Subject: F/A-18E/F -- Good to Go?
From: Chuck Spinney <cspinney@erols.com>
[Personal opinion, not representing institutional affiliation]
Thu, 02 Apr 1998 


According to reports, the F/A-18E/F is good to go.  Let's hope so, but
I think there are still some uncertainties that need to be resolved before
releasing the $2 billion for Lot II production.  Note John Douglass's
comments: 

1.  "We've now absolutely and fully been able to demonstrate that it isn't
a problem. And I'm very, very hopeful that by the end of the week we'll be
moving the second low-rate production increment on the F/A-18E/F."

"Absolutely and fully" is a sweeping uncaveated phrase implying that wing
drop is fixed.  The current solution is centered on the porous wing fold
fairing.  The Navy testers recently submitted a flight test report on this
fairing to the program office.  I have not seen it, and according to one
source (who also has not seen it), there is only one copy of the report.
He said that it was his understanding that there were some caveats in the
language of the report.  If true, Douglass's claim could be exaggerated.

1.  I think there is reason for concern.  The current version of the fix is
not an pure application of  wing porosity theory.  A pure application would
have a porous outer skin over a solid inner skin, with a quarter to half
inch of dead airspace between them.  This dead airspace would form a
plenum, and the higher static pressure behind a shock wave sitting on top
of the wing would push air through the holes into the chamber, where it
would increase the pressure, and cause the air to flow out holes under the
lower static pressure region in front of the shock.  This outflow would
thicken the boundary layer and create an aerodynamic effect similar to that
of a standard wing fence.  The key is that the phenomenon is a
self-contained, self-adapting process that naturally seeks its own balance.
 If optimised properly, wind tunnel tests suggest it might even improve
performance by reducing drag.

Unfortunately, this configuration caused an unacceptable buffet.

Now the Navy/Boeing team has modified the concept by eliminating most of
the inner skin and using the wing fold cavity as the plenum.  Ram air (air
slamming into the airplane as it moves forward) pressurizes the plenum,
entering the cavity from the front of the wing when the leading edge flap
is deflected downward, and also from the bottom of the wing.  The self
adapting feature is replaced by steady-state ram air (dynamic pressure)
flowing upward through the holes porous fairing in front the shock wave
(and maybe behind it as well).  The substitution of dynamic for static
pressure is a very primitive mutation of an elegant idea based on naturally
occurring differential static pressures--but the elegant idea will not work
for the E/F for reasons unrelated to its theory. 

The ram air mod appears to reduce the wing drop and buffet to acceptable
levels, but there is a price--ram air absorbs energy which means increased
drag and the flow through the wing may cause a lift reduction as well.  The
question is how much.  I do not believe the Navy has had enough time to
determine the magnitude of the performance penalties.  A source close to
the program told me today that the engineers at Boeing are nervous and
think they need a few more weeks to make more pressure measurements.  These
uncertainties may be a source of caveated language, if indeed, that
language exists.  

2. Douglass also said wing drop "never was a problem" ... and ... "There
isn't an airplane today that doesn't have wing drop ... the F-22's going to
have it, Joint Strike Fighter's going to have it, everyone's going to have
it,"   Both of these comments are baseless hype. 

If it was not a problem, why did the Navy work its but off trying to fix
it.  Distracting attention to the F-22 and JSF (which have problems of
their own) is a low-grade public relations tactic unbecoming to an
Assistant Secretary of the Navy.  Moreover, he has no basis for this
charge.  In fact it is utter nonsense.  As one wind tunnel expert (who is
very active in this area) told me, "If you take the time early in the
design and wind-tunnel testing phase, you should be able to correct any
lift curve anomalies and any asymmetrical characteristics.  So, I think
that an advanced fighter can be built without having wing/drop or buffet
problems (they really are closely related)."  In other words, if this plane
had been properly tested in the wind tunnel, and then prototyped, we
probably could have avoided this debacle.  

The wing drop problem exists because (1) the Navy chose to skip the
protoyping phase by hiding the fact that the E/F had a new wing in 1992,
inorder to skip Milestone I by claiming the E/F was a mirror-image
modification of the C/D, and then (2) hiding the fact that it had an
unacceptable flying defect inorder to get approval for Milestone II
production in Mar 97.  I am now convinced the roots of the problem go the
changes in the wing made to get increased range in level flight (and maybe
reduced RCS).  The designers did not increase the fuel fraction enough to
meet the range requirments.  By milking the wing design for additional
range, they made the flow fields more unstable during turning maneuvers.
If I am correct, the airplane needs a new wing design--which is now
economically impossible because a production break would be too disruptive
to the political constituency that was created when it was prematurely put
into concurrent engineering and manufacuturing development in 1992, and
then deliberately expanded by a premature production decision in 1997.