Mythbusters and explosive decompression

Also, if the test was done on the ground then of course nothing would
happen. It needs to done in a wind tunnel that is depressurized to
simulate
30K feet.

Or, you can just do what they did, and pressurize it on the ground to get
the same pressure differential.

A small bullet hole will not cause much of a structural problem--you aren't
going to see an entire fuselage section ripping off because of aerodynamic
forces. What they did with the shaped charge might cause some problems, but
otherwise the slipstream wouldn't be much of an issue (structure-wise).

I forget the exact numbers but the dynamic preasure on the nose of an jet
aircraft at cruise is about 9psi above ambient. Anyway, anybody who has
ever seen pictures of shot-up military aircraft can see that the airflow
didn't cause the skins to tear.

Mike
MU-2


"Luke Scharf" wrote in message
news
On Wed, 07 Jul 2004 17:54:59 +0000, Mike Rapoport wrote:
Gee what do you think the indicated airspeed is at M.85 at FL350?

It looks like the calibrated[0] airspeed is around 325mph:
https://ewhdbks.mugu.navy.mil/mach-as.htm

I seem to remember that Mach 0.85 is transonic, so air is actually flowing
at supersonic speeds over some parts of the airplane (nose, wings, tail?)
and subsonic over others. All kinds of stuff that I don't know how to
predict happens then. Maybe someone here is an aerodynamicist who has a
better feel for compressible flow?

But, *that* is why I am skeptical of the Mythbuster's conclusion -- it
seems to me that supersonic/transonic airflow anywhere would be a
significant consideration -- especially if the flow happens to
be over the bullet-hole.

-Luke

[0] IIRC, "indicated" airspeed isn't valid above Mach 0.3 because of
compressibility effects -- but it has been quite some time since I took
that class (and I wasn't proud of my grade) so I could be very wrong. I
have just enough education on the topic to appreciate the expertise of
people who actually know what they're talking about!

I wonder... did they just pressurize it with a low volume pressure
source or with a high volume pressure source. I remember being told that
some of the larger jetliners have enough bleed air capacity to maintain
cabin pressure with an entire window blown out. Not bad for the people
far away from the breach, but really sucks for those (literally) near
the breach.

IF this is true, then pressurizing the airliner on the ground with a low
volume source (only able to maintain the differential over the normal
leakage of the pressure vessel) then the airframe depressurizes. A high
volume source would be able to maintain cabin pressure despite the
breach of a bullet hole.

In any event, I will readily agree that the hollywood versions of
decompressions are exactly that.. hollywood.

So.. am I misinformed? or perhaps this might be pertinent/

Dave

Casey Wilson wrote:
Hi all,
I don't know if it was a rerun and has been thoroughly done over here,
but last nights episode of The Mythbusters 'busted' the explosive
decompression myth surrounding bullet holes in aircraft.
The guys sealed up a junked out cabin, pressurized it, then fired 9mm
bullets through first the skin and then a window. Nothing exciting happened.
Pretty interesting stuff.
They ended the episode by blowing a large hole in the fuselage. I was
out of the room when they set the charge so I don't know the size, shape,
etc. I did a bang up job of opening a hole.
My conviction of the miniscule damage to be caused by a sky marshall or
pilot popping off a few caps at a terrorists has been reinforced. The
diameter difference between a 9mm (.38") and a .44 Mag wouldn't make any
difference. Let's give the good guys the bigger cannon.

John Harlow wrote:
My conviction of the miniscule damage to be caused by a sky
marshall or pilot popping off a few caps at a terrorists has been
reinforced.

Some years ago an airliner landed here in Richmond after having been shot by
someone on the ground. It was a completely coincidental thing where someone
shot into the sky and just happened to hit an airliner. The news report
showed the bullet hole; other than that the plane was fine.

May or may not be relevant, depending on the following
two questions:

1. Did the bullet penetrate into any pressurized part
of the aircraft ?
2. What height (and speed) was it flying at when it was hit ?

CV

CV wrote:

John Harlow wrote:

My conviction of the miniscule damage to be caused by a sky
marshall or pilot popping off a few caps at a terrorists has been
reinforced.


Some years ago an airliner landed here in Richmond after having been
shot by
someone on the ground. It was a completely coincidental thing where
someone
shot into the sky and just happened to hit an airliner. The news report
showed the bullet hole; other than that the plane was fine.


