Wikipedia:Reference desk/Archives/Science/2015 November 29

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November 29

micro coils

microcoils advantages and disadvantages — Preceding unsigned comment added by 141.70.81.149 (talk) 13:37, 29 November 2015 (UTC)

Are you asking about micro coils for use in electronic cigarettes? -- ToE 17:06, 29 November 2015 (UTC)

The OP may be asking about the microcoils used in Nuclear magnetic resonance spectroscopy, but the question does look rather like homework. In any case, clarification is needed. Tevildo (talk) 21:07, 29 November 2015 (UTC)

Aircraft in weightlessness

How would civilian aircraft like Airbus or military planes with working engines theoretically behave in outer space weightlessness? Would they roll, bank or still be pushed forward by engines?--93.174.25.12 (talk) 19:13, 29 November 2015 (UTC)

Nothing theoretical about it, they would not do any of those things, as they all require atmosphere. Furthermore, I would strongly suspect it would explode as the pressure differential would be far greater than the plane was designed for. Fgf10 (talk) 19:56, 29 November 2015 (UTC)

The second point isn't correct - the pressure at 15 km altitude (rather more than a typical airliner's cruising altitude, but still within the range of normal aircraft) is only about 10% the pressure at sea-level, so going down to vacuum wouldn't put a significant amount of extra loading on the airframe. However, the interior of the plane would (fairly) quickly become evacuated, as it's pressurized from the outside air rather than from oxygen tanks as on a spacecraft. See International Standard Atmosphere and Cabin pressurization. Tevildo (talk) 21:22, 29 November 2015 (UTC)

Good point well made, posted off the cuff and didn't think things through. Fgf10 (talk) 22:15, 29 November 2015 (UTC)

I think our OP is confused and is conflating two completely different properties: first, objects in orbit experience weightlessness (or "microgravity") because they are essentially in freefall; and secondly, outside of a planetary atmosphere, space is a near-perfect vacuum (except for a very very tiny quantity of stray gas particles). For your reading: here is an introduction to the space environment, published by the European Space Agency, to help you frame your understanding. Here's what I consider to be a slightly better introduction to the space environment published by NASA, but our OP mentioned Airbus, so I presume they're interested in European perspectives...

Aircraft, by their very nature, are designed to depend on air flowing over their flight control surfaces for control and stability, and for the correct functioning of the air-breathing jet engines that are common to most large modern airliners. A handful of experimental and high-performance aircraft - like the North American X-15 and the Lockheed F-104 Starfighter - had reaction control systems to supplement their aerodynamic control surfaces - so those vehicles could have operated and maneuvered correctly "above the atmosphere," so to speak.

Nimur (talk) 20:26, 29 November 2015 (UTC)

Quibble: the standard F-104 had no RCS, only the NF-104A variant had. Fgf10 (talk) 21:10, 29 November 2015 (UTC)

Sp, to summarize:

• The engines require oxygen to burn the fuel - and air is used as a reaction mass - so the engines would not work.
• The control surfaces work by deflecting the airflow over the wings and tail - in a vacuum, there is no airflow - so they wouldn't work.
• The aircraft could probably withstand the increased pressure differential - so it wouldn't explode.
• But the cabin isn't air-tight, it's continually re-pressurized in flight - so the air inside would soon flow out of the aircraft and kill everyone aboard. It's not clear how long this would take. My guess is that it would take a few minutes - but it's hard to know for sure.

So the aircraft would drift helplessly, possibly spinning out of control - and everyone aboard would die within minutes. SteveBaker (talk) 02:51, 30 November 2015 (UTC)

"How would civilian aircraft like Airbus or military planes with working engines theoretically behave in outer space weightlessness?" If you assume the bolded parts mean "if the engines magically still burn despite being in a vacuum" (for example if the place carried its own air supply to feed into the jets), then the engine would provide thrust and the aircraft would move forward. (At a very basic level, a jet engine and a rocket engine work on the same principle. The difference being that a rocked carries its own oxidizer but a jet doesn't). The other points still stand - it would have no means of steering, so it would either continue in a straight line, or spin/tumble (depending on how balanced the thrusters are). Iapetus (talk) 11:09, 30 November 2015 (UTC)

Well, even if you hypothetically replace "jet engines" with "magic boxes that produce thrust," you still have a stability problem: aircraft are less sensitive than spacecraft with respect to the alignment of their thrust vectors with the center of mass. In an atmosphere, the aerodynamics serve to counter those torques; aerodynamic forces are exerted and act around the center of pressure. In a vacuum, those aerodynamic forces are all net-zero (because there is no air flow to exert a force). Spacecraft must be carefully designed so that their thrust does not induce a parasitic net torque. These concepts are described in our article on reaction control systems. An airliner, designed for atmospheric flight, is not mass-balanced for flight in near-vacuum: its engines are in the wrong place, and their thrust is pointed in the wrong direction, among the many other design problems. Nimur (talk) 12:11, 30 November 2015 (UTC)

