I first read this in a Heinlein book, I think. You are shattering my cherished illusion that RAH was always scientificly rigorous. (Although the truth is fascinating. Who knew glass was such interesting stuff?)

But the subtle second-order phase transition means that's wrong too.

One of the coolest things I've heard about recently is the creation of metallic glasses. Stronger for their weight than crystalline metal. Of course, I immediately started wondering about their musical properties...if they differ at all from crystalline metal.

Another is the use of Bernoulli's principle to explain airplane lift.

I HATE it when I believe nonsense, especially when I've been telling it to other people!

They also mention that there is one important ceramic, with a relatively low melting point, for which flow under load is really important - ice, in the form of glaciers.

The manner in which small quantities of window glass (or for that matter, sheet glass of any kind) was made in the pre-industrial/early-industrial period was by getting a blob of molten glass onto your punty (the blowing rod), and blowing out a bubble which was then flattened into a plate. This plate was then heated and spun, using the blowpipe as an axle, forcing the plate out wider & wider. When you were finished, you snipped the plate (up to 1.25m across, or larger) from the punty, annealed it to remove the stresses, and then scored it into sheets. (The punty mark at the middle of the plate is what made 'bullseye' glass.)

Early industrial-era glass was made by floating molten glass on a bed of molten tin. You then ran a heated steel roller over the molten glass, forcing it down into a more-or-less even layer. This is called 'float' or 'water' glass, and the process results in the wonderfully ripply, vaguely watery appearance of pretty much all glass produced until the early- to mid-20th century, when techniques for manufacturing plate glass as we know it today came into widespread use (our house dates to 1914, and has some original float glass windows; one of them has a central pane which is almost 1.5m across and probably almost 1m tall).

Neither the hand-blown technique nor the float glass method (the float glass method even less so) result in (many) panes of glass that are shaped as you describe. Glass wants to achieve the same level when heated - it really, really, really wants to be no more than 5mm thick or so, and will spread out on its own to a uniform level with astonishingly little persuasion. You have to coax it to get thinner than 2-3mm, but not all that much...

Glass doesn't melt like other solids - so many other substances suddenly give way from one state to another. Glass gradually phases, more like sugar candy softening under gentle heat than, say, an ice cube. I could see how observing glass under heat could lead one to believe that it is more of a funky liquid than a 'true' solid.

Glass is way cool stuff. I dabble in soft glass kiln-forming and beadmaking - but I've never blown glass.

Thus ends our first SWAG of the day.

Re: Bernoulli's principal - [puts on pilot's headset] I'm pretty sure I remember being taught that that's what causes the wind flowing over the top of a mountain to be really really bad for the structural integrity of a small plane. And also what causes carburator icing.

Two things: one, all of the pieces of glass cut from the 1.25m-diameter spun glass, except the bullseye, would be thicker on the side closer to the axis (the blowpipe). That's why older glass has a thick side and a thin side.

Second, I think you've conflated 'hot-rolled' glass (made by flattening a high-viscosity sheet of glass between rollers) and float glass, made by floating low-viscosity glass on tin. The latter is the way that sheet glass (large panes with parallel sides and excellent optical clarity) is currently made.

Oh, and Xopher - I have a sample of amorphous metal in my lab - I'll have to go whack on it when I head back. My guess is that it should have markedly different musical properties than the traditionally-used metals since it has an unusual composition and is particularly hard. But I know that assessing the acoustic properties of materials for instruments is insanely complicated, and I couldn't presume to address that side of things.

As to the 2 processes, it is quite possible that I have conflated them to a degree, though my understanding (derived largely from reading the semi-technical materials from glass suppliers - although the materials are written for non-technical audiences, and there's no guarantee that they don't contain myths or radical oversimplifications as well...) was that the float method could also produce rippled and wavy glass.

The method I learned involved blowing the "light bulb", then attaching a punty to the other side and cutting the pipe loose. Spinning/heating makes first a bowl shape as the hole from the pipe expands, ultimately a flat disc with the bullseye from the punty in the middle. There is frequently more thickness at the outer edge of the disc, though it depends on how often you reheat the blob during the bubble phase, and how evenly you blow out the bubble. Elongated bowls with deliberately thicker edges can be made by holding the punty vertical and letting the shape sag towards the floor.

Unless, of course, glassmakers use more than one sense of the word "float."

Mind the gap ;-)

I'll take miscomprehension for $200, Alex.

Carol K: Thanks for the clarification - I was unaware of the distinction.

Sun-blued (or even violetted) glass is a result of electrons getting kicked out of iron impurities in the glass by high-energy photons, and then (because glass is a very viscous liquid) not being able to migrate back to the original atom site. Houseowners on Beacon Hill in Boston are proud of this effect because it attests to the antiquity of the property. Newer glass doesn't do this, though; additives such as manganese cancel out the iron.

However, if you're interested, the book, among other things, debunks the use of Bernoulli's principle to explain lift generated by wings and gives a scientifically sound explanation for the phenomenon.

I suspect that the glass-panes-sticking-together has as much to do with moisture and microscopic grunge sticking the pieces together as anything else.

Glass discoloring over time got mentioned many years ago in, IIRC, National Geographic: they were talking about 'desert glass', where it's been out in the sun for years, and has changed color.

