Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Mon, 13 Jan 2003 13:08:11 -0500
Message-ID: <redirect-1982219@logan.com>
From: "Larry Graves" <rlg34@attbi.com>
References: <list-1982094@logan.com>
Subject: Re: [LML] wing loading
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Brian wrote:  >>If I make a wing out of solid steel, it is going to have
"wing loading" of probably a million pounds...
~~~~~~~

Brian, I'd like to take a shot at explaining the concept wing loading:

Wing loading is a term that describes a changeable characteristic.
Specifically, if I fly the same airplane at several different weights, OR
the same-weight airplane at several different G-loads, I have changed the
wing loading in each case. Wing loading describes the amount of force (or
weight) that a wing is subjected to, and as you can imagine, that force is
constantly changing. In cruise flight on a smooth day (1G) it stays
relatively constant at about the weight of the aircraft.

As soon as you maneuver or fly through turbulence, the force applied to the
wing changes and the wing loading changes. If your wing has 100 square feet
and your gross weight is 3200 lbs, the wing loading under smooth cruise
conditions is 32 pounds per square foot. If you then hit some turbulence or
wind shear and experience a momentary 3-G upload, your momentary wing
loading is 9600 lbs, or 96 lb/sq ft.

The strength or weight of a wing has nothing to do with determining its wing
loading. You can attach 3200 pounds to a kite with 100 square feet of
surface area, and it will have a wing loading identical to a L-IV at 32
lb/sq ft. (This condition will last a millisecond or two before the kite
disintegrates!)

Design engineers are concerned with gross weights and wing strength because
when wings are subjected to turbulence or acrobatic maneuvers, wing loading
goes way up, and may exceed the ability of the wing structure to take the
force. They use the term design load to describe the amount of force the
wing can safely support on a routine basis without damage, assuming a
certain gross weight.

If you exceed the design load factor (or wing loading) you will likely
damage the wing. How much damage occurs is a function of how much excess
strength is designed into the wing structure, over and above that needed to
sustain routine max wing loading. Typically, engineers will design a wing to
withstand one and a half times the design load factor before failure. This
is called the ultimate load.

"Wing load" per se has nothing to do whatever with how the wing is built,
how much it weighs or how strong it is. Obviously a strong wing will be able
to sustain higher wing loading. If you build a strong wing with 100 sq ft
surface area, and a weak wing with 100 sq ft, and attach them to airplanes
that weigh (as a result) 3200 lbs, each wing will "see" the same wing
loading. One will be safe and the other won't. In other words, if you build
two identically-shaped 100 sq ft wings, one out of steel and the other balsa
wood, attach them to airplanes and load the planes so that each weighs 3200
lbs (including wings), they will have identical wing loadings. The
designer's job is to build the wing out of a material that will support
higher forces, yet be as light as possible to allow max payload.

If you decide to load up the GW of your aircraft from 3200 lbs to, say, 4200
lbs, your wing will "see" a higher wing loading of 42 lb.sq ft at all times
in 1G cruise. In a momentary 10G turbulence hit (not unheard of) the wing
will "see" 42,000 lbs., or 420 lb/sq ft. If the designer has established
3200 pounds as a design gross weight and assumes that the wing will be able
to withstand a momentary 32,000 lb gust load without damage (10G), that same
wing may not be able to withstand 42,000 lb without damage. The length of
time a wing is subjected to high G's is important, and not always under your
control as a pilot (e.g. turbulence).

Lots more can be said about this (books in fact) but the main thing to
remember is that wing loading is a force that is applied to the wing by
adding weight to an airplane or by adding G's through abrupt maneuvering.

Hope this was more enlightening than rambling!

Best,

Larry Graves
~~~~~~~~~~~~~~