0803: "Airfoil"

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Mazuku
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Re: 0803: "Airfoil"

Postby Mazuku » Fri Oct 08, 2010 8:18 pm UTC

I got this answer from asking Google how long can a plane fly upside down.

Upside Down 747?
For a plane to fly upside down, the rules remain the same but the direction and pressure of air over the aircraft changes.

First, a pilot must change the angle of attack for the aircraft. That is, pilots must change how the wings will "attack" the air and its pressure. By changing the entire aircraft's angle on a fixed wing plane to where the nose is higher than the tail, the air is no longer hitting a completely smooth surface.

Additional pressure is added onto the bottom surface of the wings, putting massive amounts of pressure on the entire plane. With a little movement and adjustments from the pilot as the plane "flips" from the changed air pressure, the plane can maintain the upside down position--but not for long.

For most aircraft, they can't take the pressure of flying upside down. It causes additional stress to the plane's structure, especially engines and hydraulic systems.

That's why most stunt planes are constructed of heavy-duty material that can withstand the pressure. Some also have specially designed fuel tanks that shift the weight of the full back to the bottom of the aircraft to simulate the weight of a right-side up plane.

Imagine a 747 flying upside down. The hanging engines would still test gravity and put pressure on the wings to meet the ground. So if you ever board a commercial jetliner and get a chance to speak with the pilots, think twice about asking to fly upside down.
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Re: 0803: "Airfoil"

Postby airshowfan » Fri Oct 08, 2010 8:42 pm UTC

Mazuku wrote:I got this answer from asking Google how long can a plane fly upside down.


This answer is incomplete and uses technical language in imprecise ways. It strikes me as someone who doesn't know what he's talking about, but trying to hide this by writing a lot.

Mazuku wrote:how the wings will "attack" the air and its pressure


That's less meaningful (not more meaningful) than just "changing the angle of attack".

Mazuku wrote:the air is no longer hitting a completely smooth surface


What does that even mean? Why is it important?

Mazuku wrote:putting massive amounts of pressure on the entire plane


Not really any more than during normal flight. I suppose that setting the fuselage at some angle of attack will cause it to experience higher aerodynamic forces, but the airplane can handle gusts and turns (and takeoffs and landings) so it's not that serious.

Mazuku wrote:as the plane "flips" from the changed air pressure


Is he talking about the rolling maneuver, or about how the airplane would maintain inverted flight once rolled?

Mazuku wrote:the plane's structure, especially engines and hydraulic systems.


Engines and hydraulic systems aren't structure.

Mazuku wrote:That's why most stunt planes are constructed of heavy-duty material that can withstand the pressure.


The strength of the materials in aerobatic airplanes is comparable to the strength of the materials in an airliner. Sure, structure that takes compression loads during inverted flight must be made thicker (or have taller or more numerous stiffeners) so it won't buckle, but that's not the same thing at all as using some magical "heavy-duty material".

Mazuku wrote:fuel tanks that shift the weight of the fuel back to the bottom of the aircraft to simulate the weight of a right-side up plane


The fuel always goes to the bottom of the tank! That's gravity. The difference is whether the fuel can flow from the inverted-bottom (i.e. the part that's normally the top of the tank) into the engine, and in most airplanes it can't. So... the "answer" almost addresses this, but not quite. It really is very badly written.

Again, the main question (assuming that the structure and the fuel system can take inverted flight, which they should even if for just several seconds) is: Can the elevators produce enough force to push the tail down and sustain the required angle of attack, to keep the stabilizers from swinging the tail up and the nose down? The reason why a 747 can't sustain negative-g inverted flight for even a second is because the answer to that question is "No". And this answer from "Google" (well, from eHow.com) totally misses that.

That eHow.com page includes many gems that you did not include in your quoted block of text, such as

The Longer Path theory says that planes basically fly and can get off the ground when their velocity is combined with drag and lift. The wings of planes have to be large enough to push air around the aircraft. When the aircraft gains enough ground speed, the pilot directs the wings' ailerons--those moving parts on the trailing edge--to point toward the ground.


There are so many things wrong with that, I don't even know where to begin tearing it apart. My initial reaction is to want to crawl under my desk and cry a little :wink: about the state of a world that treats such nonsense as some kind of "answer".

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Re: 0803: "Airfoil"

Postby mootinator » Fri Oct 08, 2010 9:02 pm UTC

Grant10k wrote:
jpers36 wrote:
Karilyn wrote:FYI: You count to 100 on two hands using one thumb as a 5...

Forget that, I can count to 1024 on my two hands.

It's all fun and games until you get to 132.



Whoo 23! (or 29 depending on how you look at it)

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Re: 0803: "Airfoil"

Postby AeroEngineer » Fri Oct 08, 2010 9:13 pm UTC

Sorry to say, there is nothing wrong with this explanation. Except that is is not the whole story. You might think of it as the "thickness" effect. The reason what it is saying is true comes from conservation of mass. The air speeds up for the same reason the water speeds up through a small(er) opening in the flow. (Ever put your thumb over the end of a hose?) The "Wright brothers" question is the same as a sailboat sail. It produces lift by a second means, downwash (from camber, the classic foil in the comic, Clark Y, also has a net camber.) As the air flows over (and under) the wing (or sail) it is deflected down and creates lift that way. Finally a plane with a symmetrical wing can fly upside down because of the kite effect or simply angle of attack. If you ever stuck your hand out the window of a moving car, you know what that is. A sheet of plywood can fly if it is moving fast enough.
So, there are three parts to producing lift - Thickness, Camber and Kite effect. The science of airfoil design is to optimize all of these to get the most lift and least drag for a given flow (Reynolds number).

