h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

After all the good explanation from Hoovooloo, I'm not sure there's a use for my adding all of this, but ... well, this is what I learned from my Dad, who's an ex-Navy pilot ("aviator" as he reminds me) and still flying today for the US Forest Service:

Given what's been said about laminar and turbulent flow, one can imagine the way the hot air is rising in large amounts over the Earth's crust. Since the air is rising in different speeds in different areas, and since also the air is sliding around and moving parallel to the Earth's surface at the same time, and for other reasons as well, the flow of the air up and down is quite turbulent. Eddies are not only in the space-time continuum, but all around our atmosphere, in varying sizes (no one knows if all those sofas are really his, though). These eddies might looks like vorteces if we were able to see them - as I understand it, some examples of the largest and most stable (and therefore desctructive) vorteces that can form are tornados and hurricanes and typhoons.

Anyway, now that it's established that all the turbulent flow exists, how it affects birds, small planes, and large planes becomes the issue, and can get very complicated and *cough* cloudy. A bird is generally much smaller than the turbulent eddies, which can be both good and bad. It's good because the bird will be able to just go along with the flow, as it were, just like the "riding a thermal" concept. Speed is a factor as well, and this is why birds and hang gliders can catch and ride on thermal eddies in our atmosphere. It's a bit dangerous too, as sometimes the thermal can be large enough and powerful enough to carry the hapless bird/hang glider far up into the freezing cold, oxygen-depleted upper atmosphere, to come down hundreds of miles away as cold and dead as a rock. This has actually happened to people who were hang gliding, albeit rarely.

So, what about airplanes? Relatively speaking planes are much larger, heavier, and need to go quite a bit faster with a lot more effective wing area to generate the lift they need to stay aloft. For these guys, turbulence poses a different problem in that they generally cause "bumps" and the sudden changes in altitude or "dips" that make one's stomach float somewhere around one's throat. For most commercial airplanes, turbulence poses more of a discomfort problem and a hazard to the bruising and/or making sick of its passengers, but severe turbulence can seriously damage, even fatally so, the plane's working parts. It usually takes a lot for that to happen, though. Smaller airplanes are much more prone to this as they have less mass - enough to get them out of the "able to ride the eddy" category but not enough to get them into the "able to blast through the eddy" category, to make a blunt simplification of it.

I'm not sure about what causes fighter planes to suddenly wipe out when flying through the jetwash of another fighter plane, even though it was explained to me ages ago. From what I do remember, it's a far less likely occurrence than what was portrayed on "Top Gun," but that's probably obvious - Hollywood and all that. I think it has to do with the amazingly complex aerodynamics that are going on in a high speed dogfight with fighter jets like that, and the fact that they are a lot like Ferraris: very very fast, very sexy, and more tempermental than a spoiled supermodel on a diet.


- JD

There is a place in Fife where crashed Mute Swans sometimes stop the traffic after trying to overfly a road bridge which crosses a river. It was explained to me that if the direction of the breeze is exactly wrong (from the swans point of view) the bridge acts as a sort of wind break and probably disrupts the laminar flow which exists everywhere else but above the bridge. The swans can't adjust quickly enough and tumble out of the sky much to the consternation of the motorists below.
I guess it must happen elsewhere too.

On the "jetwash" question, I believe that schedulers of airshows have to be careful if very large jets (Vulcans, Concorde etc) have just put in an appearance. They are required to leave a time delay of a minute or two before the next act can perform in the same airspace.

Interesting it should be Mute Swans specifically - they're world record breaking birds for two reasons. They have more feathers than any other species (typically over 900) and they're the heaviest bird that can fly - so they'd be least able to make the kind of adjustments I was talking about, and be more likely to crash.

Funny how being big and heavy is an advantage for aircraft, but a disadvantage for birds...

H.