Flight controls and high-lift devices

Split inboard and outboard ailerons, spoilers, triple-slotted trailing edge flaps and Krueger leading edge devices — and the 747-8's fly-by-wire changes.

Boeing diagram of six 747-8 wing features — advanced airfoils, fly-by-wire spoilers and outboard ailerons, double-slotted inboard and single-slotted outboard flaps, the raked tip, flap tracks, and gapped Krueger flaps — Boeing's own breakdown of the wing's control and high-lift devices, drawn for the 747-8.Boeing

The 747 wing has to do two contradictory things: cruise efficiently at Mach 0.85, and get a 400-tonne aircraft airborne and back down at speeds a runway can accommodate. Almost every complication in its flight controls comes from that conflict.

Roll: two sets of ailerons

The 747 carries inboard and outboard ailerons on each wing, and they are not both used all the time.

At low speed both operate. At high speed the outboard ailerons are locked out and roll is handled by the inboard ailerons and spoilers alone.

The reason is structural. An aileron at the wingtip has a long lever arm — excellent for roll authority at low speed, but at high speed the force it generates can twist the wing in the opposite direction to the roll being commanded, a phenomenon called aileron reversal. Locking out the outboard surfaces once the wing is loaded avoids it.

Spoilers

Panels on the upper surface that spoil lift when raised. They serve three distinct jobs:

Roll assistance — rising on the descending wing to help the ailerons.
Speedbrakes — raised symmetrically in flight to increase drag and slow down or descend.
Ground spoilers — deployed fully on touchdown to dump lift, putting the aircraft’s weight onto the wheels so the brakes can work. Without them the wing keeps flying and the brakes have very little to grip with.

Trailing edge: triple-slotted flaps

The classic 747 wing uses triple-slotted Fowler flaps — a flap made of three separate segments that both extend rearward, increasing wing area, and deflect downward, increasing camber. The slots between segments let high-pressure air from beneath the wing energise the flow over the top, delaying the stall.

They are mechanically elaborate and heavy, and the housings for their mechanisms are the “canoes” visible under the wing.

The 747SP dispensed with them entirely: being far lighter, it managed with single-slotted flaps and no under-wing canoes at all.

The 747-8 went to double-slotted inboard and single-slotted outboard flaps, with redesigned flap tracks and fairings — simpler, lighter and quieter than the classic arrangement while achieving the same job on a better wing.

Leading edge: Krueger flaps

Along the leading edge sit Krueger flaps, which hinge down and forward from the underside to increase camber for takeoff and landing.

The 747 uses two types: rigid Krueger flaps inboard, and variable camber Kruegers outboard, which curve as they deploy.

There is a deliberate aerodynamic reason for treating the inboard section differently: it is arranged so the inboard wing stalls first. That keeps the outboard wing — where the ailerons are — flying, so the aircraft retains roll control as it approaches the stall instead of dropping a wing.

What the 747-8 changed

The 747-8’s new wing introduced fly-by-wire spoilers and outboard ailerons, replacing mechanical signalling with electrical.

Beyond saving weight, that made manoeuvre load alleviation possible — a system pioneered on the 787 which redistributes lift across the wing during unusual flight conditions, reducing the load carried by the outboard section. Because the wing never sees the peak outboard loads it otherwise would, Boeing built it lighter, saving about 1,400 lb.

The 747-8 also uses aileron droop: when the flaps extend, the ailerons deflect down and act as additional high-lift devices, improving low-speed performance and cutting noise.