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Flying a Cessna On Other Worlds: xkcd Gets Noticed By a Physics Professor

Soulskill posted about a year and a half ago | from the make-sure-your-insurance-is-up-to-date dept.

It's funny.  Laugh. 148

djl4570 writes "xkcd's 'What If' series consists of humorous takes on highly implausible but oddly interesting hypothetical physics questions, like how to cook a steak with heat from atmospheric re-entry. The most recent entry dealt with flying a Cessna on other planets and moons in the solar system. Mars: 'The tricky thing is that with so little atmosphere, to get any lift, you have to go fast. You need to approach Mach 1 just to get off the ground, and once you get moving, you have so much inertia that it’s hard to change course—if you turn, your plane rotates, but keeps moving in the original direction.' Venus: 'Unfortunately, X-Plane is not capable of simulating the hellish environment near the surface of Venus. But physics calculations give us an idea of what flight there would be like. The upshot is: Your plane would fly pretty well, except it would be on fire the whole time, and then it would stop flying, and then stop being a plane.' There are also a bunch of illustrations for flightpaths on various moons (crashpaths might be more apt), which drew the attention of physics professor Rhett Allain, who explained the math in further detail and provided more accurate paths."

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Not going anywhere... (-1, Redundant)

icebike (68054) | about a year and a half ago | (#42773619)

The upshot is: Your plane would fly pretty well,

Let me know when you get that Cessna engine started on mars.

Re:Not going anywhere... (5, Funny)

peragrin (659227) | about a year and a half ago | (#42773643)

It is a cessna engine, it doesn't run on air but on money.

Re:Not going anywhere... (2, Insightful)

Anonymous Coward | about a year and a half ago | (#42773703)

In the What-if it's explicitly stated that the gas tanks have been replaced with batteries and had an electric engine installed.

Re:Not going anywhere... (-1, Redundant)

icebike (68054) | about a year and a half ago | (#42773715)

And how low a temperature will that work at?

Re:Not going anywhere... (5, Insightful)

KeensMustard (655606) | about a year and a half ago | (#42773729)

This is covered in the simulations as well. Is there something in particular preventing you from reading it?

Re:Not going anywhere... (3, Insightful)

alphatel (1450715) | about a year and a half ago | (#42773769)

This is covered in the simulations as well. Is there something in particular preventing you from reading it?

Although I am not the poster you asked this question of, I have to admit not ever reading xkcd, having more important things on my Kindle.
Having left my e-ink display in the car, I read through what-if and if nothing else, the penny exercise had me laughing out loud. Tough to force on a rocket scientist with humor less moist than a block of dry ice, but it happens.
Thanks to / for not posting a slashvertisement and giving me the giggles.

Re:Not going anywhere... (1)

Threni (635302) | about a year and a half ago | (#42773867)

> I have to admit not ever reading xkcd, having more important things on my Kindle.

It publishes 3 strips a week, plus a what-if from time to time. It's not a book, or anything else which would compete with whatever's on your kindle for your attention, unless you're a very, very slow reader.

Re:Not going anywhere... (1)

Hotawa Hawk-eye (976755) | about a year and a half ago | (#42773963)

> I have to admit not ever reading xkcd, having more important things on my Kindle.

It publishes 3 strips a week, plus a what-if from time to time. It's not a book, or anything else which would compete with whatever's on your kindle for your attention, unless you're a very, very slow reader.

Or unless you bought the Humble eBook Bundle [humblebundle.com] back in October.

Re:Not going anywhere... (3, Informative)

jamesh (87723) | about a year and a half ago | (#42774449)

> I have to admit not ever reading xkcd, having more important things on my Kindle.

It publishes 3 strips a week, plus a what-if from time to time. It's not a book, or anything else which would compete with whatever's on your kindle for your attention, unless you're a very, very slow reader.

The bigger problem is that Friday's comic was number 1168, so if you've only just started reading now you have a lot of catching up to do. Then half way through you'll realise that if you hover the mouse over the picture some additional text pops up so you'll have to go all the way back and start again[1]. Then you need to read the blag to figure out what all the references to cancer are about.

Most of the comics can be fully enjoyed in 30 seconds or less, but some require a bit more effort...

The What-If's come out once a week and also require a bit more attention but there's only a handful of them so far.

[1] I don't know how to get hover text on my Samsung Galaxy S2... maybe kindle's can't get to it either?

Re:Not going anywhere... (4, Informative)

CynicTheHedgehog (261139) | about a year and a half ago | (#42774757)

http://m.xkcd.org/ [xkcd.org] is a better version for mobile. The title below the comic has a clickable superscript (alt text) link that will display the alt text underneath.

