Jed Rothwell wrote:Stephen A. Lawrence wrote:
Note also that autogyro analogies are likely to be misleading because the vertically oriented turbine in an autogyro . . .
You mean the propeller, right? The propellor is vertical; the unpowered rotor is horizontal.
Sorry -- I meant the "propeller" that points almost straight up, and which keeps the thing in the air. I didn't mean the propeller that points forward and keeps it moving.
IIW I meant the unpowered rotor, not the powered rotor. Whatever ... they're all turbines, after all.
. . . does no work on the plane -- a craft in level flight needs no energy to stay up. All it needs energy for is to overcome drag.
Well of course it needs energy to stay up! I don't get this statement.
Technically it does not -- no work is being done on the craft by the vertical forces because the craft is stationary on the vertical axis. Stop the plane and set it on a tower. How much _work_ is the tower doing to hold the plane up? None. Same thing for the air operating on a plane in level flight.
But of course, in practice you're correct. Some energy is actually needed, because the _force_ holding up the plane comes from accelerating some amount of air downward. But the _energy_ needed to stay up can still be made very small because no _work_ is being done on the craft by that energy. (Thermodynamics doesn't put a floor under it, AFAIK.) In the context of an electricity generating wind turbine the amount of work done just to keep it up can be ignored -- it can be made completely insignificant compared to the energy the device is converting to electricity. And when you do that, the behavior of all parts of the system is likely to be rather different from the parts of an autogyro, where the undriven rotor freewheels and there's no energy involved except that which is needed to keep it in the air.
Go back to your analogy of a soaring bird. The thing you want to think about here is glide angle -- the glide angle of any (unpowered) heavier-than-air object is never zero; such an object always falls. Indeed, that's your point, up above. But by using long wings and a streamlined design one can make the glide angle very shallow, in which case the energy which must be added to the system to keep it up becomes very small. Soaring birds gain that energy in one of two ways: Updrafts (vertical wind shear), for normal soaring, or horizontal wind shear, for dynamic soaring (the latter technique is used by albatrosses over the ocean, where the surface of the ocean is essentially homogeneous and there's no vertical wind shear to speak of). But to the casual observer it may appear that the bird is using no energy at all to remain up, because so little is required.
Note that a desire for efficient use of weight and the need to keep the tension on the cable down to something reasonable also dictate that the angle the cable makes with the ground must be as shallow as can be tolerated. At 45 degrees you've got as much vertical force on the cable as horizontal force, and the vertical force doesn't do anybody any good: the wind is only moving horizontally so you can't get work out of it by pulling _down_ on the cable. Note also that the angle in question is at the kite, not at the ground! The angle at the ground will necessarily be substantially shallower than the angle at the kite.
The energy needed to stay up comes from the forward motion spinning the unpowered horizontal rotor.
Note that the turbines in the skywindpower gadget are horizontal, like autogyro rotors. (They look horizontal to me.) The wind turns them the same way it turns the unpowered horizontal rotor in the autogyro.
I might guess that they do that to minimize drag. A turbine works at pretty much any angle, as I understand it, and the angle at which you place it depends on what you're trying to optimize.
- Jed
