Weight and Balance Formula, Can one calculate the envelope

[Joe Wasik]

Hello, I put together a quick Excel spreadsheet that calculates total
weight and total moment for a Cessna 172. However, to check the CG, I
still have to take these calculated values and place them onto the
diagram called "Center of Gravity Moment Envelope". Does anyone know
if this lookup can be performed via a calculation? Knowing this
would be very helpful.
Thank you.

[Roger Long]

See ours here:

http://www.baldeagleflyingclub.org/members/WeightBalance.xls

This sheet checks the max gross and aft CG limits. With some
experimentation, I determined that there is no way to get our individual
plane into the area where the max gross drops off. The math to do this is
fairly straightforward but I just didn't bother. There is a note telling
members to check the graph by hand if they are doing something like flying
with lead or gold bars stacked around the rudder pedals.

Feel free to modify or adapt this for your own use with no liability or
responsibility by me or our club for the results.

--

Roger Long



"Joe Wasik" > wrote in message
om...
> Hello, I put together a quick Excel spreadsheet that calculates total
> weight and total moment for a Cessna 172. However, to check the CG, I
> still have to take these calculated values and place them onto the
> diagram called "Center of Gravity Moment Envelope". Does anyone know
> if this lookup can be performed via a calculation? Knowing this
> would be very helpful.
> Thank you.

[Andrew Gideon]

Roger Long wrote:

> There is a note telling
> members to check the graph by hand if they are doing something like flying
> with lead or gold bars stacked around the rudder pedals.

Our club has spreadsheets for our aircraft that actually create the graphic.
Since I didn't write them, I don't feel comfortable posting them. But it
doesn't appear too complex a task to achieve this. A set of fixed cell
pairs define the fixed shape of the "envelope", and then a final cell pair
- computed, of course - defines the point (hopefully) within the envelope.
A line graphic is drawn from these cell pairs.

- Andrew

[Teacherjh]

>>
However, to check the CG, I
still have to take these calculated values and place them onto the
diagram called "Center of Gravity Moment Envelope". Does anyone know
if this lookup can be performed via a calculation?
<<

You can describe the envelope as a series of lines; your point will be on one
side or the other of each of those lines. So, apply a test ( p1 >+? ax+b ) for
each of those lines, and if it passes all the tests, it's in the envelope.

Jose

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

[[email protected]]

http://www.primetab.com/flying/MyNavLog.xls

The sheet marked W&B has what you need. The sheet is for a Piper Warrior, but
you can adjust it for the Cessna numbers. You will need to unprotect the sheet
before making changes, Tools/Protection/Unprotect Sheet. Input the Cessna
numbers for Basic Empty Weight, all the Arms, Unused gallons, and GPH.
Everything else is calculated. Adjust the chart.

[Roger Long]

Way too cool! I've got to figure out how to do that.

If you see the right person, please ask if I could see their spreadsheet.

--

Roger Long



"Andrew Gideon" > wrote in message
gonline.com...
> Roger Long wrote:
>
> > There is a note telling
> > members to check the graph by hand if they are doing something like
flying
> > with lead or gold bars stacked around the rudder pedals.
>
> Our club has spreadsheets for our aircraft that actually create the
graphic.
> Since I didn't write them, I don't feel comfortable posting them. But it
> doesn't appear too complex a task to achieve this. A set of fixed cell
> pairs define the fixed shape of the "envelope", and then a final cell pair
> - computed, of course - defines the point (hopefully) within the envelope.
> A line graphic is drawn from these cell pairs.
>
> - Andrew
>

[Ross Richardson]

Send me your correct e-mail address and I will send you the spreadsheet
I developed for my C-172F. It even has the graphic and a "*" to show you
where you are in the envelop. I do not have access to post is anywhere
and I cannot attach to this message. I tried to reply to the address in
your post and it bounced back.

Joe Wasik wrote:
>
> Hello, I put together a quick Excel spreadsheet that calculates total
> weight and total moment for a Cessna 172. However, to check the CG, I
> still have to take these calculated values and place them onto the
> diagram called "Center of Gravity Moment Envelope". Does anyone know
> if this lookup can be performed via a calculation? Knowing this
> would be very helpful.
> Thank you.


Ross
N7905U
180hp

[[email protected]]

You will need a bunch of "if-then" statements to define the acceptable
envelope(s). Look it up on Excel Help. Lotsa luck though as the same
guys must write the help as write the original software package.