I don't know if this leaves me any further along than when I started. Mind you, I had been happily accepting the flowing-glass factoid without ever thinking too hard about it, so I'm glad I came by.

Heh, great myth killer there.
Also, I suddenly have a desire for a "puddle of glass"...

I recall some short story from a year or two back about a guy who carved marble and his wife died and he blames the mountain for it (she fell? can't remember.) anyway, the mountain responds by having his marble slowly puddle. The story starts with a statue he just finished carving, and bits and pieces slowly "melt" and the guy is trying to figure out what the heck is going on, and by the end, there is a puddle of marble on his floor.

The cheap and almost-correct explanation based on angle-of-attack seems far more natural to me. Wings are basically ducts, canted downwards. When I put my hand out a car window at an angle to the ground striking the bottom of my hand and pushing my arm up. That's basically what wings do, except (this is the "almost-correct" part) that it turns out in practice that air bending over the top of the wing and down is the more important part of the flow. But the point is that wings drive the air down, and the plane stays up.

Here I am standing in this house, and I was sure I'd be okay if I threw the stone very slowly.

Here you go.

Also, thanks to Xopher, I now have a new toy on my wishlist: a transparasteel lute.

Glass Breakthrough at 3M

The article has a good explanation for why it's so easy to make glass from silica and so difficult to make from other materials.

On glass flow, I was told earlier this year, while I learned to break knives for microtome sectioning (so much fun!), that the knives will dull themselves over time. They're as sharp as they'll ever be right as you break them. I assumed it was a weirdness of glass, but didn't connect it to the glass-flow bit. I have heard that knives will do much the same thing, returning to a duller or sharper shape, but I haven't observed it myself.

(Then you could buy these wonders and they'd magically become worthless antiques, and very opaque in several areas, but of course that was not the point. Jeez, who'd want glass that doesn't magically change?)

Yep. See A.S Byatt's "The Djinn in the Nightingale's Eye."

Why does the asterisk point to a website about wooden shingles? I couldn't find a reference to supercooled liquid anywhere on that page.

Oh no! That means that when Kirk, Spock, and the crew comes back to get some humpback whales, Scotty won't be able to barter a secret formula for transparent aluminum for six inch stock of plexiglass. Maybe we should start a fund, have a bake sale.

Thank you.

I believe that microtome blades made of broken glass get dull because of chemical attack from the atmosphere - the sharp edge would be highly susceptible because it's almost atomically sharp (IIRC) and those atoms are hanging out there naked - they'd really rather be huddled inside with their neighbours or joined up with one of the O2 or other molecules floating around. In addition, the freshly-broken surface will have some cracks, which will grow, and will certainly accumulate cracks when you brush against them, set them down, etc. so that will take off the edge.

Actually, come to think of it, freshly-broken microtome blades, in use, may be one of the few cases where the glass flows to a practically-important degree under load. Glass under load can either break due to cracks or it can (in the absence of cracks, and under high enough load) deform, the same way that metals do. Obviously, in most situations, the breakage due to cracks wins handily over the deformation. But in this case, since the edge of the glass blade is so thin, it's possible that the stress (force divided by area) is high enough that it actually flows. I'd have to get some numbers to check that as a possibility, though.

This is an awesome book that talks about how cool freshly-drawn or broken glass is; it's one of the things that made me grow up to be a materials scientist. Giacomo, apologies for being one of the boring people.

Oh, and d'oh! I forgot to check out the amorphous metal when I was in the lab. Sorry, Xopher - I'll try to remember to check it out later.

Egads! I never thought of that! This discovery could change the course of history! Or the future. Or something like that.

Well, if you want to keep a bird in your car, that's up to you. But it's going to make driving a little difficult.

Well, okay. The "Bernouilli" and "Newtonian" views are complementary, like different frames of reference, and you can get to a correct answer either way. It's true that if you look at the direct force balance on the airplane, the reason it stays up is that the pressure on the upper surface of the wings is lower than the pressure on the bottom surface. And the reason it's lower is because the air is flowing faster there, and Bernouilli's law applies. But the next step in that logic -- why is the air flowing faster on the upper surface of the wing? -- is incredibly non-obvious. The Coanda effect comes in, and maybe some other stuff -- fluid dynamics has never been intuitive to me.

The "Newtonian" wings-are-ducts view is also only sort-of-correct, but it seems to me it is both easier to visualize and gets closer to the truth without going over, as it were.

This one drives me crazy. I'm glad that Colin Roald already started with an explanation, because I might have gone crazy if I was starting from scratch.

