Control Surfaces and Pitch Control - ATPL 081 — Principles of Flight

The stability module asked what the aeroplane does on its own; this module asks how the pilot makes it do something else. The starting point is the same frame of reference: three axes, three motions, now with a control surface assigned to each. This first lesson lays out that frame, explains the one mechanism every control surface shares (it is always an aerofoil having its lift adjusted), and then works the pitch axis in full: elevator, all-flying tail, the downwash the tail lives in, and what the CG does to the pilot's authority.

The three axes, three motions, three controls

All three axes pass through the centre of gravity:

lateral axis — runs from wingtip to wingtip; motion about it is pitch: nose up, nose down.
longitudinal axis — runs from nose to tail; motion about it is roll: one wing down, the other up.
normal (vertical) axis — stands perpendicular to the other two; motion about it is yaw: the nose swinging left or right.

Each motion has its angle:

pitch angle is the angle between the longitudinal axis and the horizontal (an attitude, measured against the horizon, and not the angle of attack, which is measured against the airflow); in a climb or descent the two part company.
bank angle φ (the lateral lesson's definition) is the lateral axis against the horizontal.
yaw angle is how far the longitudinal axis has swung, in the horizontal plane, from a reference direction (in practice: the change of heading).

One primary control commands each motion:

elevator — hinged to the tailplane; commands pitch.
ailerons — hinged near the wingtips and moving in opposition (one up, one down relative to their neutral); command roll, helped on transport aircraft by roll spoilers, the next lesson's material.
rudder — hinged to the fin; commands yaw.

The pairings echo the stability module deliberately: each control surface lives on the very surface that stabilises its axis, because the recipe for both jobs is the same: force, far from the CG.

The one mechanism: camber and local angle of attack

Every control surface works the same way, because every one of them is part of an aerofoil. There are exactly two handles on an aerofoil's lift. A conventional, hinged surface (elevator, aileron, rudder) uses the first: deflecting it changes the camber of the aerofoil it belongs to. Deflect the surface down and the aerofoil's camber increases: more lift at the same angle of attack; the entire lift curve shifts, exactly as the flap lessons showed, because a deflected control surface IS a small plain flap. Deflect it up and the camber change reverses, driving the lift down, through zero, to lift in the opposite direction.

The second handle is the local angle of attack. An all-moving surface (a tailplane that pivots as one piece, a trimmable stabiliser) changes no camber at all: it rotates the whole aerofoil and so changes the α at which that surface meets its local airflow, walking along the lift curve instead of shifting it. And even a hinged surface changes the local α story a little: deflection alters the effective angle at which the combined surface works. Camber change or α change, the result the pilot gets is the same: a different force on a surface far from the CG, which is to say a moment.

Pitch control: the elevator and the all-flying tail

The elevator applies the camber mechanism to the tailplane. Elevator up cambers the tailplane downward: its lift becomes a larger download, the moment about the CG swings nose-up. Elevator down does the opposite: less download, or an upload, nose-down. This is the working half of the balance that the longitudinal lesson built: the stabiliser holds the standing balance, and the elevator adjusts the tail's force around it whenever the pilot wants a different pitch attitude or a manoeuvre.

The all-flying tail (or stabilator) applies the α mechanism instead: the entire tailplane pivots, elevator and stabiliser in one piece. Its advantage is power: rotating the whole surface changes its force far more than cambering its rear edge can, and the surface keeps its effectiveness in high-Mach flight, where the high-speed module showed shock waves forming on the tailplane. A shock standing ahead of a hinged elevator leaves the elevator working in the separated flow behind the shock, and its deflection can no longer influence the pressures ahead of it; an all-moving surface simply rotates, shock and all. That is why transport aircraft combine a trimmable (all-moving) stabiliser for the standing balance with a hinged elevator for manoeuvring, and many high-speed and light designs use a fully all-flying tail.

The downwash the tail flies in

The tailplane does not meet the free stream: it flies in the wing's downwash, the descending flow that the 3D-airflow lessons built. The local flow at the tail arrives angled downward, so the tailplane's effective angle of attack is less than the aircraft's angle of attack alone would suggest. And the downwash is not constant: it grows with the wing's lift. Raise the aircraft's α and the wing sends the flow down more steeply, so the tail sees only part of the α increase the aircraft made. That subtraction matters twice over: it weakens the tail's stabilising response (part of every real aircraft's longitudinal arithmetic), and it means anything that changes the downwash (extending flap is the classic case) changes the tail's α and produces a pitching moment the pilot must trim away.

The CG and the elevator's authority

How much elevator a manoeuvre costs depends on where the CG sits, and the stability module already explained why: the CG sets the stability the elevator must overpower. With a forward CG, the aircraft resists pitch changes strongly, so a given response (the same pitch rate, the same g) needs a larger elevator deflection (and a larger stick force). With an aft (rearward) CG, the same response comes from less deflection. The elevator, though, has a hard limit: it can only deflect so far and generate so much force. Push the CG forward and more and more of that finite authority is spent overcoming stability; at the forward CG limit, just enough remains to do the two most demanding nose-up jobs: rotating at take-off and flaring at landing, both at low speed, where every aerodynamic surface is at its weakest. That is the control-side reading of the forward limit that the longitudinal lesson set: beyond it, the elevator simply runs out.

Thrust in the pitch balance

One pitching moment does not come from an aerofoil at all. Engine thrust acts along the engine's own line, and unless that line passes through the CG it is a pitching moment, with the engine location deciding the direction, as the longitudinal lesson used in its balance:

Engines below the CG — underslung, the transport standard: more thrust pitches nose-up, less thrust nose-down. That pairing sits behind the stall module's go-around trap.
Engines above the CG — aft-fuselage-high mounts, flying boats: more thrust pitches nose-down instead.
Engines on a line through the CG — thrust changes barely disturb pitch at all.

The elevator must hold whatever the thrust line hands it, which is why every large thrust change comes with a pitch input, and why the two are practised together.

What the exam asks

Definitions and the camber mechanism open nearly every control question in the bank. The overview, following the learning objectives:

Define the lateral, longitudinal and normal axes; define pitch, bank and yaw angles; describe the motion about each axis

Name and describe the devices that control the three motions

State that a control surface changes camber, and explain the effect

Explain the influence of local α change by movement of a control surface

Explain the working principle of the elevator and the all-flying tail

Explain the effect of downwash on the tailplane's α

Explain elevator deflection against CG location for a given response, and the forward CG limit's effect on pitch control

Describe the effect of engine thrust on pitching moments for different engine locations