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Aircraft skin is not very thick at all, yet air travel is the safest form of travel. All aviation professionals know why, but most occupants of an airliner are not professionals. How can it be explained to members of the general public that 1 mm is enough to:

  • Keep the air pressure in.

  • Keep the wings from breaking off.

  • Transport the passengers in greater safety than when they travel in their car.

I've used examples like "you can stand on a full beer can but not on an empty one". What are the secrets of aeroplane construction that makes such a seemingly flimsy construction so safe?

Update

How is a stressed skin construction explained to the general public? Lots of non-experts get quite alarmed when they are thinking of crushing a beer can.

Koyovis
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    That is a leading question. What leads you to believe that it is not possible? What is it about 2mm thick that leads you to believe otherwise? it's possible because 2mm is all that is needed. – Simon May 09 '17 at 19:18
  • Related : https://aviation.stackexchange.com/questions/12200/are-modern-airliners-still-stressed-skin-aircraft – selectstriker2 May 09 '17 at 19:22
  • @ Simon I remember telling my sister about 1-2 mm skin, her turning green, and then having to talk for a long time before she appeared to not be unsettled again. How would you explain to someone not familiar with aircraft construction? – Koyovis May 09 '17 at 19:23
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    Most aircraft skin is not that thick. For example the Lunar Module, the thing that carried astronauts to the moon, had skin that was only 0.38mm thick (0.015"). – Ron Beyer May 09 '17 at 19:25
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    I genuinely don't understand. Unless you are able to say "2mm seems not enough because xyz", without stating what xyz is, how can the answer be anything except "it's only 2mm because that's all that is needed". Most car body panels are thinner than that. Why is it safe to get into a car? – Simon May 09 '17 at 19:33
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    Q: If black boxes can survive anything, why not make the whole aircraft from the same material? A: The roads aren't wide enough for aircraft to drive on. In other words, the aircraft has to be light enough to actually fly. Any form of engineering has trade-offs, and weight vs. strength is one of those decisions. But as you said yourself, in commercial aviation the decisions have obviously been very successful from a safety point of view. (Not to mention fuel economy, construction costs etc.) – Pondlife May 09 '17 at 19:39
  • @Simon car travel is less safe than air travel, isn't it? Yet you very seldomly hear about people with Fear Of Driving. – Koyovis May 09 '17 at 19:44
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    @Koyovis I do, when I'm the passenger and my wife is driving! :) – Steve May 09 '17 at 19:53
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    Your profile says that you are an aerospace engineer, didn't they cover this in your structural engineering courses? – Ron Beyer May 09 '17 at 20:05
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    Yes, and that's exactly the point. it seems like what you are actually asking is "why do people have a fear of flying". This is an irrational fear, since if you apply logic and rationale, then the only possible conclusion is that the safest part of any journey involving flying begins when you get on the aircraft and ends when you get off it. I cannot understand why a person who is not rational would be worried about the thickness of the skin unless they explain why. – Simon May 09 '17 at 20:15
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    Another fun fact: before they used aluminum, an airplane was wood and cloth. :-) – Shawn May 10 '17 at 20:02
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    Stand on an empty beer/soda can and show them how much weight it can support. Then, while standing on it, quickly poke in both sides with your fingers and watch how quickly it collapses. Now, build a new empty can with an internal support structure (just like an airplane's) and show that it won't collapse despite a dent in the skin. That's why airplanes are so tough/safe despite the fact that their skin is so thin. – FreeMan May 12 '17 at 14:49
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    Re "How is a stressed skin construction explained to the general public?" The answer, of course, is that it's not, any more than the general public understands the physics of a cell phone or an LED. That's what engineers are for :-) – jamesqf May 12 '17 at 17:50
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    @jamesqf Yes you're right, as long as aunty does not know that there is only 1 mm between her and the troposphere on her flight to Ibiza, she'll be perfectly fine. – Koyovis May 12 '17 at 20:29
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    @Koyovis: And she'll be perfectly fine if she does know, too. – jamesqf May 13 '17 at 04:54
  • @Koyovis: But a big part of the reason why car travel is less safe than air travel has to do with all the other idiots on the roads. If airplanes had the same percentage of mid-air collisions as cars have multi-vehicle accidents, it'd be a much different story. – jamesqf May 14 '17 at 05:30
  • @jamesqf Indeed. On the other hand: per collision, an aircraft carries much more persons, travelling at speeds and altitudes that cars rarely reach. – Koyovis May 14 '17 at 08:21
  • @Koyovis But even in those conditions, stats for air travel show it to be significantly safer. Older article: http://traveltips.usatoday.com/air-travel-safer-car-travel-1581.html - In 2008: odds of dying in a motor vehicle accident = 1 in 98 for a lifetime; air and space transport = 1 in 7,178 for a lifetime. – Shawn May 18 '17 at 19:04
  • I'll add another comment so that this set of comments is flagged for automatic removal. – Koyovis Nov 25 '17 at 20:15
  • And another one. – Koyovis Nov 25 '17 at 20:16

2 Answers2

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Simple maths. Cut a cabin in two, lengthwise, and check the forces involved. The halves are held together by 4mm of aluminium, and need to withstand a pressure differential wrt to the outside over the diameter of the cabin.

Let's assume that we have a wide body plane of 6m diameter (larger than a Boeing 777), and a pressure differential of 1atm (of course, usually a plane only maintains up to 0.8atm and does not fly in a vacuum, but let's have some safety margin).

The resulting tensile stress is then about 150MPa. Aluminium only yields at 275MPa.