1. Your average jumbo jet DOES NOT produce enough thrust to keep itself aloft purely by forcing air downwards. Those are big engines, but the airplane is also very heavy. If you point a jumbo jet upright, so those big engines are working directly against gravity, the plane will not fly. (Some airplanes do have engines strong enough to do this, most obviously the Harrier Jump-Jet, which doesn't even rely on tricky vortices (and/or whatever) for lift the way helicopters do (I actually don't know anything about helicopter flight, I've just heard that "vortices" are involved, so take that for what it's worth (nothing))).
2. If your explanation begins and ends with forcing air downwards, then with the information presented in 1, you lose, you are wrong. Because there is no free lunch. Using wings to force air downward is (Newtonian) mechanically equivalent (on the body as a whole) to using jet engines to force air downward, and would require the same amounts of thrust to remain aloft.
3. Why don't those complaints stop an airplane from flying with "airfoil explanations"? Because they are using ForceA to create a pressure differential which exerts ForceB (in this case, due to the design of the wing, B > A). Which is (Newtonian) mechanically different from using ForceA to create (or cancel) an acceleration.
4. The pisser is that it's not simple. Some things in the is world are simple, airfoil flight isn't one of them. Fluid dynamics are complex (especially to somebody like me that never got down to any sort of unifying principles) and I haven't studied them [adequately]. Like Colin Roald said, you use (complicated) fluid dynamics to create a pressure differential which has creates a simple Newtonian equivalence of forces (weight & airfoil lift).

Like the "bees shouldn't be able to fly" story, the moral is that when a simple model predicts something shouldn't work, but reality demonstrates that it does, the model is leaving something out. In the case of the "Bernoulli explanation" of airfoil flight, the thing that's being left out is the creation and preservation of streamlines.

I was going to point out how it can fly despite having completely flat wings, but the heck with it. Paper airplane, wahoo!

Seconded! (And, in fact, I opened the link thinking, that's going to be him, isn't it...) I first encountered this one when I was 17, and it made sense of a lot of my A-level chemistry and physics, and then kept right on making sense of chemistry at university. (I still remember the look on my colloids lecturer's face when I finally "got it" - and blurted out "oh, like metallurgy, I see now!")

I ended up researching nanomaterials rather than the kind of things he writes about, but it's still the same basic set of principles, and I partly blame this book for my interest in history and archaeology as well - historical manufacturing techniques and the evolution of, well, Stuff.

Cassie wrote: I thought it was manganese or magnesium, one of those M elements, that turned glass purple. Yep - manganese gives purple/pink/brown, all those shades. You see it in solutions in the lab too, and I actually think of evening city skies as being "manganese-coloured" because of it.

Most glass naturally contains iron, so classically, glass in the State of Nature is green or greenish - manganese counteracts this, and more turns it mud-coloured. Cobalt turns it blue (surprisingly enough), gold turns it rosy pink or red, and if you use uranium, you get a bright vivid green. (This information brought to you by the Glass Gallery of the Victoria & Albert Museum, one of the most amazing parts of an utterly wonderful place.)

It only discredits Bernoulli if it's the only thing holding a plane in the sky. I always thought that if you just tilted the wings up, you could fly - but it would take a tremendous amount of power - its terribly inefficient. The Bernoulli effect gets you the efficiency to make it practical.

So, if flying a plane upside down is as efficient as one flying rightside up, then I'm all wet.

Mostly though, I'm just beaming with delight to have learned that viscosity is measured in units called poises.
Charming word. Call me silly, but I'm as gobsmacked as my poet-father was the first time he heard me use the word "ormoulu" casually in a sentence.

Poise. What poise.
How sticky is that glue, in poises, please?
Let me measure, en pointe.
-hee!

I'd hate to be a collector of 'canary glass' in this day and age. Can you imagine travelling through an airport with your latest finds - only to start some TSA radiation detector a-clicking?

"No, it's not a dirty bomb, dammit - it's my collection!"

It's not that the Bernoullli effect doesn't occur over a wing. It's that by itself it's insufficient to explain lift.

Bernoulli diagrams nearly all show an airplane wing that is curved on the top side and flat on the bottom. I have yet to see a wing on a real airplane (or on a bird) that is shaped that way.

Even if they were, if the Bernoulli effect were all that was holding a plane up in the air, how could stunt planes fly upside-down?

If you think of a helicopter blade as a moving wing (since that's what a propeller is, by its shape and its function of moving air), and stand under it while it's moving, you can feel with no uncertainty exactly what it is that's moving that helicopter into the air.

Bernoulli's one of my favorite effects for everything because it showed up in almost every biology class I ever took. Blood cells congregate in the center of a vessel? Tunicates, sponges, et cetera with their currents? All Bernoulli. That it helps keep planes in the air is incidental.

(And while we're at it, an enchanted plectrum that lets me play "Discipline" on it.)

Okay, I really have to get back to putting my mat sci test together, so I can go home while it is still tonight and not tomorrow morning.

Dan Layman-Kennedy: That is one hell of an oud!

I wonder what childhood memory I'm distorting?

Oh, and as I recall, float glass is floated on zinc, not tin, and that the critical difference -- aside from needing about a third the distance and it costing much less -- between float and plate glass is that with float glass, it's picked up on the rollers hot, and vertically; that way it sets fast and smoothly, rather than slowly and ripply and needing to be ground flat.

Kinda like why plagiaclases have big crystals.

vortices, rubber bands, paper clips, and a particle accelerator. Oh wait, that's how I became immortal. never mine.

4. The pisser is that it's not simple. Some things in the is world are simple, airfoil flight isn't one of them.

Well, some things really are simple, like actually flying an airplane. I like the fact that birds can fly and don't bother with the explanations. I also like that with all the complications to flying, one of the most important "instruments" is a bit yarn dangling outside in the middle of the windshield for your slip indicator. Sometimes its teh little things that count.

