CG Question

Murdoughnut

Well sized member
So I've been running hypothetical W&B situations for the new 1981 172P my club recently purchased. Basically, it's near impossible to put two people in the back pax seat without being aft CG - even when under gross. If you want to check it out for yourself, look at the "Weight and Balance Spreadsheet" on the middle of this page ... http://www.pinellaspilots.org/documents.htm

The issue seems to be the empty weight arm, which is 46.23 - a big difference when compared to the sample arm of 39 an change. If you use an empty weight arm of 39, it pulls the CG back to the middle of the envelope.

So what would explain why the EW CG is so far back? I've checked in the POH and this number was confirmed my a mechanic back in '04. The aircraft is instrument heavy, so if anything, I would have figured for a more forward CG.

Just curious - it's making me leery about sticking two women in the back.
 

Blackhawk

Well-Known Member
I would have to see what is in the aircraft. Anything in the tail? A standby battery for instruments? Still, this does seem kind of far to the aft. I would be probably go back to the equipment list, compare equipment arms with what they should actually be to make sure there are no big errors there, then redo the math. They may want to even consider ballast in the front if everything is correct.
 

Murdoughnut

Well sized member
I would have to see what is in the aircraft. Anything in the tail? A standby battery for instruments? Still, this does seem kind of far to the aft. I would be probably go back to the equipment list, compare equipment arms with what they should actually be to make sure there are no big errors there, then redo the math. They may want to even consider ballast in the front if everything is correct.
Yeah, nothing in the tail that I know of - I've sent an email to all of our club members regarding this. It's really a safety issues - guys in our club had a habit of packing our last a/c with four full sized adults, and 100-200 over gross. If these guys don't crunch the CG numbers, they could be in for a world of trouble.

I think I'll ask our mechanic as well.
 

Blackhawk

Well-Known Member
:yeahthat: I would go through the W&B with a fine tooth comb. Confirm the location of any added items. Confirm the weight of added items, especially anything in the tail. Did someone list an antena on the tail as 10 lbs. instead of 1 lb? Or did someone list the location of a radio as 160" instead of 16"? Was something put on the engine and given a positive location instead of a negative? Such as 12" instead of -12"? Confirm the original CG. If necessary, balance the airplane again. I don't think this is right, and if there were an accident the NTSB would have a field day with a W&B this far off.
 

Murdoughnut

Well sized member
:yeahthat: I would go through the W&B with a fine tooth comb. Confirm the location of any added items. Confirm the weight of added items, especially anything in the tail. Did someone list an antena on the tail as 10 lbs. instead of 1 lb? Or did someone list the location of a radio as 160" instead of 16"? Was something put on the engine and given a positive location instead of a negative? Such as 12" instead of -12"? Confirm the original CG. If necessary, balance the airplane again. I don't think this is right, and if there were an accident the NTSB would have a field day with a W&B this far off.
Yeah, I talked to one of our club officers and we're going to have it in for a look, have them re-calculate to confirm. I have a .pdf copy of the POH which has a detailed listing if every component and its arm location, so I'll run through it and see if there are any obvious errors.
 

Murdoughnut

Well sized member
Just got word that the mechanic looked at the W&B data and it was all sorts of jacked up. Weight was off by about 30lbs, and the wrong numbers were used for the CG data. We're temporarily defaulting to the W&B data that preceded the messed up one.

Thanks again for you guys insight - it was very helpful!
 

HeyEng

NAHB Doesn't Give a Crap
Good call on getting it resolved. It's not the Feds I would worry about having a field day, it would be the insurance company if heaven forbid something happened!
 
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