Yaw, Roll and Their Interaction - ATPL 081 — Principles of Flight

The stability module proved that yaw and roll never act alone; this lesson shows what that means for the pilot's hands and feet. It works through the rudder and why big aeroplanes deliberately restrict it, the ailerons and the yaw they produce in exactly the wrong direction, the family of fixes for that flaw, the spoilers that roll big jets, and finally the two secondary effects: what roll does to yaw and yaw does to roll when the pilot commands only one of them.

The rudder

The rudder is the fin's hinged control surface, and it works exactly as the camber mechanism promises: deflect it left and the fin becomes an aerofoil cambered for a side force to the right, applied far behind the CG, a yawing moment swinging the nose left. More deflection means more camber, more side force, and a larger moment about the normal axis. Deflection and moment rise together. But the moment the pilot feels is not the rudder's alone: as the nose swings, sideslip builds, and the fin's own weathercock response (the directional stability of the last module) pushes back against the rudder. The aircraft settles at the sideslip angle where the two cancel: rudder moment one way, sideslip-induced fin moment the other, net moment zero. That is why holding rudder gives a steady sideslip, not an ever-accelerating yaw: the sideslip absorbs the rudder's moment.

Rudder limiting

The rudder is sized for its hardest low-speed jobs, holding an engine failure at take-off speed and de-crabbing (swinging the nose back to the runway heading) in a strong crosswind, which makes it far more powerful than high-speed flight can tolerate. Aerodynamic forces grow with the square of speed: full deflection at cruise IAS would load the fin, and the sideslip it produces would load it further, toward and beyond structural limits: fin and rear fuselage are simply not built for full rudder at high speed, and certification does not require them to be. Commercial air transport (CAT) aeroplanes therefore carry a rudder limiter: as IAS rises, the system progressively reduces the rudder authority available:

variable stop — which shrinks the maximum deflection the surface can reach.
ratio changer — which reduces how much surface deflection a full pedal input commands.

Low and slow, the pilot has everything; fast, only the small deflections that high dynamic pressure makes sufficient, and safe.

Ailerons and the roll rate

The ailerons apply the camber mechanism in mirrored pairs: one goes down (more camber, more lift, so that wing rises), the other up (less lift, or downforce, so that wing drops). The lift asymmetry is a rolling moment about the longitudinal axis, and because the ailerons sit far outboard, a modest force difference works on a long arm. The aircraft does not simply accelerate in roll forever: as it rolls, the down-going wing meets the air at higher α and the up-going wing at lower: the roll damping that the concepts lesson called aerodynamic resistance to the motion itself. The aircraft settles where aileron moment and damping moment balance: a steady roll rate: the rate of rotation about the longitudinal axis, in degrees per second, the roll-control measure the exam asks by name.

What sets that rate:

aileron size and deflection (more of either, more moment).
IAS (more dynamic pressure, more force per degree of deflection, though damping grows too, and at very high speed the wing's flexibility starts to erode the gain).
wingspan (a long span gives the damping a long arm; large-span aircraft roll slowly).
altitude (at the same IAS the higher TAS means a given roll rate produces a smaller α change at the wingtips; the damping falls, and the steady roll rate for the same aileron input rises).
wing stiffness: the aeroelastic limit the inboard–outboard aileron section reaches.

Roll rate is the standard measure of lateral handling: fighters prize a high one, transports merely need enough.

Adverse yaw

Aileron deflection has a built-in flaw. The down-going aileron raises its wing's lift, and the induced-drag lesson taught what comes with lift: the rising wing also carries more induced drag, while the descending wing, lift reduced, carries less. Roll left, and the drag asymmetry pulls the nose right: toward the rising, outside wing, away from the intended turn. This is adverse yaw: every aileron input arrives with a yaw in the wrong direction, and the sideslip it creates works against the turn just begun. It is the first of the ailerons' adverse effects; the second family (what high speed does to an aileron on a flexible wing) has its own section below.

Curing adverse yaw

Four standard fixes, each attacking the drag imbalance or answering it with rudder:

Frise ailerons — the hinge is set back from the aileron's nose, so the up-going aileron's nose swings down into the airflow beneath the wing, adding profile drag on the descending wing to balance the rising wing's extra induced drag. The down-going aileron's nose stays faired.
Differential deflection — the up-going aileron travels through a larger angle than the down-going one: the descending wing gets extra drag from its large up-deflection while the rising wing's lift (and induced-drag) increase is kept small. Most light aircraft combine this with Frise geometry.
Rudder–aileron cross-coupling — the control system feeds a measure of rudder automatically with every aileron input, opposing the adverse yaw directly rather than reshaping the drag; fly-by-wire aircraft do this as a matter of course, mechanical interconnects did it first.
Roll spoilers — change the sign of the problem entirely: their drag acts on the descending wing, so the yaw that accompanies the roll points into the turn (the next section's subject).

