Aircraft Control Surfaces

The lever mechanism in 3D

Every control surface is the same idea: a hinged flap. Deflect it, change the effective camber of the host airfoil, and that section produces more or less lift depending on the deflection direction. The change in lift, multiplied by its moment arm to the centre of gravity, becomes a moment that rotates the aircraft.

For small deflection angles δ, the change in section lift coefficient is roughly linear:

ΔC_L = (dC_L/dδ) · δ

For typical plain trailing-edge flaps, dC_L/dδ ≈ 0.04 per degree. A 20° deflection adds ΔC_L ≈ 0.8. Multiplied by dynamic pressure ½·ρ·V², wing area S, and the surface's lever arm to the c.g., that becomes the rolling, pitching, or yawing moment the pilot needs.

The three primary axes:

ROLL  (longitudinal axis)  ←  ailerons (one up, one down)
PITCH (lateral axis)       ←  elevator (up or down on horizontal tail)
YAW   (vertical axis)      ←  rudder (left or right on vertical tail)

Surface hierarchy compared

	Primary	Secondary	Tertiary
Function	Direct attitude control	Modify lift & drag	Reduce pilot effort
Examples	Aileron, elevator, rudder	Flaps, slats, spoilers, speedbrakes	Trim tabs, anti-balance tabs, servo tabs
Pilot input	Stick & pedals continuously	Lever or switch (discrete settings)	Wheel or trim button (set-and-leave)
Operating speed	All speeds	Low speed only	All speeds
Failure consequence	Loss of axis control	Higher stall speed	Sustained pilot effort, fatigue
Typical deflection	±15° to ±30°	0° to 40° (flaps)	±5° to ±10°
Actuator demand	High bandwidth, redundant	Powerful, slow	Low force, low rate

Primary surfaces

• Ailerons. Outboard trailing-edge panels moving differentially — one up, one down. 737 ailerons span ~14% of wingspan; aerobatic aircraft 30%+ for snappier roll rate.
• Elevators. Trailing-edge panels on the horizontal stabiliser; deflecting up reduces tail lift, raising the nose. A stabilator (all-moving tailplane) gives higher pitch authority — used on most fighters.
• Rudder. Vertical stabiliser trailing edge, deflected by pedals. Critical for crosswind landings, engine-out compensation, and coordinated turn entries.

Secondary surfaces

• Flaps. Inboard trailing-edge panels that deflect down (and in Fowler types slide aft) to add camber and area. Plain, split, slotted, double-slotted, triple-slotted progressively increase C_L,max.
• Slats. Leading-edge devices that extend forward and down, generating a slot of energised air over the upper wing. Delays separation to higher AoA.
• Spoilers. Upper-wing panels that hinge up. Asymmetric use augments roll at high speed; symmetric use is a speedbrake; full extension on landing dumps lift to plant the wheels.
• Speedbrakes. Dedicated drag panels — fuselage plate (F-15), symmetric spoilers (737), or split rudder clamshell (B-2).

Tertiary surfaces

• Trim tabs. Small panels on a primary surface's trailing edge, deflected to hold steady deflection without pilot effort.
• Anti-balance tabs. Move with the primary surface, giving stick-force feedback on stabilators and large surfaces.
• Balance tabs. Move opposite, reducing hinge moment and pilot effort.
• Servo tabs. Pilot moves the tab directly; aerodynamic force on the deflected tab drives the main surface. 707 elevators used this as a hydraulic-failure backup.

Combined-function surfaces

	Aileron	Elevon	Ruddervator	Canard	Flaperon	Stabilator
Functions combined	Roll only	Roll + pitch	Pitch + yaw	Pitch (forward-mounted)	Flap + aileron	Pitch (whole surface)
Used on	Conventional aircraft	Tailless deltas	V-tails	Canard configurations	Single-spar light aircraft	F-15, F-16, MiG-21, large jets
Examples	737, A350, Cessna	B-2, Concorde, Vulcan	F-117, V-tail Bonanza	Eurofighter Typhoon, Rafale	RV-7, Rutan EZ	F-4, F-15, A380 inboard
Mixing complexity	None — single function	Linear summation of stick + roll input	Pedals + stick to both surfaces	Replaces tail entirely	Single span moves as both	One axis, one surface
Trim drag	Standard	Higher	Higher (V-tail does both jobs)	Lower (lifting tail in front)	Standard	Lower than separate elevator/stab
Stall behavior	Wing root first (designed)	Whole-wing stall	Adverse coupling possible	Canard stalls first (intended)	Whole-span stall	Pitch authority preserved

Aileron variants

• Plain aileron. Simple hinged panel. Used on light aircraft.
• Differential aileron. Up-going aileron deflects more than down-going to balance induced drag and reduce adverse yaw. Standard on most general aviation aircraft.
• Frise aileron. Hinged so that the up-going aileron's leading edge protrudes below the wing into the airstream, adding drag on the down-going wing to counter adverse yaw. Common on aerobatic and short-field aircraft.
• Inboard aileron. Mounted near the wing root rather than the tip — used on swept-wing transports at high speed because outboard ailerons can cause aileron reversal through wing twist.
• All-moving aileron / rolleron. Found on missiles and a few fighters; the entire wingtip section rotates rather than a hinged trailing edge.

Failure modes

• Adverse yaw. Asymmetric induced drag swings the nose against the roll. Fixed by Frise ailerons, differential rigging, or rudder coordination.
• Aileron reversal. At high dynamic pressure, aileron force twists the wing the opposite way; roll reverses. Early B-47 was grounded until inboard ailerons solved it.
• Rudder lock. Fully deflected rudder past its stall angle stays pinned by airflow even when pedal force is relieved. Boeing 707 prototype suffered this; fixed by larger dorsal fins and rudder limiters.
• Flutter. Coupled bending-torsion oscillation above the flutter boundary. Mass-balanced surfaces and stiffer structure prevent it.
• Jam / disconnect. Failed cable, locked actuator, or jammed pushrod leaves a surface fixed at the wrong deflection. Redundant actuation paths mitigate.
• Mass-balance loss. Surfaces are weighted at the leading edge; ice or paint accumulation shifts the balance and grows flutter risk.
• Powered control failure. Triple-redundant hydraulics, manual reversion, and hydraulic-mechanical backup ensure no single failure removes an axis.

Real-world specs

• Boeing 737. Outboard ailerons span 14% of the wingspan; triple-slotted Fowler flaps inboard; roll spoilers augment ailerons above M 0.6.
• Airbus A380. Each wing: two ailerons, three flap sections, eight spoilers, four slats. Horizontal stabiliser is a 90 m² trimmable surface pivoting ±10° for cruise pitch trim.
• F-16 Fighting Falcon. Flaperons (combined flap-aileron), all-moving stabilator, rudder. Fly-by-wire mixes commands.
• B-2 Spirit. No vertical tail; yaw from split-rudder drag panels at the wingtips. Pitch and roll from elevons.
• Eurofighter Typhoon. Foreplane canards for pitch, trailing-edge flaperons for roll, rudder for yaw. Canards stall before the main wing.
• V-tail Beechcraft Bonanza. Two angled surfaces with ruddervators for pitch and yaw. Reduced wetted area, complex mixing.