NASA Technical Paper 2256 - CAFE Foundation

shape as an indication of transition. The implication of the present observations is
that the section drag increase associated with the transition changes in propeller
slipstreams may not be as large as _lat for fixed leading-edge transition. Thus, NLF
airfoils may provide drag reduction benefits, even on multiengine configurations with
wing-mounted tractor engines.
Waviness
No premature transition was observed in any of the experiments which could be
attributed to -surface waviness even though the surfaces tested were not perfectly
smooth and wave free, These results occurred on surfaces which received no special
contour preparation; the surfaces tested represented modern production-quality
smoothness achieved in either metal or composites.
As a historical comparison, the waviness from the 1950 King Cobra test (ref. 21)
and the waviness from the Skyrocket tests areshown in figure 41. This waviness for
the King Cobra produced the minimum level of profile drag measured in those flight
experiments at Re. = 17 × 106 • A qualitative comparison of the waviness measurements
confirms _ie fact that the modern composite surface on the Skyrocket with no special
contour preparation provides a lower level of waviness than the King Cobra metal
surface, which required extensive filling and sanding to achieve the waviness shown.
This comparison illustrates the achievability , with modern fabrication methods, of
surface waviness compatible with laminar flow at-medium to relatively high Reynolds
numbers. -Conversely, the results illustrate the point that some significant amount
of surface waviness is acceptable on laminar-flow surfaces in favorable pressure
gradients of moderate strength.
Sweep Effects
The two significant wing-geometry-related phenomena which can adversely affect
laminar boundary layers on swept surfaces are crossflow instability and turbulent
contamination of the leading-edge attachment line flow (or leading-edge contamina-
tion). - Since no Obvious crossflow instability was observed on the swept wings and
winglets in these flight experiments, this discussion centers on leading-edge contam-
ination. Crossflow instability can be recognized by the existence of closely spaced
streamwise streaks preceding transition in the sublimating chemical coating.
A comparison betweeen the flight data and the spanwise contamination criterion
is presented in figure 42 for the variEze and the L0ng-EZ. The spanwise contamina-
tion criterion is summarized in reference 46 as
R 8 = 0.404 sin A _R rle
A I + (t/c) (I)
Where no spanwise contamination occurs for R 0 < 100. For various roughness condi-
tions, there may be no spanwise contamination for R o < 240. _or R 0 > 240, turbulent
contamination from any source freely propagates spanwise along the attachment
line. On the swept VariEze (A = 27 °) and the Long-EZ (A = 23 ° ) wings, the data in.
figure 42 show that R o did not exceed 100. The same was true for thewinglets on
both airplanes; R@ at the root was 51 for the VariEze and 36 for the Long-EZ. On
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