If you think of a helicopter blade as a moving wing (since that's what a propeller is, by its shape and its function of moving air), and stand under it while it's moving, you can feel with no uncertainty exactly what it is that's moving that helicopter into the air.
That is an utterly inappropriate analogy. Amusingly, Helicopters don't produce enough Newtonian reaction upforce (by pushing air down) to keep themselves aloft.
Check this out, and pay attention to the part about autorotation... with air moving UPWARDS relative to the blades of the helicopter, they still provide lift. You will become a source of great amusement to me if you suggest that the upflowing air supports the helicopter in the manner of a parachute or kite. (just making sure that you are adequately warned)

I'm perfectly content to let glass be its quirky self. It's not a fluid, but it's not a normal solid either. It doesn't "flow" per se, in most situations, but I've seen fractured glass heal itself completely before (while I didn't put it under a microscope, under a jeweler's loupe at least, it was entirely gone). I don't really care what we use to explain that, I just think it's neat. :)

(this conversation was too fun to stay away from)

Glass flow happens, of course, just not at room temperature. And since glass doesn't have a sharp melting point (like ice), there has to be an arbitrary 'fluidity threshold.' But it's determined by how easy it is to work the glass (blow, pour, whatever), not by our lifespans (although I guess there is an indirect relationship between our lifespans and how long we are willing to wait for our glass to spread out).

Here's a graph showing the viscosity of glass as a function of temperature (curve (d) is regular soda-lime silica glass). Note that the graph is the logarithm of the viscosity (1 = 10, 2 = 100, 3 = 1000, etc.) and that the viscosity goes up as you go up the y-axis - low values of viscosity = runnier.

The melting point is defined as 10 Pa.s ('1' on this y-axis, but the curves don't go that far down). The working point is defined as 10^3 Pa.s, a viscosity at which glass is easily deformed. The minimum temperature at which the glass can be handled but still hold its shape is the softening point, corresponding to a viscosity of about 4 x 10^6 Pa.s (or about 6.5 on the y-axis); the temperatures between the softening point and the working point are the working range. For soda-lime glass, the melting point is around 1400 deg C, and the working range is between about 700 and 1000 deg C.

Given the viscosity and the load (ie stress) caused by the weight of the glass, you can estimate the time for a given flow. Note, however, that the flow of glass at room temperature and under low loads is small enough to be not of practical importance, which is why temperature-viscosity curves don't normally go down to twenty degrees Celsius (and presumably why it took until 1998 to debunk this particular myth).

(I should credit my mat sci text for these criteria and numbers)

I was taking a lot of glass art classes at the time, and the idea of working in a medium that inhabited its own universe lent an added note of mystique to what was already fascinating stuff. Until an instructor debunked the pleasant fantasy by telling us the points mentioned here: old glass is uneven because of the manufacturing methods of the time, and if glass really was a supercooled liquid, then all those glass artifacts from the Ancient World should be puddles.

Now that I think about it, it's probably just as well glass isn't a mystical magical element with its own timeline. Working with glass is fraught with enough complications. Along with differences in heating and cooling temperatures, expansion and contraction rates, different vulnerabilities to heatshock, and how you can't mix glass made by different manufacturers because they all use different formulas - glass artists would probably also have to know and remember different "ooze" rates, if glass really was a supercooled liquid :)

Steel bends under weight. If you piled enough heavy stuff on top of a steel I-Beam, eventually you'd have a puddle of steel below a bunch of heavy stuff (Steel is malleable (though much less malleable than iron)). This is similar to what you're seeing when glass is bent under a weight.

The question presented in the post was, will glass deform under its own weight, and will congealed (physically indistinct) middle-section droplets deform (away from nearby upwards neighbors) under their own weight. I'm not a materials scientist, but I'll take their word for it when they say "no."

Greg London said:
I like the fact that birds can fly and don't bother with the explanations.
I spend (some) days wandering around in a stupor, too impressed with how much easier it is for animals to do things than to understand how they're done. The cognitive complexity of knowing when you're looking at a car, (much less a pencil, a computer monitor, a plate of spaghetti, or a towel crumpled up on the floor) still make me want to go back into academia and study Cognitive Science. Coordination for physical action is stranger and harder than most people imagine... by contrast, knowing how long you have until you're hit by a car is simpler than you would be taught to solve that problem in trigonometry class (but it's still harder to calculate on paper than it is to know when something will hit you).

Nabil, I wrote above about how atoms like to be bonded with other atoms. When you break a piece of glass, all the atoms on the surface are still happily bonded to the atoms below them, but they have naked bonds waving in the wind where the other surface used to be. Since glass is so brittle - that is, it barely deforms at all before it breaks - the two broken pieces can fit back together with atomic-level closeness. So if you put the pieces back together again immediately, the bonds can join up again and fuse the glass. However, if you wait any length of time, atmospheric gases will bind to the atoms (remember, those dangling bonds really don't want to be out there, and they'll bind to anything within reach) and the glass will no longer fuse.

I've never actually tried this, and now I want to go over to my lab and give it a shot...

