Weight and Balance graph profiles

[jamesm]

I suppose I should know the answer to this but for the life of me it does not compute.

So the Weight and Balance Document that came with the 1967 M20C Mooney aircraft.  Shows two graphs "Item weight" versus "moment" and "Loaded aircraft weight" versus "moment".

The Weight and Balance Document that came with the airplane gives a couple of  sample problems given by Mooney. The  Chart /graph shows about 80 and goes to about 126 and is in /1000 Lbs-Inches on the X axis. when plotted/graphed out  it looks a fallen trapezoid.

While the other documentation for other people's Mooney's and applications like Foreflight starts at 40 ish moment and the plot/graph looks like rectangle  missing the upper left corner.  also all the Weight Balance records starts around 40 ish go to about 49 inches .

My questions are:  How would you describe the difference to a DPE if you were going for a check ride.

If they ask why does one look like graphs look so different. one being a fallen trapezoid narrow at the top while the other looks like rectangle  lobbed of corner?

As far as I can tell both charts/graphs appear to both be linear may be distorted in the cut and paste process.

 I really don't have good answer to these discrepancies or perceived  discrepancies.

I am assuming the Item weight  charts starts zero  which I believe  zero Datum the goes around the center line of the nose gear truss. so that might explain why one start chart/graph starts 80.  but my math doesn't seem quite work out.  what I can't explain the difference in  shapes between the plotted Mooney and other people's Mooney's and applications like Foreflight.

 

Thanks in Advance,

James '67C

[N201MKTurbo]

Maybe because one has CG on the X axis and one has moments on the X Axis.

[jamesm]

my math seems a little  wonky. if new CG of the ~45" plus the graph Foreflight or other People's Mooney' of the same body/model. 

I  can't grasp the why the shapes are so different.

[Marc_B]

Read up on CG and MOMENT.  Moment is force x distance.  With regards to aircraft, the distances/stations are defined by a reference datum (i.e. distance in inches from the firewall...but not all reference datums are the same from manufacturer to manufacturer...)The CG (Center of Gravity) is the balance point of the aircraft or where the Center of Mass is located. 

I think some manufacturers use weight vs CG chart and some use weight vs Moment charts.  Different ways of showing related data.  Both have total weight on the Y axis, and the difference is the CG vs total Moment (CG*weight) on the X-axis.

The first chart (ITEM WEIGHT CHART) is just an easy way to calculate individual Moments (weight x station) for certain key loads (i.e pilot/pax weight, fuel weight, baggage, etc.) using the FIXED stations.  Each line corresponds to a specific station...pilot seat, rear pax seat, fuel, baggage, etc.  Basically go up y-axis to total weight of what you're calculating, trace horizontally to the load line, then trace down to x-axis to see what the Moment would be for that load at that station.

Going though the idea of how to define CG limits, what happens out of limits and the risk, how your G-loads translates into magnified forces on structures that affects this...can all get rather complicated as deep in the weeds as you want to get. 

 

 

[jamesm]

Thanks I sort see the error in my ways. Mooney Graph shows 14.5 across bottom X axis  and about  6.5 across top  X axis. 

while other application  like other People's Mooney's Excel formula  and Foreflight  shows 7.0 across bottom X axis  and about  2.5 across top  X axis. 

Sorry about I guess I need to stare  a bit longer to figure it out.

Thanks,

 

[Marc_B]

1 hour ago, jamesm said:

I can't explain the difference in  shapes between the plotted Mooney and other people's Mooney's and applications like Foreflight.

I think ForeFlight only shows a Weight vs CG chart and when you set up the aircraft profile you define the envelope by specifying weight/CG points for each of the fore, aft, and Max gross weight points.  Foreflight is calculating all the individual moments of your load out and the total CG in the background.  That's how you can enter fuel burn and it will give you start fuel weight/cg, landing fuel weight/cg, and empty fuel weight/cg points within the envelope.

If you had to do this all by hand, sometimes using a weight vs total moment chart is faster.  i.e. you just multiply weight x station and add or subtract that out from the total moment, rather than having to figure out how it changes the CG with your change and plotting it on the chart.

[Matthew P]

Go to this link, I recreated the charts so they are legible

 

[Pinecone]

Using moments saves a math step.

You multiple weights by arm to get moments.  You then add up all the weights and add up all the moments.  With a moment chart, you just go into the chart to see if you are in the acceptable range.

With a CG chart, you have the extra step of dividing the total moment by the total weight to get CG and go into the chart.

But I find the CG chart more intuitive to see where you are in relation to CG and gross weight limits.

For even more fun, the CG charts for my CAP-10 are in % of chord.