It isn't necessary that all shocks be external to the inlet, only that
stable subsonic air reach the compressor face. Some inlets are designed to
create several oblique shocks within the inlet prior to reaching a point
where an expansion of cross-sectional area (diffuser) creates a final normal
shock to decelerate the air.

R / John

Amplifying on John's well written technical description, engine thrust is
directly proportional to air pressure at the engine face. The primary
objective of a variable geometry engine inlet is to effect a maximum pressure
recovery of the air prior to arrival at the engine compressor face. The shock
wave development that John describes, especially the final normal shock wave,
accomplishes this. The pronounced effect of an inlet system that fails to
articulate is quite amazing. While I have never had an inlet system fail
during functional check flights in the F-111, a couple of my colleagues have.
In one case, the central air data computer Mach signal failed to reach both
inlets. Their F-111F barely attained Mach 1.7 in Maximum Afterburner. The F
model had the largest engines in the fleet, and could attain Mach 1.1 in
Military power on the deck, and Mach 2.5 in less than Maximum Afterburner at
altitude. They brought the jet back, maintenance repaired the problem, and
they flew it again the next day. They hit Mach 2.5 without breaking a sweet.
I haven't heard only one inlet not articulating, however, I would imagine that
the first clue would be the pilot adding rudder into the "good" engine as the
Mach increased.

The other factors, such as flow smoothing and resistance to angle of attack
excusions, do not require a variable geometry configuration. A well designed
fixed geometry inlet can accomplish these objectives.





Kurt Todoroff


Markets, not mandates and mob rule.
Consent, not compulsion.

Remove "DELETEME" from my address to reply