Stability and controllability

Static and dynamic stability on all three axes, spiral mode, Dutch roll.

16 min read · Advanced aerodynamics

After this lesson you can

  • Distinguish static stability from dynamic stability, and classify a described flight behavior using both.
  • Explain why the wing alone is inherently unstable and how the tail balances it, and predict how moving the CG changes stall speed, cruise speed, and spin recoverability.
  • Name the four contributors to lateral stability and quote the two numerical rules of thumb PHAK gives for effective dihedral.
  • Tell Dutch roll and spiral instability apart, and explain why designers generally accept the second rather than the first.

Stability and controllability are two ends of a trade-off. A very stable airplane returns to its trimmed condition on its own, which reduces pilot workload, but it also resists being maneuvered. A less stable airplane obeys the controls readily, but demands constant attention to hold a heading or an altitude.

A commercial pilot needs this trade-off for two practical reasons. First, loading moves it: center of gravity (CG) position affects longitudinal stability more than anything else the pilot controls before flight. Second, the airplanes ahead in a professional career - heavier, faster - are built around a different point on this trade-off than a primary trainer.

The foundational distinction is between static stability, which describes the airplane's initial tendency after a disturbance, and dynamic stability, which describes what happens over time after that.

The rest of this lesson

  1. 1.Static and dynamic: two different questions
  2. 2.Longitudinal stability starts with an unstable wing
  3. 3.What moving the CG really changes
  4. 4.Lateral stability: four contributors
  5. 5.Directional stability: the weathervane effect
  6. 6.When lateral and directional stability are out of balance: Dutch roll and spiral instability
  7. 7.What to remember

These 7 sections, their figures, the checks along the way and the end-of-lesson quiz come with the CPL track. Paid once, yours for good.

See what the CPL track costsEvery module 1 is free to read, no account, no card.

Key terms

Try to recall each definition before turning the card.