I suggest you read Star Smashers of the Galaxy Rangers to see how a 747 behaves as a spacecraft in space. I read it when I was a kid and I believed that 747 can fly in outer space. 175.45.116.59 (talk) 04:53, 30 November 2015 (UTC)

If space DC-8s were good enough for Xenu, they're good enough for me! --71.119.131.184 (talk) 06:50, 30 November 2015 (UTC)

Those are fictional stories; please be sure to call them out as such. They may have cultural or entertainment value, but they aren't encyclopedic references for answering science questions. Nimur (talk) 12:11, 30 November 2015 (UTC)

Lest anybody think Harry Harrison was scientifically illiterate, Star Smashers . . . was an intentionally silly parody of the Skylark stories by E.E. 'Doc' Smith from the 1920s (which were themselves somewhat scientifically absurd). {The poster formerly known as 87.81.230.105} 185.74.232.130 (talk) 15:11, 30 November 2015 (UTC)

• The OP seems to be asking what would happen to a plane if all else was normal, but the gravity were turned off. I think the use of "outer space" was intended to mean "as in outer space", not actually in outer space. If that's the case, then the plane would begin to rise as it flew, since the portion of lift used to counteract gravity would be added to any lift used for an intentional gain in altitude. At this point my knowledge of the subject fails, but I wonder if the airplane would not simply do loop-the-loops. μηδείς (talk) 02:10, 1 December 2015 (UTC)

I think that if the gravity were to be turned off with a dense atmosphere the steering (flight control surfaces) would still function. 122.109.117.69 (talk) 04:45, 1 December 2015 (UTC)

There's nothing theoretically impossible about setting up a big bag of air in outer space - for example, if you have one of those giant rotating space colonies set up with a big empty space in the middle. Weightlessness isn't airlessness! I assume under such a circumstance a plane would fly, and that it would be something like halfway between flying right-side-up and flying up-side-down. But I've read that commercial jets (which?) aren't really meant to fly upside down, so I regard the question as open and interesting. Wnt (talk) 14:11, 1 December 2015 (UTC)

Yes, if the OP's question is "Can normal aircraft fly in zero gravity?", the answer is "Yes". See Reduced gravity aircraft (which are normal aircraft flown so as to experience zero g). Tevildo (talk) 19:07, 2 December 2015 (UTC)

The wings would still cause a force perpendicular to the direction the engines were propelling the aircraft. --122.111.248.122 (talk) 13:00, 3 December 2015 (UTC)

Yes, but the aircraft will still have sufficient elevator authority to maintain level flight. See Flight envelope - what we're doing in a weightless environment is moving the envelope up by 1g, so the stall speeds (etc) will change, but the aircraft will still be controllable. I'm not sure how easy it would be to fly a helicopter in zero g, but that wasn't the original question. Tevildo (talk) 14:10, 3 December 2015 (UTC)

Blood pressure and exercise

If anxiety causes dangerous temporary spikes in blood pressure, then why can't you argue the same about exercise? Doesn't exercise also cause temporary spikes in blood pressure? And what about thrill seeking activities such as Bungy jumping. Could you not argue the same for those? 2A02:C7D:B901:CC00:9597:AF2C:2F79:8F40 (talk) 22:22, 29 November 2015 (UTC)

Well... Yes? Sure, but what's the point of your question? For an average, "healthy person", temporary increase in blood pressure caused by exercise or bungee jumping, or even some anxiety, should not be "dangerous", it's a "normal" part of life. For a spike in blood pressure to be dangerous suggests the person suffers from some preexisting condition for which high blood pressure is a risk factor. Vespine (talk) 01:14, 30 November 2015 (UTC)

The mechanisms are different. Vasoconstriction increases blood pressure while blood velocity decreases it. The temporary pre-existing condition of "alive" can change to the permanent condition of dead through many different pathways. It makes us feel better to classify out-of-the-blue deaths of otherwise healthy people to an unknown pre-existing condition but I think that is more of a "feel-good" measure. A better metric is the statistics correlating death with an activity and treating it as random chance rather than searching for causation. --DHeyward (talk) 03:47, 30 November 2015 (UTC)

If you die out-of-the-blue, and it wasn't from an external cause such as trauma or poisoning, it's not really "normal". Your body is "supposed" to be able to deal with fairly routine things like short-term spikes in blood pressure. Determining the cause allows us to try to prevent future deaths from the same cause. For instance it used to just be more or less accepted that some people would inevitably die early from cardiovascular diseases, but decades of research (such as from the Framingham Heart Study) have allowed us to learn about the underlying mechanisms, like chronic hypertension, that promote said diseases. In more acute matters, AEDs can save many lives if they're available when needed (like when you get hit in the chest at just the right moment in your heart rhythm). --71.119.131.184 (talk) 07:08, 30 November 2015 (UTC)

The problem with stress/anxiety is that the changes aren't all that temporary. That is, if it was only for a few minutes, little damage would be done. But stress that lasts for weeks, months, or years takes a toll on your body. I do, however, think it is possible to get too much exercise, where that can damage the body too. Professional bodybuilders are one example. I'd like to see stats on how long they live, compared with the average. StuRat (talk) 13:41, 30 November 2015 (UTC)