Keeping 747 wings from shaking themselves apart

Every 747 wing is built to a speed it must never exceed, and the reason is flutter. Flutter is a self-sustaining oscillation: at a critical speed, the aerodynamic forces on a flexible wing begin feeding energy into the wing's own bending and torsion faster than the structure can dissipate it, and the amplitude grows until something breaks. Getting that speed above the aircraft's dive speed, with margin, is one of the hardest problems in aeroplane design.

There are two ways to fight it, and the 747's fifty-year history runs from the first to the second.

The passive answer: mass, stiffness and balance

The traditional approach adds no electronics at all. FAA guidance on flight stability and flutter describes the three levers for a wing: torsional flexibility, aileron balance, and aileron free play, each with a limit expressed as a function of limit dive speed.

The subtle part is the aileron. A control surface that is not exactly balanced will try to rotate about its hinge in response to its own inertia, and that rotation changes the aerodynamics in a way that can drive the very mode that caused it. Balance is characterised by the ratio of the aileron's product of inertia about the wing's fundamental bending node line and its hinge line, to its own mass moment of inertia about the hinge - a parameter with a hard limit that tightens as dive speed rises.

Where the flutter mode is a simple rotation, a single concentrated ballast weight is the most efficient fix. But real flutter modes are rarely simple, and a distributed ballast that works against every mode is safer, mirroring the spanwise weight distribution of the surface. The guidance is blunt about what happens next: in most cases the flutter speeds of control surfaces are governed largely by the mass balance weights and their distribution, and improving one mode frequently degrades another.

There is also a maintenance warning buried in that document. Balance weights are attached to surfaces that flex in vibration, so they are fatigue-limited, and water, ice or dirt collecting inside a control surface can silently change its balance. Sealing and proper drain holes are a flutter requirement, not housekeeping.

The 747 got there actively, and first

The early 747s did not have to invent a lighter wing, so the passive route was the practical one - but the type was also the first production airliner to carry an active control feature of this kind. NASA research on active flutter suppression notes that a ride quality improvement system "was developed and certified for the Boeing 747 subsonic transport airplane" as the clearest early example of an active control system entering production.

A later NASA study of a 747 derivative worked through the same idea in detail, using the outboard ailerons as the control surface. Two functions were separated:

• Maneuver load control reduced the loads the wing has to survive in a turn. Deflecting the ailerons shifts the lift distribution inboard, which reduces the wing bending moment. That produces a nose-up pitching moment, and because the horizontal tail has to balance it, reducing the tail load cuts the wing design loads further. A smaller wing box follows.
• Gust load alleviation attacked the other half of the problem, using wing acceleration feedback to the same ailerons, plus elastic mode suppression of the first wing bending mode.

The control law filtered a single signal to the outboard ailerons and split it between the two functions, with the gains reduced once the flaps were extended. A wing accelerometer mounted in the outboard aileron path provided the feedback. One finding is notable for what it did not require: resizing the wing box for the reduced maneuver loads needed no added flutter material at all.

The 747-8 crosses the line

On the 747-8 the logic ran in reverse. The new 747-8 wing is a longer, more efficient structure, and the Maneuver Load Alleviation and Structural Technology system uses fly-by-wire outboard ailerons and spoilers to unload it - worth roughly 1,400 lb of structural weight, which was the point of designing the wing with the system in mind from the start.

Doing that on a freighter exposed a problem that had never before been certified. The 747-8F exhibits an aeroelastic mode that is self-excited and does not completely damp out after a disturbance - a sustained oscillation, or limit cycle oscillation. It exists only for a limited set of fuselage payload and fuel combinations inside the otherwise normal flight envelope. The mode is primarily symmetric: a 2.3 Hz sustained oscillation of the wings, engine pylons and fuselage.

The problem was that with that oscillation present, compliance with the flutter and control system requirements could not be shown. Passive methods were not enough.

OAMS, and the first active flutter control approval

Boeing's answer was the Outboard Aileron Modal Suppression System - OAMS - added to the 747-8F's fly-by-wire roll control system to damp the oscillation. It was, in EASA's words, "the first time the use of an active flight control system to control flutter is approved on a commercial transport aeroplane."

Because the approach was that new, it needed its own certification basis, and EASA Special Condition C-18 is unusually specific about what had to be demonstrated:

• With OAMS inoperative, the sustained oscillation must be shown to be stable throughout the nominal aeroelastic stability envelope and the whole flight envelope - including disturbances above the sustained amplitude of the oscillation. A flutter mode that is suppressed but goes unstable when kicked harder is not acceptable.
• With OAMS operative, the aeroplane must remain safe, stable and controllable, and the loads from the oscillation must have a negligible effect on structure and systems, including wear, fatigue and damage tolerance. Compliance had to include icing, manufacturing variations, spare engine carriage, engine-removed ferry flights, and MMEL items.
• After any failure that leaves the oscillation present, it must be stable and decay to a limited amplitude once the disturbance is removed, must not interfere with the crew's ability to read instruments or fly the aeroplane, and must not be confused with ordinary stall or high-speed buffet.

One requirement in that document stands out for its effect on operations: no MMEL dispatch is allowed with the OAMS system inoperative. No interval, no deferred rectification, no dispatch. It is the most absolute handling consequence of any system on the 747, and it is a direct measure of how seriously the limit cycle oscillation was taken.

Related

• Flight controls - the ailerons doing the work
• The 747-8 new wing - the structure OAMS protects
• 747-8 systems differences
• Engines - pylons participate in the mode