Re:Not going anywhere... (1)

jamesh (87723) | about a year and a half ago | (#42775765)

http://m.xkcd.org/ [xkcd.org] is a better version for mobile. The title below the comic has a clickable superscript (alt text) link that will display the alt text underneath.

Awesome. Tanks for the tip. I've now changed my bookmark :)

Re:Not going anywhere... (1)

ryzvonusef (1151717) | about a year and a half ago | (#42775973)

Tanks for the tip.

Another tip: xkcd comic usually get a transcript (which helps as an explainer) after a day or two.

Can I get a Tank too? I am not picky, though I wouldn't mind a nice little Sherman :D

Re:Not going anywhere... (1)

maxwell demon (590494) | about a year and a half ago | (#42776697)

I consider it a bad trend to make separate mobile web pages. The same link will be used by desktop and mobile users, but no matter what page you link to, it will be the wrong page for someone. Instead the page should display correctly for both desktop and mobile; if this is not possible with a common HTML file, just serve different files depending on whether it was accessed from a desktop or mobile browser.

Re:Not going anywhere... (1)

drainbramage (588291) | about a year and a half ago | (#42775645)

He is not a reader, he has a kindle.
In a non related issue:
Did anyone notice US teachers have begun 'boycotting' proficiency testing?

Re:Not going anywhere... (5, Insightful)

kraut (2788) | about a year and a half ago | (#42773895)

Although I am not the poster you asked this question of, I have to admit not ever reading xkcd, having more important things on my Kindle

Like slashdot?

Re:Not going anywhere... (2)

ridgecritter (934252) | about a year and a half ago | (#42773901)

As a rocket scientist, perhaps you might get a chuckle out of this xkcd: http://xkcd.com/1133/ [xkcd.com]

Re:Not going anywhere... (2)

aztracker1 (702135) | about a year and a half ago | (#42774681)

I really want that on a t-shirt... I'm a big guy, so it should work well on a 4X

Re:Not going anywhere... (2)

ridgecritter (934252) | about a year and a half ago | (#42775265)

Great idea! I'm also a big guy, but it might fit me best if I were to lay it out horizontally...):

Re:Not going anywhere... (1)

dkf (304284) | about a year and a half ago | (#42773725)

In the What-if it's explicitly stated that the gas tanks have been replaced with batteries and had an electric engine installed.

And also that this means that the plane won't fly for very long anyway, around 10 minutes. Not that this is a particular issue on many of the worlds of the solar system (unless you can also make the Cessna acid-proof, which would help a lot for Venus).

It's just what-iffery.

I got linked! (4, Interesting)

Geoffrey.landis (926948) | about a year and a half ago | (#42775401)

Wow-- I just noticed this-- I got linked!
(at the pdf report linked at the words "...The acid's no fun, but it turns out the area right above the clouds is a great environment for an airplane" in the Venus section)
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20030003716_2002108457.pdf [nasa.gov]

Re:I got linked! (0)

Anonymous Coward | about a year and a half ago | (#42775605)

Congratulations. After all these years of faithfully reading /., you finally get recognized. ;)

Seriously though, that's cool.

Re:Not going anywhere... (1)

Phaedrus420 (860578) | about a year and a half ago | (#42773755)

Yeah, that sentence was for Venus.

Re:Not going anywhere... (0)

Anonymous Coward | about a year and a half ago | (#42773807)

For the purposes of this THOUGHT experiment, the engine has been replaced with a purely electric motor. It's right there at the top of the What If article.

Re:Not going anywhere... (1)

mcgrew (92797) | about a year and a half ago | (#42773809)

Well, not without a little hacking. Just pipe oxygen into the intake. But with that little atmosphere you have more problems, like how to get lift.

Re:Not going anywhere... (-1, Redundant)

icebike (68054) | about a year and a half ago | (#42773873)

Lift?

How do you get propulsion. You couldn't possibly swing a prop big enough to provide any propulsion in the thin martian atmosphere.

The highest atmospheric density on Mars is equal to the density found 35 km above the Earth's surface. The resulting mean atmospheric pressure at the ground is only 0.6% of that of the Earth.

A prop is not going to move you in this atmosphere. You couldn't swing a standard prop fast enough, and by the time you scale up the prop diameter, the plane would spin but the prop wouldn't, because a prop that big would weight way more than the plane and would have too much inertia.

Its a silly thought exercise.

