Bjorn’s Corner: Aircraft drag reduction, Part 17

By Bjorn Fehrm

February 16, 2018, ©. Leeham Co: In the last Corner, we discussed supersonic flow and drag. Now it’s time to talk about the drag created by transonic flow on an aircraft.

Figure 1. Pressure distribution of 787 during cruise. Source: Boeing and Leeham Co.

Transonic drag

When an aircraft flies slowly, below M0.5, there is no supersonic flow on the aircraft. As the aircraft speeds up, areas where the airflow has to curve around convex surfaces will experience increased airflow speeds and lower pressure. Such areas can be on the topside of wings but also the crest of the nose of a fuselage. Figure 1 shows the (static) pressure distribution of a Boeing 787-8 during cruise.

Green areas have the lowest pressure and therefore the highest airflow speed. Then follows light blue, darker blue, which transfers to red and finally yellow. Yellow means static pressure is high and airflow speed is low.

On a recent flight to Toulouse, I sat beside an Airbus Beluga pilot. The Beluga is made from the Airbus A300-600, modified with a much larger unpressurized fuselage bubble, capable of transporting sections of Airbus airliners, such as the A350 wing, Figure 2.

Figure 2. Airbus Beluga with the large transport backpack. Source: Airbus.

For the Beluga, the wing is not the area where supersonic flow starts. It’s the forward crest of the fuselage going supersonic already at M0.65. As local areas with supersonic flow can create lots of drag, the Beluga is flown below M0.65.

For a well-designed airliner, the first area to go supersonic is the top side of the wing (green in Figure 1). The airflow speeds up over the wing as the wing curves to produce lower pressure and by it lift. The supersonic flow starts on the leading edge and grows gradually until it covers the wings crest and then spreads further back on the wing as the Mach increases.

Once past the crest, the flow gets sensitive to disturbances as the air streams against a higher pressure area as the wings taper towards the trailing edge. At some point, the supersonic flow transfers to subsonic flow through a normal shock, Figure 3.

Figure 3. Transonic flow over a wing. Source: Wikipedia.

Shocks are violent changes of density, pressure and temperature of the flow, with an increase of static pressure. This causes the flow to separate at the base of the shock, creating areas of separated flow.

The separated flow increases the drag of the aircraft. The sudden onset of drag past a certain Mach is called transonic drag and the Mach where this happens, critical Mach. 

Supercritical airfoils

NASA’s Richard Whitcomb realized the normal form of a wing profile, with maximum airspeed at about one-third of the wing cord, was not optimal to avoid transonic drag. He, therefore, shaped the wing profile to keep the maximum speed lower at the top, by not curving the top profile as much, Figure 4.

Figure 4. 1) Normal airfoil. 2) Supercritical airfoil. Source: Wikipedia.

Instead, he curved the rear bottom part of the profile, creating higher static pressure there and by it recovering some of the lift lost over the less curved top surface.

The result is a profile with lower Mach in the supersonic area before the Normal shock. The shock then is weaker, generating less pressure increase and less boundary layer disruption. Supercritical wings have lower transonic drag and higher Mach where the transonic drag starts.

In the next Corner, we’ll finish the series with some less dominant drags which appear on aircraft.


12 Comments on “Bjorn’s Corner: Aircraft drag reduction, Part 17

  1. I wish I gad taken a photograph of this transonic Shock line while flying on a Boeing 720 test bed years ago. Flying at 0.9 Mn for flutter testing of the engine and aircraft configuration allowed for a visual appearance in the Mach line above the wing. Just like the diagram!

    • Michael,
      Very cool! I had similar experiences back in the early 90’s when I was flying transcon during the summer months. Most of my flights were around midday when the sun was high. On several of them the sun was positioned just right so that a shadowgraph of the normal shock was projected onto the wing surface. I could see the combination bright/dark band clearly the entire span of the wing. It was jittering back and forth in the flow direction due to the uneven air conditions encountered. I would spend hours looking at it and would watch to see it disappear when the aircraft slowed on descent. I also wish I would’ve taken a picture but I didn’t have a camera with me on those trips.

  2. @bjorn, looking at the cockpit area in the 787 pressure distribution graphic above made me wonder how much (% of total drag) the old fashioned cockpit sections of the 737/777/A320/A330 cost vs modern cockpit sections like the 787/A350?

    on the 737Max Boeing went to a lot of effort to reloft the tail and clean up several areas of the aircraft for ~1.5% reductions in total drag (not including new winglets) but did nothing for the 1959 707 based nose design…

      • Nose areas are expensive to design, certify and build. Thats why the French Caravelle used the same subcontractor to build the same nose section from the Comet.

    • Hi Bilbo,

      I don’t know. But the edgy plane windows of e.g. the 737 or A320 do cause separations. You can see this on the marks the turbulence leaves in the paint on the aircraft at the edges of the windows. It’s an area of gradually lower pressure which follows, not an adverse pressure gradient like the aft part of the wing or the tail. Chances are the separations get squashed after a while. Such drag could be counted in excrescence drag and the total drag of such imperfections, including antennas, inlets and outlets are less than 5% of total drag.

      Modern cockpit sections with curved flush windows like the 787, A350, E-Jet, CSeries… generate virtually no additional drag.

      • The edgy windows are Not as Bad as it seems. The edge forms sort of a stationary vortex which “fills” the gap. It’s far less severe than a usual separation. If the flush windows were so much netter, A350 would have them. Also keep in mind that drag is not the only, not even the Primary design Goal for Cockpit windows 😉

    • The first graphic is obviously not an A350. It looks more like an A330 (planar windscreen sections and mixed flow engine nacelles), but there are a lot of details missing around the wing root.

  3. The SUPERCRITICAL airfoil looks like it would offer a much lower Cl, than a “normal” NACA ” laminair” high speed airfoil?

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