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OVERVIEW OF THE LANGLEY VISCOUS DRAG REDUCTION PROGRAM Jerry N. Hefner NASA Langley Research Center Hampton, Virginia
rISCOUS DRAG REDUCTION
As a result of reductions in form drag and roughness drag, skin friction
drag or viscous drag now represents a major contributor to the cruise drag of
subsonic business and transport aircraft, and hence, is considered a barrier
problem to further significant improvements in the aerodynamic efficiency of
these aircraft. In fact, viscous drag accounts for as muchas 50 percent of
the total cruise drag for subsonic transports and as muchas 35-40 percent of
the cruise drag for supersonic aircraft. To meet the challenge of reduced
viscous drag and improved aerodynamic efficiency, research in the areas of
laminar-flow control and turbulence control/drag reduction was initiated at
the NASA Langley Research Center in the mid-1970's under the Aircraft Energy
Efficiency Program and the Aeronautics Research and Technology Base. The
significance of this research is that even small reductions in viscous drag
should provide important design tradeoffs including: significant resizing
options for new aircraft designs, increased range capability without increased
take-off gross weight, increased speed and productivity, and reduced fuel
volume and cost. Only a 10-percent reduction in viscous drag could provide an
annual fuel savings of the order of $200-300 million assuming an annual civil
aviation fuel bill in the United States of $10 billion.
o Viscous (skin friction)drag is barrier problem
• Accounts for approximately
e50 percent of cruise drag for subsonic transports
o35-40 percent for supersonic aircraft
• Reductions in viscous drag provide
new aircraft designs
• Significant re-sizing options for
• Increased range capability
• Increased speed and productivity
• Reduced fuel volume/cost
MAINTENANCE OF LAMINAR FLOW FOR VISCOUS DRAC_E_C_ION The research being conducted under the Viscous Drag Reduction Program focuses on two approaches to reduce the skin friction drag. 6 Where the chord or length Reynolds numbers (Rc) are of the order of 60 x 10 or less (i.e., wings, nacelles, empennage), the maintenance of laminar flow appears most promising and provides the largest net benefits. For Reynolds numbers much larger than this (i.e., 100 x 10 _ or greater), it is unclear whether laminar flow can be maintained over relatively large surface areas of the aircraft; hence, turbulence control/drag reduction is being investigated for these surfaces (i.e., fuselage).
Three concepts are being investigated to delay the boundary layer transi- tion process and _aintain laminar flow beyond the usual transition Reynolds numbers of 4 x 10 _ or less. These include: (I) the use of favorable pressure gradient or surface shaping (natural laminar flow); (2) suction through slotted or perforated surfaces (laminar flow control); and, (3) combinations of suction and favorable pressure gradient (hybrid laminar flow control). The advantage of natural laminar flow is that it is a passive approach to the maintenance of laminar flow but it may be limited to wing sweep.angles of approximately 20 or less and chord Reynolds numbers of 20 x 10 o or less.
Laminar-flow control, using full-chord suction, will probably be required for applications where extensive l_minar flow is necessary for chord Reynolds numbers approaching 50-60 x 10 _ and sweep angles of 30 or greater. Hybrid laminar-flow control minimizes suction requirements and provides increased operational flexibility and improved off-design performance compared to natural laminar flow.
• Pressure gradient/shaping
• Suction through slotted
or perforated surfaces
Laminar flow 7
Laminar flow/_
ur u,en
-T_ Laminar flo
Laminar flow I
Natural Laminar Flow Laminar Flow Control
TURBULENCE CONTROL FOR VISCOUS DRAG REDUCTION Turbulence control/drag reduction research attempts to identify and develop highly innovative concepts employing surface micro-geometry modifica- tions to alter and control the turbulence production process and to reduce turbulent surface shear. Although the payoff from this approach to viscous drag reduction is generally much less than that for the maintenance of laminar flow, it does provide an alternative that is applicable to high length Reynolds numbers, may be retrofittable to existing aircraft, and has less operational sensitivities.