Oh no! That means that when Kirk, Spock, and the crew comes back to get some humpback whales, Scotty won't be able to barter a secret formula for transparent aluminum for six inch stock of plexiglass. Maybe we should start a fund, have a bake sale.

No, the time line is completely consistent:

A) Kirk, Spock, Scotty et al travel back in time to the 1980s.

B) A few years later, transparent aluminum is "discovered".

The bogus lift explanation that I do my best to shoot down is the one NASA labels "Equal Transit." It uses Bernoulli's principle, and only that, to explain lift. I think it's perpetuated in classrooms and textbooks because there are so many simple, dramatic classroom demonstrations of Bernoulli's principle. As others have already said at length, lift isn't that simple.

Colin Roald linked to a great explanation of lift at FIU. The NASA flight pages also have a lot of great content. Take a wander and play with their Java applets.

The one, actually. (I'm startled that I am the first person to respond to this.) The way I heard it, he was told at one point by a Publishing Person that he needed a different name for his SF stuff than his non-SF; the middle initial was his solution. Amazon tells me that the M. is in SF books; I will probably reforget this information immediately.

Do you have a reference for this, or for how autorotation actually works? The idea that rotors would continue turning in the same direction after the flow of air reverses seems entirely counter to my experience with fans.

B) A few years later, transparent aluminum is "discovered".

Why would anyone who is sitting on a discovery worth billions of dollars stick it in a drawer for a couple of years? I have operated under the assumption that as long as transparent aluminum is not available, that means Kirk and crew haven't come back in time to get the whales yet. Once it becomes available and is "discovered", then I'll know that Kirk was just here (or in time-travel speak, Kirk was just now.)

Not sticking in a drawer - going through the patent process. And probably developing the large-scale manufacturing processes, too. At which point the Monterey aquarium replaces the current version of Scotty's windowpane with a sheet of transparent aluminum and we find out how good it really is.

autogyros are aircraft with propellors on the front and a helicopter rotor on top. The rotor is only powered while the aircraft is sitting on the ground, and that is simply to get the thing spinning. Once spinning, the clutch is released, and the rotor remains unpowered for the rest of teh flight. This can be seen to be intuitively true because if the rotor were powered, there would have to be an anti-torque tailrotor to keep an autogyro from spinning while in flight.

The main rotor gets energy to spin by the airflow that goes up through the rotor, causing it to windmill. This also creates lift, which keeps you from converting into a small grease spot on the ground.

The propeller on the front of an autogyro allows you to add energy that is lost by drag, inefficiencies, and what not. It also allows you to add extra energy so that when you climb, you can maintain the same airflow up through the rotor system and keep the amount of lift.

When a helicopter loses its engine, you pull back on the cyclic like reining in a horse and drop the collective. Pulling back forces the air to go up through the rotors and dropping collective reduces drag so you don't lose rotor RPM. In a second, you've gone from powered flight with air going down through the rotors to unpowered flight with the air going up through the rotors. How the hell this works from an aerodynamic point of view, I have no clue. I am like the bird who can fly but doesn't know the math.

In a helicopter, you then trade off altitude-energy to offset your rotor-drag energy that you're losing. This allows you to autorotate unpowered safely to the ground from high altitudes. As you autorotate, you must maintain forward velocity to keep the airflow going the right way. up and through the rotors, which keeps them spinning.

If you lose an engine while you are hovering, I seem to recall that the procedure is to go cyclic forward because you have to achieve forward velocity or you're dead, and drop collective as before. I'm a little rusty on that because it's just stupid to hover at altitude.

Then when it comes to actually landing, you are basically heading towards the ground, nose down, gliding in, and around 30 feet from the ground, you pull back on the cyclic which causes the helicopter to point up, the air is suddenly pushing on the blades like a windmill, which spins them a bit faster and stops your forward velocity. The faster spinning will actually cause you to gain a bit of altitude, and when you've bled off the last of your forward velocity, you cycle forward, and land straight down, cranking the cyclic just before you crash, er, touchdown.

Take a spoon, put the scoop end on the table, and hold the handle end in the air at a 30 or 40 degree angle. turn it so the scoop end is upside down. And that approximates your ideal flight path when you lose an engine in a helicopter.

Hm, that could be. OK, so as long as transparent alumninum is not available on teh market, I can assume that Kirk and company didn't come back to get the whales over two years ago or more. He may have come back in the last two years. Or, he may not yet have come back.

The important thing, though, is that the Star Trek universe remain true.

ack! that should say "cranking the collective" not cyclic. Good grief, I hope no one had an engine out after reading that. I'm a bit rusty.

"Dropping the collective" means changing the angle of attack of the rotor blades?

Meanwhile, I remain unconvinced that this from Scott can possibly be true: "Amusingly, Helicopters don't produce enough Newtonian reaction upforce (by pushing air down) to keep themselves aloft. Check this out, and pay attention to the part about autorotation... with air moving UPWARDS relative to the blades of the helicopter, they still provide lift." Greg's comment that "you have to achieve forward velocity or you're dead" makes me question whether the airflow is actually upwards relative to the moving rotor blades, but even if the air has relative upward velocity before encountering the blade, it may still be redirected to a downward velocity in the course of the encounter. Fundamentally, if you draw a surface of integration at some reasonable distance around the chopper, *something* must balance the tonne of excess mass contained within that surface. Either somehow the chopper maintains a substantial pressure gradient that can integrate out to counter a tonne of force over an arbitrary enclosing surface, which I think is impossible, or else (1 tonne)x(10 m/s/s) of air is somehow being accelerated downward through the surface. Fluids are weird, but you still don't get a pass on Newton's laws.