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Re: 0803: "Airfoil"

Postby Hegs94 » Fri Oct 08, 2010 9:14 pm UTC

I have a question about this statement "This is a fun explanations to prepare kids for because it's common but totally wrong." I presume you are saying the original explanation given by the teacher is incorrect, (err) correct? So I have a question for you. I am taking an engineering course right now (nothing super advanced, just a Highschool course) and the current unit is designing your own rocket propelled aircraft. Literally, I was given that very explanation about how wings work. So if it isn't true, then technically could I have made a wing design with perfectly even lengths and still see it fly perfectly fine, or would it have to be at higher speeds for that?

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Re: 0803: "Airfoil"

Postby bmonk » Fri Oct 08, 2010 9:25 pm UTC

Two thoughts, before I reach the end of the comments:

1. There are other sites with bad science (and other fun things) out there, including http://www.lhup.edu/~dsimanek/home.htm.

2. The argument about lift being irrelevant if there is enough thrust is true--but don't try it with jet, turboprop/fan, or propeller engines, which use, among other means to obtain thrust, spinning airfoils to create thrust and to transfer that thrust to the air so that the vehicle may move in the opposite direction. That's what turbines and compressors do. Not to mention the blades in fans, including that cooling your computer. I think--at first thought--the only aircraft engines that don't use fans are ramjets and rockets.

Taot wrote:
littlelj wrote:
Marlayna wrote:
arbivark wrote:... "give your answer in centimeters" so the smarter kids answered 36, and we were supposed to mark it wrong.


Of course you were supposed to mark it wrong. It was wrong.

I'd have to agree - if the rubric had stated "give your answer in whole centimetres" then 36 was right, but otherwise not, since m/cm/km etc are continuous, not discrete.


I agree that 'give your answer in centimeters' tells nothing of how accurate an answer is required. Just what unit is to be used. I'd like to offer, however, opinion that the right answer to accuracy lies within what was calculated. If it was 7,5 x 4,8 = 36,0 is right, 7,28 x 5 = 36 is right, 6,62 x 5,5 = 36,4 is right etc. given that it was a school setting and the general rule with digits ( if that is the appropriate word ) is to round to the least accurate number used if not specified otherwise.


I often laugh at errors of significant digits. One text that I had included sentences like: "Climate changes significantly over hundreds of miles (160.93 km)" or "Average temperature is in the 80s [ºF] (32.22-26.67 C)." Such foolishness!

Grant10k wrote:
jpers36 wrote:
Karilyn wrote:FYI: You count to 100 on two hands using one thumb as a 5...

Forget that, I can count to 1024 on my two hands.

It's all fun and games until you get to 132.


LOL! Actually, using a balanced ternary (+0-) I can (in theory) count from -29 524 to +29 524. But it gets confusing.

Karilyn wrote:I wonder if anybody else was taught how to taught how to count to 100 on their fingers, and use the same technique to be able to do a long series of simple addition/subtraction/multiplication/division as fast as a person could read it off to you.

FYI: You count to 100 on two hands using one thumb as a 5, and the other hand having fingers be 10 with the thumb being 50.

9 on the 100 scale looks the same as 5 on the traditional finger counting scale.

I developed that system myself from abacus calculating--but some years later (1977, acc. to wikipedia) the Korean Chisenbop was introduced to the US: http://en.wikipedia.org/wiki/Chisenbop
Last edited by bmonk on Fri Oct 08, 2010 10:39 pm UTC, edited 1 time in total.
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Re: 0803: "Airfoil"

Postby Grant10k » Fri Oct 08, 2010 10:00 pm UTC

Monika wrote:
Grant10k wrote:I remember once...

You realized this in middle school? Most adults answer this riddle wrong:
You take a lump of gold, get into a boat on a closed lake and row a bit. Then you throw the gold into the water. Does the water level of the lake sink or rise?


It was a while ago, but I do distinctly remember telling the teacher we should be pressing down on the objects that float. I think she just forgot that step and didn't want to tell the class that they were taught wrong that day.

The boat thing makes sense. You could extrapolate to an extreme to make it easier to understand if someone refuses to get it. A ball of dark matter is sitting in a super lightweight tube.
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Re: 0803: "Airfoil"

Postby Snow02 » Fri Oct 08, 2010 10:31 pm UTC


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Re: 0803: "Airfoil"

Postby Time Kitten » Fri Oct 08, 2010 11:01 pm UTC

charonme wrote:
ghjm wrote:"Anything involving the engine" - "So how do gliders fly?"
"Anything involving Bernoulli" - "So how do paper airplanes fly, since their wings are completely flat?"

depends on your definition of "fly". If gliders and paper airplanes fly, then also rocks fly. As a crude simplification - gliders and paper planes don't fly, they just fall slowly :twisted:


Um.. my definition of "fly" places it as mutually exclusive to "fall" for these purposes. The paper airplanes I make have this tendency to go in a direction I have named "Up" and things that fall go in the direction "down", the two of which happen to be opposites.

