In my mind he didn't address the fundamental reason why the wing visibly oscillates while most other structures that you trust your life to like buildings, bridges, and cars do not. Airplanes are designed with a 1.5 factor of safety<p>A factor of safety is applied to a design after every load the structure will be subjected to is calculated, they multiply by 1.5 to be sure the structure will be safe. 1.5 may sound conservative but it is the smallest factor of safety someone normally encounters, spacecraft and fighter jets might use 1.2 while cars often use 3; buildings and bridges use 5. Airplanes are used much closer to the strength limit of their materials so we see them flexing.<p>Of course everything deflects under load - my strength of materials instructor illustrated this by analyzing how much an anvil compresses when a fly lands on it; answer: less than can be measured.
On a tangent, one thing that <i>slightly</i> freaks me out a bit is that on the ground, the wings hang from the plane (wings bend down); whereas in the air, the plane hangs from the wings (wings bend up).
I've been wondering why passenger planes and military cargo planes have so different layout?<p>So far my best guess is that it's related to landing gear. Landing gear makes significant portion of the total weight, you want to keep it short and compact. Engines are the heaviest things in passenger planes, so landing gear is close to engines.<p><a href="http://i.telegraph.co.uk/multimedia/archive/03137/plane_3137447b.jpg" rel="nofollow">http://i.telegraph.co.uk/multimedia/archive/03137/plane_3137...</a><p>With cargo plane the heaviest part of the plane is the cargo.<p><a href="http://cdn23.us2.fansshare.com/photos/antonovan124/antonov-an-low-level-flight-cargo-plane-wallpapers-wallpaper-256058768.jpg" rel="nofollow">http://cdn23.us2.fansshare.com/photos/antonovan124/antonov-a...</a><p>It seems bit weird that one of the most dictating thing to airplane layout is not really related to flying itself. But I could be wrong on this one. Does anybody know better?
The article's explanation is a bit simplified. It sounds like his explanation is more akin to a body freedom flutter response where the aircraft's short-period mode is coupled to the structural response. What he observed is probably closer to a more classical flutter response with the wing's bending and torsion coupled. A gust hits the wing, which increases the load, this increases the bending deformation which can also induce torsion causing a twist in the wing. At too high of a speed (the flutter speed), the response is unstable and can quickly become catastrophic. At normal speeds, the oscillation will tend to die out as the restoring force of the structure and the damping from the structure, aerodynamic loads and controls causes the response to die out. Interestingly, you can go past the first flutter speed of an aircraft with a properly designed control system (aeroservoelasticity) meaning you can get away with a lighter (read, more flexible) wing structure.
On a tangential note plane wins are quite strong themselves<p>Here's Boeing wing being stress tested:
<a href="https://m.youtube.com/watch?v=sA9Kato1CxA" rel="nofollow">https://m.youtube.com/watch?v=sA9Kato1CxA</a>
The lift varies since the aircraft flies fast through regions where the air has different velocity.<p>Think about a column of ascending air, and the plane flying quickly through that. All else being equal, the plane will accelerate vertically until it is traveling vertically at the same speed as the air column (if the column is large enough).<p>That's why flying low and fast is extremely bumpy and taxing for aircraft. Lots of air speed variations.