Yep. both cyclic and collective change angle of attack.

The cyclic changes it so there is more angle on one side than another, causing the helicopter to tilt away from the big angle (more lift) side.

The collective changes the angle on all blades simultaneously, so the helicopter goes up or down.

Rotor RPM is constant, so to go up/down you change the collective pitch of the blades. and to go forward/backward/sideways, you move the cyclic.

Hm, there's a diagram here.

When you're flying, you generally don't think in terms of integrals and gradients. You're thinking "airspeed indicator, altitude indicator, rotor RPM, engine rpm. Big sky. repeat." Meanwhile, your hands are constantly trying to balance a thousand pound spinning plate on a yardstick.

I don't know the math of autorotation, but the thinking involved in actually performing one is the function that graphs the line from "oh f**k!" at the origin and "shiiiiiiiiiiiiit" at the end point.

note the coordinate axis is labeled "plane of rotation" and "axis of rotation". The x axis is the rotor disc, the y axis is the shaft that turns it. The arrow marked "relative wind" goes from lower left to upper right. It goes up through the rotor disc.

also, the little diagram on the right shows the aircraft, the rotor disc tilted back, and the airflow going up through it.

(Note to self, remember to admire brother the next time I see him.)

later someone mentions zinc, not tin.

No one ever bothers to explain why the heck the tin is molten or why you wouldn't pour the glass on something that isn't molten but really smooth.

And I'll speculate that some game played with the cyclic is what lets you use a mostly-forward-incident-wind to keep the rotor spinning even when unpowered. Would that be the effect of "pulling back on the cyclic," which Greg says is the first thing you do when thrown into autorotation? I think it might be.

Trees pretty. Fire bad.

(someone on this board should recognize that reference and to what end it was made.)

Uhm, I think the deal is that when you're moving under power in a helicopter, you're nose down and the wind is going down through the rotor disc. When you lose power, you need to get the air going up through the disc and you pull back on the cyclic to rock the fuselage back.

Oh, and planes don't really fly. Big metal things in the air? That's just crazy. It's magic.

I'm speculating, but could it be because nothing that isn't molten is as smooth as molten metal?

As far as I know, it's normally molten tin, not zinc, that's used to make float glass (sources are the mat sci textbooks I've mentioned above, as well as Wikipedia).

Early sheet glass was, in fact, made by pouring glass on very flat sheets of metal (like iron). And the reason why you use molten metal is because a layer of liquid on top of another liquid allows you make continuous sheets of unbelievably smooth, uniform glass (as evidenced by the size and optical clarity of the glass in the average curtain wall).

SNAP!

ow.

Nope, my brain refuses to accept it. I reject your reality and substitute my own.

Jim K: The beach sand that is glass pounded down by the clangorous sea is from Book of the New Sun, not too surprisingly.

Re Bernoulli, flight, and science books: long ago, when Philip and Phylis Morrison were still reviewing books for Scientific American, they did a broad survey of science books for kids. They were less than amused by much of the material.

And as for the time lag between the arrival of the Enterprise crew in SF* and the public announcement of transparent alumin(i)um, a more stefnal explanation was covered by John Campbell decades ago in a well-known editorial, "No Copying Allowed;" it ain't easy to reverse-engineer a manufacturing process from a technology significantly more advanced than your own, even if you have a sample to hand, and in some cases, even if you have a description of the manufacturing process.

"How did the strangers with the weird personal habits make such metal? With weapons of that mighty steel we could conquer Hounslow, Putney . . . perhaps the Tootings of Bec shall fall beneath our chariots."
"Once we get the wheel thing down."
"Eh?"
"Nothing. Their jester said something about 'basic oxygen furnaces,' o Wonder of Nature."
"Then build furnaces, lout of a knavish churl! And gather as much of this 'oxygen' as you can. Perhaps it grows in peat bogs."
"You are not called Dymwyt the Wise for nothing, Your Liegeyness."

*the city, dammit.

Yeah, but Scotty had the chemical formula right there on the screen. I could see the molecular drawing floating on the monitor. If it wasn't too advanced for the software to simulate it, then it can't be too advanced to build the real deal. If it was out of our technology range, then our software would have told Scotty, "invalid operation, stack dump follows: oop". Instead, it drew it, and even labeled it "Transparent Aluminium" right there on the monitor. Therefore, I'd say there would be, at most, a 2 year lag for patent delays between Kirk-saves-whales and the appearance of transparent aluminium on the free market. It's the only valid explanation.

(Found the link while trying to identify the original culture of a modern replica cookware set made of brass/bronze; no luck yet-- a wide lipped platter, a kettle with a very narrow bottom, and two long-handled pans, one nearly flat and other other round and shallow. Do these ring any obvious bells for anyone?)

