NASA Technical Paper 2256 - CAFE Foundation

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APPENDIX
SURFACE WAVINESS ON RESEARCH MODELS
The-accurate measurement of airfoil surface waviness is important for both lami-
nar boundary-layer research and production of laminar-flow wings. The physical pres-
ence of waves on a laminar airfoil surface Can create macroscopic changes in the
local pressure gradient which can in turn trigqer transition to turbulence. The
critical amplitudes and wavelengths which can trigger transition have been empiri t
callv related to Revnolds number for a single wave in reference 27 by the equation
= 9 000c cos
k kRcl.5
where h is the double-amplitude wave height in inches, k is the wavelength in
inches, c is the wing chord in inches, and A is the wing leading-edge sweep.
multiple waves, h/k is one-third the value of a single wave.
The dial indicator (fig. AI) used for measuring surface waviness during this
investigation is mounted on a solid base with three fixed legs. A single leg is
spaced 2 in. from the paired legs, which are 0.6 in. apart for stability. The dial
indicator leg is placed at _le center. This method was selected simply to permit
comparison of modern waviness data with data from early natural laminar flow (NLF)
research, for.which_this waviness gauge design was originally used.
The procedure for making waviness measurements using a dial indicator is as
follows. For convenience in refere-ce marking, transparent tape was placed chordwise
over the line on which waviness was co be measured. Beginning at the chord leading
edge, 1/4-in. intervals were marked on the tape, and gauge deflections were recorded
at each interval. The gauge reading was then plotted versus the distance around the
surface from the leading edge. A nine-point running average (for 1/4-in. intervals)
was plotted over the raw data, because the actual airfoil surface curvature was not
accurately known. The difference between the two plots is representative of the
actual waviness. Nine points were chosen for the calculations to provide artificial
smoothing over the 2-in.-length of the dial indicator base.
There are several shortcomings which arise with this type of measurement device
and procedure used to calculate waviness. Foremost is the fact that the waviness
measured is without flight loads on the surface. With certain structures (e.g.,
those with lightly stressed thin metal wing skins), waviness in addition to that mea-
sured on the ground probably exists under flight loads. Additionally, difficulty
arises from the fact that the-center leg is deflected successively as each of the
base legs passes through a wave. This deflection yields a distorted wave with more
cycles and with both larger and smaller amplitudes than the surface being measured.
The dial resolution is one-half of 1 x 10 -_ in., and the I/4-in. in£ervals on the
wing were accurate to within 1/32 in. Swept or tapered wings can also affect inter-
pretation or meaning of the gauge readings. If the gauge is_skewed slightly from the
chord line being measured, the legs will rest at a different level and will produce
an added deflection. During the measurements on the airplanes-discussed herein, this
source of error was minimized by care in streamwise alignment of the dial indicator
For