Roll spoilers

The spoilers that the drag-devices lesson introduced come back as roll controls. For roll, the spoiler rises on one wing only, the wing that is to go down: it destroys part of that wing's lift (a rolling moment toward it) and adds drag on the same side. That drag is the reversal of the ailerons' flaw: it acts on the descending, inside wing, yawing the aircraft into the turn rather than out of it. Transports use roll spoilers in combination with the ailerons (the flight controls mix them in with aileron input, taking over more of the work as speed rises) and sometimes instead of ailerons in regimes where the ailerons are locked out or deliberately kept small. Their other advantage at high speed: a spoiler panel, hinged on the stiff forward wing box, does not twist the wing the way an outboard aileron does.

Inboard and outboard ailerons

Large jets often carry two aileron pairs. The outboard ailerons sit near the tips: a long arm, powerful at the low speeds of take-off and landing, but mounted on the most flexible part of a swept wing. At high IAS an outboard aileron's force twists the outer wing about its structure: the twist changes the wing's own α against the aileron's intent, eroding the response, and in the limit reversing it. The inboard (high-speed) ailerons sit on the stiff inner wing, arm shorter but structure unyielding: at cruise dynamic pressure their smaller moment is plenty. So the outboard ailerons are locked out beyond a given speed (typically with flap retraction), for three reasons the syllabus wants by name:

over-control — at high IAS, full outboard authority is far more than any manoeuvre needs.
exceeding structural limitations — the loads and twisting moments of a powerful surface at high dynamic pressure.
aeroelastic phenomena — flutter, divergence and aileron reversal: the interactions of air loads with a flexing structure that the limitations module returns to.

The ailerons at work

The syllabus asks for the ailerons' everyday employments, five situations and one habit of thought:

Normal flight — rolling into and out of turns, and answering every gust that lifts a wing: continuous small corrections about wings-level.
Flight with sideslip — a deliberate steady slip is a cross-control exercise: rudder holds the nose off-line, and the aileron must hold against the dihedral effect, which is trying to roll the aircraft away from the sideslip the whole time.
Crosswind landing — the wing-low method is that same slip flown to the runway: aileron into wind holds the into-wind wing down against the dihedral effect while opposite rudder keeps the fuselage aligned; the aileron stays in through the flare and touchdown.
Horizontal turns — aileron rolls the aircraft in, then comes nearly neutral, often slightly against the turn: in the turn the outer wing flies faster and lifts more, an over-banking tendency the pilot holds off with a touch of outside aileron.
One engine inoperative — the asymmetric-thrust yaw is held mainly with rudder, and the small bank toward the live engine that the technique requires is set and held with aileron, balancing the rolling moments of the deflected rudder and the asymmetric slipstream; the flight-mechanics module flies this case in full.

Flaperons and aileron droop

Two hybrids close the roll-control inventory:

Aileron droop — when the flaps extend, both ailerons deflect partly down together, curving the whole trailing edge like additional flap span, while still moving differentially about that drooped position for roll.
Flaperons — the merger taken to its end: one set of surfaces does both jobs at once, the control system summing a symmetric flap command and a differential roll command into each surface's position.

Same aerofoil physics, two orders superimposed.

The secondary effects: roll makes yaw, yaw makes roll

Finally, the syllabus asks for the two couplings as the pilot meets them (module 4's loop, seen from the controls):

secondary effect of roll — bank the aircraft and do nothing else, and the tilted lift lets it slip toward the low wing; the fin weathercocks into that slip, and the nose yaws toward the low wing: roll, left alone, produces yaw in the same direction, and the flight path curves down and around into the turn.
secondary effect of yaw — press the rudder and hold it, and two mechanisms roll the aircraft in the direction of the yaw: the outer wing sweeps forward faster and lifts more, and the sideslip the yaw creates drives the dihedral effect's roll away from the slip, which is toward the yaw. Yaw, left alone, produces roll in its own direction.

Each control's secondary effect is the other control's primary job, which is why coordinated flying uses them together, and why module 4's Dutch roll and spiral were both, at heart, these two effects chasing each other.

What the exam asks

Adverse yaw and its cures are among the most-asked control items in the bank. The overview, following the learning objectives:

Explain the rudder's working principle, the deflection–moment relationship, and the effect of sideslip on the moment

Explain why and how rudder deflection is limited on CAT aeroplanes

Explain the functioning of ailerons, define roll rate, and list the factors that affect it

Explain why aileron use induces adverse yaw, and the ailerons' adverse effects

Explain how Frise ailerons, differential deflection, rudder–aileron cross-coupling and roll spoilers reduce adverse yaw

Explain how spoilers control roll, with or instead of ailerons

Explain inboard/outboard ailerons, and why the outboard pair is locked at speed (over-control, structural limits, aeroelastic phenomena)

Describe aileron use in normal flight, sideslip, crosswind landing, turns, and engine-out flight

Describe flaperons and aileron droop

Explain the secondary effect of roll and the secondary effect of yaw