Posted for "Bill Harrelson"
<n5zq@verizon.net>:
Hello Dico,
Perhaps I can help a bit on some of your W&B
questions.
1. The “bottom front of firewall” is nothing more than a starting point for
determining where the factory
recommended datum is located. The factory
recommended datum is the tip of the spinner but since you and I may be using
different engines,
props and spinners, we’d be calculating from different
points. By computing the datum as 51.25 inches forward of the bottom front of
the
firewall we can assure that we’re all measuring our stations relative to
the same datum (reference plane). There is something wrong with the data
that
you are using in your spreadsheet. It looks to me like the datum has been
moved forward 12.625 inches from the factory recommended point
yet the arms
and CG limits remain the same as factory recommended. Is this spreadsheet for
a turbine where the spinner has been moved forward?
If this is the case you
have two choices. 1. Use the factory recommended datum, 51.25 inches forward
of the bottom front of the firewall or 2.
Change your datum to the tip of YOUR
spinner (or any other point that you choose) in which case you MUST adjust all
of your other arms and and
balance limits to reflect a reference to this new
datum.
2. The “mains retracted” number of 1088 seems to represent
the change in
moment (weight x distance) caused by retracting the main gear. I have no idea
why the nose wheel is not similarly accounted for.
This moment change is based
on the factory datum.
3. MAC is Mean Aerodynamic Chord. Notice that the “LE of MAC” sometimes
simply referred to as LEMAC is listed as 95.125. This means that the Leading
Edge of the Mean Aerodynamic Chord is located 95 1/8 inches aft
of the datum.
“MAC is 40” long” figure is given, which tells us that the MAC extends from
95.125 inches aft of the datum to 135.125
inches aft of the datum (TEMAC). A
balance envelope is commonly defined in terms of “percentage of MAC”. The
advantage of stating
the balance calculation in terms of %MAC is that it is
very similar for any conventional airplane regardless of weight or size. For
example,
if you calculated the %MAC to be 20 you’d be in the envelope, at
least as far as balance, in a Luscombe, a Lancair or a Lockheed. A %MAC of 50
would put you far aft of the balance limits whether you are talking about a
Birddog, a Beech or a Boeing. Again, there is a problem with your
spreadsheet
as it seems to be referenced to a new datum without changing all of the arms
to refer to this new datum. The %MAC limits simply do
not agree with the FS
limits (86.5 – 94.5) in your spreadsheet.
4. “Wing LE @ BL25.5 = 93.875” is simply a reference point that
may or may
not be useful to you in determining the location of various “stations” or
points at which weight is applied. In this case it tells
us that at 25 1/2
inches from the aircraft centerline (butt line or BL) the leading edge of the
wing is located 93.875 inches aft of the
datum. I believe that it’s fairly
safe to assume that we all have our wings mounted at the same location on the
fuselage. I am not sure,
however, which datum that this number is referenced
from.
I urge you to examine your basic W&B numbers carefully and
determine what you
want to use as a datum. Once you make that decision, make sure that ALL arms
and limits are referenced to that datum. You
cannot use factory arms and
limits unless you are referencing the factory datum.
In my days as an examiner I would
often notice a smile on the applicants face
when I asked for a W&B calculation. “Here’s an easy one, just a few fourth
grade arithmetic
calculations and I’ll have the answer”. Rarely would someone
be unable to compute this correctly. The smile would sometimes disappear when
I
probed a little further and asked “so what?, why do we care?” It was a rare
applicant who could correctly explain the stability and performance
tradeoffs
that define the balance limits.
Bill Harrelson
N5ZQ 320 1,950 hrs
N6ZQ IV under
construction
[I concur, everything Bill says is right on. If you're going to use the spreadsheet from the LML website (prepared for
N540TF) you absolutely MUST plug in your own stations measured from your aircraft. Since you can't physically measure things like the center of
mass for the pilot, passengers, and baggage compartment, you can deduct the station for "bottom front of firewall" from the ones shown on that
spreadsheet to determine their respective deltas, then add those deltas back onto your "bottom front of firewall" station to determine their real
stations for your aircraft. You can't shortcut physically measuring the bottom front of firewall (even though it's not directly used in the
spreadsheet, it's a critical dimension in the grand scheme of things) and the axle C/L's for the gear. Once your basic numbers are correct the
spreadsheet should operate as intended for your aircraft. <Marv> ]
From: Dico Reijers
Sent: Sunday, July 31, 2011 12:07 PM
To: lml@lancaironline.net
Subject: [LML] Newbie: Help with Weight &
Balance
Hi All,
I'm a newbie LNC4 owner and am just getting familiar with the airplane and
have a couple
questions regarding the weight and balance.
I used the sample spreadsheet (from Lancair Online's web site) and plugged in
the
#s from my weight and balance. The numbers calculated by the spread sheet
are pretty much exacly the same as per my weight and balance,
however there
are some numbers included in the sample W&B spreadsheet that are "greek" to me
(the areas in red in the PDF link
below). I am hoping someone can help me out
I posted a link to my W&B here:
http://www.cavendishcorner.com/fqqq.pdf
1.. The "bottom front of firewall" number seems to do nothing to the
calculations. Where does this number come from?
2.. The "mains retracted" number of 1088... is this the same for all IV-Ps?
Adjusting that number does make a difference on the calculations, however
that 1088 seems to make the numbers pretty much match my
current W&B. Perhaps
just a coincedence.
3.. "MAC is 40 inches long".... what does this
mean?
4.. Wing LE @ BL25.5 = 93.875..... what does this mean? Changing this
number changes the
percentage outlined in red so I should use a number related
to my serial#.
My weight and balance is located here:
http://www.cavendishcorner.com/fqqq-2.pdf
My FS limits are from 86.5 to 94.5 --- how is this range
determined?
Thanks for any help!
-Dico Reijers
Lancair IV-P
#428
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