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.
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.
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).
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.