0803: "Airfoil"

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airshowfan
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Re: 0803: "Airfoil"

Postby airshowfan » Fri Oct 08, 2010 3:59 pm UTC

Airplanes can fly upside down (i.e. airfoils can still generate lift upside-down) because the stagnation point changes.

(Unfortunately, this is compatible with the incorrect "the separated air must meet up with itself at the trailing edge, so the air taking the longer path must speed up" explanation).

Say you have an airfoil (symmetric or cambered, doesn't matter) moving through the air at some airspeed. Say that you pitch it to whatever angle of attack is necessary to supply a certain amount of lift (say, the lift force necessary to counter the weight of the airplane).

Some of the air moves over the airfoil. Some of the air moves under the airfoil. Some point on the leading edge of the airfoil will mark the spot where the air is split. That is the stagnation point. On the image, I marked it as a red dot.

Now say you turn your airfoil (e.g. the whole airplane) upside-down. Say that it's still moving with the same airspeed. Say that you pitch it to whatever angle of attack is necessary to supply a certain amount of lift (say, the lift force necessary to counter the weight of the airplane). If you have a symmetric airfoil, it will be the same angle of attack as before. If you have an asymmetric airfoil optimized for non-inverted flight, it will need a higher angle of attack to fly inverted, but that doesn't matter much right now.

inverted-airfoil-stagnation-changes.png


You'll notice that the stagnation point will be further down than it was before. On the image, I marked it as a green dot. The airfoil naturally generates a "hump" out of the upper part of its leading edge when it's upside down (just as it does when it's not upside down).

As for what kinds of airplanes can sustain inverted flight for how long, there are five things to keep in mind:

1) An asymmetric airfoil is less efficient when inverted (i.e. it will produce more drag while generating the same lift at a given speed)
2) The engine is usually mounted so that it produces thrust in the straight-forward direction when the wing is at the angle of attack that, during cruise flight, causes the right amount of lift to be generated to overcome the weight of the airplane. So if you turn the airplane upside down and then pitch the nose up in order for the wing to work, the engine will no longer be pulling straight forward, it will be pulling forward and up. So you lose some component of thrust (since the horizontal component of the thrust is now the engine thrust times the cosine of the angle through which you had to pitch the nose up for the wing to keep working). Yes, there is a tiny component of the thrust pulling upwards, and you'd think that this would help the wings out, but most airplanes have small thrust-to-weight ratios so the vertical component of the thrust during inverted flight is pretty negligible, unless you're flying an inverted high-alpha airshow pass in a ridiculously overpowered airplane.
3) Engines, the fuel tank, the oil system, and other things (and the pilot!) might not function very well when inverted.
4) Some structures will be loaded in reverse during inverted flight. The tops of the wings which used to be in compression will be in tension, the bottoms of the wings which used to be in tension will be in compression (so any struts under the wings might buckle), etc.
5) The horizontal stabilizer, which is usually required to balance the airplane, is mounted at a different angle of incidence than the wing, for stability. So when you turn the whole airplane upside down, the horizontal will be at the wrong angle of attack, and the elevators will have to work very hard in order to make up for this.

All that numbers one and two mean is that you need more thrust to sustain inverted flight. Most airplanes have some excess thrust (i.e. the engines can produce a lot more than is needed for level flight) so no real problem there.

Number three and four mean that either your systems and structures are inverted-friendly (they can work properly in negative gs), or they are not. If they're not, they'll malfunction when you sustain negative gs, even if the airplane can aerodynamically do it.

Number five is the real reason why you can't fly inverted in a typical Cessna or airliner:

inverted-airliner.png


The CG is ahead of the wing (for stability), so the horizontal stabilizer needs to push downwards (for balance), so it is set at a negative angle of incidence (i.e. it's like a little wing, but upside down). If you flip the plane upside down (and then pitch the nose up so that the wings still lift upwards), the horizontal stabilizer will now be at a very high upwards angle of attack, and will try very hard to push the tail up and bring the nose back down. The only way to overcome this is if your elevators can overcome this stabilizing force. Cessnas and airliners have big stabilizers set at a significantly negative angle of incidence (which give these airplanes great stability) and small elevators (so the pilot can't maneuver too wildly), but fighters and aerobatic airplanes have the horizontal at almost the same angle of incidence as the wing (so they're not very stable) and big, powerful elevators, sometimes the whole stabilizer is one huge elevator (so that they can pitch up and down very fast, make tight turns, etc). So the elevators in fighters and aerobatic airplanes is powerful enough, and the stabilizers are weak enough, that when you roll inverted, the elevators can overcome the force of the stabilizers and keep the airplane at that nose-up angle that it needs for the wings to work while inverted. But airliners and general-aviation airplanes, not so much.