Re:Not going anywhere... (1)

HJED (1304957) | about a year and a half ago | (#42774237)

Um, did you read the what-if it takes atmosphere into account and on each planet the plane is magically launched from a reasonable height (it does not take off)

Re:Not going anywhere... (-1, Flamebait)

icebike (68054) | about a year and a half ago | (#42774321)

He makes no sensible assertion of "taking the atmosphere into account". It it totally not believable in ANY context.
But I suppose if you grew up watching Star Trek you might be forgiven if your science education suffered as a result.

You can't achieve propulsion with a prop at a pressure altitude of 35KM AGL.

Re:Not going anywhere... (2, Funny)

Anonymous Coward | about a year and a half ago | (#42774411)

You must be great at parties.

Re:Not going anywhere... (-1)

Anonymous Coward | about a year and a half ago | (#42774721)

Did you even read it? You're obviously stupid, because UNLIKE YOU MOTHERFUCKER, he actually explains in detail what would and would not work.

Re:Not going anywhere... (5, Insightful)

Runaway1956 (1322357) | about a year and a half ago | (#42774849)

It is explained that on one world, you burn then crash - as opposed to crash and burn - and why it would happen in that order. And, on another world, you would crash, but not burn, and why.

This little "what if" is a reasonable explanation of conditions on other worlds, as we understand them, and how they would affect flight in a particular type and model of aircraft.

If the story teller were addressing an international physics conference, he might sound a bit stupid with this presentation. As he is addressing an audience of nerds, with the intent of amusing and possibly educating them - he's done an excellent job.

Re:Not going anywhere... (0)

Anonymous Coward | about a year and a half ago | (#42775027)

"You can't achieve propulsion with a prop at a pressure altitude of 35KM AGL."

Why is this? As far as I can tell the prop is just a rotating wing. As long as there is atmosphere the prop should be able to generate thrust.

Re:Not going anywhere... (3, Funny)

Qzukk (229616) | about a year and a half ago | (#42775049)

It it totally not believable in ANY context

Wow, you mean he's wrong and the Cessna would fly awesome and not just fall to the ground?

Our Cessna 172 isn’t up to the challenge. Launched from 1 km, it doesn’t build up enough speed to pull out of a dive, and plows into the Martian terrain at over 60 m/s (135 mph). If dropped from four or five kilometers, it could gain enough speed to pull up into a glide—at over half the speed of sound. The landing would not be survivable.

Glad we had you here to set things right. I'm going to get started on my plan to fly to Mars!

Re:Not going anywhere... (1)

HJED (1304957) | about a year and a half ago | (#42775933)

As someone else quoted from the what-if, which you didn't read:

Our Cessna 172 isn’t up to the challenge. Launched from 1 km, it doesn’t build up enough speed to pull out of a dive, and plows into the Martian terrain at over 60 m/s (135 mph). If dropped from four or five kilometers, it could gain enough speed to pull up into a glide—at over half the speed of sound.

At no point does he claim the plane achieves propulsion, in fact he says exactly the opposite. Remember that the word glide means that the cesna does not achieve powered flight, and "launched from 1 km" means that it is already in the air when it falls and hits the ground, completely realistic given the terms of the scenario and detailed enough for the context.

Re:Not going anywhere... (2)

Jaktar (975138) | about a year and a half ago | (#42773889)

Very first tile in illustration: rip out engine, install batteries and electric motor.

RTFA

Re:Not going anywhere... (-1, Flamebait)

icebike (68054) | about a year and a half ago | (#42773913)

Very first tile in illustration: rip out engine, install batteries and electric motor.

RTFA

And get propulsion from a prop in an atmosphere of .6 that on earth?

Use your common sense son.

Re:Not going anywhere... (2)

steppedleader (2490064) | about a year and a half ago | (#42774021)

He used a simulator for Mars that accounts for things like the effects of density differences on prop thrust, son.

What exactly are you trying to prove by refusing to read the article?

Re:Not going anywhere... (-1, Flamebait)

icebike (68054) | about a year and a half ago | (#42774045)

You can't hand-waive away physics.
What exactly do you hope to gain by propping up this stupid article?

Re:Not going anywhere... (4, Funny)

Anonymous Coward | about a year and a half ago | (#42774163)

You can't hand-waive away physics.

Sense of humor, on the other hand, is commonly waived.

Re:Not going anywhere... (1)

FirephoxRising (2033058) | about a year and a half ago | (#42774495)

Yes, you would need a rocket Cesna! Load up with LOX and rocket fuel and away you go, may need looong runway and perhaps a ski-jump to get airborne!