Twelve turbulence control concepts are currently being explored at Langley with two of these concepts (i.e., riblets and large-eddy breakup devices) producing net viscous drag reductions of the order of 10 percent.
Riblets are very small, flow-aligned, triangular grooves cut into the sur- face. If the height and spacing of these grooves are scaled properly, based on particular fluid physics properties, then net skin-friction drag reductions can be obtained; for practical flight applications riblets would typically have a height and spacing of between 0.0015-0.003 inch. Large-eddy breakup devices are thin flat plates or small airfoils immersed within the turbulent boundary layer to alter the large-scale turbulent structures within the boun- dary layer. The unique feature of both concepts is that the skin-friction reductions produced by their interaction with the turbulent boundary layer are sufficiently large, such that they more than compensate for the drag penalties associated with the particular concept (i.e., increased wetted area for rib- lets and device drag for large-eddy breakup devices).
Flow
j,,_ Tu rbu lent
boundary
layer
(_) Flow
__ Flow
llllllll/lllll/llllll/llll/l
Riblets
Large-eddy break-up devices
(longitudinal surface striations)
VISCOUS DRAG REDUCTION RESEARCH Wind tunnel, flight, computational, and systems research programs are aimed at providing the data base and design methodology necessary to ensure the techno- logy readiness of viscous drag reduction for the 1990's and to reduce the risks associated with both the near-term and far-term application to business, commuter, and transport aircraft. Other important objectives include: deter- mining the limits of applicability of natural laminar flow with regard to wing sweep, pressure gradient, Reynolds number, and disturbance environment; pro- viding the experience and data base to ensure that laminar flow can be main- tained economically and reliably in practical airline environments; and demon- strating that turbulent drag reduction is more than a laboratory curiosity and can provide significant benefits in practical flight applications.
Objectives
eData base and design methodology necessary to accelerate technology
readiness and reduce risks for application to business, commuter,
and transport aircraft
eLimits of applicability of natural laminar flow
OEconomic maintenance and reliability of laminar-flow control concepts
oFlight application of turbulent drag reduction concepts
CURRENT VISCOUS DRAG REDUCTION RESEARCH Key elements of the Viscous Drag Reduction Program are highlighted on this figure. Laminar flow research focuses on the following: developing advanced airfoils for all classes of aircraft, providing guidelines for fuse- lage and nacelle laminarization, determining allowable manufacturing toler- ances, determining the acoustic environment on transport aircraft at cruise conditions including critical noise sources and their influence on the main- tenance of laminar flow, understanding the fluid physics and providing a boundary layer transition criterion for the interaction of crossflow (CF) and Tollmien-Schlichting (TS) instabilities, establishing design criteria for perforated and slotted suction surfaces with improved tolerances to spanwise and chordwise pressure gradients, developing control concepts for separation on laminar-flow airfoils, and conducting systems research to explore the near- term application of hybrid laminar-flow control to transport aircraft. Turbu- lent drag reduction research focuses on concept invention and development, and the optimization and flight evaluation of riblets and large-eddy breakup devices.
Suction surface design
Separation control
CF/TS interactions
Advanced airfoils
Nacelle
Fuselage laminarization laminarization
Noise research
Manufactu
Turbulent drag reduction _" tolerances
Systems research
POTENTIAL PAYOFFS FOR VISCOUS DRAG REDUCTION APPROACHES Significant progress has been made in the areas of natural laminar flow, laminar-flow control, and turbulent drag reduction over the past seven to ten years and viscous drag reduction is now recognized as one of the two technolo- gies having the greatest potential for improving the performance of future aircraft (propulsion and the advanced turboprop constitute the other techno- logy). Potential payoffs could include 15 percent reductions in cruise drag for natural laminar flow applied to only wing surfaces, 20 percent reductions for laminar-flow control wings, and 5-10 percent reductions for riblets and/or large-eddy breakup devices applied to transport fuselages. These payoffs would reduce fuel volume and cost, increase the sales of business and trans- port aircraft employing viscous drag reduction, and provide the technology for global military transports and for missiles and supercruisers with increased range.