Re: Explosive decompression - how fast?

On 2008-04-27, Michael Ash <mike@[EMAIL PROTECTED]
> wrote:
> But that 300mph wind should mostly stay outside the aircraft.

Even a comparatively "mild" 100mph eddy driven by the 300 mph flow of
air outside will exert quite significant force.


> In this theory, a relatively small piece of skin detached first. The
> unfortunate flight attendent was then sucked into this hole,
> blocking it, which then caused the massive failure seen in all the
> amazing pictures. I'm not sure how plausible this whole thing really
> is

Not at all, unless it opened up *right* next to the attendant, or by
"relatively small" you mean on the order of a square metre or more
(but then they couldn't block it).

The flow speed will fall off roughly inversely with the area of the
surface of constant distance from the hole, which for at least the
first few metres should be inverse-square.  That means the drag force
of someone in that airflow falls off with the inverse fourth power.


> A bit of BOTE calculations to back it up. The wind should be roughly
> the speed of sound.

More like half to a third of that, and only at the actual hole.  But
let's be generous and say 200 m/s, which is closer to two-thirds.

If we suppose that the original hole was about 50 cm across, then it
has an area of about 0.2 m^2.  At a distance of 1 metre, the inflow
surface probably has an area on the order of at least 3 m^2, so by
simple conservation of mass the average speed of airflow across that
surface can't be more than about 13 m/s.


> But the hole in the case of the British Airways flight was not that
> big, although the pilot was pretty close to it.

As I showed above, decompression airflow forces will be roughly
proportional to the inverse fourth power of distance from the hole (in
cases where the hole is smallish compared with internal cross-section
area).  If you're half the distance from it, you'll feel about 16
times the force.


> looking at the media which prompted the post I don't see this shown
> anywhere. If the door opens approximately instantaneously it seems
> that the victim will indeed get blown out.

I already analyzed the case where the outer door is blasted open in
milliseconds.  The victim ends up drifting at about 1-3 m/s, exiting
the airlock long after the air has gone.


- Tim