The smallest scale system is purely battery power, but offers flight endurance well beyond what conventional fixed-wing can achieve. Traditional 10-ft wingspan drones carry 5-pounds for about 90 min. A comparable weight/payload TURN system can fly closer to 7 hours. This prototype is being used as a minimum viable product for an upcoming product launch.

The company was awarded an SBIR research grant from the Air Force which considered an internal combustion engine TURN embodiment. While not eternal fight, it again offers significantly extended flight endurance. The best research aircraft can fly a 250 pound payload, drawing 2000 watts of power for about five days. My research shows that an IC TURN system could remain aloft for over 30 days.

Finally, the largest scale system is striving for eternal flight while operating within the stratosphere. At 65k feet, the system is above most weather and commercial airliner traffic, and the air is thin enough to warrant a large wing fitted with solar panels. By getting the power requirements low enough, the energy collected during the day is enough to remain aloft throughout the night, thereby eliminating the need to land and refuel.

Also, in typical rotor craft the lift is highest on the outer edge of the rotor and least in the center where the airfoil speed is slowest. Does that affect where you put the payload? Is it on the edges of the wing or still in the center? At the center, if the wing is supported by the centripetal force of the wingtip motor's angular momentum, there is a huge torque in the middle if you pull it down. (much like pulling down on a suspended cable). How much deflection before you have the same problem as the stiffened wings of current efforts?

Imagine a wing that's 20+ times longer than it is deep, and is only 5% as thick is it is deep (so, for example, a 20m wide wing, that's 1m deep front-to-back, and 5cm thick top-to-bottom) - that's really hard to get stiff in bending along it's long axis, and in twisting stiffness around that long axis. This is why - as the article mentions, a sailplanes long thin wings account for 40+% of the airframe weight, when a stubby-winged but less aerodynamically efficient wing (like, say a Cessna 172) the wing might only account for 10-20%$ of the total airframe weight.

This TURN design minimises bending/twisting forces by replacing the load bearing and alignment forces with mostly end-to-end tension ones - which are much easier to resist (just load the structure up with "axial" carbon fibre...).

For the solar, truly eternal case (which isn't strictly possible because sooner or later you're going to lose a bearing or blow a capacitor or something), I think the limiting factor would be less on the aerodynamic side and more on the power-to-weight ratio you're able to achieve with solar.

A lot of that is going to depend on things like power system efficiency, latitude, and altitude which makes it hard to give a definitive yea or nay on whether this is feasible. I'm not really an expert in power systems but from an aerodynamic perspective it seems at least within the realm of possibility.

Like anything, the details of the implementation are going to make it or break it but there's not enough concrete info in the video.

Other concepts are using tube-and-wing or flying wing embodiments. But long slender wings need more material to stiffen the slender structure. Just look at NASA/AeroVironment Helios, which ripped itself apart in a modest wind. Using centrifugal stiffening as a design element within the TURN concept eliminates the aerodynamic/structural tradeoff, and permits much better airfoils than standard practice will allow... nearly three times more efficient to be precise. By reducing that much structural material the TURN system carries much more battery mass. Most HALE aircraft cap out at about 20-25% battery mass, nearly 80% of the TURN vehicle weight is allocated for energy storage.

Now outdated: Note for moderators (who I assume scan new comments): Some of this users comments have shown up dead, probably because he's a new user who made a bunch of comments quickly. It would be nice if you could come and revive them.

But, assuming that there's some point as you scale down at which perpetual flight becomes impossible, my first guess would be the surface area for solar cells at being a limiting factor.

You're also right about monitizing the business. The smallest scale system is a necessary prototyping step towards eternal flight, but it also offers a compelling competitive advantage over conventional fixed-wing drones. VTOL plus 5x the flight endurance?... who wouldn't want that.