In an old thread (from 2005) somebody gave measured (not factory)
numbers for the occupants' positions. But what I cannot find is the
moment arms for the main wheel and tail wheel. Similarly, the BGA data
sheet for the L23 lists the seat positions (where is the source for
that?) but not the wheel positions. We recently weighed our L23, but
can't compute the W&B without those moment arms. Does anybody have that
info?

Rant on: Why oh why do glider manufacturers often specify W&B in
convoluted ways that are impossible to make practical use of? I don't
need to know the %MAC, or which root rib is the datum. I need to know
how to convert the weights measured at the two wheels to the CG
location, and what is the allowed range of that location. And the
location of the seat(s), to be able to compute the W&B with any given
pilot. And in particular for a 2-seater, the practical loading
guideline is a diagram showing the 2D allowed envelope, with the front
seat occupant weight along one axis, and the rear seat along the other.
Why do I have to make my own such diagram?

On 9/23/2022 10:53 PM, Moshe Braner wrote:
> In an old thread (from 2005) somebody gave measured (not factory)
> numbers for the occupants' positions.  But what I cannot find is the
> moment arms for the main wheel and tail wheel.  Similarly, the BGA data
> sheet for the L23 lists the seat positions (where is the source for
> that?) but not the wheel positions.  We recently weighed our L23, but
> can't compute the W&B without those moment arms.  Does anybody have that
> info?
>
> Rant on: Why oh why do glider manufacturers often specify W&B in
> convoluted ways that are impossible to make practical use of?  I don't
> need to know the %MAC, or which root rib is the datum.  I need to know
> how to convert the weights measured at the two wheels to the CG
> location, and what is the allowed range of that location.  And the
> location of the seat(s), to be able to compute the W&B with any given
> pilot.  And in particular for a 2-seater, the practical loading
> guideline is a diagram showing the 2D allowed envelope, with the front
> seat occupant weight along one axis, and the rear seat along the other.
> Why do I have to make my own such diagram?

The way to level the glider before weighing is also a question to my
mind. The BGA data sheet says:

"Rear fuselage top 1000:51 rear down (points marked on fuselage side)"

We cannot find those marks. We lifted the tail (about 600 mm) until the
top of the tail cone sloped at a 1000:51 angle rear-down, and the glider
looks rather nose-down in that attitude. The 3-view drawing in the POH
shows it in a much more level-looking attitude. And the line in that
drawing that connects the bottoms of the mail wheel and the tail wheel
is (as best as I can measure off that drawing) very close a 1000:51
slope. So is that, perhaps, the reference line for leveling this
glider? That would mean lifting the tail only about 244 mm from my
estimate. (There would be some uncertainty due to tire pressure, main
wheel strut pressure, and tailwheel spring strength.)

Does anybody know the source of the data in the BGA data sheet, for the
leveling and for the front and rear seat positions?

If we can determine how to "level" the glider, we can measure everything
else.

In case you wonder why we feel the need to weigh this glider, it is
because the placard in the cockpit says the minimum solo pilot weight is
137 pounds or so. That is significantly less than the standard 154
pounds, and we don't see any obvious reason why it should be so. Most
often, gliders get more tail-heavy with age due to repairs in the tail
cone. And there is no obvious evidence of repairs up front in this glider.

On Saturday, September 24, 2022 at 9:07:33 AM UTC-7, Moshe Braner wrote:
> On 9/23/2022 10:53 PM, Moshe Braner wrote:
> > In an old thread (from 2005) somebody gave measured (not factory)
> > numbers for the occupants' positions. But what I cannot find is the
> > moment arms for the main wheel and tail wheel. Similarly, the BGA data
> > sheet for the L23 lists the seat positions (where is the source for
> > that?) but not the wheel positions. We recently weighed our L23, but
> > can't compute the W&B without those moment arms. Does anybody have that
> > info?
> >
> > Rant on: Why oh why do glider manufacturers often specify W&B in
> > convoluted ways that are impossible to make practical use of? I don't
> > need to know the %MAC, or which root rib is the datum. I need to know
> > how to convert the weights measured at the two wheels to the CG
> > location, and what is the allowed range of that location. And the
> > location of the seat(s), to be able to compute the W&B with any given
> > pilot. And in particular for a 2-seater, the practical loading
> > guideline is a diagram showing the 2D allowed envelope, with the front
> > seat occupant weight along one axis, and the rear seat along the other.
> > Why do I have to make my own such diagram?
> The way to level the glider before weighing is also a question to my
> mind. The BGA data sheet says:
>
> "Rear fuselage top 1000:51 rear down (points marked on fuselage side)"
>
> We cannot find those marks. We lifted the tail (about 600 mm) until the
> top of the tail cone sloped at a 1000:51 angle rear-down, and the glider
> looks rather nose-down in that attitude. The 3-view drawing in the POH
> shows it in a much more level-looking attitude. And the line in that
> drawing that connects the bottoms of the mail wheel and the tail wheel
> is (as best as I can measure off that drawing) very close a 1000:51
> slope. So is that, perhaps, the reference line for leveling this
> glider? That would mean lifting the tail only about 244 mm from my
> estimate. (There would be some uncertainty due to tire pressure, main
> wheel strut pressure, and tailwheel spring strength.)
>
> Does anybody know the source of the data in the BGA data sheet, for the
> leveling and for the front and rear seat positions?
>
> If we can determine how to "level" the glider, we can measure everything
> else.
>
> In case you wonder why we feel the need to weigh this glider, it is
> because the placard in the cockpit says the minimum solo pilot weight is
> 137 pounds or so. That is significantly less than the standard 154
> pounds, and we don't see any obvious reason why it should be so. Most
> often, gliders get more tail-heavy with age due to repairs in the tail
> cone. And there is no obvious evidence of repairs up front in this glider..
I’ll look at our L-23 manuals when I am at the field later this week. I’ve weighed Blanik’s before and don’t remember it being a big deal. The marks should be a small dimple in the skin.