Re:Not going anywhere... (2)

foniksonik (573572) | about a year and a half ago | (#42774603)

Woosh!!!!! This low flying humor clearly gets no propulsion in all that hot air your blowing.

Re:Not going anywhere... (0)

Anonymous Coward | about a year and a half ago | (#42775675)

You can't hand-waive away physics.

No matter how much hand-waving you do, you can't hand-waive away your virginity.

Re:Not going anywhere... (4, Informative)

Rockoon (1252108) | about a year and a half ago | (#42774185)

Icebike is proving what I have previously pointed out about him. It is not important to him that he knows what he is talking about. Knowing what you are talking about is hard.

Re:Not going anywhere... (2)

turbidostato (878842) | about a year and a half ago | (#42775117)

"And get propulsion from a prop in an atmosphere of .6 that on earth?"

There're two things to consider:
1) Of course you get prop: it's a rotating wing, isn't it? So as long as there's any atmosphere, you'll get propulsion. Maybe your question was not about "propulsion" but about "enough propulsion", which gets us into point two.
2) Who said that "enough propulsion" needs to be produced exclusively by the main rotor? In the experiment another quite porwerful prop source is included: gravity. You just take even a pig at 30 Km over the Martian surface and you'll see how it gains speed even without revolving its pig tail.

The thougth experiment was not about flying a Cessna in Mars (and other objects) but about *how* it would *try* to fly over there. See, for instance, in Jupiter it would crush, but it wouldn't crash.

Re:Not going anywhere... (1)

Anonymous Coward | about a year and a half ago | (#42775575)

First, if you are going to say that multiple times, maybe you should get it right, as it is 0.6%, not 0.6 of the atmosphere.

Second, propellers still work in such an atmosphere, just not well. When NASA was considering propulsion methods for a powered aircraft on Mars, it came down to a choice between propeller based or rocket based. The former expected to give 3-5 times the range of using rockets if powered by an internal combustion engine (requiring both fuel and oxidizer to be carried). In one particular possible case, it was a range of about 2000 km.

Of course, this is of no relevance to what was in the article being discussed, where gravity was described as more important, but I guess the article wasn't relevant to what you said either.

Re:Not going anywhere... (0)

Anonymous Coward | about a year and a half ago | (#42774097)

RTFA before writing "FIRST".
Is an electric engine with batteries covering 10 minutes of flight.

Re:Not going anywhere... (4, Funny)

Warhawke (1312723) | about a year and a half ago | (#42774177)

FTFA: The motor is electric, and the fuel tanks are replaced with Li-Ion batteries. But I'll give you style points attempting to stifle scientific hypothetical inquiry and outside-of-the-box thinking with cynical non-imaginativism. Keep it up and you might win the scientific curmudgeon of the year award!

Re:Not going anywhere... (-1, Flamebait)

icebike (68054) | about a year and a half ago | (#42774301)

How fast do you think you can spin that prop? Because with 0.6 of the earth's air density at the ground on mars, no matter how fast you spin it, you will not achieve any usable propulsion.

"Scientific hypothetical inquiry" does not dismiss physics. Childish daydreams of youth, maybe, but not science. You might just as well think outside the box about how many fairies can dance on the head of a pin.

Re:Not going anywhere... (2)

Warhawke (1312723) | about a year and a half ago | (#42774417)

Dude, read the full article. Seriously.

Re:Not going anywhere... (2, Insightful)

Anonymous Coward | about a year and a half ago | (#42774513)

One of Einstein's what-ifs when something like this. Light moves at the same speed for all observers. So imagine a space ship travelling at near the speed of light relative to Observer A. Observer B is in the space ship and shines a laser straight up and back down off of a mirror. To Observer A the laser appears to move along the top of a triangle as the ship goes by at near light speed. Observer A sees the light travel a greater distance than Observer B. Both observers see light traveling at 299 792 458 m / s. So the only way this can be true is if time is moving slower for Observer B than A.

Feel free however to point that we don't know how to make a space ship go that fast. Point out the difficulty in actually observing such an event. Deride imagination.

I agree that those thought experiments are rubbish (0)

Anonymous Coward | about a year and a half ago | (#42776005)

For one thing they are all circular, or endlessly recursive.

1) The speed of light is constant and cannot be exceeded therefore
2) ...
3) Which proves that the speed of light is constant and cannot be exceeded therefore
4) profit?

Re:I agree that those thought experiments are rubb (2)

jonbryce (703250) | about a year and a half ago | (#42776435)

The speed of light is constant and cannot be exceeded, therefore
By implication, we must have time dialation depending on frame of reference
We can work out how much we would expect that time dialation to be
We have a testable hypothesis that could potentially be disproven by experiment on board Concorde or another fast aircraft.