May or may not be relevant, depending on the following
two questions:

1. Did the bullet penetrate into any pressurized part
of the aircraft ?
2. What height (and speed) was it flying at when it was hit ?

hmmm, just thought of a third factor: The hole left by a bullet
entering the airframe may be aerodynamically very different
from one that went from the inside out.
CV

On Wed, 07 Jul 2004 20:21:51 +0000, Mike Rapoport wrote:
I forget the exact numbers but the dynamic preasure on the nose of an jet
aircraft at cruise is about 9psi above ambient.

~9psi doesn't sound too bad. I'd buy that that the skin would stay
together with that number.

I'm still curious to see if any sort of shockwave-like thing from a
the transonic flow over a bullethole would do any extra damage.

Also, someone recently was telling me that a supersonic wind-tunnel (up to
Mach 3, I believe) that they use works on 50psi -- but the system
maintains 50psi over a large area and pushes the air through a *much*
smaller throat...

Anyway, anybody who has
ever seen pictures of shot-up military aircraft can see that the airflow
didn't cause the skins to tear.

Yes, but they're built with the expectation that they'll be shot at. I'd
probably put a thicker skin on an airplane that was designed for civilian
use. I've heard stories about midair collisions where the wing of a
jet fighter literally sliced the wing of another aircraft off (with
minimal damage to the wing).

But, at only 9psi above ambient pressure, I'd buy that the skin would stay
together. But I wouldn't want to actually try it without some rigorous
testing!

-Luke

uuuuhhhh,

What about that Hawaiian Air 737 that went "convertable" mid flight and
landed relatively intact?

What about the scores of military cargo aircraft that have been shot up and
still seem to stay together (not too structurally different, and made by the
lowest bidder!!!)

Hell, even the DHL A320 took a hit by a MANPAD and made it back...




"Luke Scharf" wrote in message
news
On Wed, 07 Jul 2004 16:37:56 +0000, Casey Wilson wrote:
The guys sealed up a junked out cabin, pressurized it, then fired 9mm
bullets through first the skin and then a window. Nothing exciting
happened. Pretty interesting stuff.

One factor that they neglected to account for is that many airliners fly
at speeds approaching Mach 0.85. I'd have to see a section of aluminum
skin with a bullet-hole in it staying intact in a transonic wind-tunnel
that was running about that speed before I put much stock in their
results.

-Luke

"Mark T. Mueller" wrote in message
...

What about the scores of military cargo aircraft that have been shot up
and
still seem to stay together (not too structurally different, and made by
the
lowest bidder!!!)


The military hasn't bought aircraft that way in a very long time.

"Luke Scharf" wrote in message I seem to remember that Mach 0.85 is
transonic, so air is actually flowing
at supersonic speeds over some parts of the airplane (nose, wings, tail?)
and subsonic over others.

M.85 is called the transonic region, however, most airliners are designed to
cruise at a speed just below that speed where shock waves start to form.
Cruising with shock waves burns a whole lot of fuel. In the B-727, we
normally cruised at .77 unless it was the last leg of the week and we wanted
to catch a jumpseat home. Then we bumped it up to .88. At M.88, the
shockwaves coming off the square corners of the windscreen were so loud, we
couldn't converse with one another. We burned a bunch of extra fuel too. In
the MD-80, we normally cruise at M.76. We can bump it up, but at .81, the
rumble of shockwaves on the wings is disconcerting to the passengers. We
burn 6% more fuel just to go 10 or 12 knots faster. In contrast, the B-747
cruises at M.84 without forming shockwaves. It is designed that way.

But, *that* is why I am skeptical of the Mythbuster's conclusion -- it
seems to me that supersonic/transonic airflow anywhere would be a
significant consideration -- especially if the flow happens to
be over the bullet-hole.

Jetliners don't use thin .020" sheet aluminum for structural parts as you
would find in a Cherokee or Skyhawk. Shockwaves over a bullet hole are
insignificant. What is significant is when a force is strong enough to cause
some sheet metal to expand into the airstream and the airstream peels it
backwards. Then structural integrity is compromised, often catastrophically.

D.

"Bill Denton" wrote in message If a bullet penetrated the skin of an
aircraft, the plane could not have been more than a couple of thousand feet
high, and it would not be pressurized.

It would likely be pressurized, just not at maximum differential.

D.