Laminarisation of Supersonic Transports

Efficient laminarisation of supersonic transpons requires a hybrid approach. Figure 108 shows a concept oriented on the different physical properties:

The inner wing has high sweep angle with subsonic leading edges (the leading edge stays within its own Mach cone). Therefore the leading edge is rounded with a radius which still inhibits usage of pure Natural Laminar Row (NLF) techniques, but requires Laminar Row – Control (LFC). NLF means that laminarisation is achieved solely by suited shaping which con­trols pressure gradient and • at the leading edge – 3D divergent flow. LFC uses artificial meas­ures to alter the boundary layer flow, e g boundary layer suction, to improve the boundary layer velocity profiles w ith respect to laminar disturbance damping.

The outer wing and the empennage probably have supersonic leading edges (the lead­ing edge is outside its own Mach cone). Those leading edges usually are sharp. Possibly NLF can be used behind sharp leading edges, at least as long as profile curvature remains very small. But local Reynolds numbers grow rapidly leading to transition. In addition, curvature intro­duces pressure gradients which – on swept wings – lead to pressure gradients normal to the flow direction: this provokes some destabilising boundary layer crossflow waves. Suction can pro­long the laminar flow region. Before reaching die hinge line of deflected rudders, the boundary – layer should become turbulent in order to avoid unfavourable shock/boundary layer interferen­ces with laminar separation.

As long as the fuselage boundary layer is turbulent, a turbulent wedge extends at the wing along the intersection with the fuselage. This is a significant part of the surface of SCT – wings which have large root chord length.

In a NASA-stud/ (377J Boeing has investigated the impact of laminarisation on super­sonic transports. Table I shows the improvements due to laminanzation for a 250 passenger transport, design Mach number 2.4 and optimistic – but at least comparable – design ranges of 5000 nm and 6500 nm. The additional weight for the suction system of about 4.5 tons and the thrust reduction by 0.2% arc ncgligeable when looking at the benefits:

• reduced fuel heating, essential at Mach 2.4 over long ranges

• important weight reductions

• significantly reduced fuel burned.

This was demonstrated in a first study neglecting some snow ball effects; further improvements after optimisation arc expected

MTOW: maximum takc-of! weight OEW operational empty weight

Table 1: Laminar Flow SCT-study (377)- Mach 2 4. 250 passengers

The 6500 nm turbulent aircraft could not be realized with the assumed weight limit of 500 tons. But according to our experience, probably the weight of an SCT must even stay below 400 tons for take-off noise limitations. Over 400 tons all designs seem to diverge; especially because the absolute noise limit is reached, whereas at lower weight noise limits arc related to take-off weight.