Looking For W&B Using Arm Instead of Moment

[John T]

I'm looking for an example of a W&B worksheet using CG arm values instead of
moment values. Basically, I'm curious to see how these are done and what
the worksheets look like.

Either online or emailed examples are acceptable.

Thanks!

--
John T
tknoflyer[remove] at hotmail[remove].com
http://tknowlogy.com/tknoFlyer
_______________

[John T]

"Ron Natalie" > wrote in message
m
> You can't do it with just an arm. You need the arm plus the weight.
> And then your calculation is just going to multiply them together
> (yielding the moment).

Yes, but recent readings suggested that my understanding my have
been...incomplete.

For instance, every W&B graph I've seen (Skyhawks, Skylanes, C150s,
Warriors, Katanas, Tomahawks - all under 35 years old) have shown this graph
using only weight/moment. All the sample loading problems I've seen have
all used moment instead of arm, too.

Recent readings revealed folks referencing balance by arm instead of
moment - something I'm not used to doing or reading. It raised the question
in my mind whether there are charts that use weight/arm instead of
weight/moment.

So that's the real question: Do such charts exist? If so, where can I take
a peek at one?

Thanks.

--
John T
http://tknowlogy.com/tknoFlyer
__________

[Robert Moore]

"John T" wrote
> For instance, every W&B graph I've seen (Skyhawks, Skylanes,
> C150s, Warriors, Katanas, Tomahawks - all under 35 years old)
> have shown this graph using only weight/moment. All the sample
> loading problems I've seen have all used moment instead of arm,
> too.

In those airplanes, the "arm" is generally fixed by the location
of the seats and bagage compartment. On the graphs, the upward
sloping lines each represent a different arm. Regardless of how
it is presented, a w/b calculation always involves weights and
arms. Moment is simply weight times the arm.

Bob Moore

[Ron Natalie]

"John T" > wrote in message ws.com...

>
> For instance, every W&B graph I've seen (Skyhawks, Skylanes, C150s,
> Warriors, Katanas, Tomahawks - all under 35 years old) have shown this graph
> using only weight/moment. All the sample loading problems I've seen have
> all used moment instead of arm, too.

Which graph are you talking about? The CG envelope itself is almost always
the CG position (arm) versus weight. The other graphs (where you follow the
weight out various lines for the different positions) are just computing the moment
(multiplying the arm for you). The cessna books give you the arms you can use
in lieu of these charts if you really want.

> Recent readings revealed folks referencing balance by arm instead of
> moment - something I'm not used to doing or reading.

I'm sure you're confused. The CG point is always an arm. A moment tells
you NOTHING about the CG. A moment of 1000 could be 1000 pounds at the
datum or 1 pound 1000 units away from the datum. If you've computed the
moment, you divide that by the weight to get the arm.

< It raised the question
> in my mind whether there are charts that use weight/arm instead of
> weight/moment.

They all do. Googling for cg graph yeilds this page, which is quite representative
of just about every plane I've ever come accross...

http://www.eaa838.org/wtbal.htm

[Orval Fairbairn]

In article m>,
"John T" > wrote:

> I'm looking for an example of a W&B worksheet using CG arm values instead of
> moment values. Basically, I'm curious to see how these are done and what
> the worksheets look like.
>
> Either online or emailed examples are acceptable.
>
> Thanks!

If you can use Excel, you can easily construct your own W&B worksheet.

Put the weight of each component in column "A", its arm in column "B",
its resultant moment in column "C".

All you have to do is multiply column "A" by column "B" in coulmn "C",
add the sum of columns "A" and "C", divide the sum of column "C" by the
sum of column "A" and you have the CG in column "B".

You can get better by adding fuel in gallons, converting it to lbs in
the weight computation, oil in qts, converting it, etc.

You can build pax/fuel loading scenarios and get a real handle on what's
going on.

It is really quite elementary.

A year ago I helped a retired AA captain build a W&B program for his
plane -- it really opened his eyes!

[Teacherjh]

>>
Recent readings revealed folks referencing balance by arm instead of
moment - something I'm not used to doing or reading. It raised the question
in my mind whether there are charts that use weight/arm instead of
weight/moment.
<<

Yes, such charts exist.

I take it you are referring to the "envelope" into which you n eed to fit.
I've seen it with weight plotted against CG (arm), which I prefer as it makes
the envelope fit better on the page, and I've seen it with weight plotted
against moment, (which makes a skinny sloping envelope) There are other ways
to represent the envelope too, but in all cases you need some term involving
weight plotted against some term involving arm. The "arm" term could be arm
(CG) itself, or moment, which is just armxweight. Whichever is used, I'll call
it "balance".

To get the final point, you still need to figure out the contribution to total
weight and total balance due to each factor (pilot, passenger, fuel, etc).
This is a simple multiplication, but a multplication can be represnted on
another graph by a sloping line. This is just the old algebra y=mx+b rearing
its friendly head again. The charts are designed to be compatible, so there's
a minimum of arithmetic involved.

