*************************************************************** Okay, fixed it. Sorry about that. The two drawings above relate to the videos in previous posts. But whether the motor/generators are on top or on bottom, the previous discussion still holds true about the blades reversing direction. However..... ...further research has shown, and as evidenced in the videos, the blades tilt back way less than I thought when in generation mode. The jet stream wind is blowing across the blades at only a very small angle from being parallel with the blades. This is significant. In my original drawings, I was thinking the wind would be hitting the blades at a 45 degree angle. If that where the case, the blades would have to stop and reverse. But with the smaller angle, the blades go into "autorotation and turn the same way as when lifting. So, the blades don't actually have to stop and reverse. Google on "autorotation" for more info. Also, google on "gyrocopter" - it is actually exactly what we have when the FEG is in generator mode. So, the blades can keep spinning in their "lift" direction, that problem is solved. But it creates a new problem in that "autorotation" or "gyrocopter" mode, cannot physically take in much power from the wind. Land based turbines keep their blades facing into the wind, so the wind hits the blades perpendicular. Should be obvious that this is where maximum energy extraction occurs. But look at the second drawing above. It should also be obvious that not much energy will be extracted from the wind... even though the blades don't have to reverse.
As I see it, the airflow through the propeller at the smaller angle does not negate the need for reversing the direction of rotation (I wonder how this would pan out using a pinwheel toy held out from a window in a moving car). Reversing the blade pitch (what I implied as "pivot") is probably a requirement for avoiding the stop and reverse scenario. IAW like the pitch changes on a helicopter blade, except that I don't think helicopters reverse the pitch. The tilt would then not be restricted to a small angle, thus enhancing energy extraction. I would also worry about the stability dynamics during transition through zero pitch.
Here is a good link showing how the low angle wind on the blades doesn't reverse the blades direction: https://www.uavnavigation.com/support/kb/general/general-system-info/autorotation-helicopter-uavs I agree. But I think the issue with changing the pitch of the blades of a large FEG that weighs more than 1000+ tons and also has a drag of 1000+ tons is the stresses imposed on the mechanical pitch controllers. The controllers/mechanics would have to be very large and powerful, and would be very heavy. Don't know if that is a deal breaker.
That's more than I have time to digest, but in the case of helicopter autorotation, you can't get something for nothing. Sinking is probably the key. If you are confident in translating that to the FEG situation, that's your business. Since it's no where near a deal breaker on monster helicopters, some of which are anything but delicate, the prospects might not be dim at all.
The logistics alone in this entire concept make it unworkable. The infrastructure required is far too expensive and restrictive, the technical and maintenance issues daunting at a minimum, the competing sources of energy more efficient and far easier and the limitations are to the point of making it a pointless venture. Basically this is an interesting mental exercise and nothing more.
Having had the opportunity to explore this idea I suspect that you might be right in your evaluation. Perhaps newer materials and/or technology in the future could mitigate some of these existing hurdles.
Material science will not solve the logistics issues or the infrastructure requirements....unfortunately.
Hopefully, you can at least take the time to scan down a bit in that link and look at the vector diagrams of the blades. It is interesting. As for autorotation of a helicopter, you are correct. The helicopter must necessarily descend with forward motion to maintain the rotation of the blades. But the blades do rotate in the same direction as when they are powered. Apply the notion above to a tethered FEG with a strong wind blowing over it, as shown in the videos, and we get the gyrocopter effect. If you find time, check out the physics of a gyrocopter. Well, seems it would be the business of anyone who is interested in FEGs as shown in the Sky Windpower videos previously linked to. When the craft was up in the air and the truck started driving forward to simulate wind, the craft barely tipped back at all. I would have thought it would have tipped back about 45 degrees from horizontal. That it tipped only a few degrees from horizontal indicates that the craft was using the gyrocopter effect (which has much in common with autorotation). The largest helicopter in existence has a maximum take-off weight of 42 tons. That includes the weight of the craft and its payload. Compare that to: The premise of the original poster was that a single FEG could produce in the neighborhood of 4 gigawatts. A craft capable of that would necessarily weight over 3000 tons. This would be a craft having four 1 gigawatt generators. The following calculation is pretty rough, but it serves to illustrate the issue: In my research, I had run across some estimates on turbine drag. I remember that turning a 300 megawatt generator would produce 745 tons of drag (I can post a link but you would have to give me time to locate it.) Ratio-wise, a 1 gigawatt generator would have a drag of 2,483 tons. And the actual stress would be much higher because we also have to account for the lift in the vectors of the forces.... the lift for each rotor would be approximately 3,400 tons depending on lift angle. We are easily approaching 4000 tons on each rotor. This means that a single pitch controller on this FEG would have to withstand perhaps 80 times the stress of that experienced by the pitch controller of the world's largest helicopter. That isn't trivial. Maintenance concerns would be high as follows: https://www.globalsecurity.org/military/systems/aircraft/rotary.htm From the link: Helicopters have a large number of moving parts, designed to minimal weight and high stress (fatigue) conditions. This, combined with a low tolerance for failure, results in high maintenance. Ratios of Maintenance Man Hours (MMH) to flight hours is often 3.5 to 4.5. That is, four hours of maintenance is conducted for every flight hour. Parts are replaced well before expected failure. Many parts are machined to very close tolerances. Metal used to manufacture parts have certifications regarding the source, treatments, and inspections. 3.5 to 4.5 hours of maintenance for every hour of flight! This could probably be less for an FEG since once it is stable, it wouldn't be making a lot of abrupt adjustments like a helicopter does. Yes, of course a single FEG could be built much smaller than what the OP claimed could be done. But various "companies" have been working on these ideas for 10 plus years with minimal progress. Promises made have not been kept. And, let's say a 10 MW FEG is feasible.... It would probably be more prudent to simply build four 2.5 MW Land based turbines. Much easier and much safer.
100% in agreement. As you said, it was an interesting diversion and I learned a lot about the physics involved.
