Bjorn’s Corner: Supersonic transport revival, Part 4

By Bjorn Fehrm

August 31, 2018, ©. Leeham News: In the last Corner we discussed supersonic lift wave drag and how suddenly the length aspect ratio is more important than wingspan aspect ratio.

Now we talk about the problem of going from subsonic to supersonic flight.

Figure 1. The Supersonic drag hump. Source: Wikipedia.

Passing the Sound Barrier

The flying close to the speed of sound at the end of and after the WW II created the wave drag we’ve seen in the previous Corners. But it also created the Transonic drag we’ve discussed in the Corners about drag reduction.

Figure 2 shows the drag created by the separation of the boundary layer from a wing when the shock returning the air to subsonic speed from a supersonic region on the wing causes the boundary layer to separate.

These large separations are the cause of the shooting up of the drag just before Mach 1 in figure 1.

Figure 2 shows the separations around a typical subsonic wing (with a blunt nose), but such supersonic regions and Normal shock waves will form on many places on the aircraft when passing Mach 1, even for a supersonic aircraft with sharp leading edges on wings and fuselage.

Figure 2. The creation of Transonic drag caused by boundary layer separation. Source: Wikipedia.

The drag hump at Mach 1 is partly such separations and partly shocks forming around the nose of the aircraft/wings as detached oblique shocks, Figure 3. As the Mach increases, these shocks get more attached, and the high drag from the shocks decreases (Figure 1).

Figure 3. Shock waves formed around a T38 which has just passed Mach 1. Source: Wikimedia.

The drag increase around Mach 1 was what the first supersonic pilots felt as a “Sound Barrier”.  The engines had marginal thrust to penetrate the hump, therefore it felt like a barrier one couldn’t get through. The first aircraft flying supersonic dived through the hump and then levelled off for supersonic flight.

Today it’s still often referred to as “the Sound Barrier” as it requires extra thrust to pass through the drag hump. To pass it effectively, SuperSonic Transport (SST) aircraft must use a special climb profile and strong engines.

The climb profile for the Concorde had a level flight segment at 30,000ft where the afterburners of the engines were lit and the Sound Barrier was passed. The Aerion S2 engines were sized by the climb segment were the Sound Barrier should be passed and not by the Take-Off requirement, as for normal airliners.

In next Corner, we shall look at other aerodynamic problems created by Supersonic flight.


12 Comments on “Bjorn’s Corner: Supersonic transport revival, Part 4

  1. Presumably the sonic cruiser was supposed to be in the space just before the first kink in the graph?

    • Actually the Sonic Cruiser was designed to be at the 2nd kink in the first graph. Most modern jetliners have their cruise optimized at the first kink.

      • The second kink appears to be at about 0.5 Cd, with another around 1.1 Cd, yes? Anybody out there know what actual Mach number at which Boeing aspired to have the erstwhile beast cruise? (I fear the ‘sonic’ epithet was the result of spin, and I dare not mention higher prospective fuel-burn.).

  2. Thanks for explaining what happens as a lane or mussel passes through the sound barrier

    • BernardP – well, it certainly excited Master Mulally, for whom a glass was never half-empty. “20% faster”? Compared with M0.8, rather than M0.85 maybe. Able to connect “almost any city pair”? Well, only up to 9,500nm…(!). Perhaps it emanated from the product-development organisation’s earlier mid-1990s’ Pacific Fragmenter studies.

  3. Its worth remembering that with some wing profile and area changes along some extra stages in the Olympus engines, the Concorde had detail design done to do away with the ABs completely as they then werent needed for the push past the sound barrier at 30,000 ft.
    Showed a recurring theme of airliner design, once you have built it , only then do you have all the data on how it should really be done.

    • Any reason you could not do a shallow dive and go super sonic maneuver with lower powered engines?

      • Air traffic management using flight levels.
        Passengers not wanting to spill their champagne.
        Bad prestige due to marginal thrust, whereas prestige is a (THE) major selling point of supersonic aircraft.

        I don’t think shallow dives would help much anyway.

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