Guestimating weight and balance for a new design is one of the

harder parts of the design process.

It can't be done by comparing this one to that one.

If it could, all one would have to do to design a lighter structure

would be to compare the new design to a lighter example!

Since I've not seen any discussion about this yet...

Weight and Balance Estimation

There is a lot of smoke and mirror magic around weight and balance

because so many people misunderstand it so well!

At the heart of all of it, though, is a rotational force about a

reference point. The rotational force is called a MOMENT, and

the reference point is called the DATUM.

It doesn't particularly matter where the Datum is located, as long

as you use the same location to work the problem.

One reason to place the datum at the tip of the spinner is because

all the station numbers are then all positive. No negative distances to

confuse things.

One reason to place the datum at the main gear axles is because

the datum is station zero.zero. Multiply the weight on the wheel

times zero (the ARM is zero at the datum) and the moments for that

wheel come out to zero. Makes the arithmetic a little easier?

And, the reason to place the datum at the leading edge of the wing

it because that's where we are going to wind up anyway. The results

of our CG calculations will finally boil down to a point some given

distance aft of the leading edge, expressed in percent of chord.

The term STATION is the distance from the datum to a particular place

on the aircraft. Say, for instance, the instrument panel? Or the pilot's

belly button.

The station numbers change according to where the datum is placed.

But the instrument panel stays in the same physical location.

(would that hte pilot's belly button would as well! )

It's all about an offset from a zero point, ok?

CG range is often referred to in terms of a percentage of the wing chord.

Say 25% would be the forward CG limit, maybe 33% would be the aft limit.

So our end number actually refers to a distance aft of the leading edge.

The actual numbers will be different, depending on where the datum is located,

but they all (hopefully) point to the same place on the airplane.

The arithmetic:

weight x distance = moments

pounds x inches = pound inches

So, moments / inches = pounds

and moments / pounds = inches

So how is this used in the design stage?

Easy. And very tedious!

Rather than guessing at the whole airplane all together, we simply

take every single part of the airplane, and it's location, and work

the moments.

In the olde days, this was done with pencil, paper and slide rule.

(and that technology took man to the moon)

We have help with that these days. Computers! CAD and spreadsheet.

So let's use them...

Meet - The Spreadsheet from Hell!

Every piece of the airplane, it's weight, it's location, and the resulting

moment it contributes to the airplane.

Sound like fun?

In the example attached, I've worked a small single seat biplane.

I didn't go down to the nut and bolt level, but that can be done -

IF IT IT NECESSARY. Bolts are steel, and quite heavy for their size.

But for this class of work, it's not necessary.

Also, we are ESTIMATING, remember.

Just because the answer comes out to 8 decimal places doesn't

mean it's that accurate.

Garbage in - garbage out.

I want to direct your attention to column B, named DATA.

This cell is used in different ways in different places.

It can be the actual weight an item, such as engine, mount, pilot...

Or it can be used to calculate the weight of an item, say a wing spar tube?

Look at cell B14.

(PI()*2*0.058*0.1) calculates the weight of 1 inch of a 2"x.058 tube.

PI()*2*0.058 calculates the volume of material, and the 0.1 represents the

weight of 1 cubic inch of that material (in this case aluminum)

Column C is the quantity of these parts (how many of them are at THAT STATION)

Column D is the length of the tube, angle, piece of wood, whatever.

Column E is the station where this piece is located. For a longitudinal piece

like a longeron or diagional brace, the station you would enter is the CENTER

of the piece (that piece's center of gravity). Works ok as long as the piece

is no tapered or ?

Column F works out the actual weight of that piece, and

Column G works out the MOMENTS for that piece.

To simplify working with this monster, I've broken the parts down into

sub-assemblies, like Top Wings, Bottom Wings, struts, etc.

That brings us to Columns I and J

I is the sub -assembly weight

J is the sub-assembly moments

So that's the heart of it.

All the pieces and parts in their correct locations, accounting for

their weights and moments...

At the very bottom of the sheet is the grand total.

There is a line for TARE WEIGHT, which can be used as ballast to

bring the CG into range (for "what if" purposes). It should be

0 unless you want to add ballast somewhere.

We add up ALL the WEIGHTS, we add up all the moments.

We divide moments by weight and get the resulting center of

gravity location expressed as inches aft of the datum.

We get empty weight and empty CG.

Then we add pilot and fuel, entered in that order so that we can

see the no fuel and full fuel CG locations.

That's pretty much it for this example.

The designer would next take those CG locations and work out where

the are in percent of wing chord.

25% to 33% of the mean aerodynamic chord is a good safe starting

range. Outside that range, it's not going to be stable.

This is a biplane, so that part gets a bit messier since we have to

first figure out Mean Aerodynamic Chord, but we'll save that for a

future discussion.

My intent with this example is simply to offer budding designers

an example of how to get started on weight and balance estimation.

That's not the end of the design phase, rather it's a starting point.

