On Sun, 25 Nov 2007 07:55:05 +0000 (UTC), Bertie the Bunyip
wrote:

"Darkwing" theducksmailATyahoo.com wrote in
m:


"Bertie the Bunyip" wrote in message
.. .
Phil wrote in
news:dc605aa6-d47d-4121-bcdd-

:

On Nov 24, 2:07 pm, Bertie the Bunyip wrote:

No, you really can;'t abruptly do anything in an Airbus in the
flight mode he would have been in at cruise.


The computer won't let you? What if you need to maneuver abruptly
to avoid a collision?


That's right. It won't let you manuever more thna a given
acceleration for the flight mode you are in,particulaly at high alt
due to mahc manuevering considerations. Can't give you numbers
because I'm not flying one at the moment.


I know basically zero about high altitude flying, what are the
considerations? Does it have a lot to due with being close to the
envelope between flying and stalling in the thin air?



Not exactly. Indicated stall speed remains constant with altitude. What
does come into play is mach buffet. On the high end of the speed
envelope , obviously, you have the air accelerating around the airplane
and over the top of the wing and that gives you a buffet which destroys
lift. But if you slow the airplane down you have to increase alpha to
maintain your line of flight and the increased alpha will accelerate the
air over the top of the wing to supersonic even though the airplane is
going slower than it was in cruise.
Anything that increases the angle of attack, such as putting more weight
in the airplane will bring the minimum and maximum speed closer to your
cruise speed reducing yuor buffet margins. This means higher weights
bring the max altitude down. Loading the wing up with G either by
manuevering or an encounter with turbulence and even a forward CG will
bring the buffet on sooner, which is why some airplanes pump fuel into
the tail after takeoff once the autopilot is engaged. .
The one that comes into play here is the G consideration. They were
light, so they actually had quite a lot of G available to manuever, but
still, if you screw up at either end of the envelope, you have a big
problem. By the way, some airplanes operate with a margin of as little
as 1.25G. To give this some perspective, a thirty deg bank is 1.15 G.
These would be mostly medium long haul operators doing it to save fuel.
The margins depend on type.. most are more like 1.4 G.
If the airplane falls over, there's a good chance you'll end up through
the high end of envelope (too fast) and if that happens three things
happen to conspire to screw you. One, the center of pressure shifts back
on the wing bringing the nose down, which tends to increase speed, which
exacerbates the problem. Two, the center section of the wing is affected
more because of Area rule. the fuselage has already accelerated the air
when it meets the wing, so the center of the wing is affected more thsan
the tips, and since the center of the wing is mostly ahead of the CG the
loss of lift there brings the nose down and increases sped which
exacerbates the problem. This is mach tuck and though it isn't directly
caused by pulling excessive G it is the likely end result of an upset at
altitude.
The third factor in mach tuck is the stab. As you try pulling the nose
up as it's coming down, the increased camber of the stab (wrong way
round, f course) will accelerate the air to supersonic levels and buffet
the stab. Presto, no elevator control and you're dead.
Whatever it was that started that Egyptair airplane down over the
Atlantic, what finished them off was Mach tuck. If it develops past a
certain point there is almost nothing you can do.
So, Airbus have, for better or worse, decided to allow the airplane
itself to monitor these inflight parameters and not to allow it to do
anything too funky G wise. In reality, it hasn't worked so well. They
seem to have just as many upsets as any other aircraft.


BTW, a Cessna 172 would have these same problems if you were to get it
high enough! There are some high performance homebuilts with blowers
that need mach meters, but if you get any airplane up high enough you
have mach issues. The airplane they're planning on sending to Mars fits
into this category. It's going to be cruising at the equivelant of
somthing like a 150,000 even though it's near the surface. I thing
they're planning on a 250 knot TAS for it and that will be very tight at
those sorts of pressures.

Or maybe we should ask Anthony to check it out on his new version of
X-plane!
Version 9 is out now Anthony! Better get your order in! I'm sure the
outpouring of wisdom will wash us away like the great flood.






Bertie


Bertie and all

To add some to your data.

The B-47 had what was called the "coffen corner". At high altitude the
airspeed was just above the stall and if you increased your airspeed
you were into Mach. This required very close attention by B-47
drivers.

Big John