So if you roll a fighter or an aerobatic airplane inverted, and then push the stick, the nose will point upwards and the wings will work and you're golden. But in a typical general-aviation airplane or an airliner, if you somehow manage to roll it inverted, the nose will sink, no matter how hard you push that yoke, because the stabilizers are just too big and set at too great an angle and you don't have very powerful elevators to overcome the stabilizers' tail-up force.

Note that all this is in regards to airplanes "pulling negative gs", i.e. where the wing generates lift in the direction from the pilot's head to their butt rather than in the direction from the pilot's butt to their head. Many inverted maneuvers, like properly-executed loops and barrel-rolls, are positive-g maneuvers, i.e. the wing always works the normal way, and the pilot is always pressed against the seat and never hangs from the straps. (Whether this should be understood as a "centrifugal" effect or just as an airplane rolled inverted and pushing downwards against its own inertia... you can ask Mr Bond). The only real challenge in doing these positive-g rolls and loops is that the maneuver will have a "bottom" and a "top", and that difference in altitude means a difference in potential energy that comes right out of your kinetic energy, i.e. the airplane goes slower at the top than at the bottom (just like a roller coaster or a projectile - the engine will help a little but not much, since you have to pull off these maneuvers fairly quickly if you hope to keep the gs positive). But the range of speeds that an airplane can operate at is relatively narrow: There is a minimum stall speed below which the wings don't work, and a maximum speed dictated by engine thrust or by structural considerations. Most non-aerobatic airplanes can't do a loop because if you're flying at your maximum speed and you pull up, you'll get to your minimum speed before you get to the top of the loop, and the airplane will stall. (If you're inverted when this happens, i.e. if you make it more than 1/4 of the way around the loop and your nose has gone past the vertical, then stalling can put you in an inverted flat spin that some airplanes can't get out of. So don't try this at home). But almost any airplane can do a barrel roll, even (famously) a 707. You have to pull a couple of gs when you transition from level flight to going up the barrel roll, and then you have to pull a couple of gs again when you transition from going down the barrel roll back into level flight, but pretty much any airplane can handle that, and the altitude change will be relatively small so it won't take most airplanes from max speed to below stall speed.

EDIT: I think I was wrong in my "the elevators are probably not powerful enough to overcome the stabilizers' alpha-reducing nose-towards-the-horizon force" estimation. In my defense, I'm a structures guy. I can tell you how the airplane components are loaded very differently during -1g flight than they are during normal flight, and how they're still strong enough to take -1g flight, plus a 50% safety margin (just like the famous 50% safety margin on the 2.5 positive gs that they're supposed to withstand), and which flight conditions load which components most heavily (not ALL of them, but most of the structurally important ones). I can give you numbers about stress levels, strengths, material thicknesses, how shear beams are configured, when buckling happens, how stringers and frames are shaped and why, and how far apart they are, and so on. (Well, I'd be fired if I did, but I could). But when it comes to stability & control, what I know comes from a few classes in college, some experience flying Cessnas, some experience in Flight Simulator, and going to lots of airshows. It's a lot, but it's not as precise as what I know about structures. Anyways, over the weekend I got talking with someone who does deal with stability and control and airplane handling for a living, and he told me that an airliner should not only be able to survive a -1g manouver, in practice an airliner can actually be pushed over that hard on command. At the right speeds, with nose-down trim, etc, the elevators should be able to overcome the horizontal stabilizers' stabilizing force, and hold the airplane at the negative angle of attack necessary to sustain -1g flight. What I told him is that I've never been able to pull it off in Flight Simulator, but he insisted that it's doable. The only way to know for sure (other than taking an airliner up and flipping it, which is a bit beyond our means) is to try it in a REAL flight simulator. Now I really want to try it! But in any case, for the record, what I wrote above in this post (about how the elevators will not push down hard enough to keep the horizontal stabs from raising the tail and dropping the nose, so the wings can't be kept at enough of an angle to generate lift while inverted) is probably wrong.

EDIT 2: Yep, I was wrong.
Last edited by airshowfan on Tue Oct 12, 2010 2:28 pm UTC, edited 10 times in total.

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Re: 0803: "Airfoil"

Postby jpers36 » Fri Oct 08, 2010 4:01 pm UTC

Karilyn wrote: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.