Re:Not going anywhere... (2)

ridley4 (1535661) | about a year and a half ago | (#42774583)

Congratulations!
 
You seemed to forget the entire point of XKCD's what-if series is, in fact, taking childish daydreams and running with it. It's a bit odd, anyways, that a person who (begin rant) thinks a COTS laptop, in a shielded cabin in a magnetosphere-shielded environment using a tiny node size is every bit as radiation-hardened as a RAD750 with a 150nm node size to reduce susceptibility to smaller particles, with latchup-proof logic, parity-checked memory, etc etc. (end rant) is behaving as a physics expert to begin with.

Re:Not going anywhere... (2)

foniksonik (573572) | about a year and a half ago | (#42774625)

Flight does not require propulsion when gravity is pulling you to the center of a planetary body. It only requires lift. The examples all clearly state that the plane is dropped from a great height.

Mars plane by Boeing (5, Funny)

Anonymous Coward | about a year and a half ago | (#42773665)

"plane would fly pretty well, except it would be on fire the whole time"

I think Boeing has a plane that meets part of the criteria already.

Re:Mars plane by Boeing (-1)

Anonymous Coward | about a year and a half ago | (#42773741)

But would it be on fire? There ain't no free oxygen in the atmosphere of Venus, so how is the plane going to burn? The acid and heat will disintegrate the plane, but the combustible parts would probably turn into coal or oil instead of ash and CO2.

Re:Mars plane by Boeing (3, Funny)

mrbester (200927) | about a year and a half ago | (#42774519)

"on fire the whole time"

Typical. You go to all the trouble of flying a plane on Venus and all you get is petty criticism of minor teething troubles. There's no pleasing some people.

It's a nice analysis of a joke... (1, Insightful)

Gordonjcp (186804) | about a year and a half ago | (#42773669)

... but I think it went over his head.

X-Plane (5, Funny)

Ichijo (607641) | about a year and a half ago | (#42773705)

Your plane would fly pretty well, except it would be on fire the whole time, and then it would stop flying, and then stop being a plane.

It would be an X-Plane!

Re:X-Plane (0)

Anonymous Coward | about a year and a half ago | (#42773833)

http://www.x-plane.com/adventures/mars.html

Re:X-Plane (2, Informative)

Anonymous Coward | about a year and a half ago | (#42774191)

X-plane needs your help.

http://www.x-plane.com

https://petitions.whitehouse.gov/petition/make-patent-trolls-pay-all-costs-associated-their-frivolous-lawsuits-if-they-lose/gWPpVYMt

Ed

If your plane is on fire and not a plane anymore (2, Funny)

Lemming Mark (849014) | about a year and a half ago | (#42773711)

If your plane is on fire and not a plane anymore then you are having a bad problem. You will not fly on Venus today.

Re:If your plane is on fire and not a plane anymor (0)

Anonymous Coward | about a year and a half ago | (#42773829)

Wow, almost missed that one! http://xkcd.com/1133/ [xkcd.com]

Re:If your plane is on fire and not a plane anymor (1)

X0563511 (793323) | about a year and a half ago | (#42774209)

What's that based off of? Seems familiar.

Re:If your plane is on fire and not a plane anymor (1)

Dr Damage I (692789) | about a year and a half ago | (#42774527)

look for the xkcd comic on up goer 5

Re:If your plane is on fire and not a plane anymor (2)

DumbSwede (521261) | about a year and a half ago | (#42774303)

In case anyone misses your reference, here is XKCD's dumbed downed explanation for flying a Saturn V [xkcd.com]

This end should point toward ground if you want to go to space.

If it starts pointing toward space you are having a bad problem and you will not go to space today.

Re:If your plane is on fire and not a plane anymor (1)

PPH (736903) | about a year and a half ago | (#42774763)

You will not fly on Venus today.

I don't think your plane would actually catch fire. Melt, yes. But combust? Not enough oxygen in Venus' atmosphere.

Wrong Professor is Wrong (0, Troll)

VortexCortex (1117377) | about a year and a half ago | (#42773811)

with so little atmosphere, to get any lift, you have to go fast. You need to approach Mach 1 just to get off the ground

Sound moves at different speeds through substances with different densities. "Mach 1" is an Earth based terminology based on the speed of sound moving through our atmosphere. A vessel traveling at Earth Mach 1 speeds on Mars is not going Mars Mach 1; Mars has a faster Mach 1.

