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All aircraft, commercial built or amateur, have two critical references:

1) Lateral Level (LL) - Used to measure dihedral, perpendicular to lift and
crucial to the placement of horizontal and vertical structures, including the
positioning of the "ball" and attitude indicator.  Also used in W&B
measurements.  The usual and best position for the designer, builder and
future user is across the longerons over the spar and should be used for ALL
lateral level requirements.

2) Pitch Level (PL) - Crucially used to set wing and horizontal stabilizer
incidence, limit the lean of instrument panels, W&B measurements, fuel
calibrations, attitude indicator, static port holes, etc.  The designer has
always worked from some PL reference line established for cruise flight and
all other things related to pitch are relative to this reference.  Usually
this is the longeron (such as original slow built Lancair 320s).  If the
longeron is not PL reference, its' angle from level must be known because,
after the plane is built, it will be the only easy place to put a digital
level (or an angled wedge with a standard level) to establish the plane in PL
reference.  

3) Center Line Reference (CLR) - This is an important reference for many
things and should be marked early in the building process.  A method to do
this is to run a string thru the firewall hole and back to a center point
established at the tail (5 inches above the bottom, etc.).  Whilst the plane
is LL and PL leveled, using a plumb line and other vertical references,
permanent marking pen lines should be drawn, identifying a "straight flight"
reference, on the main spar, belly and ceiling for the later positioning of
accessories, etc.  These might include ADF antennas, remote compasses, and any
other things which could use such a reference.

Note: A really clever builder might consider permanently imbedding miniature
level indicators (stolen from a small level) in a longeron and the rollover to
make the many future upright leveling tasks quick and easy (unfortunately, I
wasn't clever).

I'm on the level,
Scott Krueger
N92EX