A designer moves an airplane's horizontal stabilizer further aft, increasing the tail moment arm, while keeping the CG location and stabilizer area unchanged. What is the primary effect on longitudinal static stability?
- ALongitudinal static stability increases, because the longer moment arm produces a greater restoring moment for a given change in tail lift
- BThe airplane becomes longitudinally unstable due to increased tail drag
- CLongitudinal static stability decreases, because the tail becomes aerodynamically less effective at high airspeeds
- DLongitudinal static stability is unaffected, since CG location is unchanged
Show answer & explanationAnswer & explanation
Correct answer: A. Longitudinal static stability increases, because the longer moment arm produces a greater restoring moment for a given change in tail lift
Longitudinal stability depends on the restoring moment the tail generates in response to a disturbance in angle of attack. Moment = force × arm length, so increasing the tail moment arm (distance from CG to the tail's aerodynamic center) increases the restoring moment for the same change in tail lift, enhancing longitudinal static stability.
Why the other options are wrong
- B. Increased drag is a secondary effect and does not make the airplane unstable.
- C. A longer arm does not reduce tail effectiveness; it increases the moment it produces.
- D. CG-to-tail distance (moment arm), not just CG location alone, is what changed and matters.
Tail Moment Arm and Longitudinal Stability
The distance from the CG to the horizontal stabilizer's aerodynamic center; a longer moment arm increases the restoring moment and enhances longitudinal static stability.
- Moment = force × arm length
- Longer tail arm increases stabilizing moment
- CG position and tail arm both affect longitudinal stability
Memory trick: A longer wrench (tail arm) turns the same force into a bigger corrective moment.