Mythbusters and explosive decompression

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 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!

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

"Luke Scharf" wrote in message
news 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).


I suspect that the skin of a preasurized airliner is thicker than the skin
on the tail of a A-10.

Mike
MU-2

"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.