Alan,

I had the same situation when I did the W&B on my ES. I ended up using the
same logic you suggest for the reference point selection. The tip of the
spinner is arbitrary, but the location of the MAS to the firewall is fixed.
I used the firewall.

As for the other arms I measured them during the weighing process. I started
with an empty AC, oil in the engine. Dropped a plumb bob from the joggle to
the ground and measured the nose and main gear offsets. I then recorded the
three scale reading.

Then I added ~ 15 gals to each tanks and again recorded the reading. I then
sat in the pilot seat and recorded, and then in the back seat and recorded.
Lastly I put a 75 # weight in the baggage area center and recorded the
reading.

Using the empty weight readings I calculated the empty weight and moment,
(remembering to include the factory specified offset in the arms for each
scale.) That allowed me to calculate the empty weight CG location and check
it against the envelope in the POH.

After adding the fuel I had a new weight and moment. The change relative to
empty allowed me to know exactly the weight of the fuel added, and the
change in moment, which allowed me to calculate the moment arm for the fuel
tanks. Continuing this way I calculated the arms for the front seats, rear
seats and baggage area. All were close to the POH, but I know these match my
plane.

I was a little surprised that the A&P whose scales I used had trouble
figuring out how this worked, even after I explained it. It just reinforced
an observation I've made that building your own plane givens you a level of
understanding many who make there living with AC don't have.

Merry Christmas

Paul Bricker
N63PB

-----Original Message-----
From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Alan
K. Adamson
Sent: Sunday, December 21, 2008 2:48 PM
To: lml@lancaironline.net
Subject: [LML] Legacy W&B

So, some of you know that I created a nice excel template to be used to do
the W&B for a Legacy.  A question has come up that I can't answer from the
information in the POH.

Is has to do with the "reference - starting point".  

Which way did you do your W&B and which way is the *correct* way.

a) Did you use the Firewall Joggle and the 64.625" ARM as the reference
point, taking all your other measurements from that and then doing the
adding and subtracting to get the actual ARM?

b) Did you use the tip of the spinner as FS 0" and measure everything from
that point?

The manual suggests that you start with the 64.625 as the reference number
and I suspect that is the correct way as that would be a closer reference to
the actual MAC, than the tip of the spinner which can vary quite a bit.
Take for example the MT prop/spinner it's a considerable different length
than the Hartzell.

So, which way is the correct way, and which did you use?

Lastly, when doing the baggage area ARM, did you use the published 141" or
did you actually measure when the ARM is for the center of the baggage area?
Or did you just back calculate it?

Thanks in advance, I'd like to include a note in my spreadsheet and I'd like
this reference for my own W&B

Alan Adamson
Atlanta, GA
Legacy FG - IO550 in carbon


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