Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Sat, 11 Jan 2003 10:49:55 -0500
Message-ID: <redirect-1980776@logan.com>
From: "Gary Casey" <glcasey@adelphia.net>
Subject: Wing loading
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Just for the fun of it, let me add my 2 cents worth:
1.  The structural and aerodynamic effects of wing loading are two
completely different topics.  How strong the wing is has no effect on the
flying qualities and vice versa.
2.  Adding weight to the wing (fuel) doesn't increase the structural
loading, so when calculating the strength the location of the weight is
all-important.  Consequently, the design capability (i.e., 4.4 G's with a
factor of safety of 2) is an approximation because it doesn't take into
account how much weight is in the wing.
3.  Increasing the official gross weight, which we are free to do, might
best be done by calling out a separate landing weight, thus preserving the
landing gear design integrity (at least on paper).
4.  Assuming that the aircraft can tolerate more of a gust load if it is
already "weightless" sort of holds air, as the airframe load depends only on
the angle of attack and airspeed.  The gust loading assumes that there is an
instantaneous change in angle of attack because of a vertical gust.
Weightlessness implies that the original angle of attack is zero, thus
giving a higher margin.  If one up gust is followed immediately by another
up gust the two could add, however.  Since the gust loading is only a
function of angle of attack changes, I suppose one can neglect the change in
tail loading as it affects the wing strength.
5.  The glide angle has little to do with the wing loading and has
everything to do with drag.  In theory, a turbine IV with a feathering prop
will have a very good glide ratio just because the prop drag can be
eliminated.  The descent rate will be high, but the glide ratio will be
very, very good.  I understand that jet transports make exceptionally good
gliders in that regard.
6.  A given gust velocity will typically impart a larger load on a "thin"
wing than a thick one because the slope of the lift/angle is steeper.
7.  High wing loading is possibly not the best factor in how "hot" a plane
is.  Span loading is probably a better measure.
8.  When assuming that the aircraft will tolerate a 4.4 G loading one has to
consider ALL the parts of the aircraft, not just the wings.  For example,
the engine mount also could break at higher G loadings.  For this reason it
could be argued that the max structural cruising speed (yellow arc) could be
raised if the aircraft is made heavier by adding fuel to the wing.  A given
gust will impart lower G loading to the structure at the heavier weight.
Maneuvering speed is, however, a fixed ratio of stalling speed so it doesn't
matter where the weight is carried.

Just a note on asymmetric wing loading - As I understand it a lot of heavier
planes have a loading asymmetry limit in their certification.  Learjets come
to mind.  If one were to burn all the fuel from one tank, as a result of a
pump or selector valve failure for example, the pilot is probably in the
test pilot mode as the aircraft has probably never been tested with
significant side/side weight differences.  And as someone wisely suggested
it would be best to add significant speed to the approach and landing phase
as an unrecoverable spin might be only a heartbeat away.

Gary Casey, PE, AA (Professional Engineer, Amateur Aerodynamicist)