It is actually possible to plot the contributions from each station as
gridlines on the same graph as the envelope, but the lines are curved if you
use arm for balance, and they are hard to distinguish from each other if you
use moment for balance. I did actually set a few up that way; it allows one to
figure out the weight and balance with no math whatsoever. But before I
finished the program to do that, I found CoPilot for the Palm, and that was
that.

Jose



--
(for Email, make the obvious changes in my address)

[John T]

Thanks, folks.

While I'll grant you that my math skills aren't nearly as polished as my
piloting skills, I did know that "weight * arm = moment". :)

The item I was looking for (included in Ron's helpful link) was a CG
envelope (thanks to Jose for the correct term) using arm instead of moment.
As I said, all the envelopes in the POHs I've used for the planes I've flown
have all used moment - not arm - in the graphical representation.

After seeing the graphic from Ron's link, I realize that I've seen similar
charts - just not in any of the POHs I've used.

Thanks again! :)

--
John T
http://tknowlogy.com/tknoFlyer
__________

[Jim Weir]

And not to belabor the point, what John wanted (and finally found) was the
good-old-days CG "box" instead of the dumbed-down parallelogram that Cessna
foisted on us starting around 1960. Seems that some marketing genius thought
that the flying public was too stupid to do a "real" weight and balance by doing
the multipication of weight*arm, adding the moments, and dividing by the total
weight to get the point of balance.

I started flying in '64, and it wasn't until I went for my A&P in '70 that I
ever saw (or used, for that matter) the good old box style CG method of
calculation.

Jim



"John T" >
shared these priceless pearls of wisdom:

->Thanks, folks.



Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

[Karl Medcalf]

I wrote a weight & balance program some time ago that will display the
envelope either way. If you're interested in looking at it, you can
find it at http://users3.ev1.net/~medcalf/avinfo.html

As noted, I accept no liability for use of the program, as you are
still responsible to properly calculate weight & balance for your
specific plane.

Enjoy.


On Thu, 30 Oct 2003 12:41:30 GMT, "John T" > wrote:

>I'm looking for an example of a W&B worksheet using CG arm values instead of
>moment values. Basically, I'm curious to see how these are done and what
>the worksheets look like.
>
>Either online or emailed examples are acceptable.
>
>Thanks!
>
>--
>John T
>tknoflyer[remove] at hotmail[remove].com
>http://tknowlogy.com/tknoFlyer
>_______________
>
>
>

Karl Medcalf - WK5M
PP-ASEL-IA (N43CS)
>

[Verbs Under My Gel]

"John T" > wrote in message m>...
> I'm looking for an example of a W&B worksheet using CG arm values instead of
> moment values. Basically, I'm curious to see how these are done and what
> the worksheets look like.
>
> Either online or emailed examples are acceptable.
>
> Thanks!

Moment is the vector product of force (weight) and displacement
(distance, or "arm"), in other words, F X S

[BTIZ]

the upward sloping line on these graphs is the computed weight x arm =
moment and then the CG is read..




"Robert Moore" > wrote in message
...
> "John T" wrote
> > For instance, every W&B graph I've seen (Skyhawks, Skylanes,
> > C150s, Warriors, Katanas, Tomahawks - all under 35 years old)
> > have shown this graph using only weight/moment. All the sample
> > loading problems I've seen have all used moment instead of arm,
> > too.
>
> In those airplanes, the "arm" is generally fixed by the location
> of the seats and bagage compartment. On the graphs, the upward
> sloping lines each represent a different arm. Regardless of how
> it is presented, a w/b calculation always involves weights and
> arms. Moment is simply weight times the arm.
>
> Bob Moore

[Robert Moore]

"BTIZ" > wrote

> the upward sloping line on these graphs is the computed weight x
> arm = moment and then the CG is read..

In my C-172 manual, the upward sloping lines have no units.
One enters the chart on the left with weight, goes over to the
appropriate diagonal line (front seat, rear seat, baggage) and
then drops down to the base line and reads moment.

Bob Moore

[John Roncallo]

Ron Natalie wrote:

>
> I'm sure you're confused. The CG point is always an arm. A moment tells
> you NOTHING about the CG. A moment of 1000 could be 1000 pounds at the
> datum or 1 pound 1000 units away from the datum. If you've computed the
> moment, you divide that by the weight to get the arm.
>

The 1986 Cessna Skyhawk manual presents the W&B data in both Weight vs
Moment and Weight vs CG.

John Roncallo

[John Godwin]

(Karl Medcalf) wrote in
:

> I wrote a weight & balance program some time ago that will display the
> envelope either way. If you're interested in looking at it, you can
> find it at http://users3.ev1.net/~medcalf/avinfo.html
>
> As noted, I accept no liability for use of the program, as you are
> still responsible to properly calculate weight & balance for your
> specific plane.
>
> Enjoy.

I've downloaded and used Karl's program. I have it running on Windows 2000
Professional and am quite impressed. I highly recommend it.

--
John Godwin
Silicon Rallye Inc.