FAA Flight Instructor Airplane (FIA)Aerodynamics and Principles of FlightHard
An airplane has strong dihedral effect but weak directional (weathervane) stability. Following a disturbance that banks the airplane, a CFI would expect the airplane to exhibit:
- APhugoid oscillation with slow pitch changes and constant airspeed
- BDutch roll with persistent yaw-roll oscillations
- CA rapid, well-damped return to level flight
- DSpiral instability, with bank angle and airspeed continuously increasing
Show answer & explanationAnswer & explanation
Correct answer: D. Spiral instability, with bank angle and airspeed continuously increasing
Spiral instability occurs when directional stability is weak relative to dihedral effect. A bank disturbance causes sideslip, the weak directional stability fails to correct the yaw quickly, and the resulting yaw-induced roll increases the bank further, leading to a slowly tightening, diving spiral rather than a return to wings level.
Why the other options are wrong
- A. Phugoid involves pitch/airspeed exchange, unrelated to lateral-directional imbalance.
- B. Dutch roll results from strong directional stability relative to dihedral effect, the opposite imbalance.
- C. Describes a stable airplane, opposite of the condition described.
Spiral Instability
Occurs when dihedral effect is strong relative to directional stability, causing a bank disturbance to develop into an ever-steepening diving spiral.
- Caused by weak directional stability vs strong dihedral effect
- Bank and airspeed increase progressively without correction
- Opposite imbalance (strong directional, weak dihedral) causes Dutch roll
Memory trick: Weak weathervane + strong dihedral = spiral dive