Re:Wrong Professor is Wrong (5, Informative)

Gothmolly (148874) | about a year and a half ago | (#42773853)

Mach 1 is the speed of sound - in that medium.

Re:Wrong Professor is Wrong (4, Informative)

WallaceAndGromit (910755) | about a year and a half ago | (#42774001)

Correct. Also, it's important to point out that the Mach number of a vehicle is a local measure of vehicle speed. As the speed of sound varies with temperature, and thus altitude, you'll find that two vehicles having the same trace ground speed but that are flying different altitudes will be at different Mach numbers. Acoustics and aerodynamics are fun.

Re:Wrong Professor is Wrong (2)

Gothmolly (148874) | about a year and a half ago | (#42775417)

Like I said - in that medium. Except you wrote something wordier so you gathered more mods.

Re:Wrong Professor is Wrong (1)

Anonymous Coward | about a year and a half ago | (#42773881)

The xkcd page said that, not the professor. I don't know if Randall meant Mach 1 on earth at some altitude, or Mach 1 on Mars. I just took it to mean "too fast for a Cessna."

Re:Wrong about speed of sound (0)

Anonymous Coward | about a year and a half ago | (#42774003)

For gases, density doesn't affect the speed. It's all about temperature (mostly) and whether the gas is monoatomic, diatomic, etc. (slightly)

sound propagates because molecules bounce off other molecules, and the speed at which those molecules move is determined by temperature.

When you get to where the gas is non-ideal, or where things are moving close to the speed of the molecules, you have to start taking into account stuff like whether the molecules are perfect little spheres bouncing around, or have a shape and can carry kinetic energy in vibration within the molecule or in the molecule's rotation.

The reason the speed of sound varies in Earth atmosphere with altitude is not because the density changes, but because the temperature changes.

Re:Wrong about speed of sound (0)

Anonymous Coward | about a year and a half ago | (#42774115)

Pssst, here's a little hint for you: Most gases actually increase their density as they drop in temperature, and it certainly does affect velocity, as anyone involved in aeronautics, ballistics(both direct fire(rifles etc) and indirect(artillery)) etc can inform you

Someone's who's had to calculate ballistic trajectories for bullets and artillery shells in ambient temperatures ranging from 50 celsius down to -55 celsius.....

Re:Wrong about speed of sound (1)

Deadstick (535032) | about a year and a half ago | (#42774397)

The temperature and composition of the gas are entirely sufficient to calculate the speed of sound. The previous poster is entirely correct.

Yes, density has a bearing on velocity -- not of sound, but of the vehicle -- because it affects drag.

Re:Wrong about speed of sound (0)

Anonymous Coward | about a year and a half ago | (#42774751)

You can increase the pressure at a constant temperature of atmospheric gas by several atmospheres and see at most a couple percent change in the speed of sound. When you get beyond 10 atmospheres, you might notices more than several percent change depending on the composition (e.g. with CO2), although with N2 and O2 you can go to 100 atmospheres of pressure at typical Earth surface temperatures before seeing much more than a 10-20% difference from constant with respect to pressure. Now if you were talking about pressures in atmospheres and temperatures down below -100 C, you would start to see big effects, but not so much with an atmosphere or less at warmer temperatures.

Re:Wrong Professor is Wrong (4, Funny)

glwtta (532858) | about a year and a half ago | (#42774101)

Not to mention when he says that Venus' upper atmosphere is "room temperature" - duh! rooms on Venus would have a very different temperature from Earth's rooms! What and idiot.

Re:Wrong Professor is Wrong (4, Funny)

Scarletdown (886459) | about a year and a half ago | (#42774215)

They also didn't point out that if attempting to fly in the Sun's atmosphere, you may last longer if you do it at night. :P

Re:Wrong Professor is Wrong (0)

Anonymous Coward | about a year and a half ago | (#42774179)

First off, the average molecular mass of the components of the martian atmosphere is about 50% higher than Earth's atmosphere, and the temperature varies from about the same to about half the absolute temperature on Earth. This basically means the speed of sound on the surface of Mars is somewhere between 30% slower and 50% slower. The pressure doesn't matter much, as at the lower pressures it is closer to an ideal gas where the speed of sound is independent of pressure. For a simple demonstration or what-if, a factor of two in that case won't matter much.

Second, the Mach number is not specific to Earth's atmosphere or even gases and can refer to just about any medium and movement within it. It is pretty common in use in plasma physics (although you would have to specify which wave velocity you are talking about, resulting in both a sound Mach number, and an Alfven Mach number).

