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4-4 <strong>ALPHA</strong> <strong>Trainer</strong><br />
REV. 0 Weight and balance<br />
Sample c.g. calculation<br />
www.pipistrel.eu<br />
Guidelines<br />
Gtotal is the total mass of empty aircraft. All calculations can be performed with aircraft empty<br />
weight and empty weight centre of gravity (c.g.), as the pilots sits directly below the centre of gravity<br />
and do not cause the c.g. to be shifted.<br />
WARNING! Both pilots’ weight do not influence c.g. or their influence is insignificant.<br />
However, baggage and/or fuel can influence the c.g. severely and may cause the aircraft to become<br />
uncontrollable!<br />
Basic CG formulas and calculation<br />
Read thoroughly. Note also that the basic c.g. at 287 mm will be used purely as an example.<br />
First, weigh the aircraft according to the procedure described in this chapter and write down values<br />
of G1 (sum of scale readings at main wheels) and G2 (scale reading at tail/front wheel). Then calculate<br />
the position of c.g. in milimeters (mm) from the datum (wing’s leading edge at wing root).<br />
Use the following formula:<br />
CG<br />
mm<br />
G1back<br />
× c G1back<br />
× 1525mm<br />
= − a = − 1020mm<br />
= 287mm<br />
G<br />
G<br />
total<br />
where:<br />
G2back is the sum of scale readings at both main (back) wheels,<br />
Gtotal is the sum of G1 and G2back (G1+G2back), a.k.a. aircraft empty weight<br />
a is the distance from nose wheel axis to wing’s leading edge,<br />
b is the distance from main wheel axis to wing’s leading edge,<br />
c = (a+b) is the sum of both distances above.<br />
Second, determine the c.g. position in percentage (%) of Mean Aerodynamic Chord (MAC) with following<br />
the formula:<br />
CGmm − R 287mm<br />
− 43mm<br />
CG% MAC<br />
=<br />
⋅ 100 =<br />
⋅ 100 = 28.%<br />
4<br />
MAC<br />
897mm<br />
where:<br />
CGmm is the position of CG in milimeters (mm),<br />
R is the difference between wing’s leading edge and MAC’s leading edge (43 mm),<br />
MAC is the Mean Aerodynamic Chord (897 mm).<br />
total<br />
Fuel and CG<br />
The fuel shifts the CG backward, however the aircraft is factory tuned so that a fuel load of fuel and<br />
luggage the CG stays forward of the aft allowable limit. Similarly, when fhe fuel quantity is empty<br />
and there is no baggage on board, the CG is backwards of the forward allowable limit. In every case,<br />
the fuel does not shift the CG out of limits.
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