From: sharpes@c-17igp.wpafb.af.mil (Civ Daniel G. Sharpes)
Newsgroups: sci.aeronautics
Subject: FSW Info
Date: 28 Jan 1993 21:00:52 -0600
Organization: UTexas Mail-to-News Gateway
NNTP-Posting-Host: cs.utexas.edu


  I wrote a series of articles on forward swept wings in '85 for the
local AIAA newsletter.  It was the capstone of four years of FSW work
I did for the Air Force.  I verified or developed theoretical
equations that would predict the stability and control characteristics
of forward swept wing aircraft.
  There are many good reasons to use forward sweep, most significant
of which is the low-speed stall characteristics.  (The best discussion of
FSW benefits is in a Popular Science that came out in '83 or '84.  It has
lots of pictures and propaganda about the glories of forward thinking.)
  Because forward sweep was used on the X-29, though, several
misconceptions about that technology have creeped in.  The first is that
forward sweep is unstable.  It's not.  Aircraft stability is determined
by where the designers put the center of gravity.  On the X-29, they
decided to put the c.g.  about 35% behind the aircraft's aerodynamic
center.  If you took the canard off, the plane was neutrally stable.
The canard destabilized the configuration, not the wing.
  The other fallacy that has crept in is wing divergence.  This error
dates back to the late 40's when Diedrich and Budiansky published a
NACA tech note (TN 1680) that said the wings would diverge and tear
off.  This is absolutely true for the condition they looked at.  If you
anchored a wing (like in a wind tunnel) and then increased the speed of
the air going past it, the wing tips would bend up, increasing lift at
the tips.  This would cause more bending, would cause more lift, would
cause ...  until the wing broke off.  HOWEVER, when the wing is on an
airplane flying in the sky, the wingtip bends up, produces more lift,
and the airplane climbs!  The wing will NOT rip off.  In fact, the
speed limit for forward swept wings isn't the divergence speed but
elevator control power!  If the pilot wants to maintain the same
altitude, he or she will push forward on the stick.  At some speed,
there simply won't be enough nose-down elevator to overcome the
increased wing lift.  This is exactly what happened on the Ju-287, the
6-jet-engine bomber and it was found during high-speed dive tests. 
When the pilot began to pull out of the dive, the stick forces were
lighter than they should have been.  The wing tips had deflected up and
were creating more lift.
  Anyway, there have been quite a few aircraft with FSWs.  These
include the Bugatti R-100 (a racer that never flew due to the Nazis), a
Swiss plane, the Pilatus SB-2 Pelican, Sweden's MFI-9B trainer and MFI
15 Safari (many of both flying today), the Czech Zlin Model 142
(certified for aerobatics in '73 by the FAA), and the prototype
Fantrainer and the HFB 320 Hansa Jet from Germany (BTW, Hans Wocke was
the designer for both the Ju-287 & the Hansa Jet).  Many gliders use
forward sweep, including some older Soviet birds.  During WW 2, the Air
Corps looked into a fuel carrying glider that would be towed behind
B-29s.  It had forward swept wings.
  My bottom line (yea, finally!) is this: trying to compare forward
sweep with aft sweep to see which one is best is opening a can of
worms.  There are too many design considerations and trade offs. 
IMHO, forward sweep should be seen as one of several design options to
produce an aircraft that meets the spec it's being built against.
  As one airplane designer has said, "There are no unique solutions, only
unique problems to solve."

     +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
     +                 _____________                         +
     +    Dan                |   "It's not a real plane      +
     +      Sharpes          |    unless you can stand       +
     +                      _|_   up inside it"              +
     +        \____________/===\____________/                +
     +             (*) (*)(  .  )(*) (*)                     +
     +                     \___/                             +
     +                                                       +
     +  Opinions expressed are mine alone, and not another's +
     +++++++++++++++++++++++++++++++++++++++++++++++++++++++++

 (BTW, I still have all my references from my AF work - four 3" three
ring binders full of wind tunnel data.  If there's interest, I can upload
the bibliography to my report, _Validation of USAF Stability and Control
Datcom Methodologies for Straight-Tapered Sweptforward Wings_,
AFWAL-TR-84-3084.)