For Relax Max: Terminal Velocity

Relax Max continued: If I may have two questions, the second is about terminal velocity. If a large object (a passenger airliner, say) reaches terminal velocity before impact, do smaller (or more aerodynamic) objects whose terminal velocity are higher (I assume) "pass" the airliner on the way down? Or does everything hit the ground at the same time?

Good question(s). As you asked, items don't have the same terminal velocity. The maximum speed an item attains in air depends on many different factors. Surface area, fluid factors, and mass are key factors, but other factors include roughness, shape, initial speed, etc. A man falling, for instance, can change his terminal velocity drastically between falling spread eagle and pulling his limbs in and falling headfirst (as skydivers do to move up and down relative to each other). A man can't change it enough not to be going too fast for landing without a parachute, but that's a different post.

Terminal velocity is effectively the speed an object obtains when the force of gravity is canceled by the opposite drag on an object so that it stops accelerating. If gravity were not involved, there'd be no terminal velocity because drag would just work to make things go slower with no counterforce. (Newtonian physics I can explain if you'd like). Initial velocity makes a difference because it adds a factor beyond gravity (and velocity has an effect on drag).

A debris field is determined by multiple factors as well: initial speed, what caused the initial breakup, and how much and what kind of debris is generated. A biplane, for instance, that lost it's rudder might have a very limited debris field, where as Columbia's debris field extended over several states. Explosions (whether combustive or pressure built) send debris forward and backward, extending the debris field. Flat low mass debris will fall slower that compact debris.

So, to answer the last, everything doesn't land at the same time - except in a vacuum as they demonstrated on Apollo 15.

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For Relax Max: "Explosive" Decompression

Relax Max asked: I'm researching an article on high altitude airplane collisions in which the integrity of the pressurized passenger cabin is suddenly compromised. I would like a more detailed explanation on the term "explosive decompression" than I am finding on wikipedia. I'm guessing it means the person can't expel the air in his lungs fast enough and so his lungs explode. Probably preferable than living two minutes before he is killed by impact with the earth. Is that what it means?

No. I've talked before about decompression on Rocket Scientist. In general, with regards to high altitudes, the effects of high altitude alone is insufficient to cause any part of a person to "explode." When movies show eyes popping out of the head or body's spontaneously exploding in vacuum, well, it's just not real. Explosion, per se, is not going to happen (unless someone insists on holding their breath). A single atmosphere is just not enough differential pressure to tear a body apart. Usually, when experts think of explosive decompression, it's a deep sea situation instead, where differential pressure can be many atmospheres instead of one and such horrible events as the Byford Dolphin calamity are possible. (Read with caution; it's gruesome)

When people describe "explosive decompression" with regards to high altitude, they are talking about the plane, not people, experiencing a pressure differential it is not designed to withstand. Not to say low pressure doesn't have effects on people. Anoxia is quite debilitating and harder vacuums can swell body parts as they did for Kittinger when during one of his high altitude balloon jumps. High enough, and the saliva can boil in one's mouth. One can be subject to the bends as a result of sudden high altitude and some forms of lung trauma or altitude sickness.

When I was a test subject, we actually trained through a simulated instance of rapid decompression, complete with fog and tests to see our reaction (and how long it took symptoms to show up) for anoxia. Great fun, that.

Decompression, however, is not a guarantee of a crash, depending, of course, one what caused the decompression. There's a nice little table in Wikipedia that lists notable aircraft incidents involving decompression. Though many ended badly, a surprisingly high number involved few if any casualties. Often the most dangerous results were debris affecting engines or stress destroying control or hydraulic lines. The pressure differential can be acerbated by the high speed of a plane so that, even after the plane's pressure has equalized (which rarely takes long), there is still a suction issue because high speed air (like that flowing outside a rapidly flying plane) is at a lower pressure than still air (like that inside the plane). It is, in fact, this differential pressure between fast and still air that allows a plane to fly by providing lift.

Additionally, anything that might extend outside the plan, like, say, a limb, will produce drag in that swiftly moving air and tend to yank the object out for aerodynamic reasons. And that ties nicely into your next question.

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For Aron: What Went Wrong?

Aron asked: One more...How does my AIAA Design/Fly/Build team avoid this from happening again the plane flew before, but it got damaged during take off. What sort of tests should we do?

Sorry I've taken so long to look at this, Aron. Wow, you're expecting me to figure out a great deal from a video. I'm also not technically an aerospace engineer in that I've never designed a plane. I'm sure you have great resources there to draw from. But, since you asked me and I am nothing if not opinionated, here's what struck me.

The problem does not appear to be the landing gear unless it was broken before you tried the first run. If it was and that's why it kept going lopsided, strengthen the gear and move on.

However, if, as it seemed, it was the repeated lopsided runs that eventually damaged the landing gear, there are a few possibilities that come to mind. First and, to my mind, most likely if the wings house the fuel tanks as they do in many planes, the tanks might not have been filled equally, throwing off the center of gravity to one side. If the center of gravity was off, this would cause turning even if the thrust from the two engines were the same. Or, if the c of g was fine, it might be that the thrust from the two engines was unequal, either a deformation of a propeller perhaps or an engine problem that let one spin at a different spin than the other.

It might also have been control surfaces out of sync (say flaps on one wing, but not the other) or a rudder turned the way it shouldn't be. It might be a deformation of one wing that changed wind resistance or lift from one side to the other or an incipient imbalance in the landing gear that went unnoticed.

Without data, the fuel inequality or control surface problem might not be detectable unless they are design issues (i.e. if they were operator error). However, a problem in the fuel tank that prevents equal filling or causes a control surface to stick my be detectable with testing.

Given the dearth of data, that's the best I can offer.

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