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Starting the 1980s, extremely powerful computational fluid dynamics software has been used to design wing sections and whole wings. I am going to focus on the Boeing 777 as an example of CFD applied to commercial airliner design since this is the aircraft that Boeing itself claims had an enormous use of CFD in design of the wing and wing-airframe integration (source). To quote, "The 777, being a new design, allowed designers substantial freedom to exploit the advances in CFD and aerodynamics. High-speed cruise wing design and propulsion/airframe integration consumed the bulk of the CFD applications". In particular, "inverse design" was first used (at Boeing at least) for the design of the 777.

Conceptually, CFD is a tool in the process of wing design, a tool that must improve attributes of the wing that ultimately then result in improvements in parameters which customers (airliners) care about. For example, one imagine the following causal chain: better CFD predictions of wing pressures --> some changed wing characteristics --> lower drag for a given lift --> lower fuel burn (which airlines care about).

My question centers around the "changed wing characteristics" part of the casual chain above. That is, I'm unsure as to what specific wing characteristics changed as a result of the application of CFD in the process of wing design. This could be changes in wing planform, thickness, t/c tapering, twist, or something else entirely. Basically, there has to be some output of the application of CFD that shows up in the actual wing itself to make the wing more efficient, and I'm curious what that output was.

Note: edited for clarity.

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  • $\begingroup$ "What I am unclear about is how the use of advanced CFD software actually changed the wing characteristics", "I'm more interested in what actual properties of the wing changed through the use of CFD." What is your actual question? These two would require very different answers. $\endgroup$ Feb 17 at 17:52
  • $\begingroup$ Sorry, but I don't understand how those two sentences are different :/. To be clear, I am interested in what physical characteristics of wings designed with intensive CFD applications changed relative to wings designed without the use of CFD that were responsible for improved L/D and thus reduced fuel burn of the former relative to the latter (eg, wing gemoetry, wing twist, t/c ratio tapering, planform changes, etc.) Does that clarify? Can also edit main question! $\endgroup$ Feb 17 at 17:58

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I'm unsure as to what specific wing characteristics changed as a result of the application of CFD in the process of wing design.

Look at the change in airfoils over span of a swept wing. A simple swept wing of constant cross section would show a marked drop in lift around its center ("Mitteneffekt") and needs to be modified in order to straighten the isobars and use all of it to its fullest potential, something that is near impossible without CFD.

Another example is the optimization of the high lift configuration of a wing. This article explains how the flap system of modern airliners has been simplified over the last decades by use of CFD tools which had previously been unavailable.

Sequence of the CFD-based high-lift design process

Sequence of the CFD-based high-lift design process, Figure 9 from AERODYNAMIC DESIGN OF AIRBUS HIGH-LIFT WINGS IN A MULTIDISCIPLINARY ENVIRONMENT.

In case of the Boeing 777 the huge GE90 engines had to be fitted to a wing similar to what had been done on the 737 and 767 before. Interactions between nacelle and wing had to be considered from low speed, high lift flight to high Mach cruise, so compromises had to be found. Doing this with CFD made that task immensely easier.

Factors influencing nacelle installation design

Factors influencing nacelle installation design, copied shamelessly from the cited paper by Dennis Berry.

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For the last twenty five years, at least, every significant development in aerodynamics has involved computational fluid dynamics.

Nobody has designed an aircraft, a wing, or even a car in the last twenty-five years without CFD being involved. (There may be some exception for some home-built ultralight somewhere in the world.)

The most specific advantage of CFD has been to reduce the cycle time of trying new things. So we can see what happens when we make a wing thicker - or thinner, or wider, or longer, or whatever. If we find that thinner wings work better in our case then we build thinner wings. Sometimes we check it by building an actual model, but in many cases we don't. In that sense we have learned what wings or fuselages or ailerons or wing mirrors (yes, car mirrors get tested by CFD) work better by using CFD.

So if you ask what we have learned from CFD, that is the same as asking what we have learned about aerodynamics. Other tools will have been involved, but CFD will have played a part.

(Incidentally learning "how to make wings thicker" is much more about structural mechanics than it is about fluid dynamics, computational or otherwise).

