Page 21 - August18T

Stability
Most airplanes exhibit some level of stability in at least two of the three axes. Almost all have built-in pitch stability. Disturbed upward or downward in pitch and then released, the airplane’s nose will oscillate up and down through two or three cycles before it returns to its original pitch attitude...not necessarily on its initial altitude, but at the same pitch attitude, angle of attack and indicated airspeed. Put another way, a pitch-stable airplane will seek the indicated airspeed (actually, angle of attack) for which it is trimmed. If it is disturbed in pitch, or if power or total drag (flap, landing gear position) changes, the airplane will pitch down or up as necessary to remain at its trimmed speed.
Most airplanes also have some level of stability in yaw. Kick a rudder pedal and release, or hit a wind shear that yaws the aircraft, and it will wallow back and forth a few oscillations before returning to straight-ahead flight.
Many aircraft are neutral in stability or even slightly unstable in roll. Enter a shallow bank and the airplane may remain banked or slowly return to approximately wings-level flight. But bank steeply enough and most aircraft will not level their own wings. In fact, in a steep turn most airplanes will continue to bank progressively more steeply. This is sometimes called the overbanking tendency, the reason it may take opposite aileron input to maintain bank once established in a steep turn.
You’ve probably seen diagrams that show the relationship between bank angle and stalling speed (see photo). What’s not often well-explained is that this relationship is only valid in level, coordinated flight. If the pilot does not resist the airplane’s
tendencies and its nose drops to seek the trimmed airspeed, the G load does not increase. In fact, the load factor will increase only if the pilot, an autopilot, or a runaway electric trim system resists the airplane’s natural tendency to change pitch if it gets off its trimmed speed. An airplane will not stall on its own. The pilot (or an automated pilot) has to actively pull against the airplane’s stability to make it stall.
Steep Turns
What happens then if the airplane enters a steep turn and the pilot provides more or less resistance than is necessary to maintain level flight? We’ll use the 60-degree bank example:
• If the pilot adds more than 2G of resistance, the airplane will climb. The nose will rise above the horizon and, if there is sufficient power, the airplane will enter a sustained
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August 2018
TWIN & TURBINE • 19