Have fun with it...

Richard

harder parts of the design process.

It can't be done by comparing this one to that one.

If it could, all one would have to do to design a lighter structure

would be to compare the new design to a lighter example!

Since I've not seen any discussion about this yet...

Weight and Balance Estimation

There is a lot of smoke and mirror magic around weight and balance

because so many people misunderstand it so well!

At the heart of all of it, though, is a rotational force about a

reference point. The rotational force is called a MOMENT, and

the reference point is called the DATUM.

It doesn't particularly matter where the Datum is located, as long

as you use the same location to work the problem.

One reason to place the datum at the tip of the spinner is because

all the station numbers are then all positive. No negative distances to

confuse things.

One reason to place the datum at the main gear axles is because

the datum is station zero.zero. Multiply the weight on the wheel

times zero (the ARM is zero at the datum) and the moments for that

wheel come out to zero. Makes the arithmetic a little easier?

And, the reason to place the datum at the leading edge of the wing

it because that's where we are going to wind up anyway. The results

of our CG calculations will finally boil down to a point some given

distance aft of the leading edge, expressed in percent of chord.

The term STATION is the distance from the datum to a particular place

on the aircraft. Say, for instance, the instrument panel? Or the pilot's

belly button.

The station numbers change according to where the datum is placed.

But the instrument panel stays in the same physical location.

(would that hte pilot's belly button would as well! )

It's all about an offset from a zero point, ok?

CG range is often referred to in terms of a percentage of the wing chord.

Say 25% would be the forward CG limit, maybe 33% would be the aft limit.

So our end number actually refers to a distance aft of the leading edge.

The actual numbers will be different, depending on where the datum is located,

but they all (hopefully) point to the same place on the airplane.

The arithmetic:

weight x distance = moments

pounds x inches = pound inches

So, moments / inches = pounds

and moments / pounds = inches

So how is this used in the design stage?

Easy. And very tedious!

Rather than guessing at the whole airplane all together, we simply

take every single part of the airplane, and it's location, and work

the moments.

In the olde days, this was done with pencil, paper and slide rule.

(and that technology took man to the moon)

We have help with that these days. Computers! CAD and spreadsheet.

So let's use them...

Meet - The Spreadsheet from Hell!

Every piece of the airplane, it's weight, it's location, and the resulting

moment it contributes to the airplane.

Sound like fun?

In the example attached, I've worked a small single seat biplane.

I didn't go down to the nut and bolt level, but that can be done -

IF IT IT NECESSARY. Bolts are steel, and quite heavy for their size.

But for this class of work, it's not necessary.

Also, we are ESTIMATING, remember.

Just because the answer comes out to 8 decimal places doesn't

mean it's that accurate.

Garbage in - garbage out.

I want to direct your attention to column B, named DATA.

This cell is used in different ways in different places.

It can be the actual weight an item, such as engine, mount, pilot...

Or it can be used to calculate the weight of an item, say a wing spar tube?

Look at cell B14.

(PI()*2*0.058*0.1) calculates the weight of 1 inch of a 2"x.058 tube.

PI()*2*0.058 calculates the volume of material, and the 0.1 represents the

weight of 1 cubic inch of that material (in this case aluminum)

Column C is the quantity of these parts (how many of them are at THAT STATION)

Column D is the length of the tube, angle, piece of wood, whatever.

Column E is the station where this piece is located. For a longitudinal piece

like a longeron or diagional brace, the station you would enter is the CENTER

of the piece (that piece's center of gravity). Works ok as long as the piece

is no tapered or ?

Column F works out the actual weight of that piece, and

Column G works out the MOMENTS for that piece.

To simplify working with this monster, I've broken the parts down into

sub-assemblies, like Top Wings, Bottom Wings, struts, etc.

That brings us to Columns I and J

I is the sub -assembly weight

J is the sub-assembly moments

So that's the heart of it.

All the pieces and parts in their correct locations, accounting for

their weights and moments...

At the very bottom of the sheet is the grand total.

There is a line for TARE WEIGHT, which can be used as ballast to

bring the CG into range (for "what if" purposes). It should be

0 unless you want to add ballast somewhere.

We add up ALL the WEIGHTS, we add up all the moments.

We divide moments by weight and get the resulting center of

gravity location expressed as inches aft of the datum.

We get empty weight and empty CG.

Then we add pilot and fuel, entered in that order so that we can

see the no fuel and full fuel CG locations.

That's pretty much it for this example.

The designer would next take those CG locations and work out where

the are in percent of wing chord.

25% to 33% of the mean aerodynamic chord is a good safe starting

range. Outside that range, it's not going to be stable.

This is a biplane, so that part gets a bit messier since we have to

first figure out Mean Aerodynamic Chord, but we'll save that for a

future discussion.

My intent with this example is simply to offer budding designers

an example of how to get started on weight and balance estimation.

That's not the end of the design phase, rather it's a starting point.

Have fun with it...

Richard

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