In the early studies of hypersonic aircraft, there was a widely held
belief that the hypersonic laminar boundary layer would prove infinitely
stable because of the heat flow out of the boundary layer. It was
thought likely that hypersonic vehicles would be the first true laminar
flow aircraft. Actually there was little substance to this belief other
than an early theoretical indication of a stabilizing effect due to heat
flaw from the boundary layer. Nevertheless it was the hopeful practice
in the early fifties to compute performance and heating for the all-laminar
case. One respected research group even suggested that hypersonic research
to reduce laminar friction deserved top priority.
Figure 5. Boundary-layer transition location on X-15 wing
compared with wind-tunnel prediction, and roughness Reynolds
number of X-15 leading-edge joint compared with critical roughness
Reynolds number .
This technical superstition has now almost entirely disappeared,
largely as a consequence of the X-15 findings (ref. 15; fig- 5). At the
Mach 6 flight Reynolds number, which was about 2.7 million/meter, wind
tunnel data for a smooth model, not including any benefits from heat
transfer indicate extensive laminar flows. But only the leading-edge
region was found to be laminar in flight. A "step" irregularity existed
behind the leading edge which, although very small, was sufficient according
to low-speed criteria to trigger transition. Thus the small surface irregularities
which have proved the nemesis of laminar flaw at lower speeds,
are apparently equally adverse on the blunt-edged wing at Mach 6.