FeatherTech II · 10:57pm Jul 18th, 2014
Hey all, I've got the next chapter loaded, but I wanted to fine-tune a few things. In the meantime, I thought I'd throw up a blog about icing just to keep you thinking about stuff. Frankly... I could go for some icing right about now, with the heat.
Icing
Q: Why is Featherprop so worked up about a little bit of ice?
Featherprop spends a lot of time worrying about ice on the aircraft's wings, and indeed, anyone who has traveled by air in the winter has heard about or seen de-icing procedures. What is not generally known is WHY the ice is so dangerous, and how different forms are dangerous in different ways. Ice on your car is annoying, but once you clean it off the windscreen you can get by with a lot of it clinging to the rest of the vehicle. Not so with airplanes- they're dependent on very specific shapes and surfaces to stay up in the air.
Icing on airframes generally can occur below 37F, theoretically stopping at about -25F, but it often still happens below that even though the official sea-level freezing point of water is 32F! Why? Well, even though the temperature gage may say 35 or 37, that does not mean ALL parts of the aircraft are above that temp-- only that the temperature probe is. The probe itself is subject to heating due to air friction, so that can cause some discrepancy, but also because it gives a near-instantaneous readout while the rest of the airframe has mass and takes time to heat up and cool down-- like pulling a mug out of the freezer, water vapor will condense and freeze on cold aircraft components.
At the other end of the scale, when it's really cold and water SHOULD be ice crystals, it's very common for water to stay in a liquid form at higher altitudes, called a supercooled state. This is because there's simply nothing for the first water molecules to form a crystal around. Think about making rock candy: Ice is a crystalline structure, just like the sugar crystals that grow into rock candy. Rock candy doesn't grow until there's something for the first crystals to coalesce on- a stick or string hanging in the sugar solution. Ice crystals in water are the same- when water is calm and there aren't any contaminants to form around, it often won't crystallize. Now, when you fly a plane through a cloud of microscopic droplets, that's a pretty big disturbance. The water droplets oblige by instantly freezing onto the airframe.
So why is icing bad? Three primary reasons: One, it makes the surface of the wing rougher. The wing depends on a smooth flow of air to push air downward. Even a thin coating of frost will prevent this smooth flow of air, reducing lift. And the slower the plane flies, the more pronounced this effect becomes. Since slower flight occurs when the plane is either landing or taking off, this can be especially critical. (It's the reason so much attention is paid to de-icing aircraft before takeoff.)
Two, that roughness creates friction and drag, slowing the aircraft down. Sometimes the ice will take on odd shapes that are very disruptive to the flow of air, even in cruise flight. To overcome this, the pilot needs to use more power, and at some point there won't be any more power available.
Finally, ice is heavy, making it necessary to generate more lift to stay up in the air. A thick coating of clear ice can add several tons of weight to the aircraft. To carry the extra weight, the aircraft has to push more air down with the wings... but with ice changing their shape and reducing their efficiency, you can very soon run out of extra lift to devote to keeping you aloft.
Not all ice is the same, either. There are three main types: Rime, clear, and mixed. Rime ice is less dangerous, often only collecting on the very leading edge, where it has little effect during cruise flight. Clear ice happens when liquid freezes onto the cold airframe, and quickly adds weight as well as changing the shape of the airfoil. Mixed ice is a combination, and often leads to the odd shapes seen here.
To a degree, all three kinds of icing can be dealt with, but no aircraft is ice-proof. Even if an aircraft is equipped with anti-icing equipment, all it does is delay the inevitable. If the pilot does not find a way to get out of icing conditions, eventually the aircraft will have too much weight and not enough lift. Sometimes this means a gradual loss of altitude, but not always.
The effect of icing is not always predictable, and when a pilot is tired, low on fuel and desperate to get on the ground, they can get distracted and try to perform a maneuver that would be fine in normal conditions, but is too much for an ice-laden airframe to achieve. If this happens at low altitude (say, during a blind let-down, when there are many different things to pay attention to), there is often not enough time to react and recover.
