So the question came up, and the dispute immediately followed, as to whether or not this tool would meet the FAA requirements (I'll not list them because I'll assume in advance anybody with something higher than a student pilot certificate knows how to find them) to satisfy the burden for flight planning under the FARs.

I've played with it a bit. It does provide for the user to input empty weight and moment.

There isn't shit in the FAA manual on weight and balance (For pilots) and even less in the AC on aircraft weighing.

Any thoughts?

https://www.myhra.org/html/ask21weightbalance.html

On Wednesday, October 6, 2021 at 7:35:38 PM UTC-7, shaun_...@hotmail.com wrote:
> So the question came up, and the dispute immediately followed, as to whether or not this tool would meet the FAA requirements (I'll not list them because I'll assume in advance anybody with something higher than a student pilot certificate knows how to find them) to satisfy the burden for flight planning under the FARs.
>
> I've played with it a bit. It does provide for the user to input empty weight and moment.
>
> There isn't shit in the FAA manual on weight and balance (For pilots) and even less in the AC on aircraft weighing.
>
> Any thoughts?
>
> https://www.myhra.org/html/ask21weightbalance.html

Go to your POH - every one should have a procedure for doing the weight and balance.

Tom

On 10/7/2021 1:35 AM, 2G wrote:
> On Wednesday, October 6, 2021 at 7:35:38 PM UTC-7, shaun_...@hotmail.com wrote:
>> So the question came up, and the dispute immediately followed, as to whether or not this tool would meet the FAA requirements (I'll not list them because I'll assume in advance anybody with something higher than a student pilot certificate knows how to find them) to satisfy the burden for flight planning under the FARs.
>>
>> I've played with it a bit. It does provide for the user to input empty weight and moment.
>>
>> There isn't shit in the FAA manual on weight and balance (For pilots) and even less in the AC on aircraft weighing.
>>
>> Any thoughts?
>>
>> https://www.myhra.org/html/ask21weightbalance.html
>
> Go to your POH - every one should have a procedure for doing the weight and balance.
>
> Tom

Alas the POH in many cases has the W&B info in a format that is totally
unusable for answering the practical question: I weigh this much, am I
within the range? Especially so for two-seaters, where the two
occupants' weights are involved but not equally. The best format is a
loading diagram graph that has the weight in each seat on the two axes
and the acceptable envelope delineated within. The 2-33 POH has such.
I've made such a diagram for our Blanik.

The tool linked in this thread mostly shows you how its calculation was
made, so you can check on it. It fails to explicitly list the moment
arms of the two seats.

Trick question: can you determine the W&B of a glider without ever
weighing the full weight of the assembled glider?

I'm with Moshe....why look at a screen and tap a bunch of times when you can look at a W&B graph stored in the side pocket. KISS
Though having the calculator on one's phone in the event the graph is loss is good planning. Better, take a picture of the graph.

Glider W&B always drifts to experience and 'Kentucky Windage' providing the answers. But, instructors have to drill procedures covering everything into students so the importance of checking becomes standard practice.
Weigh & Balance being a good example. Quick reference Charts works best is my thought.
Answer to Moshe.....Yes...put the fat guy in the front seat and if you can push the tail down everything is Jewish.
Right?
R

Consider giving SeeG a try (http://www.glideplan.com/styled-2/page12/index..html) You can create moments as needed, and there are all sorts of tools for exploring W/B. For example plot a curve of C/G vs water loading, or back calculate any moment arm from two weighings. Make multiple versions of your plane: with and without 18m tips, etc. tells you wing loading, % aft C/G, allows limits on loading and alerts you (like baggage max, and all up max wt.), etc. Created for gliders (single or dual). Free fully functional demo (just can't save results until activated).

Shameless plug by Author, Matt

On Wednesday, October 6, 2021 at 7:35:38 PM UTC-7, shaun_...@hotmail.com wrote:
> So the question came up, and the dispute immediately followed, as to whether or not this tool would meet the FAA requirements (I'll not list them because I'll assume in advance anybody with something higher than a student pilot certificate knows how to find them) to satisfy the burden for flight planning under the FARs.
>
> I've played with it a bit. It does provide for the user to input empty weight and moment.
>
> There isn't shit in the FAA manual on weight and balance (For pilots) and even less in the AC on aircraft weighing.
>
> Any thoughts?
>
> https://www.myhra.org/html/ask21weightbalance.html

On Thursday, October 7, 2021 at 4:47:14 AM UTC-7, Moshe Braner wrote:
> On 10/7/2021 1:35 AM, 2G wrote:
> > On Wednesday, October 6, 2021 at 7:35:38 PM UTC-7, shaun_...@hotmail.com wrote:
> >> So the question came up, and the dispute immediately followed, as to whether or not this tool would meet the FAA requirements (I'll not list them because I'll assume in advance anybody with something higher than a student pilot certificate knows how to find them) to satisfy the burden for flight planning under the FARs.
> >>
> >> I've played with it a bit. It does provide for the user to input empty weight and moment.
> >>
> >> There isn't shit in the FAA manual on weight and balance (For pilots) and even less in the AC on aircraft weighing.
> >>
> >> Any thoughts?
> >>
> >> https://www.myhra.org/html/ask21weightbalance.html
> >
> > Go to your POH - every one should have a procedure for doing the weight and balance.
> >
> > Tom
> Alas the POH in many cases has the W&B info in a format that is totally
> unusable for answering the practical question: I weigh this much, am I
> within the range? Especially so for two-seaters, where the two
> occupants' weights are involved but not equally. The best format is a
> loading diagram graph that has the weight in each seat on the two axes
> and the acceptable envelope delineated within. The 2-33 POH has such.
> I've made such a diagram for our Blanik.
>
> The tool linked in this thread mostly shows you how its calculation was
> made, so you can check on it. It fails to explicitly list the moment
> arms of the two seats.
>
> Trick question: can you determine the W&B of a glider without ever
> weighing the full weight of the assembled glider?