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

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ubikuberalles
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Re: 0803: "Airfoil"

Postby ubikuberalles » Fri Oct 08, 2010 4:02 pm UTC

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


The U.S. Air Force Academy? :P

Seriously, though, I was taught that in elementary school.
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Grant10k
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Re: 0803: "Airfoil"

Postby Grant10k » Fri Oct 08, 2010 4:15 pm UTC

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.

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Re: 0803: "Airfoil"

Postby negative_one_rad_theta » Fri Oct 08, 2010 4:24 pm UTC

It's good that the equal transit fallacy was finally addressed in this comic.
It's bad too, because the mere existence of such a popular non-scientific explanation for a physical phenomenon makes me feel dead inside.
It is probably the second best/worst (depending on you look at it) perversion of physics that has ever been tried.
(The worst being those gut-wrenching abuses of anything with the word "quantum" in it).

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Re: 0803: "Airfoil"

Postby Pterosaur » Fri Oct 08, 2010 4:37 pm UTC

HonoreDB wrote:"This is the way it will appear on the test."


“Will this be on the test?” is always more important than “Is this accurate?”

Steve the Pocket wrote:Someone needs to compile a list of things they actually still teach in schools.


Besides those previously mentioned, the one that I recall is the “tongue taste bud map.” I learned that in high school 15 years ago.

jonadab wrote: Columbus didn't know about it, so when he found America (err, Haiti or Cuba or whatever he actually found), he had and his compatriots no absolutely idea that the Vikings had already been to Newfoundland.


Incidentally, I discovered a new land yesterday in the woods behind my house where the deer like to nap. (The Parks Dept. probably knew about it, but they never told me.)

I shall call my new land Deervania.
Until you stalk and overrun,
You can't devour anyone.
--Hobbes

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Re: 0803: "Airfoil"

Postby Pesto » Fri Oct 08, 2010 4:57 pm UTC

cs24 wrote:The explanation is wrong because the streamlines don't meet up after the trailing edge of the airfoil: http://www.youtube.com/watch?v=6UlsArvbTeo&t=0m17s

Huzzah! Vindicated! The explanation that the two air streams would meet up at back of the wing never made sense to me, although I never openly questioned it.

Cool to see that the upper air stream does indeed travel faster than the lower air stream.

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Karilyn
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Re: 0803: "Airfoil"

Postby Karilyn » Fri Oct 08, 2010 5:25 pm UTC

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.

Binary? Yeah I never learned do to binary quickly in my head. Though admittedly I still count with my thumb as a 5 if I'm trying to tally up some whatever I'm counting. Kinda weird how it stuck with me.

Zak McKracken wrote:
Karilyn wrote:
Zak McKracken wrote:From the viewpoint of western civilisation, and thus the viewpoint of the majority of schoolbook-makers in America, it was in fact discovered by Columbus, because before that noone that he knew (and noone anyone knew that he knew and so on) knew it existed.
The viewpoint of Western Civilization isn't always the right one.
words

Image
Last edited by Karilyn on Fri Oct 08, 2010 5:36 pm UTC, edited 3 times in total.
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Re: 0803: "Airfoil"

Postby charonme » Fri Oct 08, 2010 5:28 pm UTC

picnic_crossfire wrote:Can anyone here actually answer that student's question?

A kid could be convinced/silenced (even though perhaps not educated) by this: The planes you saw flying upside down were not using this principle we just discussed here. They fly because their engines are so powerful they could even fly straight up. The primary function of their wings is not to provide lift, just to balance them and provide coordination.

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:

I believe the equal distance explanation is wrong, but not because we can find examples of planes that fly despite having different wings. This only proves that the equal distance explanation could not be the only reason why something with wings flies, it does not prove that the explanation is incorrect in case of the particular wing shape that it is applied to.

Grant10k wrote:
jpers36 wrote:Forget that, I can count to 1024 on my two hands.
It's all fun and games until you get to 132.

What's so unfunny about 132? Remember you have to start with all fingers in the 0 position to count ONE if you want to get to 1024 ;)

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Re: 0803: "Airfoil"

Postby airshowfan » Fri Oct 08, 2010 5:36 pm UTC

charonme wrote:The primary function of their wings is not to provide lift, just to balance them and provide coordination.


You know that this is incorrect, right? Just checking.

charonme wrote:This only proves that the equal distance explanation could not be the only reason why something with wings flies...


Actually, the incorrect equal-distance explanation (or, rather, the incorrect different-distances-but-supposedly-same-time explanation) CAN hold up as the only reason that something flies, even inverted, if you take into account the fact that the stagnation point changes when you turn the airfoil upside down.

