Also seen a few times errors in initial W&CG, usually because of:-
- Confusion of aft versus fwd of datum
- Occasional mix up of units.
- Occasional mix up because of two similar types using different CG datums or slightly different moment arms for standard items.
- Hate to say it, but people who just do the sums wrong, use wrong moment arms, etc. I spent a happy half day a while ago sorting out a monstrous mess made by a well known "plane weighing" limited company of the W&CG for somebody's warbird that I was consulting on. They simply applied no common sense to the CG envelope, tried to shoehorn a 2-seat tandem taildragger into a standard form designed around godnosewhat and ended up with something totally unuseable.
As skut says, this is all down to human error, lack of training and competence, and lack of proper documentation control - nothing more complex than that.
I don't know where you get 5% windage error from? Manufacturers have a stack of wind tunnel data. That is how they designed a portotype and had it certified. You can also fall back on the Lift formula and good old Sine curve.
Negative. Aerodynamics is not that simple, and wind tunnel data used for design rarely take into account either ground effect or (probably impossible to) local airflow variations within the aircraft span (such as rotor over a nearby hangar). My 5% was a back of envelope sum by me only, but I think fairly realistic.
Dr.G