"Boomer" wrote...
the first chart is an F-16 (it SAYS F-15 but a look at the stats tells you
otherwise) Notice that it's Max altitude comes BEFORE Mach1.

the second chart is for an SU-30, it also reaches max alt BEFORE Mach1.

the third chart should be a F-4 vs MiG-21 notice these planes reach max
alt
well AFTER Mach 1


Curiosity killed the cat, and I'm gonna find out why!

From a non-aerodynamicist's point of view, there are several factors at
play. Any one of them may be limiting at a specific airspeed/altitude
combination.

The short answer is that the airplane is optimized for subsonic flight
(because it spends 99%+ of its life below Mach 1), so max altitude will be
attained at a subsonic speed.

Looking at the F-15 chart, because it is the "cleanest":

Below approx 38,000', max airspeed is placarded at 800 KCAS (for
discussion purposes, roughly equal to 800 KIAS). That is a structural
airframe limitation, and the engine might be able to push it faster until it
falls apart.

At the left side of the curve is Max Lift, or stall speed, and will
equate to a roughly constant Max Lift or "Stall" Angle of Attack. If you go
any slower, you start going down.

At approx 60,000' and Mach 0.95, you hit the peak of the envelope -- max
lift, max thrust, max drag. There is only one speed where the engine can
produce enough thrust to maintain altitude, and that is at Max Lift AoA.

When the airplane is transonic (here, above Mach 0.95), drag increases
significantly, before dropping off again at approx. Mach 1.05. The only way
to go faster if you start at 60K and .95 is to descend and accelerate. At
the same time, engine thrust is likely decreasing as the intake ramps and
exhaust nozzles are not controlling the airflow to and from the engines
perfectly. As the ramps and nozzles start re-configuring, thrust falls off
somewhat. By the time the thrust has recovered at Mach 1.05, drag is
starting to increase again, so the engine thrust limits performance up to
Mach 1.4. At that speed and approx 57,000', any further climb causes thrust
to decrease to the point where altitude and airspeed cannot be maintained.