Might be possible on Titan (3, Insightful)

MichaelSmith (789609) | about a year and a half ago | (#42773843)

Load liquid oxygen into the fuel tanks. Methane comes into the engine from the atmosphere. An engine with minor modifications might be made to operate.

Oh wondrous world! (0)

Anonymous Coward | about a year and a half ago | (#42773847)

And was this Cessna 3D printed on the planet?

dirigibles? (0)

Anonymous Coward | about a year and a half ago | (#42773897)

what about dirigibles for air exploration of mars?

Re:dirigibles? (2)

MichaelSmith (789609) | about a year and a half ago | (#42773909)

This is helped somewhat by the higher density of the martian atmosphere, in relation to its pressure. The density is pushed up by the low temperature and the higher density of carbon dioxide. OTH Mars is quite windy so your vehicle will get blown around quite a bit which could create hazards.

Torque? Gyroscopic Reaction? (0)

Anonymous Coward | about a year and a half ago | (#42773993)

The Cessna Skyhawk is propeller [air-screw] driven. The power that spins the prop generates torque-reaction, and the spinning prop makes a gyroscope. Bearing frictions will try to spin the aircraft with the propeller, in rotation direction around the propeller-shaft axis, where there is atmosphere and where there is not. Where there is atmosphere the propeller-blade pushing against the atmosphere resistance will induce a torque counter-reaction in the aircraft bolted onto the torque-loaded pushing air-screw blades. Gyroscopic reaction to the torque will induce the spinning propeller (and motor-rotor) gyroscope(s) will induce a force at 90 degrees to the gyro-plane. This at every instant. In atmospheres aileron and rudder will be able to deflect resisting air to counteract, but where atmosphere resistance is absent or insufficient the Cesna will tumble, as the torque-effect on the gyro-plane continues instant to instant, in every plane that becomes the gyro-plane.
What would be needed would be a compressed-air reaction-motor to push the Cessna and, through steering thrust-nozzles, to direct thrust up, down and sideways for stabilizing, and forward for braking.

owned! (-1)

Anonymous Coward | about a year and a half ago | (#42774249)

Wow this simple prof made xkcd look like chumps! I hope that someone is fired and xkcd says they are sorry.

Not the first time he's commented on xkcd (1)

Anonymous Coward | about a year and a half ago | (#42774257)

This physicist has been reading xkcd for quite some time, actually. He has written at least one other article about it, namely the click-and-drag world.

http://www.wired.com/wiredscience/2012/09/how-big-is-the-xkcd-click-drag-world/

Re:Not the first time he's commented on xkcd (1)

cervesaebraciator (2352888) | about a year and a half ago | (#42774331)

Hmm... maybe we should do a headline FTFY:

Physics Professor Gets Hits from XKCD Readers

Orbiter & Venus - no problem :) (0)

Anonymous Coward | about a year and a half ago | (#42774437)

Orbiter [ucl.ac.uk] has no issues with fling on Venus.

I remember successfully getting from Venus surface to low orbit with the delta cliper after fighting the super dense atmosphere for half an hour while almost running out of fuel in the process. :)

Re:Orbit (1)

v1 (525388) | about a year and a half ago | (#42774887)

I was thinking more just ORBIT. You first have to ask yourself what is the height required for the mass to orbit? (this is assuming you can start the plane out at any given direction, and you know the mass of the plane)

After that, anything starting slower, lower, or weighing more will need to do some sort of powered flight to stay up.

If you start a little bit below orbital requirements, the demand will be very minor. The farther down you go, the slower you start, or the heavier you are, the more demand there will be. It's not a yes/no thing. It could be a system just off equilibrium.

Unless you have the scenario they described with the sun, where you don't hit atmosphere until you are well inside orbit (unless you have some impressive speed)

So I guess what I'm saying is that the starting conditions (speed, heading, elevation) are at least as important as the planet you are trying to fly at. Since they somewhat arbitrarily picked them, the resulting comparisons are equally arbitrary. And not by an insignificant amount.

That being said, revisit the starting conditions. If we say you must start with your speed, heading, and elevation, so that you are say, 10% below orbital speed, it becomes a question of atmosphere - "can you sustain flight?", rather than "can you pull out in time?". Can you generate enough lift to achieve... not sure what to call it... stable sub-orbital trajectory? I suppose that's the best definition of "powered flight". aka "straight and level", with an "at stable speed" thrown in for good measure.