So what can be done to fight this? There are a few basic, and some complicated, pieces of equipment to combat ice, but the ones most aircraft are equipped with are pneumatic boots, heated prop blades, and heated windshields and sensors.
The idea behind boots is simple: Hammer the ice from below and break it off. It's a nice theory, but it doesn't work out so well. Using de-ice boots is sort of an art form. When the ice is soft, they work great. When it's hard, very thin or very thick, or the boots are old and pitted, all they do is make the ice even rougher. Sometimes the ice is thick enough that the boots don't do anything at all- they just inflate a little and then suck back down (suction is used to keep them from flapping in the slipstream and tearing).
A general rule of thumb is that you'll get three good blows out of the boots, but after that they don't do much-- they'll have a lot of narrow strips of ice clinging to the portions of the boot that don't stretch as much. If you blow them too early, the thin ice will fracture but large chunks will stick, providing a catalyst for further icing. Wait too long, and the ice will be too thick and strong to break. Generally, a thickness of 1/2 to 1 inch is considered the best time to blow the boot.
The propellers deserve some consideration as well, since they're even more susceptible to icing than the wings. In general, thinner, sharper components will see more icing, and the prop blades are much thinner and sharper than the wing leading edges. Props can collect fearsome accumulations of ice... which can and are flung against the fuselage with impacts that can be clearly felt. Most prop aircraft have an "armored" section of fuselage or nose next to the prop blades I once got to see a blade shed it's entire load of ice all at once- it came off the prop and glanced off the top of my windscreen. Actually... it wasn't much fun at all. It kinda scared me.
On most aircraft, a heated electrical boot is used to keep ice from forming near the root of the blade. Older airplanes, though, used a system that sprayed fluid onto the prop blades- a little nozzle poured alcohol into a "slinger ring" inside the prop hub, which would direct it to the root of the prop blade, and it would run out along a grooved rubber pad to keep ice from forming near the hub. Further out, centripetal forces are usually enough to keep large pieces of ice from forming, but some still will, especially on slower-turning propellers. An old trick to help with this buildup is to cycle the props up and down, from max RPM to minimum cruise RPM, and try to vary the flow of air over them enough to break off some of the ice. Avoiding ice buildup can be important- an imbalanced prop can be bad for the engine, and the stress from feeling a strong vibration through the airframe isn't good for pilots either.
Windshields can be heated as well- an array of tiny wires between two glass layers provides heat, something like the defroster on a back window of a car. However, they tend to be a LITTLE more expensive-- some costing upwards of $20,000 per pane. They can break inflight, too, due to thermal stresses building up over time. It's a thumping, crunching noise that will scare the effing crap out of you when you look over and see feathery cracks spreading out.
The second time it happens, you just sort of shrug.
Now, about those fluids. Vital, precious fluids. De-icing fluids are used to remove accumulated ice, while anti-icing fluids prevent accumulation, at least for a while. Some places, to save money, will use a mix of isopropyl alcohol and hot water to remove ice, but it's worthless in terms of keeping it off- all it's good for is cheaply removing stuff that's stuck on. The two most commonly used types, though, are Type I and IV-- these are both based on propylene glycol.
That's right, antifreeze. Makes sense, huh? There's some important differences from the Prestone you pour in your car- they're engineered to biodegrade much faster, they lack some additives and feature others. One of the most important differences, though, is their aerodynamic performance. Type I, usually orange, is primarily a de-icing fluid, with limited anti-icing properties. It's much runnier, and is often mixed 50/50 with water and heated before being spayed on aircraft to melt snow and ice accumulations. They use a LOT of it, but luckily it's fairly cheap- sometimes as little as $8 a gallon! Yeah, that qualifies as cheap in aviation-- other types can cost over $50 a gallon. Keep in mind that a small regional jet can take hundreds of gallons if there's a load of snow to be cut through. I've seen an 1800gal truck depleted by de-icing four feeders and a large jet. That's... a lot of money.