Example: I throw the paper airplane horizontally, it proceeds to lift up to a height of 10-15 feet, then as it loses momentum from the throw, starts to descend, gaining velocity from the glide, lifting a number of times before it reaches the ground.

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Re: 0803: "Airfoil"

Postby airshowfan » Fri Oct 08, 2010 11:11 pm UTC

If you throw a rock fast enough, it will generate enough lift to overcome its weight, and will go "up". Anything can go "up" non-sustainably if you get it going fast enough.

The question, then, is: at a sustainable descent rate, what is the lift-to-drag ratio? By the way, it equals the glide ratio. For a rock, it's tiny, way less than one: it goes almost straight down. For a glider, it's 30 to 60: it loses one foot of altitude for every 60 feet it goes forwards. Most other things are somewhere in between.

Everything can generate SOME lift when it moves through a fluid. it's just that, all other things (size, speed, fluid density) being equal, a wing will generate a lot more lift (and a rock a lot less, and a flat plate somewhere in between).

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Re: 0803: "Airfoil"

Postby Monika » Fri Oct 08, 2010 11:15 pm UTC

Karilyn wrote:
Monika wrote:
Karilyn wrote:I wonder if anybody else was taught how to taught how to count to 100 on their fingers, and use the same technique to be able to do a long series of simple addition/subtraction/multiplication/division as fast as a person could read it off to you.

36 divided by 6 plus 5 times 2 - 2 divided by 5 minus 2 minus 2 plus 10 times 5 plus 6 divided by 8 plus 2 EQUALS:

And then everyone raises their hand, hopefully with the number 9.

FYI: You count to 100 on two hands using one thumb as a 5, and the other hand having fingers be 10 with the thumb being 50.

9 on the 100 scale looks the same as 5 on the traditional finger counting scale.

I don't get it. Can you make a drawing?

It's basically roman numbers.

Finger = I
Thumb = V
Other hand finger = X
Other hand thumb = L

I
II
III
IIII
V
VI
VII
VIII
VIIII
X

Oh.

How very confusing. I couldn't count/add/subtract/multiply that way.

Grant10k wrote:
Monika wrote:
Grant10k wrote:I remember once...

You realized this in middle school? Most adults answer this riddle wrong:
You take a lump of gold, get into a boat on a closed lake and row a bit. Then you throw the gold into the water. Does the water level of the lake sink or rise?


It was a while ago, but I do distinctly remember telling the teacher we should be pressing down on the objects that float. I think she just forgot that step and didn't want to tell the class that they were taught wrong that day.

The boat thing makes sense. You could extrapolate to an extreme to make it easier to understand if someone refuses to get it. A ball of dark matter is sitting in a super lightweight tube.
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How very neat :) .
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Re: 0803: "Airfoil"

Postby SpringLoaded12 » Fri Oct 08, 2010 11:20 pm UTC

The alt text saved me here; when I saw the first panel, I thought Randall had lost his marbles, or hired a ghost writer or something.

So I guess this comic is offensive to teachers, and we're going to get all riled up about that and talk about how we liked the first ~300 strips better, or something? That's what we do around here, right?

ARandomDude wrote:Can they really fly upside down for long durations of time (and I mean commercial planes, never mind everybody getting pissed because your plane is upside down).


According to experience with Flight Simulator 98, the plane cannot fly upside-down at all. In fact, even trying to flip it over will make the controls lock up and the plane instantly do a nosedive into the ground; as will trying to turn, switching gears, installing the game, etc.
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Re: 0803: "Airfoil"

Postby Karilyn » Fri Oct 08, 2010 11:26 pm UTC

bmonk wrote:
Karilyn wrote:I wonder if anybody else was taught how to taught how to count to 100 on their fingers, and use the same technique to be able to do a long series of simple addition/subtraction/multiplication/division as fast as a person could read it off to you.

FYI: You count to 100 on two hands using one thumb as a 5, and the other hand having fingers be 10 with the thumb being 50.

9 on the 100 scale looks the same as 5 on the traditional finger counting scale.

I developed that system myself from abacus calculating--but some years later (1977, acc. to wikipedia) the Korean Chisenbop was introduced to the US: http://en.wikipedia.org/wiki/Chisenbop

Oh hey, yeah that's the exact same thing I learned. I never heard of the name of it before. Though I don't tap on a table for it, I just hold fingers out.

Monika wrote:How very confusing. I couldn't count/add/subtract/multiply that way.

It's pretty useless for doing math, unless you are doing a large number of calculations under 100 in total in short order. I typically use it for remembering "WTF was that number I had 10 seconds ago that I can't remember anymore" instead of writing it down.

