Sorting out my W&B

[JayMatt]

Ok like the title says, I'm trying to get my W&B sorted out. 

I've looked through my POH and there are some sample problems. It puts my empty weight at 1695 with a 75.43 moment. My latest up to date weight and balance is 1738.77 with an ARM of 45.13. That is a very significant difference in ARM imo. I looked up some other WB forms online like this one http://trumpetb.net/alph/wbM20J.html and they seem to be around the same area. This online form is also way different than my example, my example doesn't match up with anything I find online. Obviously you go off the POH but I fill out a WB and i'm not in the envelope anywhere. I don't know what I'm doing wrong, I think my understanding of arm and Moment (LB-INS, / 1000) is wrong, as embarrassing as this is to admit... but I need to ask and get this right. So thanks for any help. 

 

[ArtVandelay]

Ignore the sample problem.

[jaylw314]

My suspicion is that you are conflating the concepts of arm and moment.  If your empty weight is 1739 lbs and your arm is 45.1 inches, your moment is weight x arm = 78,400 in-lbs.  The convention is to write down moment as thousands, so the your empty weight moment is written as 78.4.  That seems pretty close with what's in that sample problem.

It's confusing dividing moment by 1000, because you end up with values in the same order of magnitude as typical CG arms.  I think it's preferable to write them down so that their values are obviously distinct, or at least write moment as 78.4k rather than 78.4.  Better yet, don't bother with moment and you can do your W&B problems solely with weight and arm.

 

[PT20J]

Sample problems always seem to have an uncharacteristically low empty weight.

The original weight and arm from the factory should be the first entry on the OWNERS WEIGHT AND BALANCE RECORD page. Subsequent calculations supersede that. 

[A64Pilot]

remember WAM, weight x arm = moment

weight is mass of course, arm is where the mass is and moment is the effect on balance the mass has

Sorry if this didn’t help 

[JayMatt]

52 minutes ago, jaylw314 said:

My suspicion is that you are conflating the concepts of arm and moment.  If your empty weight is 1739 lbs and your arm is 45.1 inches, your moment is weight x arm = 78,400 in-lbs.  The convention is to write down moment as thousands, so the your empty weight moment is written as 78.4.  That seems pretty close with what's in that sample problem.

It's confusing dividing moment by 1000, because you end up with values in the same order of magnitude as typical CG arms.  I think it's preferable to write them down so that their values are obviously distinct, or at least write moment as 78.4k rather than 78.4.  Better yet, don't bother with moment and you can do your W&B problems solely with weight and arm.

 

Thank you!

[carusoam]

JM,

As you have found out…

The WnB calculations are engineering 101… easy for some engineers and a few mechanics…. But not all…

So…

Figure out if you are doing the calculations right…

Then review your log’s WnB calculations….

Often, by the time an MSer asks about his WnB… somebody has goofed up the records long before he gets to work on it…

Sometimes the zero location gets confused… it’s a bolt near the nose wheel… not something simple like a wire wall… 

 

If there has been a lot of weight added and removed…. The next conversation may be how to start anew with an official weighing…

Best regards,

-a-