The POH should have weight limits for both occupants and a way of determining the W&B given their two weights. If not, do a W&B and put a person, one at a time, in each seat. You can back calculate what the moment arm is for each seat, which is the critical piece of information you need to do a W&B with persons of different weights. Remember, weight in the passenger seat moves the CG forward, taking it out moves it backwards. This is why there is a minimum weight in the forward seat.

Yes, you can do a W&B of an unassembled glider knowing the moment arms of each component.

BTW, a moment arm is the distance from a datum point. Placing a weight at that position creates either a positive moment (weight ahead of the datum) or a negative moment (weight behind the datum). Obviously, the tail boom has the largest moment arm, so the CG is most sensitive to any weight at this point. Moral of the story: DON'T leave your tail dolly on when you launch!

Tom

PS. The moment arm for a body is an educated guess. People's bodies are built differently, so the effective moment arm is not the same. Even the type of parachute (back or seat) can make a difference. If you want to be absolutely sure do a W&B with the pilot flying in the glider. Basically, you want to know if the flown CG is safe - it can vary over a range w/o affecting safety. But a CG that is close to the aft limit is more susceptible to turbulent upsets.

On Friday, October 8, 2021 at 5:00:55 PM UTC-7, 2G wrote:
> On Thursday, October 7, 2021 at 4:47:14 AM UTC-7, Moshe Braner wrote:
> > On 10/7/2021 1:35 AM, 2G wrote:
> > > On Wednesday, October 6, 2021 at 7:35:38 PM UTC-7, shaun_...@hotmail.com wrote:
> > >> So the question came up, and the dispute immediately followed, as to whether or not this tool would meet the FAA requirements (I'll not list them because I'll assume in advance anybody with something higher than a student pilot certificate knows how to find them) to satisfy the burden for flight planning under the FARs.
> > >>
> > >> I've played with it a bit. It does provide for the user to input empty weight and moment.
> > >>
> > >> There isn't shit in the FAA manual on weight and balance (For pilots) and even less in the AC on aircraft weighing.
> > >>
> > >> Any thoughts?
> > >>
> > >> https://www.myhra.org/html/ask21weightbalance.html
> > >
> > > Go to your POH - every one should have a procedure for doing the weight and balance.
> > >
> > > Tom
> > Alas the POH in many cases has the W&B info in a format that is totally
> > unusable for answering the practical question: I weigh this much, am I
> > within the range? Especially so for two-seaters, where the two
> > occupants' weights are involved but not equally. The best format is a
> > loading diagram graph that has the weight in each seat on the two axes
> > and the acceptable envelope delineated within. The 2-33 POH has such.
> > I've made such a diagram for our Blanik.
> >
> > The tool linked in this thread mostly shows you how its calculation was
> > made, so you can check on it. It fails to explicitly list the moment
> > arms of the two seats.
> >
> > Trick question: can you determine the W&B of a glider without ever
> > weighing the full weight of the assembled glider?
> The POH should have weight limits for both occupants and a way of determining the W&B given their two weights. If not, do a W&B and put a person, one at a time, in each seat. You can back calculate what the moment arm is for each seat, which is the critical piece of information you need to do a W&B with persons of different weights. Remember, weight in the passenger seat moves the CG forward, taking it out moves it backwards. This is why there is a minimum weight in the forward seat.
>
> Yes, you can do a W&B of an unassembled glider knowing the moment arms of each component.
>
> BTW, a moment arm is the distance from a datum point. Placing a weight at that position creates either a positive moment (weight ahead of the datum) or a negative moment (weight behind the datum). Obviously, the tail boom has the largest moment arm, so the CG is most sensitive to any weight at this point. Moral of the story: DON'T leave your tail dolly on when you launch!
>
> Tom
>
> PS. The moment arm for a body is an educated guess. People's bodies are built differently, so the effective moment arm is not the same. Even the type of parachute (back or seat) can make a difference. If you want to be absolutely sure do a W&B with the pilot flying in the glider. Basically, you want to know if the flown CG is safe - it can vary over a range w/o affecting safety. But a CG that is close to the aft limit is more susceptible to turbulent upsets.

I should explain what I mean by "sensitivity." Sensitivity is the percentage change in the CG given a percentage change in a particular weight (such as the pilot's weight). Items close to the datum point, such as water ballast, will have a low sensitivity, while weights farther away, such as tail ballast, will have a much higher sensitivity.

