The X-15 Program in Retrospect: Laminar Flow

THE X-15 PROGRAM IN RETROSPECT

Laminar Flow

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 - three line graph charts showing boundary-layer transition location on X-15 wing compared with wind-tunnel

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.