Load distribution and rearward CG limit

Load distribution (TP 1102, Flight for Range and Endurance, Centre of Gravity):

• Forward only: more wing loading, higher AOA.
• Forward CG: higher tailplane AOA needed to balance (AC 700-031 App.B s.4.1(3)).
• Aft only: faster cruise.
• Neutral tail load: most efficient, but unstable; aircraft designed to carry down load on tail.
• Excessive nose-heavy loading: steering difficulty in nosewheel aircraft during landing roll and takeoff.

Rearward CG limit. Set largely by stability.

• Certification: vertical nose displacement must damp within a set number of oscillations.
• Too far aft: aircraft can climb and dive alternately, each cycle steeper, becoming unmanageable.
• CG moves aft: wing's destabilizing contribution shrinks, tail's stabilizing contribution holds.
  • At balance: neutrally stable.
  • Further aft: unstable.
• CG position sets wing lift/AOA, tail force, stabilizer deflection needed for equilibrium, so affects elevator control force.

Trap: neutral (zero) tail load is most efficient, not safest; aircraft need a down tail load because neutral load gives up stability.

Practice