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Karilyn
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Re: 0803: "Airfoil"

Postby Karilyn » Fri Oct 08, 2010 5:37 pm UTC

charonme wrote:
Grant10k wrote:
jpers36 wrote:Forget that, I can count to 1024 on my two hands.
It's all fun and games until you get to 132.

What's so unfunny about 132? Remember you have to start with all fingers in the 0 position to count ONE if you want to get to 1024 ;)

Look ma, I can count to 132!

Image

(the 1024 is implied. Otherwise you could count to 1025 ;) )
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Re: 0803: "Airfoil"

Postby HonoreDB » Fri Oct 08, 2010 5:40 pm UTC

Pterosaur wrote:Besides those previously mentioned, the one that I recall is the “tongue taste bud map.” I learned that in high school 15 years ago.


Ooh, me too, around the same time. My parents neatly refuted it that evening by sticking out their tongues and asking what the theory predicted about their respective preferences for salt.

(America was discovered by utahraptors).

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Re: 0803: "Airfoil"

Postby libra » Fri Oct 08, 2010 5:46 pm UTC

Zak McKracken wrote:
jbo5112 wrote:There is false biology being taught to defend evolution, like a human fetus having gill slits and the human tail bone is of no use, when it's an anchor point for some muscles. Yes, I'm the creationist joining the debate.
If a pair of ribs can appear in such a short amount of time, there's no reason to think that a completely unused organ couldn't disappear in a billion years.

Case in point: the human brain.

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Re: 0803: "Airfoil"

Postby charonme » Fri Oct 08, 2010 6:00 pm UTC

Karilyn wrote:(the 1024 is implied. Otherwise you could count to 1025 ;) )

You don't "count" to zero. If something is implied, it could be zero. But you could maintain that you could count to 1024 on your fingers by indeed "implying" the 1024 by having all fingers in the 0 position :)
I still fail to see why two middle fingers stop fun, while one does not :?

airshowfan wrote:You know that this is incorrect, right? Just checking.

The factual correctness of the answer to the kid is irrelevant as I pointed out before I wrote it. The only matter is whether it is capable of silencing the kid and maintaining the appearance of an "all knowing teacher" :twisted:

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Re: 0803: "Airfoil"

Postby Super_jambo » Fri Oct 08, 2010 6:17 pm UTC

You kids can't hold your fingers in more than one position?
Trinary will give you 59049... ;)

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Re: 0803: "Airfoil"

Postby sableye22 » Fri Oct 08, 2010 6:27 pm UTC

Steve the Pocket wrote:Someone needs to compile a list of things they actually still teach in schools, even just in the lower grades, that are blatantly wrong. Like that people in Columbus's day thought the world was flat (pretty sure that's what they fed me in elementary school).


They- they didn't?
Everything I know is a lie! (I feel like I'm only half-joking....) I have no idea how planes fly upside down but I know I'm in love with the teacher's little stick fists of rage in the last panel.
You don't have to spend, you just have to pretend.

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Re: 0803: "Airfoil"

Postby Grant10k » Fri Oct 08, 2010 6:28 pm UTC

charonme wrote:
Grant10k wrote:
jpers36 wrote:Forget that, I can count to 1024 on my two hands.
It's all fun and games until you get to 132.

What's so unfunny about 132?...

...

I still fail to see why two middle fingers stop fun, while one does not :?


Ok, I don't like doing this because it ruins the joke. The double middle finger that you get when you count to 132 in binary is actually far from unfunny. One could argue that it's actually the MOST funny number when counted on one's hands (585 can be funny depending on which direction you're holding your hands.)

For your entertainment: Kirby counting to 132: t('.'t)


libra wrote:
Zak McKracken wrote:
jbo5112 wrote:There is false biology being taught to defend evolution...
...there's no reason to think that a completely unused organ couldn't disappear in a billion years.
Case in point: the human brain.

Zing. Nice.

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Re: 0803: "Airfoil"

Postby charonme » Fri Oct 08, 2010 6:41 pm UTC

you're right, exactly my point :)
I was being an ass, sorry

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Mike_Bson
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Re: 0803: "Airfoil"

Postby Mike_Bson » Fri Oct 08, 2010 6:43 pm UTC

This one is the best one in a long time, I love it!

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Re: 0803: "Airfoil"

Postby ddxxdd » Fri Oct 08, 2010 6:52 pm UTC

Alright, I've had a relevant question burning the back of my mind for YEARS.

We've all been taught that the force of friction equals Normal Force times coefficient of friction, right? And that the height, width, and length of the object don't factor into that calculation, right?

