Bjorn’s Corner: Aircraft drag reduction, Part 18

By Bjorn Fehrm

February 23, 2018, ©. Leeham Co: In the last Corner we discussed transonic flow and drag. Now it’s time to finish the drag type discussion by adding some less dominant but still important drag types.

Knowing their origin will help us understand why aircraft are made like they are.

Figure 1. Low-pressure (green) shows areas with high flow speeds for the 787 during cruise. Source: Boeing and Leeham Co.

Interference drag

Interference drag is created when two surfaces, like the wing and fuselage or wing and engine nacelle/pylon, are meeting without adequate measures to avoid them interfering with each other’s airflow.

The airflow speeding up to supersonic speed as it passed over a wing was described in the last Corner. If this wing with a transonic flow over-the-top would be merged with the fuselage, without a faring creating a bridging area, the wing’s and the fuselage’s airflows would create an even faster flow at the merger area.

This creates a faster supersonic flow, followed by a stronger normal shock, causing boundary layer separation. Merging two flows without a clever transition area to keep the two flows from merging in a non-desired way and by it causing unstable flow, creates interference drag.

To avoid such merged flow fields, nacelles (which has a higher flow speed at the top) are kept at a distance from the wing’s surfaces. Wings are merged with the fuselage through a cleverly formed wing-root fairing (keeping local flow speeds down) and vertical and horizontal tails merging on top of a T-tail are merged through a bullet-shaped fairing, Figure 2. The bullet fairing separates the two flows from each other.

Figure 2. Russian IL-62 with the typical bullet fairing on the T-tail to avoid interference drag. Source: Wikipedia.

Another example is the switch from angled winglets, like on the Boeing 747-400 or Airbus A330-300, to blended winglets, like for the Boeing 737-800.

In Figure 3 we show the difference between the winglet for the present Airbus A330 and the new winglet for the A330neo (-800 and -900).

Figure 3. A330ceo and neo winglets with different shapes. Source: Airbus.

The winglet is no longer joined at a sharp angle to the wing. The wing’s high-speed flow field (violet) is smoothly continuing into the A330neo winglet. On the A330ceo, the sharp angled winglets cause interference drag at the junction of the flows (merger of violet and blue flow fields).

Miscellaneous drag

When summing-up the drag for an aircraft, there is a term often called miscellaneous drag. This is the drag associated with the air-conditioning systems inlets and outlets and the many necessary protuberances that exist on an airliner to mount air probes or antennas for navigation or communication.

The protuberances of the skin are sometimes collected in an own drag item called excrescence drag, which includes the unevenness of the skin at the heads of rivets/fasteners and joins of skin panels. Sometimes the gaps between the aircraft and the movables (flaps, ailerons, rudders) are included in the excrescence drag, sometimes these drag items have their own term in a list of an aircraft’s drag items.

In the next Corner, we’ll give some practical examples of the size of these drag types during a typical airliner mission.

16 Comments on “Bjorn’s Corner: Aircraft drag reduction, Part 18

  1. Why did most Sowjet airliners have a totally flat lower side of the fuselages while western built aircraft have quite big bellies where the wings are. Isn’t that also creating drag and possibly even negative lift as it’s shaped like an inverse wing?

    • When were they designed?
      compare to A300 A310, 767-200 bellies.

      IMU “Fat” belly fairings came up with planes like A330/A340, 777 all have much higher aerea wing profiles than previous designs.
      Also note the differences in flap track fairings and the shock bodies on Tu-95, Tu-114, some of the Convairs.
      also look at the various engine placements for the Ju-287

      see also:

  2. Sometimes it is an advantage to move antennas and satcom bulbs aft into a thicker boundary layer. For Aircrafts with different Engine options of slightly different sizes it is important to be “first” to determine the nacelle position on the wing for the others to accept.
    GE90 was second onto the 777 after the PW4074/4077 for launch Customer UAL, GE did not want that situation for the 777-300ER and after the exclusive deal could optimize the GE90-115 installation onto the 777-300ER much better.

