Centre of gravity limits

For an inherently stable aircraft, the absolute forward limit of centre of gravity movement is determined by the maximum balancing moment that the tailplane can produce while still retaining adequate control. The maximum rearward movement is limited by the onset of instability or excessive control response. In practice, unless an automatic control system is used, it could be dangerous to fly with the centre of gravity near these limits, and designers and airworthiness requirements impose a more restricted range of allowable safe centre of grav­ity movement. Great care has to be taken when loading and fuelling aircraft to ensure that the centre of gravity will remain within the acceptable range throughout the flight.

Compressibility effects

As explained in previous chapters, in the transonic speed range, the centre of lift tends to move rearwards with increasing Mach number. This has a similar effect to moving the centre of gravity forwards. It has to be corrected by raising the elevator, which effectively increases the longitudinal dihedral, and hence improves the static stability. However, as we have seen, the trim drag then rises, and the elevator control forces are increased.

The problem of the rearward movement of the centre of lift is aggravated on swing-wing (variable sweep) aircraft, because the wings move backwards as the sweep angle is increased for high speed flight. Such aircraft therefore invariably have very large tail surfaces, as seen on the Tornado in Fig. 11.12.

For a canard configuration, the rearward movement of the centre of lift at supersonic speeds is corrected by increasing the foreplane lift. Any increase in foreplane lift means that the main wing lift can be reduced, so there is little or no overall increase in trailing vortex (induced) drag. Thus, there should be less trim-drag penalty on a supersonic canard.