Other forces are carried by struts, spars and in the case of the fuselage, stringers. The skin transfers shear forces between those perpendicular stringers, but I do not think those forces are significant compared to pressure loading.

Sanchises
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•Keep the air pressure in. >> How thick is the skin of a balloon? That keeps in a lot more pressure than an airplane.

•Keep the wings from breaking off. >> That's a little more than just the skin. There are supporting beams to hold the bulk, but wrapping them in the skin strengthens that as well.

•Transport the passengers in greater safety than when they travel in their car. >> Practice, practice, practice. It's only takes a few hours to teach someone how to fly an airplane. But the hours and hours of training and flying after that are what builds a safe pilot who can properly deal with adversity. Plus, there's a lot less congestion in the sky than on the roads.

Shawn
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    I don't think a balloon has more pressure than an airplane. Up to 8 or 9 psi is common for a pressurized fuselage. – fooot May 10 '17 at 20:57
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    Your last point is good. Most of the safety is operational. Both airplanes and cars are very mechanically reliable. – fooot May 10 '17 at 20:59
  • @foot Yup. You are correct. It takes >1psi differential to inflate a balloon, which means about 15-16psi to overcome the elasticity of the balloon. But once inflated, the internal pressure is only about 1 atmosphere, with a lot of that attributed to said elasticity. However, that air isn't being let out like it is in an aircraft pressurization system. You aren't really inflating the aircraft; since you are climbing and decreasing the outside pressure, you're trying to keep the inside pressure at a human-compatible range. Regardless, your inside pressure ... – Shawn May 10 '17 at 21:28
  • ...will be higher than outside pressure, so your airplane would have a bit more pressure inside. I guess my point would be more comparable to the inflation of the balloon. The skin of the balloon can withstand a good bit more internal pressure during inflation without bursting. But an aluminum skin has a lot less elasticity. I kinda oversimplified a somewhat complex pressurization issue. :-/ – Shawn May 10 '17 at 21:33
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    Maybe you should add to your last point systematic incident and accident analysis and continuous training (in many country you pass your driving license once and it is valid for your entire life whatever the evolution of your car or your health) – Manu H May 11 '17 at 16:16
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    Your balloon analogy is false, as is your explantation in the comments. Elasticity is of negible influence in stresses in a thin walled pressure vessels (although not in a balloon, due to large deformations and high Poisson ratio in rubber). Max pressure differential in a balloon is about 0.7 psi. Finally, the aircraft skin does not care about whether air is "let out", only about the pressure differential between inside and outside. – Sanchises May 18 '17 at 11:03
  • "I kinda oversimplified a somewhat complex pressurization issue." Plus, I wasn't very clear in my explanation, I guess. 14.7 psi (sea level pressure) + about 1 psi to inflate it = 15-16 psi inside the balloon. I don't think the balloon analogy was false; just not as applicable as I originally thought. There is a bit of physics involved in balloon pressure. However, aircraft skin absolutely does care whether or not pressurization air is "let out". Since new air is constantly coming in, the old air has to be let out. That's how the pressure differential is controlled. If you continue to put .... – Shawn May 18 '17 at 18:31
  • ... air into a balloon without letting some out, it will eventually burst. So I guess a more appropriate experiment would be to measure the pressure inside an inflated balloon as it approaches the bursting point. And that would depend on the balloon's skin. Again, that's a lot more physics than the initial simple explanation I assumed. – Shawn May 18 '17 at 18:33
  • @Shawn Unless you're planning on taking a balloon to high altitude (where it will inevitably burst), I still don't see how the analogy holds - the balloon does not care about absolute pressure, only about the difference between inside and outside. And I meant to say that the skin does not care if a certain pressure is achieved through a continuous inflow or by shutting the outflow. I think you're not oversimplifying, but the opposite; balloons are actually a very tricky, highly nonlinear problem, behaving very counterintuitive (blowing in more air decreases the pressure!) – Sanchises May 19 '17 at 13:21
  • @Sanchises I think we may be getting way off into the weeds here, and maybe I misunderstand the physics of inflating a balloon, but when any pressure vessel reaches the limit of its elasticity, it will burst if the pressure differential increases and the strength of that vessel's walls is exceeded. Is that not why a high-altitude balloon will burst if it keeps going? The skin cares that the pressure differential is greater than it can hold in. ... – Shawn May 19 '17 at 18:03
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    Regardless, atmo pressure at sea level is about 14.7 psi. If you're flying at 36k ft, atmo pressure is 3.3 psi. If you pressurize your aircraft cabin to 7000 ft, that's 11.3 psi inside the aircraft. Differential is 11.3-3.3 = 8 psi. So 8 psi is being exerted on the pressure vessel.

    I'm not sure what additional pressure it would take to make a balloon burst, but I doubt it would get to 8psi before burst pressure is reached. I was wrong above. A balloon doesn't really make a good example. And I can't think of another way to explain a thin-walled pressure vessel.

     – Shawn May 19 '17 at 18:09
  • http://aerosavvy.com/wp-content/uploads/2015/05/altitude-graph.jpg – Shawn May 19 '17 at 18:09
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    @Shawn Exactly. My whole point was about "Keep the air pressure in. >> How thick is the skin of a balloon? That keeps in a lot more pressure "; so I think we're on the same page now. I can't really think of an intuitive example for a pressure vessel either - perhaps a piece of paper loaded only in pure tension is a passable example (try and rip that apart without tearing). Other small pressure vessels (boilers, etc) are generally very over-engineered to account for out-of-plane loading (dents, etc). – Sanchises May 19 '17 at 21:25