June 04, 2006

Flying Machines 

One thing I realized debating conspiracy theorists is that a lot of people out there are desperately clueless as to how airplanes work and what features they have. No wonder, then, that they will believe the most absurd claims regarding aircrafts.

Aircrafts are designed and built to fly.
This may seem stating the obvious, but in reality almost all features of airplanes (let's exclude helicopters, blimps and rockets) are a direct consequence of their intended use. In engineering, form follows function.

So let's examine in a little more detailed manner these flying machines. A plane is heavier (or better, denser) than air, so in order to make it fly an upward force at least equal to its weight must be provided. This force is calleft lift, and it's generated by the wings. The conventional explanation for lift, that uses the Bernoulli equation, is maybe a bit simplistic - but there is no doubt that a wing (airfoil) produces lift by deflecting the airflow around it.

The airflow is generated in turn by applying a horizontal force to the plane accelerating it to a considerable speed (that varies amply with plane type; jetliners need at least 200 - 300 km/h to produce enough lift) against air resistance. There are quite a few types of aircraft engines, but the propulsion means are propellers and jets. Propellers pull the plane forward by biting into the air rather like a corkscrew - in some rare cases, propellers are used to push; jets and their cousin rockets push forward by expelling a jet of high velocity gases at their aft end. Nowadays, most propellers are powered by turbine engines, but piston engines still survive - it's not the age of the Merlins anymore, tho.

A plane needs other aerodynamic surfaces to achieve a stable flight and be steered around, and these are the very basics of aircrafts. If you want to know more, there are entire encyclopaedias on the subject.

It should be clear now that airplanes need to be as light as possible (also in order to carry more payload) for their size, but also strong enough to carry the loads of normal operations (an insidious one is pressurisation) and exceptional circumstances such as turbulence. Aeronautical engineers have found an elegant solution for this problem: airplanes are made of light aluminium alloys (and polymers, these days), in the form of thin panels riveted and glued onto a rib and spar structure. Both the outer skin and frame carry part of the loads, and together they provide the required strength. This structure is used for the whole plane - fuselage, wings, tail. The outer skin is really thin, 3 - 5 mm, but to appreciate this fact you should visit the Science Museum in London and look at the slice of Boeing 747 displayed there: it's really like a flying soda beer can.

The only sizable steel parts are in the landing gear; turbine engines instead are made in great part of titanium alloys; nickel superalloys and even more advanced ceramics specially processed are used for the turbine blades. Titanium alloys have the interesting property of being as strong as steel, but with 2/3 of the density and being more resistant in certain chemical environments; titanium is not invulnerable or indestructible and when pure melts at a modest 1668 C.

Given that the interior of the fuselage is best used for essential equipment and payolad, jetliners carry most of their fuel in tanks inside the wings - though there are tanks in other locations, and even the fuselage could be used for that purpose. If you can fork out the money, manufacturers will meet almost all of your needs.

Airplanes are not inteded to be crash-resistant. It is not really because the greedy Big Air does not care about the passengers; considering the operative speed (200 - 900 km/h) and environment (up in the air) of airplanes, making them crash-resistant to the same level of cars would render them so inefficient not to be worth the hassle. And it's not that cars and other road vehicles are completely crash-resistant, anyway: road accidents kill and maim several thousand people each year in Italy alone.

We can consider the Merkava 3 main battle tank as a rather resistant vehicle, but you will also notice that it weighs 65 000 kg and travels at no more than 60 km/h, having a thick high-strength steel (and probably also ceramic) armor.

Actually, at barely 4 m wide and 9 m long a Merkava 3 is about as heavy as an MD-80 jetliner, which is 45 m long and has a 33 m wingspan. Airplanes have few analogues among earthbound structures; Formula 1 cars may be the less dissimilar, and they are not famous for being extremely resilient.

Aluminium alloys are generally less ductile than steel (fact that can be verified by bending one steel and one aluminium rod), and when airplanes impact something, they break up. Yes, this is what happens: when they crash, airplanes fragment in pieces that can be quite small if the crash occurred at high speed - and often are so badly burned up in subsequent fires to be barely recognizable.

Well, that's enough for today anyway.


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