Weight and Balance exercise

[mhorowit]

In preparation for my visit with the FSDO, I figured it might not be a bad idea to review Weights and Balance calculations.

1. A/C Spec A-696 sez: “when empty weight CG falls within pertinent range, computation of critical fore and aft CG positions is unnecessary”. That tells me that as long as I don’t exceed my gross weight, all is well. Correct?

2. Undaunted by A-696, I proceeded to work a W/B for 95822:
Given: CG Limits: 14.2-20.0
EW: #728 CG: 18.9 (includes oil)
Fuel: 12x6= #72 -9.0
Pax: 2x200= #400 23.0
Bags: #50 40.0

This gives me a GW of #1250 (#50 over), and CG of 72.9.
That CG is WAY out of limits.

Can anyone shed any explanation as to why the CG is so far out?
- Mike

[Victor Gisclair]

Re: Weight and Balance exercise

Mike i just ran your W&B and from what you give me I get
1250 # and CG = 19.4490. Lose the Bags and you get
1200 # and CG = 18.5927

[mhorowit]

Re: Weight and Balance exercise

Hi Vic - I agree with your math; BUT, the point I'm concerned about is that quoted passage in A-696 which seems to say "load it to gross and don't worry about the CG", which to me means, 'lose 50 pounds of fuel or passengers and you're good to go, you can keep the baggage'. Ideas?

[Howard Wilson]

Re: Weight and Balance exercise

Is the standard people weight still 170#? Part of the prob. might be adding the -fuel moment instead of subtracting? I'm getting 14.5 @ 1190#s with the numbers you have by (subtracting the fuel moment and w/ 170# people).Enough weight left over for a 5# trout, 3#s of fishing gear, and 2#s of fine CEEgars, all in the back please;-) Been a while since I've played with W.A.M. formula so somebody check my scrawl.

[mhorowit]

Re: Weight and Balance exercise

Well, I used #200.
I also see I calculated the motion arm wrong;
I recalculated the 'Moment'
and now have:
Empty Weight x distance = 13749
Fuel x distance = -648
Passengers x distance = 9200
bags x distance = 2000
for a total of 24311 which is then divided by 1000 to give 24.3, still out of bounds.
What am I doing wrong?I think I'm following the book instructions - Mike

[Howard Wilson]

Re: Weight and Balance exercise

Mike, divide the moment by the weight. 24311/1250=19.4 And use big people if you want to, it's your world

[lfries]

Re: Weight and Balance exercise

Mt book is at the airport but I don't think I am suppose to put 50 lbs in the baggage compartment. 46 BC12D.

[Howard Wilson]

Re: Weight and Balance exercise

I have to look at my book too, still don't think I've got it

[mhorowit]

Re: Weight and Balance exercise

Mike, divide the moment by the weight. 24311/1250=19.4 And use big people if you want to, it's your world

Howard - the moment is (length of the lever arm) x (force acting on the end of the arm), so why would I divide the moment by the weight?

I'll work the example in the Cessna book and get back to you - Mike

[mhorowit]

Re: Weight and Balance exercise

Mt book is at the airport but I don't think I am suppose to put 50 lbs in the baggage compartment. 46 BC12D.

Little sign on my sez "Limit 50#" - Mike

[Robert Lees]

Re: Weight and Balance exercise

Mike: one problem is that the EMPTY WEIGHT cg is out. Limits are 14.8" to 17.9" aft of datum. I would suggest that 728 pounds is VERY light...how confident are you of this figure?

Another problem is that you cannot use 200 # for your two occupants, because then at 1250, you are above gross weight.

If you use say 175 each, then you are OK, both gross wt and cg position.

Have a play with the attached spreadsheet, which is what I use. You can alter the data in the yellow boxes to suit your needs. It includes fore-and aft-loading conditions.

Using your figures (728#, 18.9 inches) means that when the tank gets down to 1 gallon, your c.g. goes beyond the rear cg limit. But if your empty weight was say 780#, and empty wt cg was a more realistic 17.9, then you cannot exceed the limits as long as you stay at or under 1200 lb.

Rob

[edit: spreadsheets not allowed...I will post on my website then add the link]

[edit2: Here is the spreadsheet (25kB Excel)

[mhorowit]

Re: Weight and Balance exercise

Rob - I have before me a document entitled "Operation Limitations", dated May 6, 1946 and it says:
E.W. (Computed) 728
Useful Load 472
E.C.G. 18.9" aft of datum

then later on the page:
C.G. Limits: +14.2" to +20.0"

That's my rationale for 728 as my Empty Weight.

- Mike

[mhorowit]

Re: Weight and Balance exercise

Here's what I'm looking for :"The C.G. is found by dividing the total balance moment (in inch-pounds) by the total weight (in lb.) and is expressed in inches forward (-) or aft (+) of the balance datum line."

And I found ithere

So one DOES divide by the total weight. - Mike

[Robert Lees]

Re: Weight and Balance exercise

Not disputing your sources...just that 728 (computed) is not your actual, and I mean YOUR. This means legally YOURS. Read the Type cert a bit more closely, you will see that there is a difference between the empty weight cg limits and the limits in use.

If this is related to an insurance claim, then I suggest you delete the thread.

[Wayne]

Re: Weight and Balance exercise

Rob,
I can't open your spreadsheet but would like to have a look. Would email it to me at [email protected].

Thanks Wayne