NASA Technical Paper 2256 - CAFE Foundation

max maximum
min minimum
leading edge
t transition location
U upper
w wing
free stream
Notation: --_
i.d. inside diameter ....
NLF natural laminar flow
oed.
psf ,
outside diameter
pounds (force) per Square foot
REVIEW OF PAST NATURAL LAMINAR-FLOW RESEARCH
The achievement and maintenance of NLF are the two principal challenges to its
use for performance improvement on airplanes today. Natural laminar flow is achieved
on airfoil surfaces with small sweep angles (_15 °) by- designing long runs of favor-
able pressure gradients (accelerating flow) which limit the growth of two-dimensional
disturbances (Tollmien-Schlichting (T-S) waves) in the boundary layer. The growth of
T-S waves can be aggravated, on the other hand, b_ the effects of surface waviness on
local pressure gradients and on boundary-layer velocity profiles. These effect_
reduce b0undary-layer stability and can lead to premature transition. _hus, favor-
able pressure gradients "protect" the laminar boundary layer from the effects of
limited amounts of surface waviness by counteracting the destabilizing influences of
waviness. Similar influences govern the critical sizes of other two-dimensional
protuberances such as steps and gaps in laminar boundary layers. On wings with sig-
nificant sweep, NLF is achieved by compromise between the above pressure distribution
consideration and the conflicting design requirement for less favorable pressure
gradients which limit the growth of three-dimensional disturbances (crossflow vorti-
ces) in the boundary layer. The growth rate of crossflow w,rtices is rapid in the
region of rapidly falling pressure near the leading edge. It is not presently well
understood how the interaction between crossflow vortices and T-S waves affects tran-
sition on swept wings at free-stream conditions of interest for business, commuter,
or airline transport airplanes. The technical challenge to the successful design of
such airplane wings will be to meet both of the conflicting pressure gradient design
requirements for avoidance of these two- and three-dimensiozLal instabilities.
The maintenance of wing surface conditions compatible with NLF requires that the
surfaces be kept free, in an operating environment, from critical _ounts of surface
contamination (e.g., insect debris or ice), free-stream disturbances (e.g., noise and
turbulence), and surface damage. Compared with phenomena affecting the achievability
of NLF, less is understood about the maintainability of NLF under the wide ranges of
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