  3. Bjorn, perhaps much discussed in forums, but could you please enlighten us as to why the wings on many Boeing aircrafts, especially the 787, appear to be much more flexible than those of Airbus’? If so, what would be the key differences in terms of gains and compromises of the two philosophies, including factors such as wing loading and the use of raked/blended winglet devices say if you compare the 787 and 350?

    Thanks for this insightful series.

    • Hi Jonathan,

      this is about wing design philosophy. Boeing has used inboard and outboard ailerons consistently to allow flexible wings without risking aileron reversal at high speed (the aileron twists the wingtip so the effect is reversed, therefore at high speed only the inboard aileron is used). Airbus wing center in Filton UK has used normal ailerons also for high speed, necessitating a stiffer wing to avoid aileron reversal. There are plus and minuses with both principles, to know which is the best for a certain design requires a complete wing design with a comparison of results.

  4. Thanks Bjorn, knowing very little about these things but the nose section and wing fuselage interface/transitions key areas to address. Could laminar flow surface improve these areas?

    How much improvement will there be for the 320 wing if 330N/350-like wing lets could be used if there was not the 36m CAT-C restriction? Was thinking along the lines of a wing for the often talked about 322 where going CAT-D is maybe not such a big issue.

    • Laminar flow areas build up the flow speed and therefore the low pressure more progressively to avoid the strong falling pressure areas of classical profiles. Normally it can only be realised for a forward section of a body or wing profile. It requires even more carefully designed fairings when merging aerodynamic surfaces if you want to keep the laminar flow.

      A winglets which is less inclined is more efficient, the most efficient design is the raked tip. But the span requirements increase. A recent example of the above is the revised winglets of the Embraer E175 to E175+ and then E175-E2, which has raked tips. There was no gate restriction stopping this evolution.

      • Thanks Bjorn. Except for efficiencies a 757-200 length 32x with an ~40m span and 350-like wing-tips will also look good I think.

        Hope P&W can sort out their problems. If not was wondering what thrust CFM can generate from an 81″ fanned LEAP”A” if required?

        • LEAP A engines thrust range goes up to 35,000lb for the heaviest A321s. Dont see the point of a few extra inches of diameter , which generically just means a higher BPR and better economy, not higher thrust- that would require more power from the core.

          • Thanks Bjorn, I was under the impression that the LEAP-A’s max thrust was just shy of 33 Klb.

            Was wondering what a theoretical A322’s thrust requirement will be if it needs for example 100-105T MTOW and impact of the larger wetted area due to a stretch?

            ..or will the “magic” come from a new wing.

  5. Hi Bjorn,

    Thank you for the amazing sharing on the winglets part, especially pointing out the advantage of the new design on A330 Neo.

    However, could you think of the reason why Boeing would continue to use sharp angled scimitar winglets (in 737 Max series), instead of the raked wingtips(as in the Dreamliner) or perhaps blended scimitar winglets?

    Thanks in advance 🙂

    • Hi John,

      it has 100% to do with span width limits, the 737 can’t get into the gates which have a limit of 38m unless it expands the winglets vertically. A more horizontal winglet or even raked tip, like the Naval 737 would be more efficient.

      • I am not a fan of the folding wingtip idea (like the 1st time you fly Transatlantic on a twin) but could the improvements seen with A350-like winglets be worth the effort/cost for an SA to stay in CAT-C?

        The 320 folding wingtip patent I have seen is downward folding which could have threats for damage when stationary.

        Was also wondering if the “A322” will be CAT-D as these gates will become more readily available 767 are being withdrawn?

      • Hi Bjorn, sorry this is a late one. See you mention gates with 38m limits, will it be possible for an aircraft with a 38m span to use these gates although it will be classified as a CAT-D aircraft?

        Example is the 757-200 with an 38.0m span. Lastly a new wing for an 321+/322 is often talked about, it could most likely do with an extra 2m span. Or could the change to an 30 degrees sweep (as example) for higher speed cruising and larger surface/volume that could increase fuel capacity be enough to be able to still fit into 36m?

        See the 767 sweepback is 31.5 deg while that of the A310 was 28 deg. Will be interesting to see where BAC pitch the NMA’s sweep as balance between shorter and longer haul missions.

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