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Just curious how the A350 reduces drag/fuel burn by extending the flaps ever so slightly during cruise. Everything I've read about extending flaps says that extending them pushes the centre of pressure rearwards. This causes a pitching down moment, which means you need to increase the AoA, but this affects drag and fuel burn negatively (a pitch up command increases the down force from the tailplane which has the result of increasing the effective weight of the plane, which requires an even higher AoA to compensate, which also increases the induced drag).

Here's how it works as per the (A350 Flight Deck and Systems Briefing for Pilots)

Differential Flap Setting and Variable Camber

The Differential Flap Setting and Variable Camber enable to optimize the loads and drag on the wings.
Small flaps deflections (4° maximum) either symmetrically or asymmetrically, enable to automatically:

  • Optimize the wing camber to reduce wing loads and drag
  • Perform an optimized Lateral Trim function.
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  • $\begingroup$ I don't remember the details exactly, so I'll post it as a comment. However, I think the reasoning makes sense. I once heard a talk about this while I was an intern at Airbus about this topic, and I if remember correctly it was mentioned that instead of rotating the entire plane to reach the correct angle of attack of the plane during cruise, they would extend the flaps slightly. By doing this, it was possible to keep the fuselage more horizontal, and reduce the drag resulting from the fuselage. $\endgroup$
    – ROIMaison
    Commented May 25, 2020 at 9:17

4 Answers 4

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Henning Strüber, one of the Airbus engineers behind this system, has written a paper on it.

In cruise:

This can be applied in early cruise phases to shift the center of lift more inboard and by that reducing the wing root bending moment, which can be transferred into a structural weight saving.

A plane that can be built lighter will have lower drag [for the same payload].

For a heavy and/or a hot and high takeoff:

In high-lift configuration a more outboard loaded lift distribution can be achieved to reduce induced drag during take-off.

For how that works, see here. The answer there by @PeterKämpf confirms that outboard loading requires a heavy wing relative to the whole mass (scaling laws work for an insect, but not for an albatross or an airplane). So those two regimes may appear to be contradictory: if the cruise system allows a lighter wing, then how can that lighter wing achieve more outboard lift for a heavy takeoff.

Accounting for the gust loads in cruise is the key here, which are smaller for takeoffs. (Thanks to @PeterKämpf for this insight; see comment posted below.)

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    $\begingroup$ The weight savings don't come from static loads in flight, but the expected gust loads (plus the static load, of course). Those are smaller during take-off, so more bending moment from the static lift case can be tolerated. $\endgroup$ Commented Jul 1, 2019 at 18:53
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Just complementing the answer from @ares which is quite good and refers to the main effect. I would like to refer to another "secondary" effect that implies also drag reduction.

When designing an airplane the structure is designed taking into account several factors, one of them, is the maximum load that the airplane can be exposed to.

Airbus has designed a system that during flight optimises the loads over the wing. Let's say, for example, that, without the variable cambering system we have a determined maximum load (let's say A). Using the variable cambering system the airplane is capable of reducing the load A (maybe increasing drag) to B (being B < A).

So, when designing the structure, assuming that the variable cambering system will be used, the airplane will use B as design point and not A. As B < A the size and weight of the structure with the variable cambering system will be lighter.

A lighter structure will imply less lift force needed and so less drag produced to achieve such lift. So, from a design optimization point of view, the variable cambering system is, essentially, providing new design variables which will allow a better opitmization of the airplane, reducing the drag.

Essentially, @ares has properly described the "active" mechanism, and I have described a "passive" one.

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  • $\begingroup$ Thanks Trebia! Still not sure if I'm getting the basics. If you could have a look at the questions I asked Ares I'd appreciate it! $\endgroup$
    – Speedalive
    Commented Jul 1, 2019 at 1:12
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    $\begingroup$ Your 4th paragraph looks like it got cut off: "As B"... $\endgroup$
    – FreeMan
    Commented Jul 1, 2019 at 14:11
  • $\begingroup$ @FreeMan , thanks, I used a simbol actually used everything behind as a comment. $\endgroup$ Commented Jul 1, 2019 at 20:00
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To answer your specific question: The specific ability of the A350 is to extend the inboard flaps independently of the outboard flaps and to extend them by only a few degrees.

The A350 has a high wing sweep of 35° and a high wing span. The result is, that seen from the direction of flight, the outer parts of the wings lie far behind the inboard flaps. This means, that the center of lift of the wings is actually located around where the inboard flaps are. If the inboard flaps are extended by up to 2° (the maximum possible during cruise flight), the additional lift created by them will be located close to where the center of lift was already before!

I think, this can be seen rather easy from figure 2 of the design paper linked by Speedalive. The blue line and label "inboard flap" shows where those flaps are, that can be slightly extended. Keep in mind, that those flaps are actually not just at the edge of the wing, but start already a meter or so before. If the flap is rotated by up to 2°, it produces additional lift along all of its surface, not only the edge. So the center of additional lift created by the inboard flaps alone might even be slightly before the center of lift of the whole wing.

Furthermore, as already said by others, this slight extension of the flaps by 2° creates more uplift and thus allows to reduce the pitch angle of the whole aircraft by maybe 1°. This reduces the angle of attack on the wings by the same amount, and with the typical asymmetric wings used on aircraft today, lowering the angle of attack usually leads to a slight forward movement of the center of lift.

So, putting it all together: Additional uplift is created around the back of the inner part of the wings. In exchange, the angle of attack is reduced and less uplift is created on the outer part of the wings, which actually lie behind the (slightly) extended flaps, and also on the front parts of the inner wings. Both reductions have a combined center of lift reduction, which is close to the center of the additional uplift from the flap extension.

