What aerodynamic improvements were introduced on the Boeing 787?

Question

When discussing the Boeing 787, three topics are brought up most often:

1. The new, extremely high-bypass GEnx engines
2. The shift to carbon composite construction and
3. The significant troubles Boeing encountered in manufacturing the airplane. One area I've seen less discussion is in the aerodynamic properties of the aircraft.

With that in mind, I'm curious whether Boeing made specific aerodynamic improvements to reduce drag and increase efficiency of the 787 relative to previous generation aircraft, and if so, what specific improvements were made. I recognize that it might be difficult to find this information given how recently the plane was created, but I thought I'd ask the community just in case!

Answer 1

Here is a quote from a former Boeing Technical Fellow for Aerodynamics that I got after emailing them about this very topic. I thought the community might be interested in his answer.

From the mid-90's to the mid-2000's, significant advances were made in computer-based tools for closed-loop aero shape optimization, Multi-Disciplinary-Optimization (MDO) tools for doing complex multi-dimensional trade studies across scores of design variables, structural Finite Element Analysis (FEA) optimization for structural weight and stiffness. Much of this work was done as part of attempting to meet the requirements of the HSCT (High Speed Civil Transport) through its NASA-funded "HSR" element, and the follow-on industry internal R&D efforts toward designing other "concept planes" like the Sonic Cruiser, SSBJ concepts, and Blended-Wing-Body (BWB). The supersonic airliner concepts and SSBJ in particular, required being able to optimize not just the wing shape, but the wing, body, and nacelle shapes simultaneously, optimizing to find the best possible compromise between cruise aerodynamics, transonic acceleration, subsonic cruise, and low-noise takeoff and landing performance. The very thin, supersonic wings and tails, also provided significant weight reduction, aeroelastics, and flutter challenges that acted as catalysts for the improved structures tools. The MDO capability (still evolving) helped make the right kinds of trade-offs, i.e. "optimum compromises", in the design of the 787 and 777X. The improved CFD for air-loads prediction were coupled with FEA strength and stiffness optimization and these were used to guide direct optimization of aerodynamic shapes. The advent of practical direct CFD optimization of detailed aerodynamic shapes for 787 enabled the computer to find multiple incremental drag reductions that the "pressure matching" methods used on the original 777 could not. This optimization plus the development of very strong carbon fiber primary wing structures resulted in trade-offs that determined it was better to make the wing thinner while using less sweep to get a fairly high cruise Mach of 0.85--- the resulting thinner all-carbon wing was plenty strong enough in bending, but was more flexible than previous wings.

It's not a total answer, and I wouldn't expect anyone from Boeing to give one, but nevertheless, it definitely confirms that a shift in design tools away from inverse design and towards direct lift/drag optimization led to incremental L/D gains.

I inquired a bit more about what specific flow features the optimizer was finding, and he said:

Very generally, yes, the computer-driven optimizations would seek ways to improve L/D not just drag or drag-rise. So they don't really use pressure distributions per-se as the figure of merit, rather the actual L/D that results from various shape changes. This means that in addition to trying to further reduce shock strength, they try to incrementally raise the net difference between upper and lower surface pressures thereby creating more "L" for a given "D". They also distribute the lift span-wise in a patter than does not follow the "ideal" elliptic lift distribution but rather try to create a down-wash field that allows the horizontal tail and fuselage lift to "trim" the airplane while minimizing the far down-stream wake. In a 2D airfoil sense, carrying lift farther aft on the wing (both to increase L and reduce shock strength) can cause thicker boundary layers, increasing the parasite "form drag", but the thicker boundary layers also have less local skin friction--- so the optimizer can seek the best L/D trade-off of the various drag sources (wave/shock drag+skin friction+span load induced drag+ tail and wake induced drag + wing pressure form drag), and of course do this in the presence of the nacelle and strut. It can also camber the nacelle and strut shapes, change inlet lip shapes to reduce inlet drag, and put local "blisters" or waves in wing surface, or change the shape of root fairings or wing tips to get a better combination of local pressure and the local surface slopes those pressures act on in the lift and drag directions (very much a 3D effect !). You say you have trouble picturing what would be advantageous for a full 3D optimization, but that is exactly the point--- the computer can systematically do the grunt work to investigate the effect of thousands of candidate shape variations, identify which have the most performance leverage, and recommend specific shaping trends that do not violate multi-disciplinary constraints (spar thickness for example).

Anyways, hope the community appreciates. Not going to reveal the name of the Fellow, but can confirm privately if needed.

Answer 2

I can imagine Boeing designed specific airfoils for the 787 wing, optimized for the cruise at high mach number and for each wing section individually. Carbon fiber allows you to create a more complex shape than flat sheet metal so they probably had more freedom in designing that wing. Additionally it looks to me like they managed to reduce the number of rivets on the wing surface, reducing the drag further.

They also added cruise flaps which deflect the wing flaps down in cruise ever so slightly for ideal performance adjustments. Drooping the ailerons, flaperons and spoilers helps to reduce the drag in takeoff and landing configurations. Interestingly the idle glide ratio in clean configuration of the 787 isn't that much better than other aircraft if you look at idle descent planning manuals. It works out to be a glide ratio of about 1 to 22.5 or so, similar to the 777-300ER. I assume this is caused by the slightly increased wing sweep angle compared to the 777. The aspect ratio and the rest of the overall wing geometry looks quite similar to the 777, just scaled down. But increased sweep angle helps to increase the cruise Mach-number from 0.84 to 0.85.

Other aerodynamic improvements or changes: reshaped fuselage nose, streamlined cockpit windows, windshield wipers aligned with airflow, tail cone shape, laminar flow nacelle shape, new raked wing tip, horizontal stabilizer sweep angle increased, single slotted flap compared to double slotted flap on 777 (reducing complexity and weight, smaller flap track fairings), more takeoff flap settings on the 787-10 with adjusted slat schedule.

Others changes compared to 777-300ER: bleed-air-less system and Li-Ion batteries (reducing weight), no tail skid (reduces weight and complexity) increased window size, reduced cabin pressure altitude for increase passenger comfort.