But yeah, I'll forever think in roman numerals apparently. Or in abacus as bmonk says. The wikipedia explains it better than me.
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Re: 0803: "Airfoil"

Postby airshowfan » Fri Oct 08, 2010 11:31 pm UTC

SpringLoaded12 wrote:even trying to flip it over will make the controls lock up and the plane instantly do a nosedive into the ground


The controls don't "lock up". You just don't have enough elevator authority to overcome the horizontal stabilizer's force that pushes the tail up and the nose down:

download/file.php?id=26051&t=1

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Re: 0803: "Airfoil"

Postby Vulcanis » Fri Oct 08, 2010 11:46 pm UTC

I'm surprised that no one else has mentioned the fact that it is not "Your parents are Santa" but "Santa is your parents".

This, in fact, means Santa is actually... our parents.

If we assume that we are all god's children, and Santa Claus is our parents, this then means that Santa is our one and only true god.
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Re: 0803: "Airfoil"

Postby WontonSoup » Fri Oct 08, 2010 11:55 pm UTC

I'd just like to add that I'm a second-year Mechanical Engineering student, and up until I read this comic I thought that explanation was correct... :(
Though to be fair, the last time I studied any fluid mechanics was in grade 11...

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Re: 0803: "Airfoil"

Postby thanksbastards » Sat Oct 09, 2010 12:49 am UTC

while I am a 4th year ME and thought the model was accurate enough to explain most of the lift, i didn't realise how wrong it was. someone posted a great video with pulsed markers showing flow lines and fronts, which showed all the air passing over the top of the wing beating the lower half of the air stream to the punch, so to speak. I always got in shouting matches in my own head as to WHY must the air speed up just because the distance is further, can't it just get there a bit late? this was in elementry school. Then by the end of highschool when I started doing real physics and fun math stuffs, I learned the idea of the wing passing through stationary air and it made perfect sense, so i just went with it. clearly I am getting dumber with age.

Now comes the kicker, I read this comic exactly 15 minutes prior to meeting up with my lab group to start our lab on cooefficents of lift and drag in a NASA0015 (screw you NACA[does NACA exist anymore?]) airfoil. My lab mates are a bit on the dense side unfortunately, and the one bright one is too frustrated with me argueing that we should do a handfull of corrections to our data (the angle of attack indicator was not properly zeroed, the lift guage read high, and the humidity was 50%, throwing our calculations for air density off by 17%). This is truely a Randall get out of my head and into my lab group moment.

does anyone know the Cooefficent of lift for a NASA or NACA0015 airfoile with a reynolds number of about 40-50,000? all the published data I can find starts at 80k or higher

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Re: 0803: "Airfoil"

Postby darkshade » Sat Oct 09, 2010 12:58 am UTC

airshowfan wrote:If you throw a rock fast enough, it will generate enough lift to overcome its weight, and will go "up". Anything can go "up" non-sustainably if you get it going fast enough.

The question, then, is: at a sustainable descent rate, what is the lift-to-drag ratio? By the way, it equals the glide ratio. For a rock, it's tiny, way less than one: it goes almost straight down. For a glider, it's 30 to 60: it loses one foot of altitude for every 60 feet it goes forwards. Most other things are somewhere in between.

Everything can generate SOME lift when it moves through a fluid. it's just that, all other things (size, speed, fluid density) being equal, a wing will generate a lot more lift (and a rock a lot less, and a flat plate somewhere in between).


I don't understand what you're going for with that rock example. It will go up, but decelerate downward. Lift is pretty clearly defined as a force that is normal to the freestream flow direction. Unless you find a pretty flat looking rock and throw it just right, it's not going to produce any appreciable lift, regardless of what direction it's traveling. Some people like to define lift as opposing gravity, but that's still not quite right. It's only true when an aircraft is flying steady and level (and no vertical gusts either, I guess). Plus it would define drag from a falling object as lift, which certainly isn't true.


This is a pet peeve of mine, but some of you ought to better understand just what a streamline is. If you're talking about the paths of the air particles, you mean pathlines. I guess they do coincide for steady conditions though.

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Re: 0803: "Airfoil"

Postby thanksbastards » Sat Oct 09, 2010 1:01 am UTC

oh and to fuel the why planes have trouble flying upside down thing, think of it this way. nearly all of the lift keepign a plane in the air is coming from the wings. This means that a flying plane is the same as you grabbing it by the wings and just holding it up. I find that pretty impressive from a structural stand point, think of how thin the area of the wing that attaches to the body is, relative to the weight of the body of the plane, there must be a lot of design going on under that skin. now flip the plane over, and you have the same weight, but being applied in the exact opposite direction it was designed to. Planes are light, they can't be designed to be strong from every direction without taking a massive weight penalty, and why would they?

then you have the whole keeping the engine fueld and lubed thing, which i am emarrassed to say I ahve never thought of before. Understanding mechanics makes the blue angles and such, which look cool, but after a while just fail to impress, seeem much more impressive all over again.

anyone have any commas left? I used all of mine up in that abomination of a sentance up there.

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Re: 0803: "Airfoil"

Postby monty30 » Sat Oct 09, 2010 1:13 am UTC

Oh, and what ever you do... do NOT google "Airplane wing" and look at the very first picture that shows up...

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Re: 0803: "Airfoil"

Postby airshowfan » Sat Oct 09, 2010 1:14 am UTC

thanksbastards wrote:Planes can't be designed to be strong from every direction without taking a massive weight penalty, and why would they?


Because the FAA says they should.

