I have a dream............to fly as long as I want

As I have no space for another plane (I don't really have space for the tiny Fokker V25) so I can only dream, unless of course cannibalise one to build another.
My Battery Wing plane (batteries in the wing) was an interesting exercise but it still does not fly that well even after its 6th design change but the move from LiFe to Li-ion cells opens an interesting avenue.
Li-ion cells have an even greater power density (mAh/g) than a LiPo but are limited to very low discharge rates - 1.5C max.

Now for duration flying such a low C rating is not really a problem as 'cruise' consumption is likely to be 0.5C or even less but it will limit the maximum 'get out of trouble' power that is available.

My current 'Duration' flies on 1A carrying a 7.3oz 5000mAh 2s Lipo which even at its modest 15C rating it is still capable of delivering a completely unnecessary 75A!
Using a triangular section fuselage like this:

I can arrange six 3000mAh 18650 size Li-ion cells in a 2s3p layout giving 9000mAh for an increase of just 0.7oz (total 8oz) over the 5000mAh LiPo.
Even at 1.5C the maximum current will be 13A which is the limit for the intended motor and if It can also be made to fly on 1A then the endurance will be....er.....silly!

Still dreaming but..........

could you not fit solar panels into the wing and re charge while flying
could be up for as long as the day light lasts then
10 hours in the summer.

That is exactly what I found out.
The panels used by the likes of Paul MacCready in something like Helios cost millions of dollars hence the NASA funding.
The cheap 'semi flexible' panels might be powerful enough (50W) in strong sunlight but weigh over a Kg!

Some days it all falls into place, other times it doesn't.
The beginnings of the triangular fuselage in 6mm Depron for the 'Super Endurance'.

Building a double tapered triangle is not as easy at it first appeared!
The two formers represent the limits of the battery compartment.
The 48" span wing will be fixed on the top of the fuselage.
At the moment I am undecided on the controls. For long flights a rudder gives the best fine directional control but with the battery making up nearly 60% of the planes total weight it may be advisable to retain the ailerons.

I was struggling with a suitable method to insert & remove the individual cells as they have to be charged individually.
Overall the best solution appears to be to have a hinged nose so the batteries can be simply dropped in a bit like a torch.
The motor and ESC will be in the nose and the very act of closing it will make the electrical contact with the battery.

This might not work at all!

The next bit is rather fiddly arranging 3 parallel spring contacts for the negative end of the batteries.
To get a reasonable degree of movement (all the batteries not quite the same size or weight for that matter) I decided to use coil springs but each has a 'floating' cable soldered on the end so no current actually travels through the spring wire.

The rear view shows the 3 small individual cables joined to a more substantial one that leads forward to the ESC in the nose section.

Each spring rests on a piece of hard board to spread the load into the Depron former.
I think the 3rd side of the fuselage can now be added but it will a bit of a pain if I've forgotten something!

Eric
No magnets needed.
The top hinged nose section will drop down on the positive ends of the cells and make contact.

As the nose carries the motor (and thus the thrust) it will rely on the hinge and a pair of humble 'hook and eye' fasteners to hold it in place, well that's the plan.

In conjunction with the individual batteries in my DX6i this plane would be a club safety officer nightmare - not a tagged cell in sight!

The nose section contains the motor and ESC.

The out runner is 'buried' inside the Depron 'rings' that match the folding prop spinner. There are 3 cheek fairings to be added followed by a lot of sanding to get it all to blend in smoothly.
The nose raised to give access to load the batteries.

You can just see the brass plate on the back of the nose section that makes contact with the positive ends of the batteries when the nose is down.
It does work but will the contact be good enough to drain the 3 sets of cells equally?

Just a bit more, the nose finished off.

Made a start on the new wing centre section.
The previous Battery Wing plane had a flat centre section with dihedral tips. A single centre break dihedral does give better stability which is a useful attribute on an endurance plane.

A start on the wing.
The existing 15" tip section will be extended inboard by 10" but made slightly complicated by change in wing taper and thus a 'cranked' spar join.
First the new spar.

A balsa/Depron/balsa 3mm thick sandwich with the spars themselves increasing in depth towards the root.
The ribs added to the 2mm Depron lower skin and the aileron servo wire fed through.

After some final profile sanding the top skin will be added flush either side of the spar.
All the components are symmetrical so 2 sets were cut out at the same time. The other wing half should build a bit quicker.

As I hoped the other half wing has been quicker to build.
The LH wing is complete, the RH needs the servo lead extending.

Next the tail feathers

The wing completed and resting on the fuselage.

It looks like the complete plane should be about to 15oz all up with the batteries making up just over half that, which is a similar ratio to my 5 hour endurance plane.

There is some question as the true capacity of these batteries with some only seeing 1/3 of that marked.
I suspect due to a relatively high internal resistance they only show a decent capacity with a very low discharge rate but even than I would be surprised if a hard cell Li-ion could really achieve a better power to weight ratio than as a soft LiPo.