Re: airplane structural terms

Date:         13 Feb 99 02:25:46 
From:         Don Stauffer <>
Organization: honeywell
References:   1
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Okay, I'll take a crack at all but one.

boeing707 wrote:

> Hi everybody.
> I've come across a few terms relating to the fuselage and wing
> structure of airplanes that I dont understand. Can somebody please
> define the following terms:
> 1. Mean Aerodynamic chord (relating to wing structure)

This is for a wing which has a planform which is NOT a rectangle.  The
chord is the distance from the leading (front edge) of the wing at any
distance from the fuselage measured parallel to the centerline of the
aircraft, to the trailing edge.

A rectangular wing would have its chord equal everywhere along the wing.
One could compute an average geometrical chord with simple geometry on a
non-rectangular wing.  The mean aerodynamic chord would be very similar to
this geometrical average chord for all practical purposes.

> 2. Percent of mean aerodynamic chord (%MAC) (i think this relates to
> wing loading)

This is the location of the center of gravity.  Rather than refer to
station, to be defined below, it is referred to a percentage of chord
because this number has a great deal to do with the stability of the
aircraft.  Lets say an aircraft has a chord of five feet.  Lets say the
center of gravity for longitudinal (fore/aft) balance is at a point one
foot back from the leading edge.  The CG is then at 20% of chord.  The
aircraft will go unstable if a different weight distribution moves the CG
back beyond a certain critical percentage of the chord, depending on the
airfoil, the tail, and other design features of the aircraft.

> 3. Wing incidence

We mentioned above that the chord is the line from the forward most
extension of the leading edge to the very trailing edge.  Not only is the
distance important, but the angle between this line and the centerline of
the aircraft is important.  The airfoil must be at a certain angle to the
oncoming air at a given condition of flight.  If the wng has no incidence,
the fuselage now points upward at various "angles of attack".  We can keep
the fuselage level during flight by making the wing have the proper angle
to the air by adding the necessary "wing incidence."

> I've also seen some blueprints for the 707 (the greatest airplane ever
> made :)  ) and saw two abbreviations that i did not understand:
> 1. STA (relating to fuselage) and how are the STA numbers determined?

This is an old shipbuilding convention.  A ship or an aircraft has three
reference axes.  The planes perpendicular  to the longitudinal axis are
are called 'stations'.  These are just reference planes when we talk about
drawings, etc.

While no one FORCES the designers and draftsman to do this (and there have
been aircraft that do NOT use this convention), the convention even from
the earliest days of building ships by lofting (drawings) has been that
the stations are measured from the very nose of the ship or plane.  So
STAtion 100 means the station is located 100 inches from the extreme
nose.  Aircraft designers usually use inches for measurement, shipbuilders
(I think) still use feet.

> 2. WPL (relating to wing) and how are the WPL numbers determined?
> I appreciate any help you may give me.

You got me on WPL

Don Stauffer in Minneapolis
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