# The position of the cg of the airplane usually is

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Chapter 2 / Exercise 1
Sensation and Perception
Brockmole/Goldstein Expert Verified
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
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Chapter 2 / Exercise 1
Sensation and Perception
Brockmole/Goldstein 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.
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