Weight and Balance - a micro-rant

I have a sample size of TWO, so take this or leave it.
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 incorrect. When leveled per the POH, the factory distance figure was spot on.
Just sayin'

Originally Posted by Uncle Fuzzy[_2_]

I have a sample size of TWO, so take this or leave it.
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 incorrect. When leveled per the POH, the factory distance figure was spot on.
Just sayin'

Same "previous owner" for both ships? Now that would be an interesting twist! I assume the measurement units were consistent between the factory and previous owner...just thinkin'!

Brad.

On Wednesday, August 28, 2013 2:30:05 AM UTC-7, Uncle Fuzzy wrote:
I have a sample size of TWO, so take this or leave it.

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 incorrect. When leveled per the POH, the factory distance figure was spot on.

Just sayin'

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

Originally Posted by Uncle Fuzzy[_2_]

On Wednesday, August 28, 2013 2:30:05 AM UTC-7, Uncle Fuzzy wrote:[color=blue]

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!

Brad.

Brad Alston wrote, On 8/28/2013 9:36 PM:[color=blue]
'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".

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
http://tinyurl.com/yb3xywl

On Thursday, August 29, 2013 11:38:23 AM UTC-7, 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".



--

Eric Greenwell - Washington State, USA (change ".netto" to ".us" to

email me)

- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm

http://tinyurl.com/yb3xywl

The weight distribution between the main wheel and the tail wheel/skid changes with the aircraft attitude because the vertical CG is above the tire contact patch. Like the others have said. Follow the manual.

Craig

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

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