IIRC, there was an experiment some years ago which conclusively showed that the different sound properties of instruments made of different materials were only due to differences in shape coming from the manufacturing process. To prove this, the experimenter made concrete flutes which were very precisely machined so as to sound like a wooden flute.
(Googled it, the researcher was John Coltman)

They said, "You have a brick guitar,
You do not play things as they are."

The man replied, "Things as they are
Play slide upon the brick guitar."

And they said then, "But play, you must,
A tune beyond us, as we am,

A tune upon the brick guitar
Of things as-is, with DRM."

"I cannot roll the world, though round,
Although I oil it as I can.

"I sing a hero's funneled head,
A mighty axe, no heart contained,

"Although I oil him with this can
He still gets rusty in the rain.

"If strumming bricks should trouble you as stark,
Let me tell of the swans, that they live in the park;

"Say it is the serenade
Of one whose instrument got laid."

-- with all the usual apologies.

There's a way-cool exhibit at the Hiller Aviation museum on the SF (the city) peninsula about a proposal, during the early days of the Apollo program, to recover Saturn V booster stages by parachute and reuse them. To avoid dunking the stages in the ocean, they would be plucked out of the air by a *giant* helicopter, with 100-yard-long blades (IIRC) spun by pairs of large jet engines at the tips; the blades spun at something like 10 RPM. They have a video made from the "demo" film, which, in those pre-CGI days, used wooden models hung on wires...

Oh, and metallic glasses? Also known as "microcrystalline" metals. Made by rapid solidification of metals; one way is to shoot a jet of metal against the rim of a rapidly-spinning disk. One of many jokes about the National AeroSpace Plane project (aka the Full Employment for Computational Fluid Dynamicists program) is that, while lots of aerospace designs require Unobtanium to work, the NASP needed Rapidly Solidified Microcrystalline Unobtanium.

1) At Wiscon 14, Iain Banks explained the 'M' as being a marketing decision by his publisher. At a later con, his publisher denied that and said the M (for Menzies) was inserted for the SF novels to placate his family (Banks is not his family name). I still tend to think it was a Stupid Publisher trick, but then I work in the industry. ;-)

2) My Google-foo is failing me, but I distinctly remember an incident in Milwaukee where a heavy lifting helicoptor was transporting an air-conditioning unit for a new building and lost the rear rotor. The pilot (Viet Nam vet) was able to not only use the remaining controls to guide the cargo to a nearby sandpile and drop it without significant damage, but then guide the helicoptor itself to a softish landing (copter destroyed but pilot and all bystanders unharmed).

John

Oh my god. I must see. You don't have a link, do you?

OK, will people please stop posting stuff that hurts my brain? I'm still a little dizzy trying to imagine a window pane brownie in the oven with a molten tin crust and a filler made of two thousand degree glass and it all coming out perfectly smooth.

So, I swear I just saw a program on TV about making violins and these guys were shaving the wood to certain thicknesses because the top wood panel in a violin is built to vibrate and amplify the sound. Now I'm trying to picture a concrete violin and it's hurting my brain.

It wouldn't work; violins are strings, not woodwinds.

With woodwinds, it really is the vibrating column of air, rather than the vibrating instrument, which is why plastic clarinets and bagpipes -- Delrin is wonderful stuff -- sound every bit as good as the wooden ones. (Also cost less and break less....) This is also why the concrete flute would work fine.

A concrete violin, or any other stringed instrument where the body of the instrument has to vibrate to produce the tone, wouldn't work well at all.

All the blind tests done with bagpipes in the last decade or so would indicate that people really can't tell.

I don't know if this is true of clarinets, but parallel experience would suggest that it really matters if the listener knows what the instrument is made of.

There might not be any high-end plastic clarinets being made, either, which -- in terms of manufacturing precision and care -- really matters.

er, OK, then what instruments have the body vibrate to produce the tone? Is it just stringed instruments? Everything else makes sound by a vibrating column of air? I could have a concrete trumpet? I suddenly have the odd sensation of knowing just how little I know about how musical instruments work.

Drums & other percussion instruments? (Guessing here...)

The instruments that involve a column of air would be woodwinds and horns...and do calliopes count even though it would be steam not air?

Are pipe organs considered 'wind' instruments? I've forgotten what the mechanics are that make the sound for these...

Well, first: you mean the percussion subclass of ideophones, since membranophones (drums) use the body as a reverb cavity.

Also...I'm dubious that guitars and lutes are in that category. I don't think a guitar made of rosewood sounds the same as a guitar made of pine (or plastic). Different materials can have different resonant properties even as a chamber. I want to see his data. Also, what's true for a high-pitched instrument like a flute may be less true, or not true at all, for a deep-toned one like a contrabassoon, or even a guitar.

Julie L.: bizarre as it may seem, you can borrow a platinum flute from someone who already has one. There's a reason Varèse titled his piece Density 21.5.

A carbon-fiber stringed instrument, on the other hand, works just fine, and has the benefit of looking like Darth Vader's violin. (Apparently they don't sound as good as the very best wooden instruments, but they sound much better than wooden instruments in the same price range).

