Desirable CG Position
Proper distribution of mass (i. e., weight) over the aircraft geometry is key to establishing the CG. It is important for locating the wing, undercarriage, engine, and empennage for aircraft stability and control. The convenient method is to first estimate each component weight separately and then position them to satisfy the CG
Aerodynamic center range
in flight »i Vcn ground
I j-i – rotation stability – maximize
trim crag – minimize
ground maneuver margin – maximize
Figure 8.1. Aircraft CG position showing stability margin location relative to overall geometry. A typical aircraft CG margin that affects aircraft operation is shown in Figure 8.1.
The aircraft aerodynamic center moves backward on the ground due to the flow field being affected by ground constraints. There is also movement of the CG location depending on the loading (i. e., fuel and/or passengers). It must be ensured that the preflight aftmost CG location is still forward of the in-flight aerodynamic center by a convenient margin, which should be as low as possible to minimize trim. Where the main-wheel contact point (and strut line) is aft of the aftmost CG, the subtending angle, в, should be greater than the fuselage-rotation angle, a, as described in Section 7.6. The main wheel is positioned to ease rotation as well as to assist in good ground handling.
Advanced military combat aircraft can have relaxed static stability to provide quicker responses. That is, the margin between the aftmost CG and the in-flight aerodynamic center is reduced (it may be even slightly negative), but the other design considerations relative to the undercarriage position are the same.
Initially, locations of some of the components (e. g., the wing) were arbitrarily chosen based on designers’ past experience, which works well (see Chapter 6). Iterations are required that, in turn, may force any or all of the components to be repositioned. There is flexibility to fine-tune the CG position by moving heavy units (e. g., batteries and fuel-storage positions). It is desirable to position the payload around the CG so that any variation will have the least effect on CG movement. Fuel storage should be distributed to ensure the least CG movement; if this is not possible, then an in-flight fuel transfer is necessary to shift weight to maintain the desired CG position (as in the Supersonic Concorde).
Fuel loads and payloads are variable quantities; hence, the CG position varies. Each combination of fuel and payload results in a CG position. Figure 8.2 shows variations in CG positions for the full range of combinations. Because it resembles the shape of a potato, the CG variation for all loading conditions is sometimes called the “potato curve.” Designers must ensure that at no time during loading up to the MTOM does the CG position exceed the loading limits endangering the aircraft to tip over on any side. Loading must be accomplished under supervision. Whereas
10 20 30 40
Percent wing MAC (typically 10 to 50%)
(b) Range of CG variations – vertical limits
passengers have free choice in seating, cargo and fuel-loading are done in prescribed sequences, with options.
It has been observed that passengers first choose window seats and then, depending on the number of abreast seating, the second choice is made. Figure 8.2 shows the window seating first and the aisle seating last; note the boundaries of front and aft limits. Cargo – and fuel-loading is accomplished on a schedule with the locus of CG travel in lines. In the figure, the CG of the OEM is at the rear, indicating that the aircraft has aft-mounted engines. For wing-mounted engines, the CG at the OEM moves forward, making the potato curve more erect.
For static-stability reasons, it must be ensured that the aircraft has a static margin at all loading conditions. With the maximum number of passengers, the CG is not necessarily at the aftmost position. Typically, the CG should be approximately 18% of the MAC when fully loaded and approximately 22% when empty. The CG is always forward of the neutral point (i. e., the aircraft’s aerodynamic center, established through CFD and wind-tunnel tests). The aerodynamic center is assumed to be 50% of the MAC and must be iterated until the final configuration is reached.
Figure 8.2 represents a typical civil aircraft loading map, which indicates the CG travel to ensure that the aircraft remains in balance within horizontal and vertical limits. Loading starts at the OEM point; if the passengers boarding first opt to sit in the aft end, then the CG can move beyond the airborne aft limit, but it must remain within the ground limit. Therefore, initial forward cargo-loading should precede passenger boarding; an early filling of the forward tank fuel is also desirable.