Anyway, Type I is great for getting rid of stuff, but not so good at preventing it from accumulating, especially if precipitation is still coming down- it gets diluted easily and runs right off the aircraft. That's when you turn to the expensive stuff, Type IV. This green goop is much sticker and thicker, so it stays on longer. There's a limit to how sticky you want it, though. Think of ripples on a pond, caused by a breeze. Those ripples represent friction and disruption of airflow- when that happens over a wing, it's bad. So de-icing fluids are engineered to stick to the wing until a certain point, called the "shear speed," and then break away and fly off. Many small aircraft can't use Type II fluid because it has a much higher shear speed, and so would cause airflow disruption during a critical portion of flight-- exactly what you're trying to avoid!
Other ways of de-icing are simpler or more complicated. You can toss the plane in a heated hangar for a while, or a new technology that's coming out uses brief microwave pulses to melt it off rapidly, but doesn't penetrate the aluminum skin. The aircraft taxis in the barn, the overhead panels zap it, and then, if needed, it gets a light coat of anti-icing fluid. There's a HUGE cost savings, because the equipment uses fairly low power RF, and you're not pouring hundreds of gallons of fluid on the ground.
"Yes yes," I can hear you say, "That's all well and good, but what does it have to do with Featherprop?" Well, it has a lot. From what we see in the Frostmane, there aren't any fancy ways of getting rid of ice, and not much in the way of protective equipment. Here's what he's got: The Twin Trotter has pneumatic boots on the wings and tail, a heated panel on Featheprop's windscreen, and rudimentary heating pads on the prop blades. There isn't any real de-icing capability anywhere- it all depends on keeping the aircraft from icing in the first place or melting it off afterwards.
Ooh, icing! Pinkie Pie loves icing!
While I can't attest to icing experiences in flight (my one experience flying a small plane was under my brother's watchful eye, on a calm day, with nothing to worry about--for obvious reasons; I'm no pilot), I do remember a particularly nasty winter storm in which snow would freeze on the hood of my truck, then, when it got heavy enough, slide forward, where it got caught in the airstream off the edge of the hood, and then it got flung back into the windshield. Very distracting.
To my knowledge, the only danger of icing you missed was giant chunks of ice being ingested by the engines. There has been at least one passenger jet (with tail-mounted engines) that was lost that way. Too lazy to look it up right now, but I think it was SAS and it went down in a forest in Sweden.
Another excellent informative blog, and I look forward to the next one!
2297957 You're right, I glossed over that. It actually does affect Featherprop, because he's flying a turboprop. I've got some pics of ice vanes like he's have, I'll add them to the post later. Three is one photo up there that gives me shivers-the one where the engine inlet has tons of ice on it.
Mostly I wanted to get this up so folks could read and ask questions. I've got a few stories if the right ones get asked!
2298008
Well, I'll say that while your level of pony tech is far outside what I prefer (still, they're all OCs, so no reason why it couldn't be set years in the future of the show), I'm a sucker for aviation stories, man (or pony) vs. nature stories, and well-researched stories, and so far you're batting 1.000 in those categories.
Which is to say, if you write it, I'll probably read it.
This was a good read. I grew up watching jets get iced in Alaska, but this gave me quite a bit of info I didn't have, thanks!
2298021 For the most part, i agree with keeping tech mostly agrarian, but if we look at the show, there's electricity, rotary telephones (in Manehattan), balloons, metal (which implies mining, smelting/smithing, and industry), and most importantly, airships. The view I took is that magic supersedes a lot of those processes, because it's cheaper and easier (and unicorns would have a vested interest in keeping competition from taking root).
Take magic out of the picture, like for gryphons, and there's motivation to innovate. That's the head-niche I worked in- the tech is there because it's needed to survive in a harsh environment, but when you take it to say canterlot, it'd be looked at as ludicrous and laughable; why struggle with a badly transmitter when a dragon can send a message easily? It's still limited. I didn't want it to be an easy solution, but to bring its own set of challenges and obstacles. Every innovation needs to bring new problems or it's boring.
