jselfridge

Sorry about that. It's all I had to work with. The TURN concept recently won a local business pitch competition, with a video posted on their website. Slightly (just slightly) better production quality.

https://757pitch.org

1. The payload likes to stay around 5-10% of the total vehicle weight (otherwise the tether sags to steeply). I've generated sizing models from 5 pound payloads on up to 250 pound payloads for the Air Force SBIR research.

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).

In steady wind, it needs to spin faster to maintain its wind robustness, which does consume more power... but that's common to all aircraft. Generally speaking, the it is robust to wind gust velocity about 25-30% of the wing speed.

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!

DARPA already looked at that through ISIS. The problem is the massive volume of gas within the airship. Even the lightest winds require a substantial amount of power to travel or station-keep. Google is using weather balloons, but they don't have any active controls, so they drift aimlessly away from the equator, and then need to be manually picked up after about a month.

You're right about the surface area. Altitude is the limiting factor. In the stratosphere, the air is thin enough to warrant large wings, which have lots of area for solar cells. I attempted to design a system for the Air Force at 10-15k feet, but the wings are just to small to collect any meaningful solar energy. That's why we shifted to an internal combustion engine design for that mission.

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.

You're right... there's no beating entropy. But the limiting case seems to be the battery charge/discharge cycles, which floats around the 500 range. So that's close to a year-and-a-half flight.

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.

The research is using a spiral development process, where flight data from small scale prototypes are used to validate existing simulation and dynamic models, and then those models are used as design tools for the next largest embodiment. So right now, there are three different scale systems which serve different demographics.

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.

I'm the originator of the TURN concept, and owner of the company. I can answer any questions you guys have about the vehicle.