NASA Technical Paper 2256 - CAFE Foundation

the swept strakes of both the VariEze (A = 61 °) and the Long-EZ (A = 51°), R e
exceeded I00. However, small regions of laminar flow were still observed near the
leading edges of bo_% strakes. Relaminarization by rapid flow acceleration might
have been responsible for the short laminar runs observed on the strakes. Calcula _
tions, by the method of reference 46, of conditions necessary for relaminarization to
occur show that _e necessary flow acceleration may have been present on the upper
surface of the VariEze and Long-EZ strakes within about l-percent chord of the lead-
-ing edge* On the Long-EZ, on the very short inboard strake (A = 64 ° ) where
R8 _ 240, no laminar flow was recorded by the chemical pattern. At the leading-edge
break between A = 64 ° and A = 51 o , the leading-edge contamination from the 64 °
swept region did not propagate onto the 51 ° swept region in spite of the fact that
R e varied from 127 to 148 for this region. (See fig. 42(b).)
On the Learjet wing (A = 17°), R A did not exceed 100 in spite of the extremely
high unit Reynolds number during the t_st. On the Learjet winglet, where R 0 varied
from 151 at the root to 75 at the tipduring the tests, it could not be ascertained
whether spanwise contamination was present on the portions of the winglet which were
turbulent. This uncertainty was caused by excessive roughness in the form of screw
heads and a step which caused transition in some regions of the leading edge.
Even if spanwise contamination were present at the test condition where
R = 3.08 × 106 ft -I, at typical cruise (R = 0.87 x 106 ft-1), the values of R_
would drop to 80 at the winglet root and 40 at the tip, thus ensuring no spanwlse
contamination. In fact, at the aforementioned Learjet cruise unit Reynolds number on
a surface swept 40 °-, the leading'edge radius could be as large as 1.5 in. and still
keep R 8 < 100-for no spanwise contamination.
This observation implies that, in general, on certain relatively large lifting
surfaces, spanwise contamination at high-altitude cruise may not be a serious con-
cern. As an example, on the Gulfstream Aerospace GIII airplane (chosen for its large
size in the business jet class), at an altitude of 45 000 ft and at M = 0.85,
R@ varies from 80 at the wing r6ot-to 68 at the tip, precluding spanwise contamination..
AS a final example of operations below the spanwise contamination criterion,
R@ for the DC-10 winglet (ref. 47) varies from 64 at the root to 40 at the tip for
M = 6.82, at a cruise altitude of 35.000 ft, and at a cruise unit Reynolds number of
about 1.9 × 106 ft-1. - Based on these observations, it appears that for certain
important potential applications, spanwise contamination need not be a concern for
relatively large lifting surfaces.
Insect Debris Contamination
The effect of contamination on NLF wings by insect debris is an important con-
sideration in NLF airfoil design as well as in the operation of airplanes with
laminar-flow wings. These considerations, as well as insect population characteris-
tics, are discussed In some detail in the literature (refs. 48 to 54). In practice,
the seriousness of insect debris contamination will-likely be dependent on airplane
characteristics and mission. If needed, active methods of insect protection such as
porous, fluid-exuding leading edges may serve the purposes of both insect and ice
protection. The ice-protection performance features of such systems are discussed in
reference 55, and the ability of wetted leading edges to protect against insect
debris contamination is discussed in references 56 and 57.
For a representative insect debris contamination pattern accumulated in flight
on the Bellanca Skyrocket, only 25 percent of the insects caused transition at sea
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