Numbers become more critical as your ability to generate lift is lowered, or as that 10% below orbit is raised. At some point it will become a question of whether or not you have time to pull out of the dive. (or whether it's even possible, assuming there's no ground) And then we get into what your "ceiling" would be - the highest altitude you can climb to, where you finally level out while trying to climb, equilibrium of climb. It's interesting to ponder that most craft have TWO ceilings... one is their orbit, and the other is somewhere below that. So what we may really be asking here is, "does the craft have a ceiling other than orbit?"

Although that article does dig a bit deeper than xkcd did, it's still quite a long way from the goal.

Picking nits (3, Interesting)

PPH (736903) | about a year and a half ago | (#42774821)

If that professor wants to pick nits with xkcd, the path an object follows while falling in a vacuum isn't a parabola. Its an ellipse. In most cases, the ellipse intersects the surface of the body being orbited in what is typically referred to as a crash. But if one is considering dropping the object (with some forward velocity) above a small enough body, the distinction becomes important.

Re:Picking nits (1)

PAKnightPA (955602) | about a year and a half ago | (#42775989)

Are you sure this is right? It has been a while since I took physics, but I tried to derive this. It would seem x = t, y = C - kt^2 which is definitely a parabola... What am I missing? I'm genuinely curious...

Re:Picking nits (-1)

Anonymous Coward | about a year and a half ago | (#42776069)

That's a nice, juicy, and tight asshole you have there. It looks like it's waiting for a cock to take a vacation inside it. Does that cockyness by chance belong to me?

Re:Picking nits (0)

Anonymous Coward | about a year and a half ago | (#42776127)

IIRC, you need to model the planet as a single gravitational point. Your equation has a simplifying assumption, namely that gravity only operates along the Y axis.

Re:Picking nits (0)

Anonymous Coward | about a year and a half ago | (#42776237)

That's a simplified equation assuming a flat Earth with constant gravity, where F = mg. That works perfectly fine when you're talking about something really big that you're relatively close to. But, the more accurate equation is of course F = G m1 m2/r^2, and that gives you an ellipse.

Re:Picking nits (1)

Anonymous Coward | about a year and a half ago | (#42776159)

If you want to nitpick, the curve of a free falling projectile is a conic section. Depending on initial conditions it may be a circle, an ellipse, a parabola or hyperbola.

Re:Picking nits (1)

Donwulff (27374) | about a year and a half ago | (#42777031)

Actually I would like to nit picks with the professor in that the starting conditions of the flight are specifically not stated. As the professor himself says, "Randall doesn’t explicitly state the starting conditions for the Cessna, so let me guess that it starts off 1 km above the surface with a speed of 60 m/s." With different values for the starting speed, different results will be obtained. In the graph where he shows Randall's and his calculated trajectories in one, he's specifically not "provide more accurate paths", but is in essence pointing and laughing at "Look how ridiculously off Randall's math is".

But he's WRONG. Naturally they're both ridiculously, utterly wrong because I'm so much smarter than either of them. It's obvious even to a child the starting speed should be 0m/s, and thus the trajectory for all of the cases without atmosphere will resemble the trajectory of a rock falling from the sky. (Although, are we drawing the trajectory in reference to the planetoids surface, in reference to the planes starting point, or perhaps in distant Earth's reference frame? In each case the trajectory will be different, and as the professor himself notes, "Oh – you might notice that I have not looked at the radius of curvature for the planets. You can do that for a homework assignment if you wish.")

Steak Drop (1)

martin-boundary (547041) | about a year and a half ago | (#42775137)

For those who didn't RTFSteak blog entry, here's a summary of the analysis:

Drop the steak from orbit, it's the only way to be sure.

I love xkcd :)

I'm hoping the prof is watching (1)

the_Bionic_lemming (446569) | about a year and a half ago | (#42775789)

On a drunken beerday many many years ago, i postulated that a cessna flung at .99 c (just under the speed of light) striking the earth would probably destroy it.

I was ridiculed, maybe rightly, but if we're running physics and math here, what would the effect be?

Re:I'm hoping the prof is watching (2)

slimjim8094 (941042) | about a year and a half ago | (#42775911)

This other what-if actually addresses this pretty well: http://what-if.xkcd.com/20/ [xkcd.com]

What If? (1)

ProzacPatient (915544) | about a year and a half ago | (#42775977)

I love this series. The scenarios that he works out are so absurd it's hard not to be laughing the whole time while reading (and visualizing) Randall's explanation. I had a hard time keeping it together while imaging a giant rain drop dropping down on one house or imaging someone dropping a steak from space for the purpose of cooking.

But Why... (0)

Anonymous Coward | about a year and a half ago | (#42776269)

I for one, think that its utterly fantastic that you spent so much time disproving a cartoon.

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