A modern fly-by-wire transport jet is designed with 'relaxed static stability,' meaning the natural aerodynamic center is very close to or slightly behind the CG at certain flight conditions. What is the primary reason manufacturers design aircraft this way, relying on the flight control computers to provide artificial stability?
- ATo increase the natural spiral stability of the airframe without computer input
- BTo eliminate the need for a horizontal stabilizer entirely
- CTo increase the airplane's critical Mach number by altering the center of pressure
- DTo reduce trim drag and improve cruise fuel efficiency by minimizing the tail-down force needed
Show answer & explanationAnswer & explanation
Correct answer: D. To reduce trim drag and improve cruise fuel efficiency by minimizing the tail-down force needed
With relaxed static stability, the CG is positioned further aft (closer to the neutral point), reducing the download the tail must generate to trim the aircraft in cruise. This reduces trim drag and improves fuel efficiency; the reduced natural stability is compensated for by the flight control computer, which provides artificial stability and handling qualities that meet certification requirements.
Why the other options are wrong
- A. Relaxed static stability reduces, not increases, natural stability; the computer compensates for this reduction.
- B. A horizontal stabilizer is still required for pitch trim and control even with relaxed stability design.
- C. Critical Mach number is governed by wing design (sweep, airfoil), not by CG/stability margin design choices.
Relaxed Static Stability (Fly-by-Wire)
An aircraft design approach where the CG is placed near or aft of the natural aerodynamic center to reduce trim drag, with the flight control computer providing artificial stability augmentation.
- Reduces tail download needed for trim, saving fuel
- Requires fly-by-wire flight control computers for artificial stability
- Trades some natural stability for efficiency, compensated electronically
Memory trick: Let the computer hold what nature loosened