What others have already pointed out: At the beginning of the cruise flight, the fuselage is usually heavy, as the center tank is still filled with fuel. So more force needs to be transferred from the wings to the fuselage. If the additional force is created by increasing the angle of attack (or by flying at a lower altitude in denser air), it is created along the whole length of the wings and the whole structure needs to be able to carry that additional force over the full length of the wings. With the slight extension of the inboard flaps only, the additional uplift is created on the inner wings instead, close to the fuselage. So the maximum weight, that the outer wings have to carry for extended periods of time, is reduced. They don't need to be so strong, so less material can be used there, saving on aircraft weight.

Trim Tanks

I tend to disagree with others, that claim, that this "cambered wing" is a good replacement for the trim fuel system in A330 and A380. They are different systems to optimize fuel efficiency: The "cambered wing" is about reducing the angle of attack during heavy flight phases. It actually can also be used later, for example, when ATC asks the pilot to climb 1000 feet higher than what would be optimal for the current weight to avoid other traffic. Then again, flying higher, the aircraft would either had to increase the angle of attack and thus also the drag, or use the slight extension of the innermost flaps as a more efficient alternative!

Trim fuel, on the opposite, is about managing the center of gravity and let it get closer to the center of lift. When flying close to ground during initial climb and final approach, a large separation between these two points is essential for stable flight, especially during upsets like crosswinds and similar. During cruise flight, less separation is required and trim tanks can help to reduce it. During take off and landing, even more separation is required, and this is achieved by extending all flaps (not only the innermost ones) by a large angle (and not only by 2°), which substantially moves the center of uplift backwards.

On the other hand, the high wing sweep of the A350 allows also some trimming by exploiting the different fuel tanks of the wing only: Pumping fuel to the wings' outward tanks (or just consuming the fuel from the inner tanks, first) will move the center of gravity substantially to the back.

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Briefly speaking, there at two types of drag: profile drag and induced drag. Profile drag can be said to be composed of viscous drag and shock-wave drag (these two often interact). Induced drag is a result of the local effective velocities on the wing due to the -vortex- circulation. I don't know what is your background and thus I won't go more in depth. I will just mention that, if you want to simplify things, profile drag can be seen as the drag that a 2D airfoil would have, while induced drag is a measure of the efficiency of a wing geometry. In reality, these two types of drag may strongly interact when the aspect ratio of wings is small.

For example, it is well known that elliptical spanload distributions are optimal (when induced drag is the objective). This 'elliptical spanload' can be reproduced if you design a wing of i) constant chord with elliptical distribution of twist, ii) elliptical distribution of chord with constant twist, iii) a combination of twist and chord that gives an elliptical distribution.

What AIRBUS says, is that we can vary the chord in-flight to optimize spanload and root-bending moment. What they actually do, in simple terms, is to optimize for induced drag and possibly control, i.e. stability and flight controls by changing the chord distribution. Now, if you change camber, you will also optimize for profile drag (because camber is a property of the 2D airfoil) and twist (in the sense of aerodynamic twist). For commercial jets, flying in transonic conditions, this means that the camber is modified to reduce shock-wave drag loss.

Clarification: Aerodynamic twist refers to changes in camber along the span. It is often more efficient to change the airfoil shape along the span instead of using the same airfoil at a different angle.

If you want more details, please let me know.

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  • $\begingroup$ Thanks Ares. I'm still not sure if I completely understand. How is the A350 is able to reduce induced/parasite drag by extending flaps/increasing camber? My understanding is that flaps increase downwash which decreases the effective alpha of the wing, giving an additional, downstream-facing, component to the aerodynamic force acting over the entire wing which tilts the lift vector back and causes induced drag. Same with the slight increase in wing chord/area + decrease in aspect ratio.. As far as I've read in pilot resources, extended flaps don't add to fuel efficiency at all. $\endgroup$
    – Speedalive
    Commented Jul 1, 2019 at 1:03
  • $\begingroup$ In the A330 they have a stab trim tank which keeps the CG within 2% of the aft limit and I can see how this saves fuel.. It is well known that an aft CG saves fuel due to the reduced tail down force (effectively making the plane lighter) which reduces the required alpha for a given weight. This reduces induced drag, increases TAS, and decreases fuel burn. With the A350, I understand that they burn the center tank fuel first for a similar effect, but the slight extension of flaps moves the CP rearward and increases downwash which, from what I understand, has an opposite and negative effect. $\endgroup$
    – Speedalive
    Commented Jul 1, 2019 at 1:08
  • $\begingroup$ Obviously, this is not the case in reality, because if it had a negative effect, they wouldn't have designed such a system. So I'm curious what concept I am misunderstanding or missing. Currently studying for my ATPL exams and I keep getting hung up on this question when I'm reviewing principles of flight. I won't be asked this on an exam or anything, but it bothers me that what I'm taught in ground school can't explain how this system improves the 350's aerodynamics. $\endgroup$
    – Speedalive
    Commented Jul 1, 2019 at 1:11
  • $\begingroup$ @Speedalive Too many comments/questions! I'll try to answer them... So, "How is the A350 is able to reduce induced/parasite drag by extending flaps/increasing camber?" I think I have described the whole process in my answer. If you are not convinced I have to bring the equations in, but I guess this is too much. The thing is that this induced velocity field depends on what's happening on the whole wing. Thus, playing with the camber and chord distribution you can 'tailor' your spanload to each specific flight condition. $\endgroup$
    – ares
    Commented Jul 1, 2019 at 19:10
  • $\begingroup$ @Speedalive If you wanna understand this better, a good way to start is Prandtl's lifting-line theory, assuming you have a basic background in aerodynamics. $\endgroup$
    – ares
    Commented Jul 1, 2019 at 19:13

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