Fortunately the X-15 structure was designed conservatively to cope
with turbulent heating. Hypersonic turbulent heating rates were recognized
from the outset as an area of special importance for flight measurements
because of the weakness of the available semi-empirical prediction methods
and the almost complete lack of reliable hypersonic wind-tunnel data for
turbulent flow. It should not have been any cause for real surprise,
therefore, when the X-15 flight results of 1961 (ref. 16) showed a marked
departure from the predictions available at that time, averaging about 35
percent below the van Driest and the Eckert T' predictions (refs. 17, 18)
for the low wall temperature conditions of these flights (fig. 6).
Figure 6. Turbulent heat-transfer data for wing
of X-15 compared with prediction method.
At first this result was received with disbelief by most fluid mechanics
specialists, who are by nature skeptical of flight measurements. The
result has been thoroughly substantiated., however, by repeated measurements
at several locations on the airplane together with local flow f ield surveys
to aid in analysis of the data. The important highlighting of this weakness
of the prediction methods by the X-15 stimulated comprehensive studies in
a variety of ground facilities. New cold-wall data have been obtained
which fully confirm the X-15 results. Improved prediction methods are
being sought and new investigations of the structure of the cooled turbulent
boundary layer are in progress (ref. 19, for instance). This is an
excellent example of one of the greatest values of an exploratory research
airplane - the highlighting of an important problem and the stimulating
of ground-based research for its solution.
