The position of the CG of the airplane
usually is expressed in inches from the
datum. Since a measurement from the datum
is an arm, the term CG arm also is used to
describe the location of the CG. To find the center of gravity of an object or a group of
objects, the moments of all the parts are added, and this total is divided by the total
weight of the parts. Figure 8-28 shows how to do this using the seesaw example.
Figure 8-28.
Using the
left end of the seesaw as
the datum, the position of
the CG is calculated
using the children's
weights and the weight of
the seesaw itself.
8-35
CALCULATING THE
POSITION
OF THE
CG

##### We have textbook solutions for you!

**The document you are viewing contains questions related to this textbook.**

**The document you are viewing contains questions related to this textbook.**

Expert Verified

When looking at figure 8-28, you could probably guess that the CG was over the fulcrum,
since the seesaw was in balance. Now, try to calculate the CG location using two new
children with different weights. [Figure 8-29]
Figure 8-29.
Where is the CG? You
should get a total moment of 12,800
and a total weight of 180 pounds, for
an answer of 71.1 inches.
You can use the same technique to find the CG of an airplane. Substitute the weight and
moment of the airplane for the weight and moment of the seesaw. Combine it with the
weight and moment of the pilot to obtain the new CG location, as shown in figure 8-30.
Figure 8-30.
Multiply the pilot's weight by the
distance from the datum to get her moment.
The weight and moment of the airplane are
found in its weight and balance documents.
To find the CG location, divide the total
moment by the total weight.
8-36
C H A P T E R
8
A I R P L A N E
P E R F O R M A N C E

SHIFTING WEIGHT TO MOVE THE CG
Returning to the example of the children on the seesaw from figure 8-29, to rearrange the
children so that the seesaw balances, you should move the CG to the fulcrum. To do this,
move the larger child, Susan, toward the
datum so that her weight acts through a
shorter
arm,
reducing
her
moment.
[Figure 8-31] Of course, all of this can be
described mathematically, as you will
learn later.
8-37
W E I G H T A N D
B A L A N C E
S E C T I O N
B
Figure 8-31.
Shortening the arm reduces the moment. As
Susan moves toward the datum, her moment is reduced, which
moves the CG toward the fulcrum until the seesaw is balanced.
DETERMINING T
O
TAL WEIGHT
AND CENTER OF GRAVITY
In the seesaw examples, you used what is called the computation method. It demon-
strates the principles of weight and balance most thoroughly. Using the computation
method for airplanes requires multiplying and adding up large numbers, and there are
plenty of opportunities to make mistakes, even if you use your calculator. To simplify the
process, many manufacturers provide tables and/or graphs in the POH. You should be
able to use all three methods (computation, table, or graph), since weight and balance
information in your POH may be in any of the different formats.