Weight and Balance - a micro-rant

At 09:30 28 August 2013, Uncle Fuzzy wrote:
I have a sample size of TWO, so take this or leave it. =20
In BOTH the Speed Astir and Janus C log books, some previous owner had
made=
pen-and-ink' changes to the distance from the main wheel to the tail
wheel=
.. In BOTH cases, the factory figure was correct, and the ink changes
incor=
rect. When leveled per the POH, the factory distance figure was spot on.
Just sayin'

I am really struggling to understand your point. The distance between the
mainwheel axle and the tailwheel axle is always going to be the same
irrespective of the position of the glider. I suppose you could get minor
differences because of expansion but I would have thought that they were
not measureable.
The distance between two points on the ground, described by a line drawn
perpendicular and passing thought the axle centres will of course vary
depending on the attitude of the glider in relation to the ground.
Both measurements are needed.

What you say is true...but the op points out that the inked in W&B used
what you said in your first paragraph, when they should have used the
second........

Over the years I have watched and laughed.... mechanics, glider pilots,
home builders etc. really can't seem to do an actual weighing...

One funny example I've seen more than once... is doing a glider weighing
OUTDOORS!!!!

Gliders and airplanes are like women...they seem to somehow gain a pound or
two each year!

Cookie





At 00:31 29 August 2013, Don Johnstone wrote:
At 09:30 28 August 2013, Uncle Fuzzy wrote:
I have a sample size of TWO, so take this or leave it. =20
In BOTH the Speed Astir and Janus C log books, some previous owner had
made=
pen-and-ink' changes to the distance from the main wheel to the tail
wheel=
.. In BOTH cases, the factory figure was correct, and the ink changes
incor=
rect. When leveled per the POH, the factory distance figure was spot
on.
Just sayin'

I am really struggling to understand your point. The distance between the
mainwheel axle and the tailwheel axle is always going to be the same
irrespective of the position of the glider. I suppose you could get minor
differences because of expansion but I would have thought that they were
not measureable.
The distance between two points on the ground, described by a line drawn
perpendicular and passing thought the axle centres will of course vary
depending on the attitude of the glider in relation to the ground.
Both measurements are needed.

On 29/08/2013 10:31, Don Johnstone wrote:
At 09:30 28 August 2013, Uncle Fuzzy wrote:
I have a sample size of TWO, so take this or leave it. =20
In BOTH the Speed Astir and Janus C log books, some previous owner had
made=
pen-and-ink' changes to the distance from the main wheel to the tail
wheel=
.. In BOTH cases, the factory figure was correct, and the ink changes
incor=
rect. When leveled per the POH, the factory distance figure was spot on.
Just sayin'

I am really struggling to understand your point. The distance between the
mainwheel axle and the tailwheel axle is always going to be the same
irrespective of the position of the glider. I suppose you could get minor
differences because of expansion but I would have thought that they were
not measureable.

No. Gliders with trailing link suspensions have a variable distance
between the axles depending on mass and attitude.

The distance between two points on the ground, described by a line drawn
perpendicular and passing thought the axle centres will of course vary
depending on the attitude of the glider in relation to the ground.
Both measurements are needed.

No. UF is right. Only the distance between perpendiculars is needed.
The actual distance between axle centres is irrelevant. Whoever defaced
his book needs to go back to weighing school.

The process we go through to find a glider's CG position only finds a
'y' axis figure. By defining the attitude when the weighing is done, we
remove a degree of freedom which makes the process much simpler - but
accurate attitude setting is essential.

GC