Tom

On 10/8/2021 8:10 PM, 2G wrote:
> On Friday, October 8, 2021 at 5:00:55 PM UTC-7, 2G wrote:
>> On Thursday, October 7, 2021 at 4:47:14 AM UTC-7, Moshe Braner wrote:
>>> On 10/7/2021 1:35 AM, 2G wrote:
>>>> On Wednesday, October 6, 2021 at 7:35:38 PM UTC-7, shaun_...@hotmail.com wrote:
>>>>> So the question came up, and the dispute immediately followed, as to whether or not this tool would meet the FAA requirements (I'll not list them because I'll assume in advance anybody with something higher than a student pilot certificate knows how to find them) to satisfy the burden for flight planning under the FARs.
>>>>>
>>>>> I've played with it a bit. It does provide for the user to input empty weight and moment.
>>>>>
>>>>> There isn't shit in the FAA manual on weight and balance (For pilots) and even less in the AC on aircraft weighing.
>>>>>
>>>>> Any thoughts?
>>>>>
>>>>> https://www.myhra.org/html/ask21weightbalance.html
>>>>
>>>> Go to your POH - every one should have a procedure for doing the weight and balance.
>>>>
>>>> Tom
>>> Alas the POH in many cases has the W&B info in a format that is totally
>>> unusable for answering the practical question: I weigh this much, am I
>>> within the range? Especially so for two-seaters, where the two
>>> occupants' weights are involved but not equally. The best format is a
>>> loading diagram graph that has the weight in each seat on the two axes
>>> and the acceptable envelope delineated within. The 2-33 POH has such.
>>> I've made such a diagram for our Blanik.
>>>
>>> The tool linked in this thread mostly shows you how its calculation was
>>> made, so you can check on it. It fails to explicitly list the moment
>>> arms of the two seats.
>>>
>>> Trick question: can you determine the W&B of a glider without ever
>>> weighing the full weight of the assembled glider?
>> The POH should have weight limits for both occupants and a way of determining the W&B given their two weights. If not, do a W&B and put a person, one at a time, in each seat. You can back calculate what the moment arm is for each seat, which is the critical piece of information you need to do a W&B with persons of different weights. Remember, weight in the passenger seat moves the CG forward, taking it out moves it backwards. This is why there is a minimum weight in the forward seat.
>>
>> Yes, you can do a W&B of an unassembled glider knowing the moment arms of each component.
>>
>> BTW, a moment arm is the distance from a datum point. Placing a weight at that position creates either a positive moment (weight ahead of the datum) or a negative moment (weight behind the datum). Obviously, the tail boom has the largest moment arm, so the CG is most sensitive to any weight at this point. Moral of the story: DON'T leave your tail dolly on when you launch!
>>
>> Tom
>>
>> PS. The moment arm for a body is an educated guess. People's bodies are built differently, so the effective moment arm is not the same. Even the type of parachute (back or seat) can make a difference. If you want to be absolutely sure do a W&B with the pilot flying in the glider. Basically, you want to know if the flown CG is safe - it can vary over a range w/o affecting safety. But a CG that is close to the aft limit is more susceptible to turbulent upsets.
>
> I should explain what I mean by "sensitivity." Sensitivity is the percentage change in the CG given a percentage change in a particular weight (such as the pilot's weight). Items close to the datum point, such as water ballast, will have a low sensitivity, while weights farther away, such as tail ballast, will have a much higher sensitivity.
>
> Tom

Right. Except, to be pedantic: Items close to the *CG*, such as water
ballast, will have a low sensitivity. The "datum point" is totally
arbitrary, if you re-do the calculation with a different datum point
(calculating the correct "arms" and CG range relative to that new datum
point) you will get the exact same result (% of CG range). Indeed for
some gliders the official datum point is the tip of the nose!

Curious fact: items far from the CG affect the CG location to the same
extent whether a heavy object right at the CG is present or absent.

Regarding doing the W&B of a glider without ever weighing the full
weight of the assembled glider, I mean a full W&B from scratch.
Following, of course, the factory guidance on moment arms etc. How?
Hint: What you do have to do is measure the weight at the tail of the
assembled (and leveled) glider. But you don't have to weigh the weight
at the main wheel, if you do something else instead. Spreadsheet on
request...

And yes people come in different shapes, and the best way to determine
whether a given pilot (with a given chute) is within range (if in doubt)
is to weigh the tail with the pilot seated (and the glider leveled).

On Friday, October 8, 2021 at 5:48:20 PM UTC-7, Moshe Braner wrote:
> On 10/8/2021 8:10 PM, 2G wrote:
> > On Friday, October 8, 2021 at 5:00:55 PM UTC-7, 2G wrote:
> >> On Thursday, October 7, 2021 at 4:47:14 AM UTC-7, Moshe Braner wrote:
> >>> On 10/7/2021 1:35 AM, 2G wrote:
> >>>> On Wednesday, October 6, 2021 at 7:35:38 PM UTC-7, shaun_...@hotmail..com wrote:
> >>>>> So the question came up, and the dispute immediately followed, as to whether or not this tool would meet the FAA requirements (I'll not list them because I'll assume in advance anybody with something higher than a student pilot certificate knows how to find them) to satisfy the burden for flight planning under the FARs.
> >>>>>
> >>>>> I've played with it a bit. It does provide for the user to input empty weight and moment.
> >>>>>
> >>>>> There isn't shit in the FAA manual on weight and balance (For pilots) and even less in the AC on aircraft weighing.
> >>>>>
> >>>>> Any thoughts?
> >>>>>
> >>>>> https://www.myhra.org/html/ask21weightbalance.html
> >>>>
> >>>> Go to your POH - every one should have a procedure for doing the weight and balance.
> >>>>
> >>>> Tom
> >>> Alas the POH in many cases has the W&B info in a format that is totally
> >>> unusable for answering the practical question: I weigh this much, am I
> >>> within the range? Especially so for two-seaters, where the two
> >>> occupants' weights are involved but not equally. The best format is a
> >>> loading diagram graph that has the weight in each seat on the two axes
> >>> and the acceptable envelope delineated within. The 2-33 POH has such.
> >>> I've made such a diagram for our Blanik.
> >>>
> >>> The tool linked in this thread mostly shows you how its calculation was
> >>> made, so you can check on it. It fails to explicitly list the moment
> >>> arms of the two seats.
> >>>
> >>> Trick question: can you determine the W&B of a glider without ever
> >>> weighing the full weight of the assembled glider?
> >> The POH should have weight limits for both occupants and a way of determining the W&B given their two weights. If not, do a W&B and put a person, one at a time, in each seat. You can back calculate what the moment arm is for each seat, which is the critical piece of information you need to do a W&B with persons of different weights. Remember, weight in the passenger seat moves the CG forward, taking it out moves it backwards. This is why there is a minimum weight in the forward seat.
> >>
> >> Yes, you can do a W&B of an unassembled glider knowing the moment arms of each component.
> >>
> >> BTW, a moment arm is the distance from a datum point. Placing a weight at that position creates either a positive moment (weight ahead of the datum) or a negative moment (weight behind the datum). Obviously, the tail boom has the largest moment arm, so the CG is most sensitive to any weight at this point. Moral of the story: DON'T leave your tail dolly on when you launch!
> >>
> >> Tom
> >>
> >> PS. The moment arm for a body is an educated guess. People's bodies are built differently, so the effective moment arm is not the same. Even the type of parachute (back or seat) can make a difference. If you want to be absolutely sure do a W&B with the pilot flying in the glider. Basically, you want to know if the flown CG is safe - it can vary over a range w/o affecting safety. But a CG that is close to the aft limit is more susceptible to turbulent upsets.
> >
> > I should explain what I mean by "sensitivity." Sensitivity is the percentage change in the CG given a percentage change in a particular weight (such as the pilot's weight). Items close to the datum point, such as water ballast, will have a low sensitivity, while weights farther away, such as tail ballast, will have a much higher sensitivity.
> >
> > Tom
> Right. Except, to be pedantic: Items close to the *CG*, such as water
> ballast, will have a low sensitivity. The "datum point" is totally
> arbitrary, if you re-do the calculation with a different datum point
> (calculating the correct "arms" and CG range relative to that new datum
> point) you will get the exact same result (% of CG range). Indeed for
> some gliders the official datum point is the tip of the nose!
>
> Curious fact: items far from the CG affect the CG location to the same
> extent whether a heavy object right at the CG is present or absent.
>
> Regarding doing the W&B of a glider without ever weighing the full
> weight of the assembled glider, I mean a full W&B from scratch.
> Following, of course, the factory guidance on moment arms etc. How?
> Hint: What you do have to do is measure the weight at the tail of the
> assembled (and leveled) glider. But you don't have to weigh the weight
> at the main wheel, if you do something else instead. Spreadsheet on
> request...
>
> And yes people come in different shapes, and the best way to determine
> whether a given pilot (with a given chute) is within range (if in doubt)
> is to weigh the tail with the pilot seated (and the glider leveled).

