Other Jimness > Cinder Spires Books
Sails?
Shecky:
--- Quote from: wrangler on October 23, 2015, 11:47:36 PM ---I can see density and inertia coming into play when we talk about the time it may take to accelerate to the speed of the air mass upon departure from the surface, but eventually an aircraft will reach that speed. Every airplane that flies utilizes that speed in calculating its flight path.
If you wish to postulate a mass so large that the airship will not reach the speed of the air mass during the time of its flight, then I'd expect that any sails would have negligible effect in imparting additional speed to that airship.
--- End quote ---
"a mass so large" = any nongaseous object. And when a heavier-than-air aircraft that relies on lifting surfaces to remain aloft matches the speed of the air mass it's in, that's called stalling and ends *very* uncomfortably for all involved. All of which means it's a completely inapplicable example, as the airships in TAW do *not* rely on lifting surfaces to remain aloft; the lift crystal is what negates gravity (to a controllably variable extent), while the sails provide the motive force. And the coupling between the air current and the airship can never achieve 100%; it's physically impossible when the differences in density are measurable. Furthermore, to address your comment about inertia applying only until windspeed is matched (which, as I've shown, never happens, but for the sake of argument), you're assuming not only a perfectly smooth laminar flow but an absolutely static vector value (i.e., the air current never changes velocity or direction or meets another air current, etc.). This is more of a practical consideration than one that addresses the fundamental principle I'm trying to convey to you, but there it is, all the same.
Try this on for size: Remember those little "paratrooper" toys from many years ago (i.e., essentially just an action figure with a toy parachute attached to it)? Go outside when the air masses are moving (i.e., the wind is blowing), unfold the parachute and place the action figure in your hand without constraining it. It *will* be pulled out of your hand, but it will never equal the speed of the wind.
This is, of course, a horribly rough illustration, but it *is* illustrative. Sails *can* propel an entirely airborne object, and that object will not quite match the speed of the air current it's in.
wrangler:
--- Quote from: Shecky on October 24, 2015, 03:28:26 PM ---"a mass so large" = any nongaseous object. And when a heavier-than-air aircraft that relies on lifting surfaces to remain aloft matches the speed of the air mass it's in, that's called stalling and ends *very* uncomfortably for all involved. All of which means it's a completely inapplicable example, as the airships in TAW do *not* rely on lifting surfaces to remain aloft; the lift crystal is what negates gravity (to a controllably variable extent), while the sails provide the motive force. And the coupling between the air current and the airship can never achieve 100%; it's physically impossible when the differences in density are measurable. Furthermore, to address your comment about inertia applying only until windspeed is matched (which, as I've shown, never happens, but for the sake of argument), you're assuming not only a perfectly smooth laminar flow but an absolutely static vector value (i.e., the air current never changes velocity or direction or meets another air current, etc.). This is more of a practical consideration than one that addresses the fundamental principle I'm trying to convey to you, but there it is, all the same.
Try this on for size: Remember those little "paratrooper" toys from many years ago (i.e., essentially just an action figure with a toy parachute attached to it)? Go outside when the air masses are moving (i.e., the wind is blowing), unfold the parachute and place the action figure in your hand without constraining it. It *will* be pulled out of your hand, but it will never equal the speed of the wind.
This is, of course, a horribly rough illustration, but it *is* illustrative. Sails *can* propel an entirely airborne object, and that object will not quite match the speed of the air current it's in.
--- End quote ---
The point I made (perhaps not clearly enough) regarding airplanes was that they move with the air mass, in addition to their progress through it, and that this movement must be taken into account in determining the flight path over the ground. Of course this would be inapplicable regarding the comparison you mention, and which I didn't make.
I'm willing to concede that theoretically the mass may never reach 100% of the speed of the air mass, but approach it asymptotically. But practically speaking, I consider it close enough to make sails pointless for motive force for at least a large mass. As long as you have a force (the motion of the air mass) acting on a mass, it will continue to accelerate.
Regarding static wind speed, yes, that was assumed to simplify the example; the effect of changes on a very large mass was considered negligible for the purposes of that example.
Shecky:
--- Quote from: wrangler on October 24, 2015, 04:39:45 PM ---The point I made (perhaps not clearly enough) regarding airplanes was that they move with the air mass, in addition to their progress through it, and that this movement must be taken into account in determining the flight path over the ground. Of course this would be inapplicable regarding the comparison you mention, and which I didn't make.
I'm willing to concede that theoretically the mass may never reach 100% of the speed of the air mass, but approach it asymptotically. But practically speaking, I consider it close enough to make sails pointless for motive force for at least a large mass. As long as you have a force (the motion of the air mass) acting on a mass, it will continue to accelerate.
Regarding static wind speed, yes, that was assumed to simplify the example; the effect of changes on a very large mass was considered negligible for the purposes of that example.
--- End quote ---
It's not close enough. I'll grant that moving with the air current cuts down on the drag from the main body, sure. But there will always be far more coupling between sails and air than between hull and air, and when the ship as a whole has reached its maximum speed in the air current, that gap (although certainly reduced) will never entirely zero out...and in the meantime, the ship is still moving (ergo, sails as motive force work), and there will still be at least breezes on the deck. Gale-force winds, not so much, but current shifts will increase perceived windspeed on the deck.
Just because it works differently from ships on the sea in no way means it doesn't work. To address your original post:
--- Quote from: wrangler on October 22, 2015, 01:53:33 AM ---So how do sails propel these airships, since there's no "wind" within an air mass?
Perhaps it's revealed later? I just started this book.
--- End quote ---
...they do propel the ships. And there is perceived air movement.
knnn:
Wrangler still makes a very good point though. How do you "tack to wind" in an airship? In water, it's the large resistance of the water together with a keel that allows you to keep your ship with sails at an angle to the wind. If your ship is completely in the air, what keeps the ship from turning into the direction of the wind?
Griffyn612:
--- Quote from: knnn on October 25, 2015, 12:36:46 AM ---Wrangler still makes a very good point though. How do you "tack to wind" in an airship? In water, it's the large resistance of the water together with a keel that allows you to keep your ship with sails at an angle to the wind. If your ship is completely in the air, what keeps the ship from turning into the direction of the wind?
--- End quote ---
That would only be a problem if the sails were deployed when propelled against the wind by other power, wouldn't it? It's not like they'd just leave the sails up all the time.
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