7
$\begingroup$

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.

Improve this question
$\endgroup$
  • 10
    $\begingroup$ 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. $\endgroup$ – Simon May 9 '17 at 19:18
  • 5
    $\begingroup$ 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? $\endgroup$ – Simon May 9 '17 at 19:33
  • 6
    $\begingroup$ 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.) $\endgroup$ – Pondlife May 9 '17 at 19:39
  • 10
    $\begingroup$ Your profile says that you are an aerospace engineer, didn't they cover this in your structural engineering courses? $\endgroup$ – Ron Beyer May 9 '17 at 20:05
  • 4
    $\begingroup$ @Koyovis: And she'll be perfectly fine if she does know, too. $\endgroup$ – jamesqf May 13 '17 at 4:54
12
$\begingroup$

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.

Improve this answer
$\endgroup$
7
$\begingroup$

•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.

Improve this answer
$\endgroup$
  • 1
    $\begingroup$ I don't think a balloon has more pressure than an airplane. Up to 8 or 9 psi is common for a pressurized fuselage. $\endgroup$ – fooot♦ May 10 '17 at 20:57
  • 3
    $\begingroup$ Your last point is good. Most of the safety is operational. Both airplanes and cars are very mechanically reliable. $\endgroup$ – fooot♦ May 10 '17 at 20:59
  • 2
    $\begingroup$ 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) $\endgroup$ – Manu H May 11 '17 at 16:16
  • 2
    $\begingroup$ 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. $\endgroup$ – Sanchises May 18 '17 at 11:03
  • 1
    $\begingroup$ 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. $\endgroup$ – Shawn May 19 '17 at 18:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.