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I have searched and even studied plans from Airbus itself, but I was not able to confirm the following and would like to ask if the information I have is correct.

TLDR version so you don't have to read all the text: Does A380 really have the center of gravity positioned BEHIND the aerodynamic force center?

(What I know so far): For any passenger aircraft, without (almost any) exception, the center of gravity is always closer to the nose of the aircraft than the aerodynamic forces resultant. This is to provide an auto-stabilising force for the aircraft in case it stalls for whatever reason. Since the airplane is "nose heavy", it would eventually establish itself "towards the air flow" and after some height drop, should at least partially regain normal flight mode. Also it prevents the aircraft from going into a flat loop. While this setup is safe, it also means that the tail wing needs to correct for the center of gravity and "pull down" (omitting the duck planes now), which means it does not help to create lift, instead reducing the total lift. This increases fuel consumption.

(What I need to confirm): During my UL theory training, one of the instructors told us that A380 (and all the new passenger planes driven mostly by computers alike) will have the center of gravity behind the aerodynamic center, which enables to use the huge tail wing to actually create lift instead of decreasing it and thus lowering the fuel consumption dramatically. While this is great from economical and ecological point of view, it would only work while the computers look after the plane. It would become so unstable that there is no way a human could fly it "by hand".

I understand that if the computers go down on a "fly-by-wire" airplane completely, the joystick/rudder becomes just a piece of plastic anyways, but a computer that constantly looks after the stability of the plane seems much more "error ready" than a simple wire between a cockpit and a servo.

Modern jet fighters have CoG behind the aerodynamic center and it is said that you only stall them once. There is no way to recover a stall like you would do with your good old Cessna. It strikes me that something like that would get approved for a civilian aircraft.

Thank you for your inputs. I was not able to find any resource concerning A380's CoG.

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    $\begingroup$ The idea that no human could control an inherently unstable aircraft is a myth. While some fighter aircraft are designed to be extremely unstable, making human reaction times too slow, I would not expect an aircraft with such a large mass moment of inertia to be unstable under human feedback. $\endgroup$
    – Sanchises
    Commented Jan 25, 2017 at 12:03
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    $\begingroup$ @user27978 What matters is not how heavy the plane is but how unstable it is. The argument in your comment is very similar to "A heavy object must fall extremely quickly because of the huge gravitational force acting on it." Well, no, because the huge force also has to accelerate a huge mass. Mass cancels out. $\endgroup$
    – David Richerby
    Commented Jan 25, 2017 at 15:35
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    $\begingroup$ Moments scale linearly with size, mass moments of inertia quadratic, so rotation is slower. Mass cancels out. Of course, mass is cubic to size, area quadratic, so one needs disproportionally large control surfaces (note the massive tail fin on an A380). So, a large aircraft will not depart as quickly from forward flight (large phase margin), but will require harder control actions to get it back in line (gain margin). $\endgroup$
    – Sanchises
    Commented Jan 25, 2017 at 15:53
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    $\begingroup$ Of course, none of the above actually answers the question, but unstable systems are not the worst thing that can happen in engineering, by far. Compare with the B52's anhedral to reduce roll stability - too much stability equals very slow maneuverability. $\endgroup$
    – Sanchises
    Commented Jan 25, 2017 at 16:02
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    $\begingroup$ No time right now for an exhaustive answer, but the fact alone that net force from both wing and stabiliser are upwards does not automatically make the aircraft unstable. $\endgroup$
    – Waked
    Commented Jun 7, 2017 at 15:50

2 Answers 2

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No. Positive lift on the tail does not necessarily mean that the center of gravity needs to be aft of the aerodynamic center of the aircraft. Only when the lift per area of the tail surface is higher than that of the wing will the center of gravity move into the unstable region.

the center of gravity is always closer to the nose of the aircraft than the aerodynamic forces resultant

Again, no. The center of gravity needs to be at the same lengthwise station as the center of pressure in trimmed flight, otherwise the aircraft would not be trimmed and start to pitch. With low-mounted engines there might be some thrust moment which must be canceled by a slightly forward location of the center of gravity, but this effect is small.

Next, the auto-stabilisation is not only helpful when stalling. It also provides a smoother ride in gusty conditions and makes it easier for the pilot to fly the aircraft.

Since the aspect ratio of the tail surfaces is smaller than that of the wings, lift there is bought more expensively, so it will be bad for performance if the tail produces more lift per area than the wing. Negative stability is only helpful for supersonic and highly manoeuverable airplanes. Airbus does not do that, and besides, the A380 would not be certified with negative stability.

What it does have, however, like all Airbus aircraft since the A310, is relaxed static stability. The center of gravity is still ahead of the neutral point, but less so than in conventional airliners. This does indeed help to lower drag, and the reduced (but still positive!) stability is enhanced by electronic means.

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  • $\begingroup$ If the CoG is still ahead of the neutral point is the horizontal stabilizer on an A380 indeed producing positive lift during level flight? $\endgroup$
    – TomMcW
    Commented Jun 17, 2017 at 21:24
  • $\begingroup$ Yes. Less per area than the wing, but still positive. Only when you move it further forward there is a point when lift on the tail is zero. Then the center of gravity is where the center of pressure of the wing-fuselage combination is. Now move it even more forward, and lift on the tail turns negative. $\endgroup$
    – Peter Kämpf
    Commented Jun 17, 2017 at 21:34
  • $\begingroup$ Wouldn't any lift of the horizontal stabilizer other than zero cause the aircraft to pitch about the CG? (regardless of the location of the CG) $\endgroup$
    – Michael Hall
    Commented Oct 5, 2018 at 0:18
  • $\begingroup$ @MichaelHall: There are pitching moments from the wing which need to be balanced by lift (positive or negative) at the tail. Only when all wing lift acts at the lengthwise station of the cg will zero lift at the tail be needed for steady flight. $\endgroup$
    – Peter Kämpf
    Commented Oct 10, 2018 at 4:23
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I asked the question to an A380 pilot today and he confirmed that the stabiliser has positive lift, and that therefore the CG is behind the centre of lift of the main wings. According to him the A380 Has fuel trim that transfers fuel from tail to main tanks. He used to fly A330 and stated that direct law in A380 is easier and more stable than in the A330.

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    $\begingroup$ If A380 is more stable in direct law—and in fact since it is flyable in direct law at all—it must be positively stable. That makes the answer a clear and resounding NO. $\endgroup$
    – Jan Hudec
    Commented Jun 17, 2017 at 8:38
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    $\begingroup$ Note that the tail might be providing positive lift, at least with CoG at the aft limit and high speed. But that does not yet make the plane unstable—the lift would have to fly at higher coefficient of lift than the main wing for that. It does not. That is only done on fighters. $\endgroup$
    – Jan Hudec
    Commented Jun 17, 2017 at 8:40

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