Hm, so I recall a compony that built electric guitars out of metal (aluminum, I think). A guitar buddy of mine thought that the electric pickup in a wood guitar might move with the wood, but that it wouldn't move in metal, and thought it might sound different. Alternates were that the vibration of the string would go down the metal body a lot more and could drastically alter the way the strings vibrated, would vibrate the pickup, and ultimately, teh way the guitar sounded.

We could never test the theory, because the metal guitars were too expensive for college students.

I liked the idea of a metal guitar just because the thing would be well near indestructable, and wouldn't shift adn warp as you went from 10 degree below and dry air in you car in the winter, to 70 and humid in somebody's apartment.

I guess I'm having a hard time seeing how to separate out the 2. If the body of the drum contains the column of air, how does that differ from the body vibrating? And wouldn't the material from which the body was constructed potentially change that vibration?

(Not questioning you - just thinking out loud (thinking in pixels?).)

Ever look at a vibraphone? Both principles are active there. The ideophonic part is the actual "key" of the vibe, the part struck. Beneath it is a sounding chamber, partially closed by a rotating disk. As the disks spin on an axis, this changes the length of the column of air, causing the characteristic vibrato sound of the vibe.

The chambers themselves only contain the column of air. The "keys" are vibrating themselves, and transferring part of that vibration to the column of air.

Oh my god. I must see. You don't have a link, do you?

Sadly, it's not online to my knowledge. You'd have to go to the
Hiller Aviation Museum to see it -- the museum is definitely worth a stop if you're ever in the area. They have lots of other exotic helicopters and aircraft, some one-of-a-kind.

There's a mention of the Saturn recovery plan online here
with slightly different numbers than I recalled, but no pictures, alas.

Wood is liquid because it has phloem.

The Saturn V booster recovery proposal struck me as a desperate "how can we use helicopters in the space race" notion.

Definitely worth a trip if you're going up 101.

See also Guitars with Metal Resonators: Dobro.

Well that's dissappointing, I wonder if its too late to cancel the order...

I think it's good odds that in order to have enough mass to be playable, the carbon fibre instruments have to be much, much stronger than the minimum structural strength requirements to string one.

Just think, in a decade or so, they'll probably be able to do carbon nanotube musical instruments.

I recall seeing a proposal for "behind enemy lines" recovery for downed airmen and special ops. The grunt would inflate a weather balloon with a couple hundred (thousand?) feet of line attached to it. This would clear any trees and what not. Then, a C-130 would come along, low and slow, with a big, v-shaped cable catcher on it's nose. The initial tug on the guy on the ground would be straight up (in the proposal, at least), so it would pull the guy straight up and out of the trees. The initial tug would also be small and get progressively larger as the aircraft went further along. Then once the guy is fully off the ground and moving at the same speed as the aircraft, they'd winch in the cable and bring the guy in through the back loading ramp.

Apparently, this approach would solve the "aircraft exposed to enemy fire while hovering to pick up the grunt" problem.

I don't know if it was scrapped because the initial force was actually bigger than expected and they'd end up winching in a torso with no arms or legs, or if somebody realized that being behind enemy lines with a big ass balloon hovering over you telling everyone "helpless downed airman here" was a bad idea.

But it was good fiction.

The guitar I rememeber seeing was made entirely out of aluminum, including the neck. THat is generally the bit that warps and shifts with changes in temp and humidity. The guitar had no body to speak of, it was just a long, skinny, bar-shaped thingy, made entirely out of aluminum.

I did some googling and can't seem to find anything that looks it. I do recall pondering whether being able to control the warp of the neck through the truss rod (a bolt going through the neck) is something that could not be replicated on an all aluminum neck. Maybe it turns out aluminum doesn't work well for allowing minor adjustment. I know some guitarists are very sensitive to the way their rig is set.

I have a les paul electric, a yamaha acoustic, and an electric travel guitar (I can't remember the brand), so I don't really need another one. I might have replaced my travel guitar with an all alumninum electric, but oh well.

Col. Allison Brooks, then Commander of the ARRS, and A3C Ronald Doll participated in the first human testing of the Fulton surface-to-air two-man recovery kit at Edwards AFB, California in May 1966. Recovery kits were designed for one and two-man recoveries, but eventually proved impractical for most rescue purposes. By 1996 the 8th SOS was the only unit in the world that maintained crew proficiency in the use of the Fulton recovery system, and had been prepared to launch if called upon since the late 1960's. A fatal accident in 1982, the only fatality in 17 years of live pick-ups, damaged the credibility of the personnel pick-up system within the special operations community. That, along with the increased availability of long-range air-refuelable MH-53J Pave Low and MH-47E Chinook helicopters, and tightening budgets, caused AFSOC to deactivate the capability in September 1996.

Evidently, the Fulton was an evolution of an equipment-recovery system the US developed during WW2... Come to think of it, I recently saw something on the History Channel or the Military Channel showing C-47s swooping down at insanely low altitudes with hooks to snatch the tow lines of gliders and yank them airborne again. Evidently this was used for medevac (!!!).

This could go on and on.

Part of the colour differences in window glass USED to be because they found that a small amount of lead added to the mix gave a clearer glass (still used in ornamental "lead crystal" cut glass. The lead gave the purple tint, rather than the green tint of flint glass.