Figure 3-16. The MAC is the chord drawn through the geographic
center of the plan area of the wing.
CG MAC
Trailing edge mean
aerodynamic chord
Leading edge mean
aerodynamic chord
TEMAC
LEMAC
Datum
where the datum is located.
Center of Gravity (CG) and Mean
Aerodynamic Chord (MAC)
Center of Gravity (CG)
In addition to overloading or drastically reducing the
aircraft’s weight, the distribution of weight is also a concern.
When aircraft equipment is changed, the person making the
equipment change must make an entry on the equipment list
indicating items added, removed, or relocated; the date of
the change; and the person’s name and certification number
in the aircraft’s maintenance records.
For the purpose of weight and balance computations, the CG
of an airplane is an imaginary point about which the nose-
heavy (–) moments and tail-heavy (+) moments are exactly
equal in magnitude. If suspended from the CG point, the
aircraft would have no tendency to rotate nose up or nose
down. The CG of the loaded aircraft can range fore and aft
within certain limits that are determined during the fl ght test
for type certific tion. These limits are the most forward- and
rearward-loaded CG positions at which the aircraft meets the
performance and flight characteristics required by the FAA.
Any loading that places the CG outside the limits for a
particular aircraft seriously impairs the pilot’s ability to
control the aircraft. For example, it is more difficult to take
off and gain altitude in a nose-heavy aircraft, and the aircraft
tends to drop its nose when the pilot reduces throttle. It also
requires a higher speed to land safely. A tail-heavy aircraft is
more susceptible to stalling at low speed, which is a concern
during the landing approach.
On small airplanes and on all helicopters, the CG location
is identified as being a specific number of inches from the
datum. The CG range is identified in the same way. On larger
airplanes, from private business jets to large jumbo jets, the
CG and its range are typically identified in relation to the
width of the wing.
Mean Aerodynamic Chord (MAC)
The width of the wing, or straight-line distance from the
leading edge to the trailing edge, on an airplane is known as
the chord. If the leading edge and the trailing edge of a wing
are parallel, the chord is equal at all points along the entire
length of the wing. The average length of the chord, or MAC,
of a tapered wing is more complicated to define. The MAC,
as seen in Figure 3-16, is the chord of an imaginary airfoil
that has the same aerodynamic characteristics as the actual
airfoil. You can also think of it as the chord drawn through
the geographic center of the plan area of the wing.
Usually listed in the aircraft’s TCDS when it is required for
weight and balance computations, the MAC is established
by the manufacturer, defining its leading edge (LEMAC)
and trailing edge (TEMAC) in terms of inches from the
datum. [Figure 3-16] The CG location and various limits
are then expressed in percentage of the chord or percent
MAC. In order to relate the percent MAC to the datum, all
weight and balance information includes two items: the MAC
length in inches and the location of the LEMAC in inches
from the datum. For the purpose of simplicity, most light-
aircraft manufacturers express the CG range in inches from
the datum; transport-category aircraft CGs are expressed in
percent MAC.
The relative positions of the CG and the aerodynamic
center of lift of the wing have critical effects on the flight
characteristics of the aircraft. Consequently, relating the
CG location to the chord of the wing is convenient from
a design and operations standpoint. Normally, an aircraft
has acceptable fl ght characteristics if the CG is located
somewhere near the 25 percent average chord point. This
means the CG is located one-fourth of the distance back from
the LEMAC to the TEMAC. Such a location places the CG
forward of the aerodynamic center for most airfoils.
The weight and balance data of the airplane in Figure 3-17 states
that the MAC is from stations 144 to 206 and the CG is
located at station 161.
MAC = TEMAC – LEMAC
MAC = 206" – 144"
MAC = 62"
Figure 3-18. Formula for determining the CG expressed in percent
MAC.
CG inches % MAC = Distance aft of LEMAC × 100
MAC
=
= 27.4
17 × 100
62
Figure 3-17. Large aircraft weight and balance calculation diagram.
CG = 27.4%
CG = 161
MAC
TEMAC
LEMAC
Datum
CG = 27.4%
CGCG 1= 161161
MAC
Station 144" Station 206"
Figure 3-19. Formula for determining the CG in inches from the
datum.
CG inches from datum = LEMAC + MAC × CG % MAC
100
= 144 +
= 160.9
62 × 27.4
100
CG = 161"
In order to find the percent MAC, firs determine the distance
of the CG from LEMAC.
CG – LEMAC = Distance from LEMAC
161" – 144" = 17"
The location of the CG expressed in percent MAC is
determined using the formula found in Figure 3-18. The CG
of the airplane is located at 27.4 percent MAC.
It is sometimes necessary to determine the location of the
CG in inches from the datum when its location in percent
MAC is known.
The CG of the airplane is located at 27.4 percent MAC.
MAC = 206 – 144 = 62
LEMAC = station 144
Determine the location of the CG in inches from the datum
by using the formula found in Figure 3-19.
The CG of this airplane is located at station 160.9 inches aft
of the datum. It is important for longitudinal stability that the
CG be located ahead of the center of lift of a wing. Since the
center of lift is expressed as percent MAC, the location of
the CG is expressed in the same terms.