Document
RIBLETS FOR AIRCRAFT SKIN-FRICTION REDUCTION
ii 72 _3
Michael J. klalsh NASA Langley Research Center Hampton, Vi rginia ABSTRACT Energy conservation and aerodynamic efficiency are the driving forces behind research into methods to reduce turbulent skin friction drag on aircraft fuselages. Fuselage skin friction reductions as small as 10 percent provide the potential for a 250 million dollar per year fuel savings for the commercial airline fleet. One passive drag reduction concept which is relatively simple to implement and retrofit is that of longitudinally grooved surfaces aligned with the stream velocity. These grooves (riblets) have heights and spacings on the order of the turbulent wall streak and burst dimensions. Present paper summarizes riblet performance (8 percent net drag reduction thus far), sensitivity to operational/application considerations such as yaw and Reynolds number variation, an alternative fabrication technique, results of extensive parametric experiments for geometrical optimization, and flight test applications.
RIBLETS Local skin friction reductions as large as 8-10 percent have been experimentally measured in the wind tunnels at NASA Langley Research Center.
The research has reached a stage where flight applications are near term. The riblets are now available on a thin vinyl film with adhesive backing. This film could be applied to new as well as existing aircraft. Data indicate that the film could be applied to both fuselage and wing surfaces. The 8-percent local skin friction reduction would therefore translate into an 8-percent reduction in the turbulent viscous drag of the aircraft, or approximately 4 percent aircraft drag. An added advantage of the riblet drag reduction technique is that it is passive.
Bottom line -
8 percent aircraft viscous drag reduction
for CTOL aircraft, retrofittable and passive
WHAT ARE RIBLETS?
Riblets are streamwise surface striations that are aligned with the local free-stream velocity. The typical cross section of an optimum riblet is shown on the slide. The optimum and most practical riblet has sharp valleys and sharp peaks. The purpose of the riblets is to modify the near-wall structure of the turbulent boundary layer. The spanwise surface variation down in the cross section imposes a strong spanwise viscous force that creates a wall slip Iayer.
Riblet - streamwise surface striation
Cross section _ NVVV_
Purpose - Alter turbulence dynamics near the wall by
imposing/utilizing strong spanwise viscous
forces to create a wall slip layer
APPLICATION ISSUES RESOLVED The work at Langley Research Center has resolved many of the application issues. The present paper discusses test results on optimum riblet geometry, maximum drag reduction, riblets scaling for drag reduction at flight conditions, yaw sensitivity, and a practical application technique.
• Optimum geometry for maximum drag reduction
• How much drag reduction?
• Scaling (wind tunnel to flight)
• Yaw sensitivity
• Retrofittable application technique
RIBLET MODELS TESTED Determination of optimum riblet geometry required parametric testing.
This slide indicates the range of shapes studied. The test results show that the symmetric v-groove gave the maximum drag reduction performance. The peaks and the valleys of the grooves should be relatively sharp for maximum drag reduction.
Model
Description
AAA
Symmetric V-groove
Rectangular
.A_A.
Spaced triangular
Right angle rib
/VV_ Peak curvature
Valley curvature
Peak and valley curvature
Notched peak
Spaced V-groove
Unsymmetric groove
Oblique V-groove
OPTZNUM R[BLET This slide presents net drag data for an optimum riblet surface that was tested in the low-speed wind tunnel at Langley. The drag data is referenced to the drag of a flat plate reference. The horizontal coordinate is the spacing of the grooves in wall coordinates, s+. The parameter s+ is related to the free-stream velocity and skin friction as shown in the equation on the slide. The maximum drag reduction of 8 percent was found to be insensitive to upstream history and free-stream+velocity as long as the physical dimensions of the riblet were sized to an hsand + : 15 at the operating free-stream velocity of interest. The parameter h + is the height of the groove in wall coordinates, This scaling of the drag reduction in wall coordinates allows the extrapolation of the low-spEed wind tunnel results to a flight environment. The equations for s and _ shown on the slide indicate that the physical size of the riblet for drag reduction is only a weak function of Re , the Reynolds number based on distance along the aircraft fuselage (I/I_ power). Therefore, the riblet dimension could be fixed for a particular aircraft rather than varying drastically along the fuselage.