The datum point is not arbitrary - all of the moment arms provided by the manufacturer are referenced to it.

You are just restating what I already wrote, so what is your point, to make yourself look smart?

Tom

Moshe is correct that the datum point is arbitrarily picked, though specified by the manufacturer or whoever did the official W&B. Most will pick a point somewhere near the CG, but it is not necessary to do so. Could be the nose, could be the tailwheel. He is also correct that "sensitivity" depends on the moment arm from the CG, not the datum. Those might be nearly the same, or might not be.
On Friday, October 8, 2021 at 10:15:22 PM UTC-7, 2G wrote:
> On Friday, October 8, 2021 at 5:48:20 PM UTC-7, Moshe Braner wrote:
> > On 10/8/2021 8:10 PM, 2G wrote:
> > > On Friday, October 8, 2021 at 5:00:55 PM UTC-7, 2G wrote:
> > >> On Thursday, October 7, 2021 at 4:47:14 AM UTC-7, Moshe Braner wrote:
> > >>> On 10/7/2021 1:35 AM, 2G wrote:
> > >>>> On Wednesday, October 6, 2021 at 7:35:38 PM UTC-7, shaun_...@hotmail.com wrote:
> > >>>>> So the question came up, and the dispute immediately followed, as to whether or not this tool would meet the FAA requirements (I'll not list them because I'll assume in advance anybody with something higher than a student pilot certificate knows how to find them) to satisfy the burden for flight planning under the FARs.
> > >>>>>
> > >>>>> I've played with it a bit. It does provide for the user to input empty weight and moment.
> > >>>>>
> > >>>>> There isn't shit in the FAA manual on weight and balance (For pilots) and even less in the AC on aircraft weighing.
> > >>>>>
> > >>>>> Any thoughts?
> > >>>>>
> > >>>>> https://www.myhra.org/html/ask21weightbalance.html
> > >>>>
> > >>>> Go to your POH - every one should have a procedure for doing the weight and balance.
> > >>>>
> > >>>> Tom
> > >>> Alas the POH in many cases has the W&B info in a format that is totally
> > >>> unusable for answering the practical question: I weigh this much, am I
> > >>> within the range? Especially so for two-seaters, where the two
> > >>> occupants' weights are involved but not equally. The best format is a
> > >>> loading diagram graph that has the weight in each seat on the two axes
> > >>> and the acceptable envelope delineated within. The 2-33 POH has such.
> > >>> I've made such a diagram for our Blanik.
> > >>>
> > >>> The tool linked in this thread mostly shows you how its calculation was
> > >>> made, so you can check on it. It fails to explicitly list the moment
> > >>> arms of the two seats.
> > >>>
> > >>> Trick question: can you determine the W&B of a glider without ever
> > >>> weighing the full weight of the assembled glider?
> > >> The POH should have weight limits for both occupants and a way of determining the W&B given their two weights. If not, do a W&B and put a person, one at a time, in each seat. You can back calculate what the moment arm is for each seat, which is the critical piece of information you need to do a W&B with persons of different weights. Remember, weight in the passenger seat moves the CG forward, taking it out moves it backwards. This is why there is a minimum weight in the forward seat.
> > >>
> > >> Yes, you can do a W&B of an unassembled glider knowing the moment arms of each component.
> > >>
> > >> BTW, a moment arm is the distance from a datum point. Placing a weight at that position creates either a positive moment (weight ahead of the datum) or a negative moment (weight behind the datum). Obviously, the tail boom has the largest moment arm, so the CG is most sensitive to any weight at this point. Moral of the story: DON'T leave your tail dolly on when you launch!
> > >>
> > >> Tom
> > >>
> > >> PS. The moment arm for a body is an educated guess. People's bodies are built differently, so the effective moment arm is not the same. Even the type of parachute (back or seat) can make a difference. If you want to be absolutely sure do a W&B with the pilot flying in the glider. Basically, you want to know if the flown CG is safe - it can vary over a range w/o affecting safety. But a CG that is close to the aft limit is more susceptible to turbulent upsets.
> > >
> > > I should explain what I mean by "sensitivity." Sensitivity is the percentage change in the CG given a percentage change in a particular weight (such as the pilot's weight). Items close to the datum point, such as water ballast, will have a low sensitivity, while weights farther away, such as tail ballast, will have a much higher sensitivity.
> > >
> > > Tom
> > Right. Except, to be pedantic: Items close to the *CG*, such as water
> > ballast, will have a low sensitivity. The "datum point" is totally
> > arbitrary, if you re-do the calculation with a different datum point
> > (calculating the correct "arms" and CG range relative to that new datum
> > point) you will get the exact same result (% of CG range). Indeed for
> > some gliders the official datum point is the tip of the nose!
> >
> > Curious fact: items far from the CG affect the CG location to the same
> > extent whether a heavy object right at the CG is present or absent.
> >
> > Regarding doing the W&B of a glider without ever weighing the full
> > weight of the assembled glider, I mean a full W&B from scratch.
> > Following, of course, the factory guidance on moment arms etc. How?
> > Hint: What you do have to do is measure the weight at the tail of the
> > assembled (and leveled) glider. But you don't have to weigh the weight
> > at the main wheel, if you do something else instead. Spreadsheet on
> > request...
> >
> > And yes people come in different shapes, and the best way to determine
> > whether a given pilot (with a given chute) is within range (if in doubt)
> > is to weigh the tail with the pilot seated (and the glider leveled).
> The datum point is not arbitrary - all of the moment arms provided by the manufacturer are referenced to it.
>
> You are just restating what I already wrote, so what is your point, to make yourself look smart?
>
> Tom

