Stability, Control & Ground Effect

Stability is the airplane's tendency to return to a steady flight condition after a disturbance. It comes in two forms and acts about three axes.

• Static stability is the initial tendency after a disturbance. Positive static stability returns toward equilibrium; neutral stays displaced; negative diverges away.
• Dynamic stability describes the motion over time. Positive dynamic stability means oscillations dampen out and die down.

Most training airplanes are designed with positive static and positive dynamic longitudinal stability so they are forgiving and self-correcting.

• Longitudinal axis (nose to tail): roll, controlled by ailerons.
• Lateral axis (wingtip to wingtip): pitch, controlled by the elevator.
• Vertical axis (top to bottom): yaw, controlled by the rudder.

Longitudinal (pitch) stability is heavily influenced by the center of gravity (CG). A CG that is too far aft reduces stability and can make the airplane dangerously difficult to recover from a stall; a CG too far forward increases stability but raises stall speed and control forces.

Four effects tend to yaw a single-engine prop airplane left, most noticeably at high power and low airspeed (takeoff/climb):

1. Torque reaction — engine torque rolls the airplane opposite prop rotation.
2. P-factor — the descending propeller blade (on the right) takes a bigger bite at high AOA.
3. Spiraling slipstream — corkscrewing airflow strikes the left side of the vertical fin.
4. Gyroscopic precession — most significant in tailwheel aircraft when the tail rises.

Right rudder counteracts these tendencies.

Within about one wingspan of the surface, the ground interferes with the wingtip vortices and downwash, reducing induced drag. The airplane feels like it 'floats.' Two hazards: lifting off in ground effect before reaching a safe climb speed (the airplane may settle back as you climb out of ground effect), and floating in the flare on landing.