On Tue, 29 Mar 2005 06:17:42 -0600, "John Carrier"
wrote:

of the center of pressure, that kind of thing. Maybe a Phantom driver
could
explain it better.

The aerodynamic center shifted forward abruptly as you were decelerating
through about .95 IMN. As the aero center shifts forward, the stabs
downward trim force becomes greater and a pitch up occurs. (This is rather
typical transonic behavior, although it varies from jet to jet.)

In the F-4's case, if you were pulling 6 G or so, you'd suddenly find
yourself around 9 G during this transient. At medium/high altitudes, the
airframe would give a hint that this was about to happen with a subtle
buffet cue. You could reduce your back stick just as the aircraft dug in
and maintain your G without exceeding it. If you were low (say 5,000',
higher IAS for .95) the buffet cue wasn't there and it could sneak up on
you.

I experienced the low altitude manifestation once and use the incident as an
illustration of the effects (big time overstress) of transonic pitch up for
my aero lecture.

R / John


Walt used the term "G-dig", but I always heard it called "Mach
tuck"--(coincidentally we had a guy in the squadron with last name
Tuck, so his call sign became Mach -- rather than the more
conventional "Friar".)

Most jets of the period really couldn't command a lot of G when
supersonic--the slab simply didn't have enough authority. So, if a
fight was engaged in the supersonic speed range, guys trying to get as
much turn rate as possible would have a yard of stick pulled into
their gut. When the airplane decelerated through the Mach, that slab
prepositioning when it went sub-sonic would then command a whole lot
more AOA and G. Overshoot of the allowable G limit was damn near
inevitable.

One solution was to ask the back-seater to keep on eye on the mach for
you and call when it looked like you were going to transition. Good
situational awareness also helped--you knew your altitude, your entry
airspeed, your attitude and your tactical position relative to the
adversary. Predicting when you were going to go sub-sonic was then a
function of art rather than science for the experienced guys.


Ed Rasimus
Fighter Pilot (USAF-Ret)
"When Thunder Rolled"
www.thunderchief.org
www.thundertales.blogspot.com