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  • $\begingroup$ Thanks! Definitely recognize that CFD is now essential and that it has allowed wing designers to reduce their cycle times and try new things. I'm curious about whether there is any identifiable trend in the things that were tried that ended up working and thus made wings more efficient over time. Maybe there isn't, and so there is no way to uniformly characterize the physical output of the application of CFD. But I'm curious if there is a trend. Does that clarify the question? $\endgroup$ Feb 17 at 18:09
  • $\begingroup$ One of the huge advantages of CFD is that you can try many more things. So lots of the things that were tried were tried because CFD let us do it (rather than having to build a physical model and run it in a wind tunnel). So there isn't a "CFD let us try X" trend, there is "CFD let us try X, Y, Z, A B... and X ended up working." $\endgroup$ Feb 17 at 18:46
  • $\begingroup$ I suppose I meant to ask, what are examples of X - the things that ended up working - in your analogy above. Sorry for the confusion! $\endgroup$ Feb 17 at 20:26
  • $\begingroup$ Just wanted to follow up here @DJClayworth :)! $\endgroup$ May 9 at 18:56
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The by far most influential effect of CFD on aviation design lies in its ability to process humongous amounts of data allowing the visualization of what how air performs around certain solid shapes.

One typical CFD 'invention' (if you can call it that) is insight in how seemingly insignificantly small objects can cause surprisingly large scale effects, both advantageous as well as disadvantageous. As a result, if CFD really 'changed' anything in wing design, it's the notion for detail.

Another typical CFD fingerprint, is the design of what I would call 'whole shape aircraft' for lack of a better term. The functions of airlift, directional control, load capacity, stability and versatility in flight have become more and more one single result, rather than tasks divided over separate components. Like in the space shuttle. If you want to make a brick fly, CFD will show you how.

Hold your hand out of a driving car window and you can feel something is happening, but only CFD can show you exactly what that is. It can also show you what is happening to the air behind your hand, or behind the car even. CFD can tell you how a dead fish can still swim upstream. It gives insight into things that would otherwise just be mysterious.

Last but not least, CFD can be used in actual flying. I haven't seen it on manned aircraft yet, but I recall seeing an indoor drone demonstration, in which CFD was used to make a drone instantaneously adapt flight controls to damage or malfunction. This would be very helpful on for instance twin engine jet fighters that suffer loss of one engine. In that case CFD can be used to determine the damage and what control adjustments are needed to alter the aircraft's performance to still answer to the input of the pilot or even execute an autonomous safe RTB.

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  • $\begingroup$ @rclocher3 Since this is the only comment: Certain fish when released in a water current shortly after dying, proved to swim upstream strictly due to the elasticity and shape of their bodies. A movement not unlike that of a sailing vessel, but in this case entirely natural. CFD shows how that is possible. $\endgroup$
    – user55607
    Feb 24 at 5:52
  • $\begingroup$ For anyone else who is curious about the "dead fish swimming upstream" remark, I looked it up and found this article. Unfortunately the animated GIF suffered link rot. The article referred to Beal, D. N. et al., 2006, 'Passive propulsion in vortex wakes', Journal of Fluid Mechanics, vol. 549, pp. 385-402, doi.org/10.1017/S0022112005007925 . $\endgroup$
    – rclocher3
    Feb 24 at 16:14
  • $\begingroup$ Thanks @Berend, I think your answer illustrates why it's so tough for a non-insider to appreciate how CFD has improved the wing of aircraft. As you note, the resultant changes to the wing aren't very noticeable, like a huge shift in airfoil shape or planform configuration, but a plethora of minor little details. I'd love to know what those details are, but hard to find out. $\endgroup$ Feb 24 at 20:39
  • $\begingroup$ @interested22 The result is in the details, but the difference is in the general approach. CFD doesn't 'understand' anything. It doesn't generate expectations. It merely does the job of calculating all known parameters in as much detail as possible, separate for every pixel. I worked on trying to prove how bees fly supersonic with the help of CFD. $\endgroup$
    – user55607
    Feb 24 at 21:04
  • $\begingroup$ @interested22 Some examples of CFD based enhancement to wings are things like vortex generators and certain details in the trailing edge of wings and surface profile. The invention of winglets is pre CFD, but their optimization isn't. Belly fins are CFD products too , but that is hardly wing design. $\endgroup$
    – user55607
    Feb 24 at 21:34

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