Don't worry, I will never, ever write in a computer, car, television, or even a portable music player. Records and speakers are canon, but not digital media!
2298518
Conflicting headcanon time, yay!
Disappointingly short reply:
t.
While I agree that there are many examples of schitzo-tech, some of them can be explained away, and some of them are older than you'd think (for example, in the US, the first motion picture predates the first power plant). Also, some of their 'technology' can be handwaved away by saying "it's magic, I ain't got to explain s
<prepares to be dogpiled by angry responses>
While my interpretation of the show's tech level isn't the same as yours . . . as a reader, I don't care.
I'm willing to go hand-in-hand with you into Featherprop and Espresso's world, and you don't have to explain how the hay FP can hold a control yoke with hooves, or how pony earphones might work. A good story doesn't need to worry about those pesky little details, and a good author knows that the audience might very well come up with a better mental image of it than he ever could (Celestia knows I've used that to my advantage in my own stories).
.
Honestly, I'm not worried. Project Sunflower II (can't remember the actual title) has tablet computers . . . but they work with the story. Me--I try to keep my tech at roughly Civil-War era, and brush some of the show's more memorable canon cannons under the rug [and it's worth noting that most of my readers are willing to play along]. We're both writing for an unpredictable fandom, and we've gotta take the bad (inconsistent canon) with the good (ponies!).
Finally, since--in my opinion--it ultimately comes down to character--had you written the same story, but with interstellar rockets instead of puddle jumpers--I'd still be reading it.
I feel that I may have lost the point of this reply . . . if that's the case, you can blame Comrade Popov.
One thing to consider too is that clear ice builds up fast and does it in a way that can sneak up on you. We flew back to the airport once from Kansas City where my roommate was piloting, and all I was doing was looking out the window and shining the light on the wings for the whole trip, because we couldn't climb above the ice (climb up, get covered in ice before you get out, descend rather violently) and we couldn't go down much farther (they discourage you from driving your Cessna 172 down I-70 for some reason). Any ice and we descended a hundred feet or so. It melted off and we'd sneak up a hundred feet or so. Since a flying aircraft is actually colder than the surrounding air where the lift is generated, 32.1 degree air loaded with water hits a 30 degree airplane and you can *halve* the lifting ability of the aircraft in minutes while the weight skyrockets. Hence, yours truly with a flashlight.
You know, there are times I wonder how I survived college.
2298606 See, the problem is that you weren't going fast enough to get frictional heating of the skin. :D
That's my theory, at least. I will say that, when you DO have boots, they're much more effective when you're moving at the top of the yellow arc. When I used to fly in Oregon, I didn't worry so much about ice at cruise speed, but I wanted it off before the approach. Staying high and fast until close in served several masters, and 200kts of slipstream helps strip it off quite well!
Joking aside, that sounds like a nasty trip. There's a couple pics up there from my personal experience, but the worst I saw was the flight with the shaky Chieftain wing. The boots weren't doing a thing, and the ice kept building after that. I have a photo with big ice nodules on the windscreen, too. Eventually I was at approach speed on high cruise power, but it was smooth and I knew I had a warm layer to duck into, so I stayed high to take advantage of a good tailwind.
2298552
But... but that's part of the fun- I have those explanations! They're in my head! it's all so beautiful and clear in here.
... Actually, I think the desire to explain all the details is partly an ego-stroking thing, sort of wanting to hear how clever you are. And while some creativity is good and praiseworthy, it's still got to be secondary to the story. Lots of people create highly involved, intricate, unique and intensely personalized worlds, but if they're not populated by characters or stories that bring us into them, they're sterile and dead. Which is why
a comment like that means so much- hearing that from someone whose writing you enjoy and skills you respect is really one of the best things about this whole endeavor. Thank you!
2298871 "...it's all so beautiful and clear in here."
Particularly since you bonked your bean and let all the light in