It's been so long since I looked at the maintenance manual for my ASH26E, I couldn't
remember where it's datum point was, so I had to look it up: it is the wing LE at the
fuselage, an easy to use and find point. Some day I will have a flight computer so smart,
all I will have to do is give it the basic weights and it will do the heavy thinking for
me. My dream is the glider has force sensors on the main and tail wheels, so I don't even
have to give it any weights! But then, I suppose, the flight computer would feel obligated
to scold me when I "arrive" rather suddenly during the landing.

On 10/8/2021 11:02 PM, jfitch wrote:
> Moshe is correct that the datum point is arbitrarily picked, though specified by the manufacturer or whoever did the official W&B. Most will pick a point somewhere near the CG, but it is not necessary to do so. Could be the nose, could be the tailwheel. He is also correct that "sensitivity" depends on the moment arm from the CG, not the datum. Those might be nearly the same, or might not be.

--
Eric Greenwell - USA
- "A Guide to Self-launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1

To answer the ops question on whether his W&B tool would satisfy the FAA, I answer no unless it is in the FHB.
The glider manufacturer would need to list the tool as an approved method. Thinking of course that the tool is an app. and not just a math calculator.
No way for the Field Inspector to determine the accuracy of the tool and he has been trained to never accept responsibility as has his cronies back at the office.
I mentioned above creating and placing a graph in our trainers. I also included on the backside a sample W & B with all the numbers and data in the event a DE or FAA wanted the long version. E=MC2 and all that.

R

On 10/9/2021 8:50 AM, Eric Greenwell wrote:
> It's been so long since I looked at the maintenance manual for my
> ASH26E, I couldn't remember where it's datum point was, so I had to look
> it up: it is the wing LE at the fuselage, an easy to use and find point.
> Some day I will have a flight computer so smart, all I will have to do
> is give it the basic weights and it will do the heavy thinking for me.
> My dream is the glider has force sensors on the main and tail wheels, so
> I don't even have to give it any weights! But then, I suppose, the
> flight computer would feel obligated to scold me when I "arrive" rather
> suddenly during the landing.
>
> On 10/8/2021 11:02 PM, jfitch wrote:
>> Moshe is correct that the datum point is arbitrarily picked, though
>> specified by the manufacturer or whoever did the official W&B. Most
>> will pick a point somewhere near the CG, but it is not necessary to do
>> so. Could be the nose, could be the tailwheel. He is also correct that
>> "sensitivity" depends on the moment arm from the CG, not the datum.
>> Those might be nearly the same, or might not be.
>

The wing LE at the fuselage is easy to think about, but hard to locate
with enough precision, given the rounded fairing, etc. More
importantly, it is hard to accurately measure the distance (along the
long axis of the leveled glider) of any other given point from the
datum. So what we use is the factory-given distances for the important
points (such as the tailwheel), and close-enough estimates for others
(such as the instrument panel - each instrument has its CG at some
unique distance behind the panel anyway).

Since I am light weight and have to worry about the CG being too far
aft, for every glider I fly I set up a spreadsheet that computes the
W&B, based on all available information (POH, latest official W&B sheet,
etc). After that it's easy to account for added equipment or ballast,
etc. In most cases, going over the info carefully, I found errors in
previous calculations. A negative sign dropped, a typo in a moment arm,
etc. When the difference is between 30% and 35% of the CG range, no
harm done. When it's between 98% and 103% of the CG range, it's serious.

