Re: airplane structural terms

Date:         13 Feb 99 02:25:49 
From:         Ken Ishiguro <kenish@earthlink.net>
Organization: EarthLink Network, Inc.
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Here are some answers to your questions:

1. Chord is an imaginary straight line drawn through the wing from the
leading edge to the trailing edge.  Since the wing tapers, the mean
chord is an average length and position.  The aerodynamic chord is a
rather complicated calculation (as opposed to the geometric chord).

2.  %MAC refers to where the center of gravity falls along the chord
when the aircraft is loaded.  There is a forward MAC and aft MAC limit,
and the CG must fall between these points.

3.  Wing incidence is the angle (viewed from the side) that the wing is
attached to the fuselage.  In other words, if the leading edge were held
in one place and the trailing edge were moved up or down, the angle of
incidence would be changing.  Obviously, most wings are securely bolted
to the fuselage, and the AI is fixed (except for the Boeing Tiltrotor).
An "all flying" horizontal tail would be an example where angle of
incidence is variable.

BTW, angle of attack is the angle the wing is meeting the airflow.  If
you were to suddenly pull the aircraft nose-up, angle of attack would
increase, while angle of incidence would stay the same (unless the
pull-up tore the wings off)  :-)

4.  STA is "station", also can be FS for "fuselage station".  This is
the coordinate system used to locate items or points on the aircraft.
FS0 is usually at the nose, or just ahead of the nose.  It the goes in
inches back to the tail.  Inside an aircraft structure, you will often
find ribs, beams, etc. marked with the station number.  One complication
is on "stretch" aircraft.  In order to keep station numbers consistent
between models (a certain gizmo is always at FS1200 regardless of
model), some manufacturers come up with screwball station number systems
to accomodate the stretch.

5.  WPL-  I think you mean WBL, which is Wing Butt Line (no, a butt line
is not the Rockettes)!  BL0 is the centerline of the fuselage.  Boeing
uses LBL and RBL for left and right of centerline in inches, but WBL's
on the wing.  Douglas uses negative numbers to the left and positive to
the right, with no Wing BL's.

Vertical coordinates are the Water Line (WL).  On a 747, WL200 is the
top of the cabin floor, which places WL0 slighly below the tires.  IIRC,
Douglas places WL0 at the window line, and uses positive or negative
coordinates.

Like lots of other aircraft stuff, FS, BL, and WL is from the nautical /
shipbuilding industry.

Ken Ishiguro