will pay to explore and test what design of structures give the best weight to strength ratio for the size you intend.
In most cases modellers have 'adequate' power so plane structures tend to be of the 'plenty' strong enough category rather than the lightest possible for the required duty.
In principle you have to ask for each part is is over strength for its duty and would a different material (not necessarily stronger) give a weight benefit.
The bigger the span the more important material selection, weight analysis and testing becomes.

Yep. Hence the acknowldgement of a somewhat 'iterative' approach. As per previous posts, I'm building a few test sections of key components to see how they play out. A pretty much incremental process. Failure is *always* an option for this kind of gig

It'll be fine line between strong enough not to fail but no more weight than absolutely necessary... like a lot of other design challenges. Racing trimarans are a good example. People think trimarans are less stable because "they're always flipping over and breaking". Yeah, nah. *Much* more stable than a monohull. But if you're pushing them 10/10ths then you're going to cross to the wrong side of the line. Like comparing an F1 car to your average sedan. Both are cars... Although in this case, for the solar glider, it's weight/performance not speed/performance.

I've got control of quality to a large degree, time's not an issue, budget's not tooo much of a contraint and the resources are Ok. Not a bad project environment! I'm really enjoying it so far. But oh such a long way to go!! Stay tuned.

 

[quorneng]

Wulfkeeper
It might be worth looking at the construction techniques of super light planes particularly where money was no object like the Airbus Zepher and the manpowered types.
I note that the Zepher team chose to use a Mylar type skin over a sketchy but high strength framework rather than using a 'solid' skin. I am pretty sure with the technical resources available this type of structure was chosen because it gave the best strength to weight ratio.

With my own experiments to find the most practical form of super light weight home build used a sheet foam skin. Unlike a film it is self supporting and does add to the overall strength but importantly on a like for like basis it is only about 1/10 the weight of balsa.
I will follow progress with interest.

 

[WulfKeeper]

Wulfkeeper
It might be worth looking at the construction techniques of super light planes particularly where money was no object like the Airbus Zepher and the manpowered types.
I note that the Zepher team chose to use a Mylar type skin over a sketchy but high strength framework rather than using a 'solid' skin. I am pretty sure with the technical resources available this type of structure was chosen because it gave the best strength to weight ratio.
View attachment 212577
With my own experiments to find the most practical form of super light weight home build used a sheet foam skin. Unlike a film it is self supporting and does add to the overall strength but importantly on a like for like basis it is only about 1/10 the weight of balsa.
I will follow progress with interest.

Yeah, that's the approach the French solar e-glider project took too. I'm also borrowing from lightweight planes (and boats), I'm going to try a cross laminate balsa 'skin' (scroll back to 17 July). Self supporting but almost hollow (just a few ribs). The strength is the tension you get from the cross lamination.

It'll also be 'spaced' with CF tape straps at the ends to stop the ailerons inducing twist. Even the ribs won't be structural. Just enough of them to hold the bottom surface in shape with some super lightweight stringers.

Well... that's the idea. I rarely build strictly off-plan ... even when I do have plans! (I think my SunBird mods are still on this forum somewhere) It's what I enjoy most

 

[Jackson T]

I know you're not short on feedback, but I just thought I'd add my 2c...
When I was thinking about solar planes I thought about solar cells on the boom too. For the cells on the boom to 'pay off' the power gain must outweigh the drag gain. A skinny carbon fibre boom like on f5j gliders would have so much less drag than a triangle big enough for the C60 cells, considering all that wetted area and any side gusts flowing around the sharp triangle corners. My personal conclusion (if I ever made my own) was that the drag and weight (CG?) penalty would outweigh the increase in number of solar cells.
BTW, I like the idea of storing batteries in the wings! As well as reducing the fuselage size it spreads the flight loads out along the wing, reducing the bending moment at the root. After all, if the weight was distributed evenly with the wing area there would be no bending moment!
Have you considered a twin tailboom design? It would have much less rudder torsional loads to deal with and would make a top mounted horizontal stab (for more solar cells) easier.
Keep up the great work!

 

[WulfKeeper]

I know you're not short on feedback, but I just thought I'd add my 2c...
When I was thinking about solar planes I thought about solar cells on the boom too. For the cells on the boom to 'pay off' the power gain must outweigh the drag gain. A skinny carbon fibre boom like on f5j gliders would have so much less drag than a triangle big enough for the C60 cells, considering all that wetted area and any side gusts flowing around the sharp triangle corners. My personal conclusion (if I ever made my own) was that the drag and weight (CG?) penalty would outweigh the increase in number of solar cells.
BTW, I like the idea of storing batteries in the wings! As well as reducing the fuselage size it spreads the flight loads out along the wing, reducing the bending moment at the root. After all, if the weight was distributed evenly with the wing area there would be no bending moment!
Have you considered a twin tailboom design? It would have much less rudder torsional loads to deal with and would make a top mounted horizontal stab (for more solar cells) easier.
Keep up the great work!

G'day Jackson. Input welcomed. Esp from a veteran (it's on YT!) big glider builder such as yourself

Yeah, with you on the tail cells, it was a serious consideration. A marginal call for sure. A lot of similar projects skip them. Our mate Spike at Bearospace has them in a build he's got on the bench in a vid and SolarImpulse2 went for them.

Weight with that triangular 3x12mm CF tubes and 2mm ply bulkheads is surprisingly light (I'll get it on the scales). Twin boom... Hmmm. I've got a very SI2 style tail feather config in mind... the triangular section helps with that. Oh yeah... gotta knock up one of those to test the construction too...

Still umm'ing and err'ing on the battery loaded spar. Going to be a PITA to build (wire). We'll see on that score too!

Thanks for the comments and support you lot. Much appreciated.

 

[quorneng]

Apologies for chipping in again but as Jackson T says batteries in the wing does make sense.
Not solar powered but this does demonstrate the principle.
The fuselage is a tube just the diameter of the motor and only long enough to house the ESC and the radio.

The wing centre section box spar housed 4 x 18650 Li-ion cells in a line but arranged as 2s2p. I elected to use these as they can be permanently built in and charged in situ. Note the built in on/off switch in the leading edge and the charging jack hole behind.
It flew pretty well but I made the mistake of buying cheap cells that had less than 1/5 of their claimed capacity so it could fly for just under 1 hour rather than the anticipated 5+ but the principle certainly worked.

 

[Piotrsko]

Comments: there are thermals/lift at night, that's how you get night thunderstorms. They are just uncommon and weak. Flying area will need much consideration ,perhaps as much as the build.

Mylar comes in rolls, sticky or plain. Doculam is a trade name, and it is thermoset so it heat shrinks but there are other purveyors.

Thinking more: max speed approaches stall speed at altitude so when Major Payne broke the glider speed record, he was on the edge of stall for most of his flight averaging 130 kts.

 

[WulfKeeper]

Apologies for chipping in again but as Jackson T says batteries in the wing does make sense.
Not solar powered but this does demonstrate the principle.
The fuselage is a tube just the diameter of the motor and only long enough to house the ESC and the radio.
View attachment 212699
The wing centre section box spar housed 4 x 18650 Li-ion cells in a line but arranged as 2s2p. I elected to use these as they can be permanently built in and charged in situ. Note the built in on/off switch in the leading edge and the charging jack hole behind.
It flew pretty well but I made the mistake of buying cheap cells that had less than 1/5 of their claimed capacity so it could fly for just under 1 hour rather than the anticipated 5+ but the principle certainly worked.

Chip in's most welcomed!

Did you spot weld the cells? I'm a bit reluctant to go down that path... I *think* I can do it without killing myself