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may be affected with some loads,
such as when hoisting.
Passenger seats occupy the whole
floor space evenly; this loadspreading
principle needs to be
borne in mind with freight (cargo is
best distributed like passengers
would be), which makes it easier to
provide decent restraint on each
pack, because access areas to exits
above and around the cargo are
needed for when the load has moved
after an emergency stop. You may
find it helpful to line the floor with
something waterproof if you're not
sure what you're carrying—people
sending packages don't generally
know about Dangerous Goods and
may send items that leak something
'orrible all over the place. Loads
must be restrained with nets or
straps (or a combination of both)
and must distribute the load over
available fixtures, such as seat
attachments. The range of C of G for
most helicopters is a short distance
fore and aft of the main rotor mast.
The reference datum is an imaginary
point from which all calculations
start and where some C of G ranges
are expressed (for example, 106" aft
of datum). Mostly, it is at, or slightly
forward of, the nose, but can be at
the rotor mast of a helicopter. On
large aircraft, C of G limits may also
be expressed in terms of % MAC, or
The Mean Aerodynamic Chord, which is
the average distance from the leading
to the trailing edges of the wing, or
the chord of an imaginary
rectangular wing with the same
centre of pressure, that is easier to
use than a swept back one:
LEMAC is the distance from the
datum to the leading edge of the
MAC, at the front, and TEMAC is
the distance to the trailing edge, at
the back. LEMAC is therefore 0%
MAC and may also expressed as a
distance aft of the datum. So, the C
of G will lie somewhere between
LEMAC and TEMAC, depending
on the weight and configuration.
This formula calculates the %MAC
(use the same units):
%MAC = CG-LEMAC x 100
MAC
Find the conventional C of G first,
then divide its distance aft of
LEMAC into the MAC (TEMAC
minus LEMAC). The reason you
need to know the %MAC settings is
because some jets have their
horizontal stabiliser trim settings
marked in this value (the figures are
a product of the C of G and flap
setting). Others, such as the 737
have them marked in units of nose
up trim, and you will need to look in
the trim tables to get the settings for a
given C of G. To convert %MAC
figures back to an arm (for C of G
change – see below), first convert
the C of G as %MAC to C of G in
ins aft of LEMAC:
CG (aft LEMAC) = CG%MAC/100 x MAC
82 Operational Flying
Then just add the figure obtained
above to the distance from Datum
to LEMAC.
The arm is the distance from the
reference datum to the area in
question, such as a passenger seat or
the fuel tank. It may be measured in
Imperial or Metric units, and you
must use the same ones. The
expression station may also be used.
To get the C of G of an aircraft, you
multiply the weight of each item in it
by the arm to get the moment, or the
amount of leverage that item
contributes.
The aircraft itself will have an arm
and a moment from when it was last
weighed, and this is where you start.
You can find it in the weight and
balance schedule (usually in the flight
manual), and it may be varied if you
add or take off various items of
equipment, such as the hook or hoist
in a helicopter.
Because you might end up using very
long numbers, sometimes you use a
moment index, the result of dividing
the moment by 1,000 to make the
figures more manageable. Here is a
simplified typical calculation for a
Bell 206 helicopter (the principles
are the same for larger machines):
Item Wt Arm Moment
Aircraft 1881 116.5 219137
Front pax 185 65 13000
Rear Pax 185 104 19240
Baggage 50 147.50 7375
Fuel 310 110.7 34273
Total 2611 112.22 293025
The total C of G for takeoff is
112.22, obtained by dividing the total
moment figure (293025) by the total
weight (2611). This particular
machine's fuel has a variable CG
range, meaning that it has one all on
its own (that is, the arm will change
with fuel weight), so the figure of
110.7 will change with the amount,
for which check the flight manual.
Also look for a graph like this:
The procedure is to multiply the
weights by the arms to get the
moments, and divide the total
　
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