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?)
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).
Great answers, thanks for the insight! Responses to the responses:
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.
Comments
Cool concept!
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?
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).
Great answers, thanks for the insight! Responses to the responses:
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.