An Introduction to Passive vs. Active Laminar Flow Control for Aircraft Wings
By Van Warren MS CS, AE
Note to the reader: All diagrams and references are hot-linked to their sources for convenience.
Introduction
In our exploration of passive versus active laminar flow we must concentrate our
focus on the boundary layer, that region between the freestream flow moving at v
and the layer of molecules anchored to the surface of the wing by van der Waals
forces with a velocity of 0. These latter molecules satisfy the “no-slip” condition. This
is shown in the following NASA slide:
What is not shown in the slide is that the boundary layer thickness is different for
laminar and turbulent flows. Laminar flows have a thinner boundary layer than
turbulent ones. Laminar means less drag for the wing. Thus a laminar boundary
layer is greatly desirable.
We will start by examining a paradox. In his excellent book on The Fluid Dynamics of
Drag, MIT Professor Ascher Shapiro asks the question, “Why does a dimpled golf ball
travel farther than a non-dimpled golf ball of the same size?”
Prof. Shapiro’s example highlights the importance of understanding the nature of
laminar versus turbulent fluid flow. It is also critical to understand the flight regime
in which we are operating. Are we a sailplane or a fighter jet? The graph above
illustrates this paradox. The golf ball is operating in a low Reynolds number
environment. If golf balls were fired from guns like musket balls, smooth golf balls
might travel farther. An excellent problem to study would be, “How fast would Tiger
Woods have to hit a golf ball for it to lose its dimples?”
It is helpful to recall the definition of Reynolds number. According to NASA GRC, the
conservation of mass in three dimensions, a change in velocity in the streamwise
direction causes a change in velocity in the other directions as well. There is a small
component of velocity perpendicular to the surface which displaces or moves the flow
above it. One can define the thickness of the boundary layer to be the amount of this
displacement. The displacement thickness depends on the Reynolds number which is
the ratio of inertial (resistant to change or motion) forces to viscous (heavy and
gluey) forces and is given by the equation:
Re 
 vl

where
 = density of the fluid
v = freestream velocity
l = characteristic dimension, for example thickness
 = viscosity of the fluid
It is key that we recognize that a golf-ball is always operating in a “stall”, that is with
a significant area of separated flow.
Under normal flight conditions a wing is not operating in a stall, yet we must keep its
behavior in a stall in mind for safety in design reasons.
One key to understanding the golf ball case is that a turbulent boundary layer is
thicker than a laminar boundary layer. However the turbulent boundary layer
remains attached to the ball reducing the bluff body wake significantly.
A thick boundary layer by its very nature implies more drag than a thin one. In the
sphere example, the advantages of the flow remaining attached around the back of
the ball outweight the gains due to laminar flow.
For this reason, in compromised situations, dimples, “boundary layer trips”, “vortexgenerators” or other mechanisms are installed to insure that the flow remains
attached as long as possible. The aft surfaces of some aircraft are dimpled for this
reason. Most automobile sun-roofs have a trip just ahead of the sunroof to prevent
low frequency rumbling due to a laminar flow field. Aircraft like the 727 have vortex
generators to encourage the flow to remain attached in near-stall conditions. This
along with airfoil and wing design prevent precipitous changes in lift in off-design
flight configurations.
Passive Boundary Layer Control - A Gloster Javelin showing
three sets of vortex generators located along the outer portion of the wing
You may have observed the stream of fluid on a ski boat that encounters a bump or
defect in the hull. The flow aft of this is always turbulent and extends in a growing vshape that contaminates the laminar flow around it. In aircraft and racing cars a
special inlet called the NASA inlet minimizes the induced turbulence from necessary
penetrations of the cabin or hull.
Summary
Engineering is about trade-offs. If the benefits of drag reduction are outweighed by
system complexity, cost, or weight, then we are forced to accept passive solutions.
Active boundary layer suction is implemented at the cost of engine power, fallibility
(bugs, dirt, dust), cost (holes and ducting). Active boundary layer suction turns an
aircraft into an air filter, and the overhead of keeping that filter clean becomes an
important aspect of the overall cost/benefit equation.
Reynold’s Number
A key indicator of whether the flow is likely to be laminar or turbulent is the
Reynold’s number.
Viscosity
Passive Laminar Flow
Active Laminar Flow
Visualizing Flow
Aerodynamic Efficiency
Delaying Transition from Laminar Flow
Other Considerations
Pitching Moment Stability
Precipitous Change in Drag and Lift Coefficients
Graceful Degradation
Delaying the Onset of Turbulence.
Passive Laminar Flow means to insure that
Active Laminar flow control
Active LFC must be used to achieve laminar flow across larger distances from the
leading edge. The main means of achieving active LFC is to remove a portion of the
turbulent boundary layer with a suction or blowing mechanism that uses porous
material, slots in the wing, or tiny perforations in the wing skin (see figure8).
Figure 8: F-16 used in the LFC study
NASA used F-16XL in the LFC study were modified to operate a suction system that
pulled the turbulent boundary layer through a porous sheet of titanium that was the
upper wing surface in the test area.
Passive laminar flow
Passive laminar flow can be achieved in the wing design process, but the laminar
condition is normally very small in relation to the wing's cord and is usually confined
to the leading edge region. The North American P-51 Mustang fighter of World War II
fame had a first design of passive laminar flow wing, but laminar conditions existed
for only a very short distance past the leading edge. Passive laminar flow can also be
created on an existing wing by altering the cross-sectional contour of the lifting
surface to change the pressure gradient. Both of these laminar conditions are called
natural laminar flow.