Most documentation gives percentage weight distrubtion between main and nose gear for ramp weight. I'm curious if this weight distribution would be the same for takeoff weight.
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$\begingroup$ Is %MAC a relationship between the nose gear and the main gear for ramp weight? Perhaps... to determine maximum force exerted by each wheel. For takeoff, enroute and landing operations, most airplanes use a relationship between the leading edge and trailing edge of the wings. Thus 0% MAC puts the CG at the leading edge of the wing. Personally, can't imagine why they wouldn't use the same formula for ramp weight. It would provide the details necessary without extra pilot workload. $\endgroup$– wbeard52Sep 1, 2021 at 16:19
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2 Answers
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The difference would be from fuel burned during taxi, which is generally a very small fraction of the total fuel, so the impact on CG would be tiny. And, since burning fuel tends to bring the CG more into, rather than out of, limits, the "who cares" impact is essentially nil.
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$\begingroup$ So burning fuel moves the center of gravity forwards, towards the nose? $\endgroup$ Sep 1, 2021 at 0:56
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1$\begingroup$ Depends where the CG is in relation to the center of the fuel tank being burned (which might not be in the same exact place depending on which tank is being burned right now). I'm not certain that there won't be exceptions to a particular generalization in that regard. For the 737-800, as best I recall, adding fuel to the wing tanks moves the CG in one direction, then adding fuel to the fuselage tank (filled last, burned first) moves it in the other. $\endgroup$– Ralph J ♦Sep 1, 2021 at 1:06
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2$\begingroup$ Swept wing a/c with center tanks, the CG moves aft as the center is burned, then back forward as the wings are burned. With no center tanks, it just moves forward. On a straight wing a/c with wing tanks, the shift is negligible, very slightly forward as the fuel's CG in the wing box is a bit aft of the plane CG. $\endgroup$– John KSep 1, 2021 at 1:29
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$\begingroup$ Thanks guys, awesome responses. During taxi, do you know which tank is being used, so as to determine which direction the center of gravity is likely going in? $\endgroup$ Sep 1, 2021 at 1:32
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6$\begingroup$ Just for the heck of it, I ran a weight and balance for a 747-400 with a ramp fuel load of 200000 lbs, a taxi fuel burn of 2000 lbs, and an otherwise empty airplane. It moved the ramp CG from 27.9% MAC to 27.8% MAC. That equates to 0.3278 inches. $\endgroup$– TerrySep 1, 2021 at 5:55
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Great answer above. Acceleration also plays a negligible task regarding this as fractions of fuel are forced to settle backward in each fuel tank; this somethimes causes more noticeable spills under the wing of some overfilled airliners only when they start to roll for take off - as fuel is forced backwards and follows the trailing edge swept angle outwards until they reach the vents... and leaks.
answered Sep 1, 2021 at 19:01