|
Background
The discipline of aerodynamics
includes three distinct areas of interest: the
fundamental understanding of flow physics and basic
fluid phenomena, the experimental and computational
prediction and analysis of aerodynamic applications,
and flow control to enhance the aerodynamic performance
of aircraft. The concepts and mechanisms that permit
flow control are among the most important products
of the modern aerodynamicist, and they will lead
to new paradigms in the aerodynamic design of future
aircraft. To enable the identification and development
of flow control concepts, the researcher will generally
have expertise in all three of the foregoing components
of aerodynamics. The key to flow control, however,
is a thorough understanding of the fundamental
physics of three-dimensional, high Reynolds number
aerodynamic phenomena including vortical flows,
boundary-layer transition and turbulence, and flow
separation.
Langley Research and Development
Activities
The rich history of Langley
research on flow control concepts includes pioneering
research for concepts such as boundary-layer control
for high-lift, upper surface blowing and the externally
blown flap for short takeoff and landing capability,
supercritical airfoils, winglets, and laminar flow
control and numerous turbulence control concepts
(e.g., mass injection, riblets, vortex generators,
passive porous surfaces) for skin friction drag
reduction and/or separation control. Although tremendous
progress has been made in flow control, more effective
and versatile concepts are constantly being explored
to enhance the aerodynamic performance and reduce
the costs of future civil and military aircraft.
Currently, research on flow control has been directed
at several specific objectives: fundamental studies
of the relative efficiency and optimum configuration
for passive control devices such as vortex generators,
assessments of unconventional advanced passive
concepts such as passive porosity, development
and evaluation of emerging active flow control
concepts including steady and unsteady flow control
concepts, and the development of advanced actuators
and sensors for active control—especially
microelectromechanical systems (MEMS). Unfortunately,
many of the more recent advances in flow control
concepts have not yet been incorporated in current
civil aircraft because of lack of maturity, risks,
and concerns over unknowns regarding the costs
associated with aircraft component redesign and
manufacturing. Cost concerns today require that
all new technologies must “buy” a way
onto the production aircraft.
One of the most widely applied
concepts for flow control is vane-type, passive
vortex generators that transfer high-energy fluid
outside the boundary layer to the surface region
inside the boundary layer. First introduced in
1947, vortex generators consist of a row of small
plates or airfoils that project normal to the surface
and are set at an angle of incidence to the local
flow to produce an array of streamwise trailing
vortices. These devices are used to energize the
boundary layer such that boundary-layer separation
is eliminated or delayed, and this can be used
to enhance wing lift, improve control effectiveness,
and/or tailor wing buffet characteristics at transonic
speeds. Many commercial transports utilize vortex
generators to enhance wing aerodynamic performance
over an enlarged flight envelope. Air travelers
can readily view vortex generators that are normally
arranged in a spanwise direction on the upper surface
of the wing or the empennage of modern transports;
single vortex generators can also be found on the
sides of the fore and aft sections of the fuselage
and on engine nacelles.
Microvortex Generators
Although aircraft designers
have made wide use of relatively large vortex generators
(VGs) to solve numerous flow control problems,
the relative size of the auxiliary vanes can unfavorably
impact the performance of aircraft. Conventional
VGs usually produce residual drag through conversion
of aircraft forward momentum into unrecoverable
turbulence in the aircraft wake. Therefore, the
design and implementation of a passive, effective
VG configuration that prevents flow separation
for critical flight conditions yet imposes little
or no drag penalty on the aircraft is a formidable
challenge to the aerodynamicist.
Led by John C. Lin, a team of Langley researchers
dramatically improved the characteristics of VGs
by developing smaller microvortex generators (MVGs)
to produce streamwise vortices that more efficiently
transfer momentum within the boundary layer. Langley’s
research on MVGs began as a fundamental investigation
of boundary-layer separation control in the early
1990s. Within that fundamental objective, researchers
attempted to determine the minimum effective size
for vortex generators. Langley organized an aggressive
experimental program to obtain detailed information
on the mechanism by which vortex generators reenergize
the turbulent boundary layer and prevent separation.
The resulting optimization to a sub-boundary-layer
scale provided a major breakthrough in the fundamental
understanding of the nature of vortex generator
flow control and potential applications. The initial
laboratory experiments were conducted in the Langley
20- by 28-Inch Shear-Flow Control Tunnel.
Following the exploratory
tests, Langley discussed the results of the MVG
research with the aircraft industry, and this peaked
industry’s interest in the MVGs quite significantly.
A cooperative investigation with McDonnell Douglas
in 1991 focused on the impact of MVGs on the high-lift
performance of a flapped wing model in the Langley
Low-Turbulence Pressure Tunnel (LTPT). The model
was a McDonnell Douglas two-dimensional, single-flap,
three-element airfoil. The use of MVGs to eliminate
flow separation enabled the flap configurations
to be more aggressive than conventional design
would permit. The results showed that the more
aggressive design with MVGs dramatically enhanced
aerodynamic performance including a 10-percent
increase in lift, a 50-percent decrease in drag,
and a 100-percent increase in lift-to-drag ratio.
For commercial transport aircraft, these positive
aerodynamic effects could lead to improved landing
performance with the simpler (more economical)
single-flap design and, more importantly in many
instances, to reduced approach noise (i.e., less
engine power to achieve the same lift). Another
practical benefit of using the MVGs for high-lift
applications is that they are small enough to be
stowed with the flap at cruise and hence do not
increase the cruise drag.
In addition to industry interests
in applications to commercial transport aircraft,
the Langley-developed MVG concept has also been
applied by the general aviation industry to enhance
performance and high-lift characteristics. In a
cooperative investigation with Gulfstream Aerospace
Corporation, tests were conducted in the LTPT to
improve the Gulfstream V high-lift geometry using
microvortex generators in 1994 and 1995. In addition,
during flight tests conducted in 1996 and 1997
by Gulfstream, the microvortex generators outperformed
conventional vortex generators for controlling
shock-induced separation. The Gulfstream V now
incorporates MVGs on the outboard upper surfaces
of its wing for enhanced cruise performance. With
the MVGs installed, the Gulfstream V was able to
achieve a higher maximum cruise speed, extend its
operational range capability, and exhibit better
controllability. The enhanced aerodynamic performance
provided by MVGs allowed Gulfstream to meet their
technical goals and assure a timely and successful
product. As previously discussed, the Gulfstream
V aircraft has set numerous domestic and world
speed and performance records and was named the
winner of the 1997 Collier Trophy presented by
the National Aeronautic Association.
Microvortex generators mounted
on flap of two-dimensional high-lift model in
the Langley Low-Turbulence Pressure Tunnel. View
is looking upstream.
Gulfstream V uses microvortex
generators on outer wing for enhanced performance.
Microvortex generators on
flap element of Piper Malibu aircraft.
Close-up view of microvortex
generators.
Another highly successful
application of MVG technology involved a transfer
of design information to the New Piper Aircraft,
Inc., in 1996. New Piper applied the MVG concept
to enhance the low-speed, high-lift characteristics
of the Malibu Meridian aircraft. MVGs were mounted
along the leading edge of the wing trailing-edge
flap to enhance flow turning and avoid separation.
This technology enabled the Piper Malibu Meridian
aircraft to easily pass the FAA certification requirements
for stall speed (below 61 knots), which it had
previously not met.
|