How strictly does it need to maintain angular velocity as it goes around?
What would the maximum speed in still air be? Some fraction of the rotor tip speed?
Does the tilt-wing configuration (presumably for landing and perhaps takeoff) set the constraint on payload mass fraction, as that will require more structure than the tension-stabilized rotary mode? Or is the payload mass fraction relatively insignificant compared to the battery and solar cell mass?
Safety: is there a way for it to gracefully fail? Transition from 4 blades to 3, and let the failed one dangle down as you descend? Would this be even remotely possible?
It's more important that each wing maintain equal spacing between one another. But the angular rate of the entire system can be adjusted to accommodate the current operating condition.
I've evaluated models that can travel at 68 knots. Not terribly fast, but the intended application is to stay in one spot for as long as possible.
The tilt-wing configuration is actually dated. The L/D ratios of the wing would require the motors to be excessively oversized for takeoff and landing only. The design has been revised to have a single motor on the outboard section, and the system begins its rotation while on the ground prior to takeoff.
With three rotors it is still possible to stabilize the central hub, assuming the three can support the additional weight of the lost rotor arm. And with my controls background, I can't wait until I have more time to investigate this failure mode mitigation option!
1. Any preliminary numbers for total weight and/or disk loading?
1a. How much weight budget is left for payload while still being past the break-even point, considering you'll need enough battery to weather nights and clouds?
2. What kind of numbers are you assuming for flux on the wings? In my experience these numbers are always supremely disappointing compared to theoretical maximum once you take into account angle of irradiance due to time of day, season and latitude, plus the unpredictable effects of cloud cover.
3. Is there any particular benefit to having each of the wing sections decoupled from the structure and attached by tether vs. just extending the airfoil and meeting at a hinge or universal joint?
4. Controllability during takeoff looks like it's going to be a shit show :) Any proof-of-concept results there? Does the h-stab span the entire wing section, or only just behind the props? If it's the former, does that mean the solar cells have to flex?
5. I don't know if the illustration right above the Flight Operation heading is representative, but if it is, I expect a pretty big drag hit from those tethers. (Brings me back to q3 - why not just make that more airfoil?)
6. How fast are we expecting this thing to spin?
2. I have a look up table that plots energy capture for various latitudes vs day-of-year. It accounts for azimuth angle of the sun and duration of sunlight, where the integral of the area under the curve accounts for reduced collection at sunrise and sunset. To put it in perspective, operating on the worst winter day at 55 deg latitude is 15x harder than flying at the equator.
3. That produces triangular span loading (common to helicopters) which is not nearly as optimal as an elliptical span load distribution (common to gliders). Inboard sections just add weight and drag, without generating that much lift. The tether also has drag, but it's only 6% of the total system drag.
4. Agree. Takeoff has been completely revised and the transformational component has been abandoned. Now there is only a single motor on the outboard tip, and the system spins prior to takeoff. So the control laws for the retracted state are nearly identical to the extended state, just with a different set of gain values.
5. To build upon Q3, the tethers do have a very high Cd value (circular cross section is about 1.2), but they are extremely thin (small frontal area), and because the system rotates, the average velocity is 40% of the wingtip (which makes a huge deal for the V^2 in the drag equation).
6. At the largest scale, it takes nearly 40 seconds to make a full revolution. This slow rotation really helps to reduce the overall power requirements (P=VD).
2. Definitely squares with what I remember of the problem. Seems like accommodating the most extreme operating conditions would compromise the design so strongly that it's better to just constrain the envelope it's intended to work inside. If you can reach continuous operation at "reasonable" latitudes during a large portion of the year, that could still be a strong value proposition.
3+5: Gotcha, so I just wasn't appreciating how small the tether cross-section really is.
4. Ah, makes sense - that sounds much cleaner. 4 flying wings trying to do a VTOL while attached to a weight seemed a little crazy.
1+ 6. Wow! So this is one big boi. Pretty cool. I would have pictured a much faster rate of revolution.
Sure, we all have the eye on the industrial horizon here.
But what would a "the streets have their own uses for things" application of the TURN concept look like?
For instance, I build RC planes for fun, and am a huge fan of Kline-Fogleman airfoils, for their simplicity if not economy .. and I can imagine a low-cost TURN implementation based on KF airfoils which might be more viable at the LoRa/WAN levels of scale. Something easily deployable, cheap/disposable/replaceable, on a much smaller scale of industry - yet widely available.
(It mentions a unit with a five pound payload flying for around four hours, comparing it to a conventional platform with a similar on-board battery which was able to remain aloft for around an hour.)
Added: OK, it is all generated by the centripetal force caused by the mass of the wing units. So fast spinning and lots of cable/parasitic drag to get the wings reasonably level. Seems like a lot of wing area for a small payload. How is this better than just a wing large enough to support the payload with cables to distribute the load across the wing length?
Added2: This scheme still has the problem that the downwash from each wing will affect the other wings, just like in the helicopter case.
What if instead of teathers, you just had two large circles of composit, as a hub and a rim, that were stiff enough to keep all wing positions but stiffer than a flimsy string - and thus keeping formation?
Paragliders can have asymmetric collapses of the airfoil due to turbulence, but usually it's no big deal (other than being a little scary). Catastrophic loss of tension is avoided because there's still airflow over the rest of the glider, which remains pressurized. All the pilot needs to do is maintain safe directional control until the full wing re-pressurizes. You feel a sudden loss of pressure and then an abrupt surge of pressure, then things normalize.
