Spoilers and Speed Brakes - ATPL 081 — Principles of Flight

The last two lessons taught devices that add lift. This lesson teaches the opposite family, and it exists because a clean modern wing is, at times, simply too good. An aircraft that glides fourteen metres forward for every metre of height cannot descend quickly without gaining speed; a wing that keeps lifting after touchdown holds its weight off the wheels just when the brakes need it; and at high speed the ailerons alone can struggle to roll the aircraft crisply. The answer to all three is a device that destroys lift and adds drag on command: the spoiler, and its symmetric use as a speed brake.

What a spoiler does

A spoiler is a flat panel lying flush in the wing's upper surface, hinged at its front edge so it can rise into the flow. Raised, it attacks the wing exactly where the wing does most of its work: the upper surface, whose suction carries most of the lift. The flow cannot follow the raised plate: it separates at the panel's edge, and behind it the ordered, fast, low-pressure flow is replaced by a slow, churning wake. The suction over that stretch of wing collapses, so lift falls at once; and the plate standing in the stream is a textbook blunt body, so form drag rises at the same moment. Compare the philosophy with a flap's: a flap reshapes the wing to generate more lift; a spoiler deliberately ruins part of the wing so it generates less. The name is literal: it spoils the flow.

Roll spoilers, speed brakes, lift dumpers

The same panels do three different jobs, depending on which wings raise them, how far, and when:

Roll spoilers — raised on one wing only, to roll the aircraft: the spoiled wing loses lift and drops. Used with (or partly instead of) the ailerons, they offer two advantages the aileron cannot match: the drag they add acts on the down-going wing, so the aircraft is pulled into the turn instead of yawing away from it: favourable (proverse) yaw, where the aileron gives adverse yaw; and their effectiveness survives at high speed, where deflecting an aileron can twist a flexible wing enough to undo its own rolling moment: a problem the control module examines as aileron reversal.
Flight spoilers (speed brakes) — raised part-way on both wings symmetrically in flight: no rolling moment, just less lift and more drag for the whole aircraft. This is the speed-brake function this lesson's second half develops.
Ground spoilers (lift dumpers) — every panel to full deflection at touchdown: the wing's remaining lift is dumped, the aircraft's full weight settles onto the wheels, and the wheel brakes (which grip in proportion to the load on the tyres) can finally do their work. The added drag helps the deceleration; putting the weight on the wheels is the real purpose.

Spoilers on the two graphs

Raising spoilers redraws the two working graphs in the opposite direction to everything in the high-lift lessons. On the lift curve, the wing produces less $C_L$ at every angle of attack: the curve is displaced downward, and its maximum falls with it. A reduced $C_{L\,max}$ means, through the stalling-speed formula, that the stalling speed increases while the spoilers are up. On the polar, both changes pull the wrong way at once: less lift and more drag push the curve down and to the right, and the lift-to-drag ratio collapses: the origin tangent touches the spoiled polar along a much shallower line. That collapse is not a side effect: it is the point. A spoiler is the pilot choosing a worse $C_L/C_D$ on purpose.

Speed brakes in the flight phases

Used symmetrically, the panels become speed brakes: on most jet aircraft the wing spoiler panels themselves, on some types dedicated boards on the fuselage or tail. What they are for follows one phase at a time:

Descent — they solve the too-good-wing problem: an efficient aircraft asked to go down steeply can only trade height for speed, unless extra drag absorbs the energy: with speed brakes out the aircraft descends much more steeply at the same indicated airspeed. The extreme case is the emergency descent after a pressurisation failure, flown with full speed brake precisely so that height can be shed as fast as possible without running into the speed limits.
Approach — partial speed brake lets the crew lose speed and height together: energy management when air traffic control keeps the aircraft high or fast.
Landing roll — the same panels go to full deflection as the ground spoilers of the previous section.

Which drag do they add? Not induced: nothing about the raised plate makes more lift or stronger vortices. A speed brake is a bluff plate held against the stream: separated wake, unrecovered pressure: form drag, a member of the parasite family. In coefficient terms it raises the parasite drag coefficient of the whole configuration, and the drag force that coefficient produces grows with the dynamic pressure, with $v^2$ , which is why speed brakes bite hardest exactly where they are most wanted, at high speed. And on the polar, symmetric speed brake is the previous section's spoiler story applied to both wings: the curve moves down and to the right, and the lift-to-drag ratio is sharply reduced.

Speed brakes and the drag curve

Put that onto the total drag curve of the total-drag lesson. The parasite curve rises everywhere; the induced curve does not move. The total therefore rises, and the crossing point between the two families, which the total-drag lesson showed is the minimum drag speed, slides toward lower speed: the strengthened parasite curve now equals the unchanged induced curve earlier. With speed brakes out, $V_{md}$ decreases. One practical consequence comes with it: the speed-stable region (everything faster than the minimum drag speed) now reaches further down the speed scale, so a slow, draggy configuration is also a slightly better-behaved one in speed-keeping.

Rate and angle of descent

The descent numbers follow from the ratio this lesson keeps attacking. In a glide (or any idle-thrust descent) the angle of descent is set by the lift-to-drag ratio: the worse the ratio, the steeper the path. Speed brakes cut the ratio directly, so at a given airspeed the aircraft flies a steeper descent angle, and a steeper angle flown at the same speed along the path means the height is disappearing faster too: the rate of descent increases as well. That pair (steeper and faster down, without touching the airspeed) is the whole reason the handle exists.

What the exam asks

This lesson is 'describe the device, then move the graphs the other way'. The overview, following the learning objectives:

Describe the aerodynamic functioning of spoilers: roll spoilers, flight spoilers (speed brakes), ground spoilers (lift dumpers)

Describe their effect on the lift curve and the stalling speed: $C_L$ down at every α, $C_{L\,max}$ down, stalling speed up

Describe their influence on the polar and the lift-drag ratio: both sharply degraded

Describe speed brakes and the reasons for using them in the different phases of flight

State their influence on the polar and the lift-drag ratio

Explain how speed brakes increase parasite drag: a bluff plate's form drag, growing with speed squared

Describe how speed brakes affect the minimum drag speed: $V_{md}$ decreases

Describe their effect on the rate and angle of descent: both increase at a given airspeed