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Flying a Cessna On Other Worlds: xkcd Gets Noticed By a Physics Professor
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."
The upshot is: Your plane would fly pretty well,
Let me know when you get that Cessna engine started on mars.
Sig Battery depleted. Reverting to safe mode.
"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.
... but I think it went over his head.
It would be an X-Plane!
Any sufficiently unpopular but cohesive argument is indistinguishable from trolling.
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.
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.
Wow, almost missed that one! http://xkcd.com/1133/
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.
http://michaelsmith.id.au
And was this Cessna 3D printed on the planet?
what about dirigibles for air exploration of mars?
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.
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.
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.....
What's that based off of? Seems familiar.
For large sets, this will be our guide even unto death, for the LORD will work for each type of data it is applied to...
Wow this simple prof made xkcd look like chumps! I hope that someone is fired and xkcd says they are sorry.
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/
Letter To Iran
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.
Orbiter 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. :)
look for the xkcd comic on up goer 5
"Cursed is he who rises early in the morning..." Isiah 5:11
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.
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.
Have gnu, will travel.
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.
Have gnu, will travel.
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.
I work for the Department of Redundancy Department.
Drop the steak from orbit, it's the only way to be sure.
I love xkcd :)
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
http://www.geoffreylandis.com
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?
_ _ _ Go for the eyes Boo! GO FOR THE EYES!
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.
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?
I for one, think that its utterly fantastic that you spent so much time disproving a cartoon.
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.
The two body problem has been solved for hundreds of years and it is one of the foundational results in physics. A lack of familiarity with it is damning. Being pedantic and obnoxious while proving you have no idea what you are talking about is unforgivable.
Use Google Reader; when viewing the comics in Reader you can longpress the image for alt text.
The "Tracker" video analysis app download page TFA links to is down. In fact the whole server www.cabrillo.edu is down.
Coral cache: http://www.cabrillo.edu.nyud.net/~dbrown/tracker/ -- even the installer packages are cached!
On Mars the Cessna wouldn't have enough lift, so you'd make a plane with a much better power-to-weight ratio by using thin carbon fiber delta wings to increase the effective area of the lift surfaces.
Or use a digirible.
Tubby or not tubby. Fat is the question
One mistake in the explanation text : Mars and Titans are not the only bodies whose surfaces we have pictures of: there is also Venus. The Russians sent a number of probes there, which took a number of pictures :
http://www.mentallandscape.com/c_catalogvenus.htm
What if it was made of sodium?
Confucius say, "Find worm in apple - bad. Find half a worm - worse."
What?
And that's not what I meant. It was in a flight manual or something. Various conditions that ended with "you will not be X today"
For large sets, this will be our guide even unto death, for the LORD will work for each type of data it is applied to...
I did not know that, very interesting. Thank you.
http://xkcd.com/1053/
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.'
Exactly like my first wife. The sex was great at first, and she was totally on fire, then the sex stopped and she stopped being a human being!!!
Sure enough, the cow costume was hanging up next to the superhero outfit and sailors uniform. (S,Spud)