1.20 - S+ - oo 1.15 su cv_-f/2 u 0 o -.I 0 o° I.I0 c_-f --' Rex O D/D 0 FP 1.05 [] 1.00 .95
ooo
I I .9O I I I I 0 I0 20 40 50 60 3O S+
VINYL FILM RIBLET
The previous slide indicated the riblet scaling parameters. The next step to achieving a flight-capable device was to find a practical fabrication technique. The method advocated by :3/,I Co. was to make the riblets on a vinyl film with an adhesive backing. This slide shows drag data obtained with a vinyl riblet compared to a riblet which had been machined on an aluminum surface. Due to the manufacturing process, the aspect ratio (h/s) for the vinyl riblet was somewhat smaller than that for the aluminum riblet. The reduced aspect ratio of the vinyl riblet resulted in a smaller drag reduction than that obtained with the aluminum riblet. The data are significant in that drag reduction was obtained on a surface suitable for aircraft application.
Material h(mm) s(mm) h+at s+=15
1.20
O Aluminum 0.41 0.47 13
12Vinyl 0.29 0.47 9.3 0 0
1.15
0 0
o o
1.10
o o°
1.05
D/DFp
1.00
.95
I I I I I I
.90
10 20 30 40 50 60
S + ORIGINAL PAGE IS OF POOR QUALITY FLIGHT TEST FILM The scaling parameters have been used to determine the physical dimensions of a riblet film suitable for drag reduction at flight conditions. The slide shows a micro-photogragh of a riblet film to be flight tested in July 1985. The physical height and spacing of the riblet is 0.0013 inch. Despite the overall physical size the film still has sharp peaks and valleys.
EFFECT OF YAW Another aspect of a flight application is the yaw sensitivity of the riblets. All data shown previously have been for zero angle of yaw. This slide shows the vinyl film at 0°, 15 ° , and 30 ° yaw. There is little change in the maximum drag reduction at 15°; however, all drag reduction is lost at 30 ° yaw. In a riblet application it would be important to keep the grooves aligned with the flow within 15 °. For cruise the near surface streamlines are usually in this less than 15 ° range for CTOL aircraft.
Z_ 1.30 - Model a 1.25 - O 13M 15 ° El ° 1.20- Zi 13AA 30° A EP O _ 1.15 - '% _.
o
1.00_ I 0 10 20 30 40 50 s-t-
SUMMARY OF R[BLETPERFORMANCE
The riblet drag reduction appears to result from the spanwise velocity gradient @u/@z There is a thickening of the viscous near wall region that moves the turbulent fluctuations further from the wall. This whole process reduces the turbulent production magnitude. The basic turbulence interaction process is unaltered. A maximum drag reduction of 8 percent is obtained with a sharp peak v-groove having a space and height 0(15+). The drag reduction is present up to 15 ° angles of yaw; however, by 30 ° yaw, the drag reduction is lost. Vinyl riblets provide a simple method for application to new aircraft as well as aircraft presently flying.
• Spanwise geometry causes a _)U/_z
• Thickens viscous region near the wall
• Moves turbulent fluctuations further from the wall and
reduces turbulence production magnitude
• Turbulence interaction/production processes unaltered
• Basically, provides a "slip layer" at surface
mMaximum drag reduction for riblets 8 percent
*Optimum geometry is V-groove with "sharp" tips
• Optimum size is 0(15+)
• Insensitive to yaw angles up to 15°
• Vinyl riblets make aircraft applications simple and retrofittable
REQUIRED AND ONGOING RESEARCH Ongoing research at Langley is presently focusing on increasing the riblet drag reduction performance by increasing the aspect ratio (h/s) of the riblet geometry. Present data indicate that the drag reduction increased with aspect ratio with a maximum value of one tested thus far. Further research will attempt to increase the aspect ratio above one. Now that the scaling of the riblet drag reduction has been firmly established, the wind tunnel results can be used to extrapolate to flight conditions. Two sets of flight tests are planned in the near future: a subsonic test at M : .7 on the fuselage of a Learjet; and a M : 2.0 test on a flight test fixture mounted on the underside of the fuselage of an F-IO4G aircraft at Dryden Flight Research Facility.