By that I mean: Because, for the sake of safety, the FAA has a huge list of conditions that every certified airplane must survive without damage, and one of them is negative-g flight. Be it a pilot-induced push-over avoid a collision, as a result of a sudden and strong down-draft, or caused by severe turbulence, airliners do experience negative gs once in a while, so the FAA says that this should not tear them apart, and the manufacturers must show (by analysis and/or by tests - there's a lot of negotiation) that any new airplane can take it before the FAA certifies it.

And it's not that hard. During normal flight, the top of a wing is in compression and the bottom is in tension, and the root is bending up. During negative-g flight, the top is in tension and the bottom is in compression, and the root is bending down (which is "up" from an absolute point of view). So as long as both sides can handle both kinds of stress, you're ok. In fact, I wonder whether any structural components on the wing are sized by the negative-g condition, i.e. are stressed more highly by negative gs than, say, by a high positive-g turn. I think that if a wing can take +3.75g, then it must be able to take -1g without too much difficulty! (Except for wings of high-wing single-engine airplanes that have struts. It seem like they'd buckle under negative gs unless they're made extra stiff to take that. Most of the time they're in tension and they don't have to be very strong at all for that, a wire could do it).

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Re: 0803: "Airfoil"

Postby rhhardin » Sat Oct 09, 2010 1:15 am UTC

Bernoulli reverses cause and effect in any case. The air goes faster because it ran down a pressure gradient from high to low pressure; the speed didn't cause the pressure change but rather responds to it.

Airplanes fly by throwing air downwards. What airfoil shape does that with the least forward drag at the design speed is a matter for the Navier Stokes equations to figure out.

Having thrown air downwards, there's missing air above the wing, and low pressure results. That gives you speeded up air there that rushes to fill in.

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Re: 0803: "Airfoil"

Postby thanksbastards » Sat Oct 09, 2010 1:46 am UTC

I like the simple wikipedia solution given ^^ while I'm sure it glazes over a few things it really does seem right enough to be taught to someone who only needs to understand it at that level while avoiding obvious lies like "the air meets up after the wing in the same place for cocktails"

now to continue on my point, a plane is designed to sustain flight with the wing loaded at ~1g with a lot of fluctuation, that is far from a static load so all kinds of fatigue stress must be accounted for . While I am sure a plane CAN fly upside down, I think of that saying "a good pilot can land anywhere ONCE." It is meant to survive such an emergency, but how many times it is meant to I am sure depends on the plane. I would not be surprised if a 747 that was inverted MUST be removed from service after such an event. I really doubt those engine pylons, if that is what they are called, are meant to hold the engine UP with all the loads swaying the back and forth for any extended period of time. How much aluminium is used in stuff like that anyways?

of course irealize I am arguing with a guy whose name is airshowfan, and neither of us have numbers to back up our armchair engineering, but I feel pretty confident that if a plane were strong enough to sustain inverted flight, it would be redesigned to save that weight.

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Re: 0803: "Airfoil"

Postby airshowfan » Sat Oct 09, 2010 2:10 am UTC

thanksbastards wrote:I realize I am arguing with a guy whose name is airshowfan, and neither of us have numbers to back up our armchair engineering


I have numbers. But if I shared them, I'd be fired :wink:

I also have the FAA requirements which airplane structure is required to meet, and those I can share, because they're public. FAR 14 (Airworthiness Standards) Part 25 (Commercial Transport airplanes) Subpart C (Structures) lists all the conditions that an airliner must survive without permanent damage. One of them (here's the text from Section 337, and the diagram) is a negative one g maneuver; basically the loads that would be encountered during sustained inverted flight. So yes, the pylons can hold the engines up, not just down. I know for a fact that some of the certification testing on Boeing airplanes include negative-g pushovers, where the pylons have to push the engines away from the wings. This is done at least on the static test airframe and I think it's done in flight as well (although I'm not sure they go all the way to minus one in flight).

if the plane goes OVER this minus-one-g limit that is required by the FAA, then yes, it has to be inspected thoroughly at least, and might be removed from service at worst, unless the manufacturer went out of their way to over-engineer an airplane and give it an even higher negative-g limit, and that would indeed be excessive and not worth the cost. But minus one g, the airplane does have to be able to take, because the FAA says so.
Last edited by airshowfan on Sat Oct 09, 2010 2:23 am UTC, edited 1 time in total.

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Re: 0803: "Airfoil"

Postby Chalnoth » Sat Oct 09, 2010 2:16 am UTC

So, an explanation of this has probably been posted, but whatever, here's mine:

First, let's examine the case where the air that flows over the top of the wing exactly meets the air that flows over the bottom. This is known as "laminar flow". What is the lift in this situation?

Well, consider: the speed of the air going over the top of the wing means that the pressure is decreased by an amount proportional to the extra distance moved. But this pressure is applied over the surface of the wing, so even though the pressure is lower, the area is larger, so the total force on the wing is exactly the same from the air going below the wing and above it!

So if the wing actually experienced laminar flow, the aircraft would fall like a rock.

What must happen instead is the air on the top of the wing must actually move faster than the air going across the bottom. One way this picture can be simplified is to decompose the airflow around the wing into two components: laminar flow (which makes no contribution to the lift), and circular flow (which provides lift). In fact, it is relatively easy to prove that the lift the wing experiences is directly proportional to the amount of circular flow around it.

