![]() ![]() We place a 200-lb weight on the beam, 150" from the datum line.We place two 100-lb weights on the beam, one is 50" from the datum line, the other is 90" from the datum line.This means the center of our beam is at (180 / 2) + 10 = 100". We use 10" forward of the beam as our arbitrary datum line.We have a 180" beam that weighs 50 lbs. ![]() What if we don't have a fulcrum and need to determine where the center of gravity is? We need to know: the distance of each object from a common point, called the datum, the weight of each object, and the weight and length of the object itself. On a larger jet that are several other concepts to wrestle with and everyone assumes the knowledge exists. We all seem to be weight and balance experts with a Cessna 150 or Piper Cherokee because that's where we learned the concepts and it seems most textbooks are aimed to the private pilot audience. If the CG is too far either way, the aircraft cannot be controlled in pitch. There exists a range of CG positions which the horizontal stabilizer is capable of countering. The aircraft's "center of lift" exerts an upward force, so the center of gravity (CG) must at all times be forward of the center of lift to keep the aircraft in stable flight. Why? An aircraft with a conventional tail-mounted horizontal stabilizer uses that stabilizer to exert a downward force. In either case, you should have complete understanding of the concepts and a very good idea of where your airplane's range of acceptable centers of gravity lie. ![]()
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