My guess is that something if similar happens here, it'll happen asymmetrically, and the props might keep airflow over the other 3 wings, which are still flying. They maintain stability while the wing that went floppy drops, regains its tension, and then resumes normal flight.
Last: I'd wager that they plan on deploying these in stable air, at very high altitude.
In addition, what are these eternal flight numbers based on in terms of latitude, and season? In the video, he states that they were looking at internal combustion @ 30 days. What about the solar approach? Would 18 hours a day of light be enough? 12? 6?
Can anyone here call out whether or not this looks like a viable thing?
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.
They compare the TURN concept to satellites in the beginning, mentioning only positives by comparison - of which there are indeed many (cheaper, better comms). One thing I didn't see mentioned was the failure mode between the two. A satellite that ceases to function will stay in orbit, until this orbit decays enough for it to burns in the upper atmosphere. If one of the rotors on this device fails, it will just come crashing down to earth.
And also, in typical neo-luddite fashion, I'm fearful for the surveillance applications of this device. Even if staying airborne isn't eventually completely free (eternal flight) with this system, the ability to stay up for 9 hours at a time on a few Li-Ion cells opens up a world of possibilities, not all of which are great. If NOAA can afford to put a number of these out in the Atlantic to get better atmospheric readings, surely cities, states, city-states, and others can afford to put a number of these above their populace.
However, progress is inevitable, and the technology can't always be blamed for it's potentially negative applications. I think this is an amazing project - but it will require some self-control to not let this push us further into a surveillance state.
"The first unmanned aircraft of its kind to fly in the stratosphere, Zephyr harnesses the sun's rays, running exclusively on solar power, above the weather and conventional air traffic. It is a HAPS: a High Altitude Pseudo Satellite, able to fly for months at a time, combining the persistence of a satellite with the flexibility of a UAV."
This isn't a new idea, at all. You have to go back to the late 1780's to find a time before the use of balloons as recon platforms:
There's a big difference between a person going up in a balloons for a few hour to scout a military locations and a swarm of drones constantly monitoring a city in peacetime.
> And also, in typical neo-luddite fashion, I'm fearful for the surveillance applications of this device. Even if staying airborne isn't eventually completely free (eternal flight) with this system, the ability to stay up for 9 hours at a time on a few Li-Ion cells opens up a world of possibilities, not all of which are great.
The Department of Customs and Border Protection already operates a fleet of tethered airships. These seem to be a much simpler solution to "eternal flight."
It might not be incorrect, but tethered means you are connected to something. In this case, that something is the aircraft itself. Self is not the first thing most people are going to think of when they hear the word tethered. Especially since, as you point out, there are already aircraft that are tethered to the ground.
These aren't commercially available though. Twice the performance of currently available automotive batteries is a major advantage for aircraft applications.
To make a comms network you'd need them relatively evenly spaced out to maintain coverage over a wide area. You'd also need them to have beefy and power hungry broadcast/receiver systems. How well can they scale with the solar energy?
Overall a very cool idea though!
At a certain point, "perpetual" also starts to run into problems of mechanical failure. Then again, if these things are cheap enough and/or able to limp home in the case of a snapped cable or failed prop motor, maybe it still works out.
Then again, I'm just a programmer. I recognize I have no grasp of the power requirements for that kind of project.
The inexpensive nature of them seems a major draw. I wonder if they can detach cables and have the wings fly home on their own while having the center parachute down?
But we are hardly able to keep conventional solar glider UAVs in the air, I suspect that adding the drag of a tether moves the goalposts far out of reach.
Basically, the whole thing takes off like a helicopter, before turning the wings on their sides.
Similar ideas have been suggested using balloons:
An array of wind-powered drones could scale well beyond what's possible with a traditional wind turbine, since most of the torque gets applied at the wingtips. There might be advantages with inconsistent wind gusts too, since the rotor could wind up in strong wind and unwind in weak wind. Startup costs would be negligible since a few drones could start the rotor and more could be added as needed. It might even by safer for birds if a drone array turns slower than a fixed rotor.
Something like this is sorely needed for bringing broadband to other parts of the world. Best of luck to Justin in this endeavor. I hope he succeeds wildly!
I've been playing with geodesic cellular kites, and "eternal" flight is certainly possible. If the ratio of surface area to mass is great enough it's actually hard to stay on the ground. Cellular kites scale. I would like to eventually make flying buildings, etc...
Check out "Tensairity" https://en.wikipedia.org/wiki/Tensairity
They basically combine the existing principles of control-surface based helicopter blade pitch control (e.g. Kaman K-Max) and blade tip propulsion, then drive them to the UAV-exclusive extreme where the individual blades gain complexity until they are a coordinated team of autonomous planes while the "body" of the helicopter along with all the complicated mechanical linkages to the blades is reduced to simple tethers between the blades and a central connection point where a stationary payload could be attached.
I suspect that they are overestimating the structural savings relative to something like the helios glider (where mass was already distributed along the wing) and underestimate tether drag.
The big difference is that a new type of aircraft won't fly at all if it's as bad an idea as solar roadways, whereas the latter could be built mile after mile until the world runs out of stupid money, no matter how bad it really is. If nothing else, the idea of "tethered uni-rotors" deserves a place in the canon of cool sci-to technologies that could theoretically work. They would fit in quite nicely in the future segment of Seveneves for example.