On Saturday, October 9, 2021 at 5:50:27 AM UTC-7, Eric Greenwell wrote:
> It's been so long since I looked at the maintenance manual for my ASH26E, I couldn't
> remember where it's datum point was, so I had to look it up: it is the wing LE at the
> fuselage, an easy to use and find point. Some day I will have a flight computer so smart,
> all I will have to do is give it the basic weights and it will do the heavy thinking for
> me. My dream is the glider has force sensors on the main and tail wheels, so I don't even
> have to give it any weights! But then, I suppose, the flight computer would feel obligated
> to scold me when I "arrive" rather suddenly during the landing.
> On 10/8/2021 11:02 PM, jfitch wrote:
> > Moshe is correct that the datum point is arbitrarily picked, though specified by the manufacturer or whoever did the official W&B. Most will pick a point somewhere near the CG, but it is not necessary to do so. Could be the nose, could be the tailwheel. He is also correct that "sensitivity" depends on the moment arm from the CG, not the datum. Those might be nearly the same, or might not be.
> --
> Eric Greenwell - USA
> - "A Guide to Self-launching Sailplane Operation"
> https://sites.google.com/site/motorgliders/publications/download-the-guide-1

Not long after I first started work at the, then, Pacific Northwest Laboratory a scientist who was a pilot came by and asked us about doing exactly that. The idea didn't go anywhere because the cost was just too high to justify it. An accurate W&B has to be done in a hangar because any breeze will make the readings invalid. I watched one Alaska bush pilot program where the judged the W&B by the tail not dropping to the ground once loaded. I had one CFI ask me, at a BFR, how to improve fuel mileage. Answer: load a lot of baggage on the plane to move the CG to the aft. I reminded him about CG limits and he became silent.

The datum point may seem to be arbitrary, but it isn't. All the W&B calculations are based on it. Yes, the manufacturer can chose where they locate it, but once chosen that is it and you MUST use that point for all future measurements. We did a W&B on my new 31Mi which involved measuring the actual moment arms of the main and tail wheels. This was done empty and with me in the glider with my parachute.

One thought: the W&B is most sensitive to the tail weight. So, once you have done an accurate W&B you can do a quick check on any changes by just measuring the tail weight. This involves lifting the tail to the flying attitude (which is a real PITA for the 31Mi!), but after knowing what the weight is in that position and how much the weight changes when resting on the ground you can just use the ground weight to detect any changes that would warrant doing another full W&B.

Tom

On 10/9/2021 2:00 PM, 2G wrote:
> ...
> The datum point may seem to be arbitrary, but it isn't. All the W&B calculations are based on it. Yes, the manufacturer can chose where they locate it, but once chosen that is it and you MUST use that point for all future measurements. We did a W&B on my new 31Mi which involved measuring the actual moment arms of the main and tail wheels. This was done empty and with me in the glider with my parachute.
>
> One thought: the W&B is most sensitive to the tail weight. So, once you have done an accurate W&B you can do a quick check on any changes by just measuring the tail weight. This involves lifting the tail to the flying attitude (which is a real PITA for the 31Mi!), but after knowing what the weight is in that position and how much the weight changes when resting on the ground you can just use the ground weight to detect any changes that would warrant doing another full W&B.
>
> Tom
>

I'd bet that W&B on the 31Mi did not involve "measuring the actual
moment arms of the main and tail wheels", rather, measuring the weights
there, and multiplying by the arm lengths as given by the factory.

We agree that once the manufacturer chose the (arbitrary) datum point,
it is best to stick with it. But if that datum point happens to be far
from the CG, it's what's close to the (loaded) CG, not the datum point,
that the W&B is less "sensitive" to.

And good point about weighing the tail as a good check on things, short
of a full W&B. I am not sure how accurate is it doing that on an
apparently calm day outside, but sometimes that's all we can manage.

On Saturday, October 9, 2021 at 12:30:07 PM UTC-7, Moshe Braner wrote:
> On 10/9/2021 2:00 PM, 2G wrote:
> > ...
> > The datum point may seem to be arbitrary, but it isn't. All the W&B calculations are based on it. Yes, the manufacturer can chose where they locate it, but once chosen that is it and you MUST use that point for all future measurements. We did a W&B on my new 31Mi which involved measuring the actual moment arms of the main and tail wheels. This was done empty and with me in the glider with my parachute.
> >
> > One thought: the W&B is most sensitive to the tail weight. So, once you have done an accurate W&B you can do a quick check on any changes by just measuring the tail weight. This involves lifting the tail to the flying attitude (which is a real PITA for the 31Mi!), but after knowing what the weight is in that position and how much the weight changes when resting on the ground you can just use the ground weight to detect any changes that would warrant doing another full W&B.
> >
> > Tom
> >
> I'd bet that W&B on the 31Mi did not involve "measuring the actual
> moment arms of the main and tail wheels", rather, measuring the weights
> there, and multiplying by the arm lengths as given by the factory.
>
> We agree that once the manufacturer chose the (arbitrary) datum point,
> it is best to stick with it. But if that datum point happens to be far
> from the CG, it's what's close to the (loaded) CG, not the datum point,
> that the W&B is less "sensitive" to.
>
> And good point about weighing the tail as a good check on things, short
> of a full W&B. I am not sure how accurate is it doing that on an
> apparently calm day outside, but sometimes that's all we can manage.

How much do you want to bet?