When designing/flying an aircraft, however, how is this circular flow produced? Basically, it's through the angle of attack: that is, the angle at which the wing intercepts the oncoming air. One thing that's nice about this is that it means you can construct an aircraft that is aerodynamically stable. That is, when the nose pitches down, the amount of circular flow, and thus lift, increases, pushing the plane upward, while if the nose pitches up, the amount of lift decreases. An aircraft that is aerodynamically stable can be flown at a particular altitude at a given speed without worrying about touching the stick, even without autopilot. Even if the pilot accidentally bumps the stick, the aircraft will just oscillate up and down around that altitude (if the pilot wants to stop the oscillations, they have to be careful to push/pull the stick at the right time, or they'll just end up making the oscillations even bigger!).

If the pilot of an aerodynamically-stable aircraft wants to fly at a different altitude, they can either change their airspeed, or change what is known as the "trim". The trim of an aircraft is set by the relationship between the angle of attack of the front wings and of the tail wings, and usually adjusted by changing the angle of attack of the tail wings. Many aircraft these days have computer-assisted flight controls that automatically adjust the trim to keep the airplane at the current altitude at all times.

airshowfan
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Re: 0803: "Airfoil"

Postby airshowfan » Sat Oct 09, 2010 2:23 am UTC

Chalnoth wrote:the pressure is decreased by an amount proportional to the extra distance moved


Actually, the pressure change is proportional to the change in the square of the speed.

But that's a minor nitpick. Overall, you're right. And that's a good way to expose why the equal-distances thing doesn't work, about how the projected area (the horizontal component) of the upper surface is the same as that of the lower surface. I hadn't thought about it that way. Neat.

talkeetna
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Re: 0803: "Airfoil"

Postby talkeetna » Sat Oct 09, 2010 3:18 am UTC

The lift of an airfoil can be described by the COANDA effect, best visualized with a spinning beach ball (or soccer ball, courtesy of Jef Raskin).

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jmorgan3
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Re: 0803: "Airfoil"

Postby jmorgan3 » Sat Oct 09, 2010 3:36 am UTC

Chalnoth wrote:First, let's examine the case where the air that flows over the top of the wing exactly meets the air that flows over the bottom. This is known as "laminar flow". What is the lift in this situation?

That is not what "laminar flow" means. See here for the real definition.
Chalnoth wrote:Well, consider: the speed of the air going over the top of the wing means that the pressure is decreased by an amount proportional to the extra distance moved. But this pressure is applied over the surface of the wing, so even though the pressure is lower, the area is larger, so the total force on the wing is exactly the same from the air going below the wing and above it!

Even if it was true that pressure drop was proportional to distance covered, the area that matters is the planform area: the area of the surface when projected onto the plane normal to lift direction, which is the same for both surfaces of the airfoil.
Chalnoth wrote:So if the wing actually experienced laminar flow, the aircraft would fall like a rock.

Very wrong. See, for example, laminar flow airfoils, which are designed to only experience laminar flow.
Chalnoth wrote:What must happen instead is the air on the top of the wing must actually move faster than the air going across the bottom. One way this picture can be simplified is to decompose the airflow around the wing into two components: laminar flow (which makes no contribution to the lift), and circular flow (which provides lift). In fact, it is relatively easy to prove that the lift the wing experiences is directly proportional to the amount of circular flow around it.

Replace "laminar" with "irrotational" and "circular flow" with "circulation" and you are quite right.
Chalnoth wrote:When designing/flying an aircraft, however, how is this circular flow produced? Basically, it's through the angle of attack: that is, the angle at which the wing intercepts the oncoming air. One thing that's nice about this is that it means you can construct an aircraft that is aerodynamically stable. That is, when the nose pitches down, the amount of circular flow, and thus lift, increases, pushing the plane upward, while if the nose pitches up, the amount of lift decreases. An aircraft that is aerodynamically stable can be flown at a particular altitude at a given speed without worrying about touching the stick, even without autopilot. Even if the pilot accidentally bumps the stick, the aircraft will just oscillate up and down around that altitude (if the pilot wants to stop the oscillations, they have to be careful to push/pull the stick at the right time, or they'll just end up making the oscillations even bigger!).

When the nose pitches down, the lift decreases. Stability in pitch is achieved by placing the center of gravity of an aircraft forward of the resultant of the lift force. That way, if the plane is nudged in the nose-down direction, the lift force will decrease. That decreases the nose-down aerodynamic pitching moment, causing the plane to pitch back up.
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Faranya
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Re: 0803: "Airfoil"

Postby Faranya » Sat Oct 09, 2010 3:51 am UTC

bmonk wrote:
Taot wrote:
littlelj wrote:
Marlayna wrote:
arbivark wrote:... "give your answer in centimeters" so the smarter kids answered 36, and we were supposed to mark it wrong.


Of course you were supposed to mark it wrong. It was wrong.

I'd have to agree - if the rubric had stated "give your answer in whole centimetres" then 36 was right, but otherwise not, since m/cm/km etc are continuous, not discrete.


