3302. Some Considerations about Aircraft Center of Gravity

Paper

Abstract

Why is the center of gravity so important for an aircraft? Handling qualities, stability, loads depend on center of gravity position. Unacceptable domains exist where flying the aircraft could be hazardous, difficult, and even dangerous. An aircraft has to stay within a center of gravity domain and therefore has center of gravity limits. In general operation mode, a passenger civil subsonic aircraft only has longitudinal center of gravity limits depending on the aircraft weight and the flight phase: take-off, flight, or landing. If lateral and vertical limits exist, they are not usually reached in operation on passenger civil subsonic aircraft. This paper gives an overview of this technical subject: the aircraft center of gravity.
First preliminary definitions are given: center of gravity, reference axis, reference chord, center of gravity calculation, center of gravity diagram, certified center of gravity limits, aircraft loading, and operational center of gravity limits.
The origins of these longitudinal, lateral, and vertical limits are then explained, such as the determination and control of the aircraft center of gravity.
The longitudinal limits include handling qualities, loads, and loadability. We examine the impact that limits have on the aircraft definition. Then loadability considerations in the preliminary phase are considered. A loadability index is defined to measure the loadability of an aircraft and to permit comparison between different configurations and aircraft. The impact of fuel management and fuel transfer is analyzed, as well as how limits can be redefined for derived aircraft. On supersonic aircraft, longitudinal limits also depend on speed (mach number). This property is illustrated by the example of the Concorde. The center of gravity determination and control during operation are shown.
Lateral limits have properties similar to the longitudinal ones but generally are such that they do not lead to operational constraints.
Vertical limits generally appear on cargo aircraft. The example of an A300-600 Super Transporter (Beluga) is shown with the presentation of its limits and the measure of vertical CG for the empty aircraft.
Finally the certified weight-center of gravity domain has to be explored during flight tests. The flight test installation must allow achieving any point of this domain. A340-500/-600 flight test installations are shown.
In conclusion, the apparently simple technical subject, aircraft center of gravity, is not so simple and is very large, so it was not possible in one paper to go deeper in some points (operational limits, handling qualities) and also some other points were not treated (CG variation due to fuel, with pitch and roll angles, in static, in dynamic: fuel slosh effect; flexible aircraft; risks of exceeding the limits). But nevertheless the objective was to give a good understanding of what is behind the aircraft center of gravity, from the aircraft definition, architecture of CG control by airlines in operation, to explain where the limits came from, to show that the incorporation of loadability during preliminary design is a necessity of the aircraft sizing process, what is done by the manufacturers to optimize the aircraft, if there are loading constraints where they came from, and the benefits for the aircraft.