Well then why is it that wider tires on cars and trucks provide better braking power? Or is the fact that "wider tires procures better braking power" a myth?
I'm waiting for someone to say something worth sigging...

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Re: 0803: "Airfoil"

Postby jshilliday » Fri Oct 08, 2010 6:55 pm UTC

Speaking of preparing your children, here's another opportunity. Sooner or later, a teacher will tell your child's class that "The moon does not rotate on its axis; it always has the same side facing the earth." If the teacher will allow it, your kid should say "OK, you be the Earth, just stand there." And then she faces the teacher and moves in the appropriate circle around her, saying "Now I'm facing the blackboard, now I'm facing the windows, now I'm facing the back of the room, and now I'm facing the door. See, I'm turning around my own axis like the moon does." It's amazing how many people will watch the demonstration and STILL insist the moon doesn't rotate.

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Re: 0803: "Airfoil"

Postby Monika » Fri Oct 08, 2010 7:06 pm UTC

Grant10k wrote:I remember once, in middle school science class, we were measuring the volume or densities (I forget which) of different objects (marbles, cork, paperclilps, plastic cubes, etc.) and I noticed something fundamentally wrong with the way we were measuring.

We were dropping the objects into water to measure how much the water rose, but if the object was lighter than water, it would float on top, and if it were heavier, it would sink to the bottom. Hypothetically, a cork stopper with a lead core (so it sinks) would give different results than the pure cork, and if it had a uranium core, it would give the same results as the lead core. In essence, we were measuring the density of object lighter than water, and the volume of objects heavier than water.

When I brought this concept to the teacher, as I remember it she realized the flaw, but had the class continue with the experiment anyway. I don't think there was enough time left in the class to void everyone's results and start over, but I was still bummed.

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?



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?
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Re: 0803: "Airfoil"

Postby airshowfan » Fri Oct 08, 2010 7:07 pm UTC

Floating objects displace their weight in water (which is a lower volume). Sinking objects displace their volume in water (which is a lower weight). Right?

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Re: 0803: "Airfoil"

Postby Monika » Fri Oct 08, 2010 7:09 pm UTC

airshowfan wrote:Floating objects displace their weight in water (which is a lower volume). Sinking objects displace their volume in water (which is a lower weight). Right?

Right. When you throw out that gold the water level sinks, because gold has a higher density than water. Most people say "unchanged".
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Re: 0803: "Airfoil"

Postby airshowfan » Fri Oct 08, 2010 7:17 pm UTC

Yeah, that makes sense. While on a boat, the gold is displacing a lot of water (its weight), but once in the water the gold is displacing less water (its volume).

Once the gold goes overboard, the boat hull displaces less water corresponding to the weight of the gold, while the gold itself displaces some water (not as much, i.e. not enough to make up for the decreased displacement of the boat hull) corresponding to its volume.

I had to think about that for a minute. (But I would not have said "unchanged"!)

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Re: 0803: "Airfoil"

Postby davebrown42 » Fri Oct 08, 2010 7:40 pm UTC

ddxxdd wrote:Alright, I've had a relevant question burning the back of my mind for YEARS.

We've all been taught that the force of friction equals Normal Force times coefficient of friction, right? And that the height, width, and length of the object don't factor into that calculation, right?

Well then why is it that wider tires on cars and trucks provide better braking power? Or is the fact that "wider tires procures better braking power" a myth?


Basically a myth. Wider tires do increase rolling resistance, so if you have no brakes at all, wider tires will slow you down slightly faster. :P

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Re: 0803: "Airfoil"

Postby Karilyn » Fri Oct 08, 2010 7:58 pm UTC

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
Gelsamel wrote:If you punch him in the face repeatedly then it's science.

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Kline-Fogleman airfoil, longer path underneath.

Postby wbeaty » Fri Oct 08, 2010 7:59 pm UTC

Be aware that Kline-Fogleman and supercritical airfoils fly just fine. Their upper pathlength is the short one.

airshowfan wrote:Actually, the incorrect equal-distance explanation (or, rather, the incorrect different-distances-but-supposedly-same-time explanation) CAN hold up as the only reason that something flies, even inverted, if you take into account the fact that the stagnation point changes when you turn the airfoil upside down.


Nope, that completely underestimates the value for lift (besides misleading everyone about the nature of lift-production.) If you track the flowing air using smoke-pulses (or using a CFD simulation,) you find that the parcels divided by the leading edge will never rejoin. The upper parcels greatly outrace the lower ones. I.e. the air velocity is everything, while the surface path lengths are basically irrelevant.
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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
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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.
Image

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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