I agree that 'give your answer in centimeters' tells nothing of how accurate an answer is required. Just what unit is to be used. I'd like to offer, however, opinion that the right answer to accuracy lies within what was calculated. If it was 7,5 x 4,8 = 36,0 is right, 7,28 x 5 = 36 is right, 6,62 x 5,5 = 36,4 is right etc. given that it was a school setting and the general rule with digits ( if that is the appropriate word ) is to round to the least accurate number used if not specified otherwise.


I often laugh at errors of significant digits. One text that I had included sentences like: "Climate changes significantly over hundreds of miles (160.93 km)" or "Average temperature is in the 80s [ºF] (32.22-26.67 C)." Such foolishness!


I always hated the significant digit system. It is all well and good for small numbers to avoid an illusion of precision, but it falls apart with large numbers. Like the 4 000 000*1.15=4 000 000 mentioned before. Even if it is avoiding some false precision, I normally wouldn't consider 600 000 of something to be negligible...

As for the answer on the test, if there was no precision conventions given, I wouldn't call it wrong. However, I wouldn't call them 'smarter' for expressing it to a lower precision, they might have just been lazy or careless.
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VapidFrobie
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Re: 0803: "Airfoil"

Postby VapidFrobie » Sat Oct 09, 2010 5:04 am UTC

Yay! An XKCD containing aeroscience!

I'm the guy who sits quietly in the back of every class, folding just about every piece of paper in his backpack, trying to perfect the art of paper airplanes. I've made some custom designs that come close to the old world record, but not too close. Still got some work to do. I also recreate real jets. My crown jewel is the F-14 with working swingwings that I did to win a bet. This is more than my favorite branch of science, it's the closest thing I have to a passion. That, along with the Santa Claus line, is why this is now my new favorite XKCD of all time.

And yes, that kid's question is something that has bothered me for a long time. Based solely on my experience with paper aircraft, I'm thinking it has more to do with the shape of the wingtips and the trailing edge of the control surfaces.

Azkyroth wrote:Where do people live where they discuss physics at even this rudimentary level at an age where kids still believe in Santa?


Definitely not here in the California school systems. I learned that and much more from TV. My education would be horribly lacking if not for PBS Kids.

darkshade
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Re: 0803: "Airfoil"

Postby darkshade » Sat Oct 09, 2010 7:06 am UTC

Faranya wrote:
bmonk wrote:
Taot wrote:
littlelj wrote:
Marlayna wrote:
arbivark wrote:... "give your answer in centimeters" so the smarter kids answered 36, and we were supposed to mark it wrong.


Of course you were supposed to mark it wrong. It was wrong.

I'd have to agree - if the rubric had stated "give your answer in whole centimetres" then 36 was right, but otherwise not, since m/cm/km etc are continuous, not discrete.


I agree that 'give your answer in centimeters' tells nothing of how accurate an answer is required. Just what unit is to be used. I'd like to offer, however, opinion that the right answer to accuracy lies within what was calculated. If it was 7,5 x 4,8 = 36,0 is right, 7,28 x 5 = 36 is right, 6,62 x 5,5 = 36,4 is right etc. given that it was a school setting and the general rule with digits ( if that is the appropriate word ) is to round to the least accurate number used if not specified otherwise.


I often laugh at errors of significant digits. One text that I had included sentences like: "Climate changes significantly over hundreds of miles (160.93 km)" or "Average temperature is in the 80s [ºF] (32.22-26.67 C)." Such foolishness!


I always hated the significant digit system. It is all well and good for small numbers to avoid an illusion of precision, but it falls apart with large numbers. Like the 4 000 000*1.15=4 000 000 mentioned before. Even if it is avoiding some false precision, I normally wouldn't consider 600 000 of something to be negligible...

As for the answer on the test, if there was no precision conventions given, I wouldn't call it wrong. However, I wouldn't call them 'smarter' for expressing it to a lower precision, they might have just been lazy or careless.


I don't think you understand how the "significant digit" system operates.If 4E6 is only measured to one significant digit, then 4E6*1.15 is no more accurate than 4E6. Yes, 600E3 seems like a rather "significant" error roundoff, it's within the error assumed by 1E6+/-1E6. That's more of a testament to how ineffective one significant figure is than anything else. The actual value, which is generally dependent on whatever units you're using, is irrelevant.

Badly Shaved Monkey
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Re: 0803: "Airfoil"

Postby Badly Shaved Monkey » Sat Oct 09, 2010 7:13 am UTC

OK, registered at last to post a comment.

And that comment is to admit that I thought I got the joke on first reading, but when I read this thread I realise I 'got' it for the wrong reason.

I read the opening text box of the first panel, "Handling a student who challenges your expertise with an insightful question" positioned next to the diagram together with the additional text below the airfoil as explaining how, if a teacher handles this right, then both the teacher and the class speed their flow, gain lift and fly.

There have been 144 posts in this thread already, so I think I might be the only person who got the wrong joke.

larvacea
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Re: 0803: "Airfoil"

Postby larvacea » Sat Oct 09, 2010 7:19 am UTC

VapidFrobie wrote:...I'm thinking it has more to do with the shape of the wingtips and the trailing edge of the control surfaces.