Note that you can nest "if-then" statements (I recall up to 7 in my
Excel) so that in one long statement you could establish a valid
envelope with up to 7 separate boundaries.

An interesting challenge! Good luck.

[Casey Wilson]

> wrote in message
om...
> You will need a bunch of "if-then" statements to define the acceptable
> envelope(s). Look it up on Excel Help. Lotsa luck though as the same
> guys must write the help as write the original software package.
>

I have an Excel worksheet with NO if-then statements anywhere.
I used the data from the pilot's handbook to determine the corners
of a graph that represents the W&B envelope. Then I built a table
duplicating the W&B factors from the handbook along with the calcs for the
moment arms. Down at the bottom where the weights and moment arms are
totaled, I transfer them to the graph using an asterisk symbol to represent
the single data point.
Would you please give me an example of a couple if-thens you used,
please?

[Peter Duniho]

"Roger Long" > wrote in message
...
> Way too cool! I've got to figure out how to do that.
>
> If you see the right person, please ask if I could see their spreadsheet.

It's not terribly difficult.

I've copied the weight & balance spreadsheet I use for my airplane here:
http://www.nwlink.com/~peted/Weight%20and%20Balance.xls

It is for a specific aircraft (Lake Renegade), and would have to be
significantly modified for a different make/model. If used with the same
make/model, pretty much all that would need to be changed is the empty
weight, of course.

The spreadsheet includes a chart that plots the CG for the entered load, and
shows the envelope for both takeoff and landing W&B limits. Passenger and
baggage loads are entered in pounds, fuel loads are entered in gallons.

It should be simple enough to look at the spreadsheet to learn the basic
techniques for putting it together. Then you can apply those techniques for
the make/model you're interested in.

Pete

[Joe Wasik]

Ross, Thanks for your note and your offer to send me a
spreadsheet. Hopefully I'll be able to figure out the
math principles behind the formulas. Based on the number
of responses, there is hope for what is needed. I'll have
a close look at the posts when I get home.

My email to you bounced as well. I guess no one uses real
email addresses anymore. Try this one, it'll be valid for
a couple of days: jwasikusa "@" dublinhills "dot" com.

Thanks.

Ross Richardson > wrote in message >...
> Send me your correct e-mail address and I will send you the spreadsheet
> I developed for my C-172F. It even has the graphic and a "*" to show you
> where you are in the envelop. I do not have access to post is anywhere
> and I cannot attach to this message. I tried to reply to the address in
> your post and it bounced back.
>
> Joe Wasik wrote:
> >
> > Hello, I put together a quick Excel spreadsheet that calculates total
> > weight and total moment for a Cessna 172. However, to check the CG, I
> > still have to take these calculated values and place them onto the
> > diagram called "Center of Gravity Moment Envelope". Does anyone know
> > if this lookup can be performed via a calculation? Knowing this
> > would be very helpful.
> > Thank you.
>
>
> Ross
> N7905U
> 180hp

[[email protected]]

Excel uses only an IF statement. I have used it in an unrelated large
scale water flow situation, but found it messy to use. I was thinking
that a series of them could be used to end up with a yes (OK) or no
answer, but I like your concept better of showing where you are on the
actual CG/GW range.

In addition you could show the effect of burning off fuel as a line.
I don't think any 172s will present a problem in this area, but some
others like Bonanzas can.

My experience is that it is hard to exceed the CG range of a 172, but
easy to exceed the gross weight.

[Peter Duniho]

> wrote in message
om...
> Excel uses only an IF statement. I have used it in an unrelated large
> scale water flow situation, but found it messy to use.

Most likely because you are not sufficiently familiar with Excel. The fact
that you think that "Excel uses only an IF statement" supports this
likelihood. It has hundreds of worksheet functions, not counting all of the
add-ins that provide even more functionality.

I don't know what calculations are involved in a "large scale water flow
situation", but I doubt it really required exclusive use of nested IF
statements.

> [...]
> In addition you could show the effect of burning off fuel as a line.
> I don't think any 172s will present a problem in this area, but some
> others like Bonanzas can.

You could draw a line, but why bother? All you really need are the two
end-points. I've never seen a W&B graph that had a convex curve, which
would be required to allow the takeoff and landing weights to fall inside
the envelope, while still having an intermediate state outside the envelope.

Pete