see video here, cued to 3:52, showing a passenger’s view of an A380 wing during final approach and landing. Near the tip of the wing there are three - count 'em, three - ailerons, all moving independently of each other. In fact, a great deal of the time they seem to be acting in opposition to each other.
Why three? I’ve seen the question asked elsewhere, but not answered well.
What benefit is offered by three independent ailerons on each wing instead of just one, especially given a fly-by-wire control system?
WAG- it’s a freaking gigantic plane with freaking gigantic wings. I’d guess using only a couple large ailerons would produce a lot of stress on them and their mountings during flight. Having additional control surfaces reduces the stress and fatigue of any one aileron and allows for more flexibility and fine control of the aircraft. Operating different ailerons in different combinations can affect the degree and rate of roll, and having a computerized and FBW system means that the complexity of adding a third aileron is reduced compared to traditional cable systems.
Given the fly-by-wire control, I would expect multiple ailerons per wing, not one per wing as your “given a fly-by-wire contol system” comment implies you think is more proper.
The pilot isn’t expected to manually control each in a fly-by-wire setup. The computer interprets what the pilot wants to do and it moves each as needed to accomplish it best.
I don’t think the ailerons are actually opposing each other; to me, it looks as though the center aileron simply has a bit of time lag.
The A380 has two hydraulic systems to operate flight controls and two electric systems. My guess is that two of the ailerons are powered hydraulically and one electrically (or vice versa) and the two different (elec vs. hyd) systems have a little different time lag-especially when being moved at high rates.
Fly-by-wire certainly presents an opportunity for multiple ailerons, but what is the benefit? What do three ailerons accomplish that one cannot?
Fine control has been suggested. This is useful in high-speed flight, but given that this is fly-by-wire, the movements of the control surfaces can be made as small as desired by the software, regardless of pilot inputs.
Redundancy for the sake of fatigue/cycle reduction has been suggested, yet in the video all three ailerons are always moving; it’s not like they each take turns, reducing the total # of cycles on each one.
A definite disbenefit is that the complexity incurs additional manufacturing costs, maintenance/inspection costs, and aircraft weight (=increased fuel costs, reduced cargo/passenger weight capacity).
I believe this is the case with the Boeing 747. See video here, cued to 1:40, showing 747 ailerons in action during takeoff. Two ailerons, one inboard and one outboard. My understanding is that they work together during low-speed flight to provide the needed level of authority, but that the outboard aileron is not active during high-speed flight. On this aircraft, one never sees inboard and outboard ailerons working against each other, but this is not the case on the A380, as shown in the OP video: the three small outboard ailerons, no inboard ailerons, and those three ailerons all seem to be moving entirely independently of each other, often in opposition. It sure looks like the same roll couple could be achieved just by moving one control surface a small amount, rather than flapping all three of them to large/opposite excursions.
Opposing aileron deflection would act as speed brakes. You’d want the outboard one to deflect upward, and the inboard one downward to help avoid tip stalling.
The force exerted on each control surface is proportional to the air pressure and surface area. F=PA. By having a smaller area, each aileron experiences a smaller force than if it were bigger. This force needs to be opposed by the servos and motors/etc that move the control surface, and so if the force is large, you need more powerful - and likely larger - control mechanisms. The electrical power that drives these comes from the plane’s engines, so if they need to be higher powered, then the engines need to generate more power as well, which add weight and cost, etc. Having three sets of smaller systems makes a lot more sense than one or two sets of larger ones in this context.
Similarly, the forces exerted on the panels and fittings holding the aileron in place are also smaller, so stress, strain, fatigue etc are smaller and failure of the fittings is less likely to occur (these depend on # of load cycles, but also on the magnitude of the load). It’s easier to maintain a fitting than it is to replace/repair one. The loss of one aileron when you have only 2 is a bit more serious than losing one if you have 3.
Overall, you have smaller, longer-lasting (I think aileron fittings are intended to be life-limited on a plane, but I’m not sure) control surfaces with fittings exposed to lower loads and drive mechanisms that don’t need to generate as much force, requiring lower power from the engines, so more power can go towards thrust/other onboard systems. I think those trade-offs are a net design advantage. The finer control in roll is an added benefit - the three surfaces can fine-tune the forces/moments causing the roll much better than only 2 can do, giving more control and (presumably) a smoother ride.
One possible advantage of independent operation of multiple ailerons would be control of wing behavior.
Wings on large modern airliners are designed with quite a bit of flex, you can see the movement during flight - even old 747’s had a couple of meters of vertical movement at the wing tips. I would imagine there would be torsion as well.
Independent ailerons could effect control of such deflection, which may be helpful during a rough landing.
WAG: Won’t there be an element of *lateral *air displacement when an aileron is deployed? In that case, a computerised multi-aileron system will present a more optimised profile than a “dumb” one as it adapts to the changing dynamics.
If all three actuators are attached to one aileron, the aileron has to be sturdy enough not to break if one actuator gets a mind of its own and decides to oppose the others–which means added weight. With one aileron per actuator, no such problem exists.
The problem of how many flight control surfaces to have and how many systems should be powering each one is a tough one, and even the products of one company (say, Boeing, for example) may have different design solutions.
For instance, the 757 and 767, designed at roughly the same time, have different aileron layouts: The 757 has a single aileron on each wing, with two hydraulic systems powering each aileron; and the 767 has an inboard and an outboard aileron on each wing, with two hydraulic systems powering each.
I’m referring not to the flaps (the lift-enhancing devices deployed along most of the trailing edge of the wing throughout the video). I’m referring to the trio of ailerons (flight control surfaces) outboard of the last flap. They are labeled “ailerons” in this schematic drawing. In the video, their crazy dance begins in earnest around 4:00.
Load allevation doesn’t function at low speed/altitude. They help with wing flex, also at speed. They act as flap assists. As pointed out, the disadvantages far outweigh these minor benefits.
The flight control system is massively redundant, extremely complicated, and very expensive. So are A380 crashes. The reason there are six ailerons is filght safety, period.
Basically, 3 ailerons for safety/ease of manufacture, plus differential action for the multiple functions in addition to roll control which they provide.
From here:
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In the roll axis, the A380 has the same handling qualities and envelope protection as other FBW Airbus aircraft. Roll rate is commanded by lateral displacement of the sidestick. As well as moving the three ailerons and six of the eight spoilers on each wing to attain the desired rate of roll, the flight control system automatically commands rudder deflection to minimise sideslip. Unlike the 747 and 777, which have two ailerons per wing (one mid-wing and one near the tip) all three ailerons on the A380 are located outboard of the outer engine pylon. At speeds above 240kt indicated the outermost aileron is locked out, the remaining two more than sufficient to attain desired roll rates. Each aileron is commanded independently and, due to structural aeroelasticity, may be out of step with its wing mates – deflecting up, for example, while the wing itself is rolling up. All of this is transparent to the pilot, with the system providing control in the roll axis that is precise and predictable.**
It looks like the ailerons are tied into the spoilers for roll rate.