The efficiency of the process has something to do with the trailing edge of the wings; google "Kutta condition" to get started on the explanation. Paper airplanes are great for exploding aerodynamic myths. Can paper airplanes fly "upside down"? Why yes, yes they can. Imagine that.

Sails (as in, sailboats) are another great place to see this in action: a zero-thickness wing with sharp leading and trailing edges, and it generates lift. Golly gosh.

Fight2Fly
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Re: 0803: "Airfoil"

Postby Fight2Fly » Sat Oct 09, 2010 7:24 am UTC

All that math and science is all good and fun... but don't forget to factor in the fact that the precious fuel system may not work inverted for very long. :roll: Heck, the plane I fly was designed to fly upside down and it can only sustain inverted flight for about 3 minutes at a time due to the fuel system.

Perhaps I am lost in the pages of discussion, but is the question about whether the wing could create enough lift to sustain inverted flight, or whether the airframe itself could withstand the forces associated with it?

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ManaUser
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Re: 0803: "Airfoil"

Postby ManaUser » Sat Oct 09, 2010 7:33 am UTC

So on the airplane thing, would it be fairly accurate albeit simplistic to say that a wing works by a combination of pushing air down in front and "pulling" air down behind? Obviously as an object moves, it create increased pressure in front and decreased pressure behind it. The shape of the wing means that the pressurized air is below the wing and the depressurized air is above it.

dragoneye1589
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Re: Thank you

Postby dragoneye1589 » Sat Oct 09, 2010 7:39 am UTC

Zak McKracken wrote:This comic is a gift. I'll print it out and stick it right to our wind tunnel.
wbeaty wrote:
dragoneye1589 wrote:The only answer I've ever gotten that I have been satisfied with is that fluid flow is so complex (see Navier-Stokes) that lift isn't fully understood even by those with PhD's in the subject.

VERY TRUE!

Hmm... not quite. I think what's dazzling many is the circulation/airfoil flow thing, but you don't actually need that to explain lift.
All that's needed is actually momentum equilibrium: I'm throwing something downwards, so I get an upwards force (I'm also slowing it down some, so it comes at the cost of drag).



See, with the conservation of momentum explanation, you have to treat the air around the airfoil as a control volume in order to measure the change in momentum across the boundaries and therefore the force applied. All you have done is simplified the problem down to a black box that does something to the air to change its momentum. This is not a valid explanation of how lift occurs, but rather an explanation of the lift force we expect to see based on simple physics.

rangerdean
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Re: 0803: "Airfoil"

Postby rangerdean » Sat Oct 09, 2010 8:26 am UTC

On the "discovery of America".

Since you can "discover" a great restaurant in a part of town you don't usually visit, why is it wrong to say Columbus discovered America? The Basic Human Decency hair-splitting doesn't change the fact that "America" as we know it surely would not exist today had Columbus not "discovered" it when he did.

So, a definition (dictionary.com):

dis·cov·er
   /dɪˈskʌvər/ Show Spelled[dih-skuhv-er]
–verb (used with object)
1.
to see, get knowledge of, learn of, find, or find out; gain sight or knowledge of (something previously unseen or unknown): to discover America; to discover electricity.
2.
to notice or realize: I discovered I didn't have my credit card with me when I went to pay my bill.
3.
Archaic . to make known; reveal; disclose.

****
So indeed, Columbus did "discover" America (or a small part of it), as did the Norse, and, strictly speaking, as did the "natives" that were here when Columbus arrived (there were no peoples in Greenland when the Norse arrived there as I understand it).

However, I believe it is a misuse of "discover" to say the natives discovered it, in the context that it is generally used to describe explorers. It was more like a gradual arrival into previously non colonized (by humans) area that took thousands of years to occur. Individuals and tribes likely "discovered" America in <10 mile increments. Which is not to say Columbus actually saw much more, but the knowledge of the day (about the size of the earth and measuring distance and time) and the technology of transportation (sailing ships and horseback) certainly made the explorers much more efficient than the native colonizers, and they had a much better grasp of the significance and potential of the discovery.

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Stanistani
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Re: 0803: "Airfoil"

Postby Stanistani » Sat Oct 09, 2010 8:51 am UTC

ysth wrote:
Stanistani wrote:
squareroot wrote:OT: I made two xkcd threads in a row!!! WHAT NOW!

What now? It goes quietly away. :(

Why is that?
<snip>

I guessed wrong. :)

Richard
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Re: 0803: "Airfoil"

Postby Richard » Sat Oct 09, 2010 11:02 am UTC

A little (imprecise) experiment to demonstrate the coanda effect and lift (or diffusers on race cars). Turn on your tap and dangle the backside (convex) of a spoon near it. Slowly move the spoon into the stream. When the spoon meets the stream it will be pulled further into the stream by the reaction to the water slung off the tip. I never really bought that bernoulli shit. He may make my carburetor work but I don't think he can hold an airplane in the air. I have seen a picture of a plane flying above clouds and it clearly makes a groove.

The1exile
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Re: 0803: "Airfoil"

Postby The1exile » Sat Oct 09, 2010 11:43 am UTC

I like this comic. It's been a while since we've had a genuinely clever physics one.
mosc wrote:
Endless Mike wrote:The military wrote custom PS3 software and bought a bunch of them for some very specific application.

A modern warfare lan party, duh.


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