Longitudinal Stability Summary

Longitudinal Stability

  • Effect of Throttle: Closing throttle reduces downwash; insufficient force at "T" lowers stabilizer.

  • Power Impact: Increased power can cause the nose to rise due to counteracting tail load.

    • Low thrust line increases nose-up tendency.

  • Aircraft Speed: Decreased speed reduces airflow and downwash, accentuating nose heaviness which can pitch down more.

  • Force Balance: CG-CL-T acts as lever; upward force at CL, downward force at CG and T.

  • Stabilizer Angle: Fixed horizontal stabilizers set for optimal stability at design cruising speed.

    • Balanced load at cruise; varying loads at low/high speeds affects stability.

  • Center of Lift: Moves with angle of attack changes; forward with increased AOA and aft with decreased AOA.

  • Design Consideration: Most aircraft designed with wing's center of lift aft of CG for nose heaviness, requiring downward force on stabilizer for balance.

  • Static Stability Factors:

    1. Wing's location relative to CG.

    2. Horizontal tail location relative to CG.

    3. Tail surface area/size.

  • Static Stability Mechanism: For restoring moment after nose-up, wing and tail moments must change to ensure nose returns down during instability.