Weight and Balance - a micro-rant

On 30/08/2013 04:38, Eric Greenwell wrote:[color=blue]
Brad Alston wrote, On 8/28/2013 9:36 PM:
'Uncle Fuzzy[_2_ Wrote:
;844103']On Wednesday, August 28, 2013 2:30:05 AM UTC-7, Uncle Fuzzy
wrote:-

No, different 'previous owners. The measurements that they 'inked'
into the log books were accurate - with both wheels on the hangar
floor. Unfortunately, that's not what the POH calls for. Both manuals
specify 'level' and the Janus, (if I remember) actually gives a
distance. from the floor to the tailwheel. (which, conveniently
corresponds to the height of the box the scale comes in.

Is he difference significant? I doubt it. But I'm kinda' OCD about
specifications (also why I won't use RG-58 for L band (yet another
rant.
LOL-[/blue]

What?...you mean we should read AND understand the POH!? Hmmmm...when
you are throwing your person into the air and trusting in laws of
physics and someone else's engineering, I would think accuracy counts!
But then again, that's just me thinkin' again!

The difference in the two measurements is small (1% or so) and therefore
irrelevant. The critical measurement that will be strongly affected by
measuring in the wrong attitude is the distance of the main wheel axle
from a datum point; in the case of my ASH 26 E, that datum point is the
leading edge of the wing at the root. A 1" error in this measurement
means a 1" error in the location of the CG - a substantial error on wing
chord of only 33".

You're right - but it's actually worse than that, Eric. On my glider
the distance from datum to axle centre is only about an inch. The
percentage error in this measurement (especially given the usual
conditions under which it's obtained ) is likely to be quite high. Also
the solution to the equation is a small difference between two large
numbers. The whole process is very error-prone.

GC