9.4 Axes, Stability, and Flight Controls

ACS PA.I.G.K1a (primary flight controls) and PA.I.G.K1b (secondary flight controls) sit on the same three axes PHAK Chapter 5 uses to teach stability. If you cannot name which surface rotates the airplane about which axis, the rest of the systems oral is noise.

Three axes, three primary controls

The longitudinal, lateral, and vertical axes all pass through the CG. The airplane rotates about one or more of them whenever you change attitude.

The vocabulary trap is swapping the motion name with the axis name. Longitudinal stability is about pitch, but the rotation is about the lateral axis. Lateral stability is about roll, about the longitudinal axis. Say it twice.

Ailerons. Wheel or stick right: right aileron up, left aileron down. The left wing’s camber and AOA increase, lift increases, that wing rises.

Elevator. Aft yoke: trailing edge of the elevator up, the tail is pushed down, the nose pitches up, wing AOA rises. A stabilator is a one-piece slab that pivots as a whole; an anti-servo tab moves in the same direction as the slab to add feel and discourage over-control. A canard is a forward surface that produces up-lift, not the tail download of a conventional stabilizer.

Rudder. Left pedal: rudder trailing edge left, tail pushed right, nose yaws left. Rudder authority rises with speed and with propeller slipstream. It does not make the airplane turn by itself in the PAR picture; the horizontal component of lift in a bank makes the turn. Rudder keeps the nose aligned with the relative wind so the turn is coordinated.

Adverse yaw, and why the rudder is not optional

The down-going aileron is a high-lift device. More lift means more induced drag on the rising wing. The up-going aileron reduces lift and drag on the descending wing. The drag imbalance yaws the nose opposite the roll: that is adverse yaw. It is worst at low speed / high AOA, where aileron deflections are large and the surfaces are less effective. PHAK Chapter 6: apply rudder in the direction of the turn to cancel it. “Step on the ball” or “rudder toward the low wing” is the same sentence.

Designers also build the wing to help:

• Differential ailerons. The up aileron travels farther than the down aileron. Extra drag on the descending wing offsets the induced-drag spike on the rising wing.
• Frise-type ailerons. The up-going aileron pivots on an offset hinge so its leading edge protrudes below the wing into the relative wind, adding form drag on the descending wing. A slot often helps the down-going aileron stay attached.
• Coupled aileron–rudder interconnects (with a spring you can override to slip) and flaperons appear on some types.

You still need your feet. Differential and Frise reduce adverse yaw; they do not repeal it.

Secondary controls: flaps, trim, spoilers

Secondary systems make the airplane easier to fly or change the wing’s lift/drag for a phase of flight. They are not required to rotate the airplane about an axis the way the primary three are.

• Flaps (plain, split, slotted, Fowler) increase camber and, for Fowler, area. Lift and induced drag both rise. Used to lower stall speed and steepen the approach. Leading-edge slots, slats, flaps, and cuffs delay tip or leading-edge separation so the wing can reach a higher AOA before it stalls.
• Trim (tabs, balance tabs, anti-servo tabs, ground-adjustable tabs, movable stabilizer) relieves continuous control force. Trim to an attitude and power setting; do not fly the airplane with the trim wheel. Aft CG and a sensitive stabilator make trim a safety item, not a convenience.
• Spoilers dump lift and add drag. On a glider they control glide path; on a jet they also help roll and put weight on the wheels after touchdown. Few PAR trainers have them, but the ACS still expects the name.

Static versus dynamic stability

Stability is the airplane’s inherent tendency to return toward the original flight path after a disturbance. Maneuverability is how readily it can be flown into a new path and take the stress. Controllability is how it responds to the pilot. Those are three different words on the test.

Static stability is the initial tendency, the first moment after you let go:

• Positive static: nose (or bank, or heading) starts back toward the original condition.
• Neutral static: it stays at the new condition.
• Negative static: it starts farther away.

Dynamic stability is what the motion does over time after that initial tendency:

• Positive dynamic: oscillations dampen.
• Neutral dynamic: they continue at the same amplitude.
• Negative dynamic: they diverge.

The desirable combination is positive static and positive dynamic. Positive static plus negative dynamic is a divergent oscillation — the airplane starts home and then overshoots worse each cycle. Longitudinal phugoid (slow speed/altitude wander at nearly constant AOA) is usually easy to trim out. A short-period pitch oscillation is not something you “ride.”

Longitudinal (pitch) stability depends on CG location relative to the wing and tail, tail volume, and downwash on the stabilizer. Reduce power and downwash, and a stable airplane pitches down toward a new trim speed — the PHAK speed-stability story.

Lateral (roll) stability is built with dihedral (the low wing in a sideslip sees a higher AOA and lifts itself), sweepback, high-wing keel effect, and weight distribution. Directional stability is the vertical tail acting as a weathervane. Too much directional stability and too little dihedral produces spiral instability (the graveyard-spiral tendency). The opposite mix produces Dutch roll. Trainers are usually a little spiral-unstable and easy to keep level.

CG rewrites stability and the stall

Because the tail’s restoring moment is force × arm, sliding the CG aft:

• shortens the tail arm,
• reduces longitudinal stability (lighter stick, easier to over-rotate, more willing to depart),
• lowers stall speed slightly (less tail download, so the wing carries less than weight + download),
• can leave you without enough elevator to pitch the nose down in a stall.

Sliding the CG forward does the opposite: more stable, higher stall speed, heavier elevator, and a firm nose-down break. Forward-CG landings can run out of elevator in the flare. Both ends of the envelope are AFM limits, not preferences. The weight-and-balance chapter will make you compute the number; this chapter asks you what the number does.

Scenario: Maya’s first steep turn

Maya rolls left with her feet on the floor. The left aileron goes down, the left wing makes more lift and more induced drag, and the nose yaws right — adverse yaw — while the ball skids. The turn is uncoordinated at high load factor, which is the accelerated-stall / spin-entry picture from 9.2. She adds left rudder as she adds left aileron (and right rudder as she rolls out, because adverse yaw reverses). After the maneuver she retrims; she does not hold 10 pounds of back-pressure with her fist. If she had loaded the baggage compartment past the aft CG, the same pull would have felt lighter, the nose would have been happier to keep rising, and the stall recovery in the next lesson would have been the one PHAK warns you may not finish.