Tom

On 10/9/2021 11:00 AM, 2G wrote:
> On Saturday, October 9, 2021 at 5:50:27 AM UTC-7, Eric Greenwell wrote:
>> It's been so long since I looked at the maintenance manual for my ASH26E, I couldn't
>> remember where it's datum point was, so I had to look it up: it is the wing LE at the
>> fuselage, an easy to use and find point. Some day I will have a flight computer so smart,
>> all I will have to do is give it the basic weights and it will do the heavy thinking for
>> me. My dream is the glider has force sensors on the main and tail wheels, so I don't even
>> have to give it any weights! But then, I suppose, the flight computer would feel obligated
>> to scold me when I "arrive" rather suddenly during the landing.
>> On 10/8/2021 11:02 PM, jfitch wrote:
>>> Moshe is correct that the datum point is arbitrarily picked, though specified by the manufacturer or whoever did the official W&B. Most will pick a point somewhere near the CG, but it is not necessary to do so. Could be the nose, could be the tailwheel. He is also correct that "sensitivity" depends on the moment arm from the CG, not the datum. Those might be nearly the same, or might not be.
>> --
>> Eric Greenwell - USA
>> - "A Guide to Self-launching Sailplane Operation"
>> https://sites.google.com/site/motorgliders/publications/download-the-guide-1
>
> Not long after I first started work at the, then, Pacific Northwest Laboratory a scientist who was a pilot came by and asked us about doing exactly that. The idea didn't go anywhere because the cost was just too high to justify it. An accurate W&B has to be done in a hangar because any breeze will make the readings invalid. I watched one Alaska bush pilot program where the judged the W&B by the tail not dropping to the ground once loaded. I had one CFI ask me, at a BFR, how to improve fuel mileage. Answer: load a lot of baggage on the plane to move the CG to the aft. I reminded him about CG limits and he became silent.

It was a joke ;^) I don't see any value to getting a W&B from onboard sensors, as it
would be cheaper and more reliable to use calculations with a set of normal W&B numbers.
It's not like we're an airliner filled with passengers and luggage of unknown weights.

--
Eric Greenwell - USA
- "A Guide to Self-launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1

On 10/9/2021 4:52 PM, 2G wrote:
> On Saturday, October 9, 2021 at 12:30:07 PM UTC-7, Moshe Braner wrote:
>> On 10/9/2021 2:00 PM, 2G wrote:
>>> ...
>>> The datum point may seem to be arbitrary, but it isn't. All the W&B calculations are based on it. Yes, the manufacturer can chose where they locate it, but once chosen that is it and you MUST use that point for all future measurements. We did a W&B on my new 31Mi which involved measuring the actual moment arms of the main and tail wheels. This was done empty and with me in the glider with my parachute.
>>>
>>> One thought: the W&B is most sensitive to the tail weight. So, once you have done an accurate W&B you can do a quick check on any changes by just measuring the tail weight. This involves lifting the tail to the flying attitude (which is a real PITA for the 31Mi!), but after knowing what the weight is in that position and how much the weight changes when resting on the ground you can just use the ground weight to detect any changes that would warrant doing another full W&B.
>>>
>>> Tom
>>>
>> I'd bet that W&B on the 31Mi did not involve "measuring the actual
>> moment arms of the main and tail wheels", rather, measuring the weights
>> there, and multiplying by the arm lengths as given by the factory.
>>
>> We agree that once the manufacturer chose the (arbitrary) datum point,
>> it is best to stick with it. But if that datum point happens to be far
>> from the CG, it's what's close to the (loaded) CG, not the datum point,
>> that the W&B is less "sensitive" to.
>>
>> And good point about weighing the tail as a good check on things, short
>> of a full W&B. I am not sure how accurate is it doing that on an
>> apparently calm day outside, but sometimes that's all we can manage.
>
> How much do you want to bet?
>
> Tom
>

Suppose you actually measured those distances. Did you get the exact
same numbers as in the POH? If not, which did you believe?

On Saturday, October 9, 2021 at 2:31:32 PM UTC-7, Moshe Braner wrote:
> On 10/9/2021 4:52 PM, 2G wrote:
> > On Saturday, October 9, 2021 at 12:30:07 PM UTC-7, Moshe Braner wrote:
> >> On 10/9/2021 2:00 PM, 2G wrote:
> >>> ...
> >>> The datum point may seem to be arbitrary, but it isn't. All the W&B calculations are based on it. Yes, the manufacturer can chose where they locate it, but once chosen that is it and you MUST use that point for all future measurements. We did a W&B on my new 31Mi which involved measuring the actual moment arms of the main and tail wheels. This was done empty and with me in the glider with my parachute.
> >>>
> >>> One thought: the W&B is most sensitive to the tail weight. So, once you have done an accurate W&B you can do a quick check on any changes by just measuring the tail weight. This involves lifting the tail to the flying attitude (which is a real PITA for the 31Mi!), but after knowing what the weight is in that position and how much the weight changes when resting on the ground you can just use the ground weight to detect any changes that would warrant doing another full W&B.
> >>>
> >>> Tom
> >>>
> >> I'd bet that W&B on the 31Mi did not involve "measuring the actual
> >> moment arms of the main and tail wheels", rather, measuring the weights
> >> there, and multiplying by the arm lengths as given by the factory.
> >>
> >> We agree that once the manufacturer chose the (arbitrary) datum point,
> >> it is best to stick with it. But if that datum point happens to be far
> >> from the CG, it's what's close to the (loaded) CG, not the datum point,
> >> that the W&B is less "sensitive" to.
> >>
> >> And good point about weighing the tail as a good check on things, short
> >> of a full W&B. I am not sure how accurate is it doing that on an
> >> apparently calm day outside, but sometimes that's all we can manage.
> >
> > How much do you want to bet?
> >
> > Tom
> >
> Suppose you actually measured those distances. Did you get the exact
> same numbers as in the POH? If not, which did you believe?

Hmmm, I guess you DON'T want to bet after all. The 31Mi manuals DON'T list these moment arms. Instead, the MM says this:

Distances "a" and “b” are not constant due to the landing gear suspension. Therefore, these must be determined whenever the weight of the glider changed during weighing!