Charlie

On Sunday, September 25, 2022 at 4:27:46 AM UTC-4, kuzi...@gmail.com wrote:
> On Saturday, September 24, 2022 at 9:07:33 AM UTC-7, Moshe Braner wrote:
> > On 9/23/2022 10:53 PM, Moshe Braner wrote:
> > > In an old thread (from 2005) somebody gave measured (not factory)
> > > numbers for the occupants' positions. But what I cannot find is the
> > > moment arms for the main wheel and tail wheel. Similarly, the BGA data
> > > sheet for the L23 lists the seat positions (where is the source for
> > > that?) but not the wheel positions. We recently weighed our L23, but
> > > can't compute the W&B without those moment arms. Does anybody have that
> > > info?
> > >
> > > Rant on: Why oh why do glider manufacturers often specify W&B in
> > > convoluted ways that are impossible to make practical use of? I don't
> > > need to know the %MAC, or which root rib is the datum. I need to know
> > > how to convert the weights measured at the two wheels to the CG
> > > location, and what is the allowed range of that location. And the
> > > location of the seat(s), to be able to compute the W&B with any given
> > > pilot. And in particular for a 2-seater, the practical loading
> > > guideline is a diagram showing the 2D allowed envelope, with the front
> > > seat occupant weight along one axis, and the rear seat along the other.
> > > Why do I have to make my own such diagram?
> > The way to level the glider before weighing is also a question to my
> > mind. The BGA data sheet says:
> >
> > "Rear fuselage top 1000:51 rear down (points marked on fuselage side)"
> >
> > We cannot find those marks. We lifted the tail (about 600 mm) until the
> > top of the tail cone sloped at a 1000:51 angle rear-down, and the glider
> > looks rather nose-down in that attitude. The 3-view drawing in the POH
> > shows it in a much more level-looking attitude. And the line in that
> > drawing that connects the bottoms of the mail wheel and the tail wheel
> > is (as best as I can measure off that drawing) very close a 1000:51
> > slope. So is that, perhaps, the reference line for leveling this
> > glider? That would mean lifting the tail only about 244 mm from my
> > estimate. (There would be some uncertainty due to tire pressure, main
> > wheel strut pressure, and tailwheel spring strength.)
> >
> > Does anybody know the source of the data in the BGA data sheet, for the
> > leveling and for the front and rear seat positions?
> >
> > If we can determine how to "level" the glider, we can measure everything
> > else.
> >
> > In case you wonder why we feel the need to weigh this glider, it is
> > because the placard in the cockpit says the minimum solo pilot weight is
> > 137 pounds or so. That is significantly less than the standard 154
> > pounds, and we don't see any obvious reason why it should be so. Most
> > often, gliders get more tail-heavy with age due to repairs in the tail
> > cone. And there is no obvious evidence of repairs up front in this glider.
> I’ll look at our L-23 manuals when I am at the field later this week. I’ve weighed Blanik’s before and don’t remember it being a big deal. The marks should be a small dimple in the skin.
>
> Charlie

Hopefully this can help. I can supply a spreadsheet via direct email.
Bill Tisdale

Airframe constants: Factory weighing May 31, 2000
Empty CG range 25.75 to 26.75" From Type Certificate. 26.25" +/- 0.5"
Max Gross weight 1,124.0 Lb From POH. Page 2-3
MaxPayload 427.00 Lb Calculated from Max all up weight - glider empty weight
Max Seat 242 Lb From POH. Page 2-4
Max baggage 22 Lb From POH. Page 2-4
Removable Ballast 33 Lb From POH. Page 2-4
In Flt Fwd CG limit 4.397 In From POH. Page 2-5
In Flt Aft CG limit 12.783 In From POH. Page 2-5
Empty Weight 697 LB From POH Empty Weight & Balance. Page 6-5
Empty CG % MAC 67.3 % 67.3% MAC From POH Empty Weight & Balance. Page 6-5
Mean Aerodynamic Chord MAC 4.11 FT From POH. Page 1-3
Dist from MAC to ref plane -6.95 in From Blanik Maintence mnual section 8 Page 3
Empty CG 26.24 in Calculated from MAC & empty CG % MAC

Item Weight Arm Moment
Empty weight 697.00 26.24 18,290.92 Weight & moment Arm from above
Front Pilot 242 lb Max 190 -48.6 -9,234.00 Moment Arm from Type Certificate Data Sheet
Ballast 33 Lb 0 -48.6 0.00 Ballst 33 lb from POH above
Rear Pilot 242 lb Max 130 -6.7 -871.00 Moment Arm from Type Certificate Data Sheet
Baggage 22 Lb Max 0 0.00 Max baggage From POH
Total weight: 1,017.00 Total Moment: 8,185.92

Total Weight 1,017.00 Max all up weight: 1,124.0 Lb. Max all up weight From POH above
Total Payload 320.00 Max Payload: 427.0 Lb. Max payload from POH above
In Flt CG 8.05 Range: 4.397" to 12.783" W/O wing tips. From POH. Page 2-5
Wing tips add 12 Lb & move CG aft 0.493"