Details of these upcoming flight tests will be discussed on the next two slides. Tests are also planned in the 7' x 10' tunnel in 198_. These tunnel tests will be conducted at high U _ Q , , and Re (100 x 10 ). Also to get the riblet film ready for fleet app,,ca_ion, the i_ollowing areas will have to be examined: durability, porosity, ultraviolet sensitivity, and clogging.
The need for a porous film will be discussed further in a later slide.
• Improved performance (increased H/S)
• Flight tests
• Learjet fuselage at M = .7 (July 1985)
• Dryden, M = 2 (May - June 1985)
• High Uod Qod Rex (100 x 106) tests in 7' x 10' tunnel (1986)
• Long duration flight readiness
• Durability
• Porosity
• Ultraviolet sensitivity
• Clogging
TURBULENT DRAG REDUCTION FLIGHT RESEARCH
T h i s s l i d e shows a photograph o f a L e a r j e t Model 28/29 a i r c r a f t t o be
The r i b l e t f i l m w i l l be t s t e d a t
used f o r r i b l e t f l i g h t t e s t s i n J u l y 1985.
Drag
Mach numbers up t o 0.8 and Reynolds number p e r f o o t up t o 3.0 x 10 f .
r e d u c t i o n performance w i l l be measured w i t h a d r a g balance and a boundary
l a y e r rake. Two r i b l e t f i l m s w i l l be t e s t e d : one w i t h h = s = 0.003" and t h e o t h e r h = s = 0.0013".
FLIGHT TEST FIXTURE ON F-1046
T h i s s l i d e shows t h e f l i g h t t e s t f i x t u r e mounted on t h e u n d e r s i d e o f t h e f u s e l a g e o f an F-104G a i r c r a f t a t Dryden F l i g h t Research F a c i l i t y . The t e s t f i x t u r e can p r o v i d e d a t a f o r Mach numbers 0.4 t o 2.0. A d r a g balance and a boundary l a y e r rake a r e mounted on each s i d e o f t h e f i x t u r e . T h i s t e s t f a c i l i t y w i l l p r o v i d e d a t a showing t h e s u p e r s o n i c drag r e d u c t i o n performance The t e s t s a r e p r e s e n t l y scheduled f o r May-June 1985.
o f t h e r i b l e t s .
NET EFFECT OF RIBLET APPLICATION This slide summarizes the net effect of a riblet application. The riblet film has application to civilian as well as military aircraft. The riblet has the potential to reduce turbulent skin friction on the aircraft fuselage by 8 percent, This skin friction reduction on the fuselage and wing is associated with a displacement thickness reduction (8*) that results in a 6 percent reduction of the afterbody form drag. The riblet film itself, if made porous, could reduce the 5 percent of fuselage drag which is due to leakage from the pressurized cabins and the roughness drag of the aircraft by 3 percent. The summation of the riblet drag reduction benefits is a possible 16 percent reduction in fuselage drag or a 4 percent reduction in the net drag of the aircraft. In addition the riblet film would provide corrosion resistance and could be used as a substitute for paint. Many of the markings and decorations on aircraft flying now are made out of a vinyl film. Wing application benefits would be additive to these.
-CTOL -Tanker •Civilian OWUlitary-Transport (C130, C141, C5) -SST -Patrol (P3) Source Percent drag reduction for fuselage Control of wall turbulence, 8% direct CF reduction 6_ reduction upstream of 6% reduction of afterbody afterbody region form drag 5% reduction due to modification Converting jet leakage into of leakage from pressurized cabins porous injection Smoothing due to surface film 3% roughness drag reduction Total fuselage drag reduction = 16% Total aircraft drag reduction = 4°1o • Additional benefits • Corrosion resistance (cost reduction) • Substitute for paint (colors molded into film) (cost/wt. reduction)