If they did, I would obviously believe the measured values.

Tom

While is is customary to pick a datum that is somewhere near the CG (for example the leading edge of the wing root), the manufacturer can pick an arbitrary point - just needs to be one that can be reliably located. There is a good argument that the nose would be best, because all the moments have the same sign and there are no dropped sign errors.
On Saturday, October 9, 2021 at 8:59:51 AM UTC-7, Moshe Braner wrote:
> On 10/9/2021 8:50 AM, Eric Greenwell wrote:
> > It's been so long since I looked at the maintenance manual for my
> > ASH26E, I couldn't remember where it's datum point was, so I had to look
> > it up: it is the wing LE at the fuselage, an easy to use and find point..
> > Some day I will have a flight computer so smart, all I will have to do
> > is give it the basic weights and it will do the heavy thinking for me.
> > My dream is the glider has force sensors on the main and tail wheels, so
> > I don't even have to give it any weights! But then, I suppose, the
> > flight computer would feel obligated to scold me when I "arrive" rather
> > suddenly during the landing.
> >
> > On 10/8/2021 11:02 PM, jfitch wrote:
> >> Moshe is correct that the datum point is arbitrarily picked, though
> >> specified by the manufacturer or whoever did the official W&B. Most
> >> will pick a point somewhere near the CG, but it is not necessary to do
> >> so. Could be the nose, could be the tailwheel. He is also correct that
> >> "sensitivity" depends on the moment arm from the CG, not the datum.
> >> Those might be nearly the same, or might not be.
> >
> The wing LE at the fuselage is easy to think about, but hard to locate
> with enough precision, given the rounded fairing, etc. More
> importantly, it is hard to accurately measure the distance (along the
> long axis of the leveled glider) of any other given point from the
> datum. So what we use is the factory-given distances for the important
> points (such as the tailwheel), and close-enough estimates for others
> (such as the instrument panel - each instrument has its CG at some
> unique distance behind the panel anyway).
>
> Since I am light weight and have to worry about the CG being too far
> aft, for every glider I fly I set up a spreadsheet that computes the
> W&B, based on all available information (POH, latest official W&B sheet,
> etc). After that it's easy to account for added equipment or ballast,
> etc. In most cases, going over the info carefully, I found errors in
> previous calculations. A negative sign dropped, a typo in a moment arm,
> etc. When the difference is between 30% and 35% of the CG range, no
> harm done. When it's between 98% and 103% of the CG range, it's serious.

On Sunday, October 10, 2021 at 1:41:48 AM UTC-4, jfitch wrote:
> While is is customary to pick a datum that is somewhere near the CG (for example the leading edge of the wing root), the manufacturer can pick an arbitrary point - just needs to be one that can be reliably located. There is a good argument that the nose would be best, because all the moments have the same sign and there are no dropped sign errors.
> On Saturday, October 9, 2021 at 8:59:51 AM UTC-7, Moshe Braner wrote:
> > On 10/9/2021 8:50 AM, Eric Greenwell wrote:
> > > It's been so long since I looked at the maintenance manual for my
> > > ASH26E, I couldn't remember where it's datum point was, so I had to look
> > > it up: it is the wing LE at the fuselage, an easy to use and find point.
> > > Some day I will have a flight computer so smart, all I will have to do
> > > is give it the basic weights and it will do the heavy thinking for me..
> > > My dream is the glider has force sensors on the main and tail wheels, so
> > > I don't even have to give it any weights! But then, I suppose, the
> > > flight computer would feel obligated to scold me when I "arrive" rather
> > > suddenly during the landing.
> > >
> > > On 10/8/2021 11:02 PM, jfitch wrote:
> > >> Moshe is correct that the datum point is arbitrarily picked, though
> > >> specified by the manufacturer or whoever did the official W&B. Most
> > >> will pick a point somewhere near the CG, but it is not necessary to do
> > >> so. Could be the nose, could be the tailwheel. He is also correct that
> > >> "sensitivity" depends on the moment arm from the CG, not the datum.
> > >> Those might be nearly the same, or might not be.
> > >
> > The wing LE at the fuselage is easy to think about, but hard to locate
> > with enough precision, given the rounded fairing, etc. More
> > importantly, it is hard to accurately measure the distance (along the
> > long axis of the leveled glider) of any other given point from the
> > datum. So what we use is the factory-given distances for the important
> > points (such as the tailwheel), and close-enough estimates for others
> > (such as the instrument panel - each instrument has its CG at some
> > unique distance behind the panel anyway).
> >
> > Since I am light weight and have to worry about the CG being too far
> > aft, for every glider I fly I set up a spreadsheet that computes the
> > W&B, based on all available information (POH, latest official W&B sheet,
> > etc). After that it's easy to account for added equipment or ballast,
> > etc. In most cases, going over the info carefully, I found errors in
> > previous calculations. A negative sign dropped, a typo in a moment arm,
> > etc. When the difference is between 30% and 35% of the CG range, no
> > harm done. When it's between 98% and 103% of the CG range, it's serious..

It does not matter what tool is used to calculate the in flight CG.
What is important is:
1- the weighing that establishes the EWCG is done as defined in the approved flight manual including masses, positions, and levelling. All stuff in the glider must be noted and listed.
2-The table that is used to calculate the flight CG uses that information exactly and uses the location of other masses as defined in the flight manual.
The rest is just calculation. Many manuals provide tables that make it easier to determine if one is within range but make it harder to determine where within the range.
A quick weighing of the tail wheel empty, properly levelled, is a good sanity check on a ship in unknown condition. Comparing this with the last available weighing makes it easy to determine if a full weight and balance is needed.
UH