Natural Laminar Flow NACA 6-Series Airfoils –Developed by conformal transformations, 30 – 50% laminar flow –Advantages: Low drag over small operating range, high C lmax –Disadvantages: Poor stall characteristics, susceptible to roughness, high pitch moment, very thin near TE –Drag bucket: pressure distributions cause transition to move forward suddenly at end of low-drag C l range –Minimum pressure at transition location NACA Report No. 824
Natural Laminar Flow NACA 6A-Series – % laminar flow –Eliminated TE cusp –Essentially same lift and drag characteristics as 6- series NACA Report No. 903
Natural Laminar Flow NACA : x tr upper = , x tr lower = NACA A: x tr upper = , x tr lower = XFOIL
Natural Laminar Flow NLF Airfoils –Aft-loaded airfoils with cusp at TE (Wortmann or Eppler sailplane airfoils) –Front-loaded airfoil sections with low pitching moments (Roncz-developed used on Rutan designs or canards) –Also NASA NLF- and HSNLF-series, DU-, FX-, and HQ- airfoils –Inverse airfoil design based on desired pressure distribution, capitalize on availability of composites –Low speed and high speed applications –Codes used for design include Eppler/Somers and PROFOIL –Up to 65% laminar flow –Drag as low as 30 counts 1. NASA Contractor Report No , Lutz, “Airfoil Design and Optimization,” Garrison, “Shape of Wings to Come,” Flying NASA Technical Memorandum 85788, 1984.
Natural Laminar Flow: Case Study SHM-1 Airfoil for the Honda Jet Lightweight business jet, airfoil inversely designed, tested in low-speed and transonic wind tunnels, and flight tested Designed to exactly match HJ requirements –High drag-divergence Mach number –Small nose-down pitching moment –Low drag for high cruise efficiency –High C lmax –Docile stall characteristics –Insensitivity to LE contamination Fujino et al, “Natural-Laminar- Flow Airfoil Development for the Honda Jet.”
Natural Laminar Flow: Case Study (Continued) Requirements –C lmax = 1.6 for Re = 4.8x10 6, M = –Loss of C l less than 7% due to contamination –C m > at C l = 0.38, Re = 7.93x10 6, M = 0.7 –Airfoil thickness = 15% –M DD > 0.70 at C l = 0.38 –Low drag at cruise Fujino et al, “Natural-Laminar- Flow Airfoil Development for the Honda Jet.”
Natural Laminar Flow: Case Study (Continued) Design Method –Eppler Airfoil Design and Analysis Code Conformal mapping, each section designed independently for different conditions –MCARF and MSES Codes Analyzed and modified airfoil Improved C l max and high speed characteristics Transition-location study Shock formation Drag divergence Fujino et al, “Natural-Laminar- Flow Airfoil Development for the Honda Jet.”
Natural Laminar Flow: Case Study (Continued) Resulting SHM-1 airfoil –Favorable pressure gradient to 42%c upper surface, 63%c lower surface –Concave pressure recovery (compromise between C lmax, C m, and M DD ) –LE such that at high α, transition near LE (roughness sensitivity) –Short, shallow separation near TE for C m Fujino et al, “Natural-Laminar- Flow Airfoil Development for the Honda Jet.”
Natural Laminar Flow: Case Study (Continued) Specifications: –C lmax = 1.66 for Re = 4.8x10 6, M = –5.6% loss in C lmax due to LE contamination (WT) –C m = at C l = 0.2, Re = 16.7x10 6 (Flight) –C m = at C l = 0.4, Re = 8x10 6 (TWT) –M DD = at C l = 0.30 (TWT) –M DD = at C l = 0.40 (TWT) –C d = at C l = 0.26, Re = 13.2x10 6 (TWT) –C d = at C l = 0.35, Re = 10.3x10 6 (WT) Fujino et al, “Natural-Laminar- Flow Airfoil Development for the Honda Jet.”
Laminar Flow Control stabilize laminar boundary using distributed suction through a perforated surface or thin transverse slots plenum chamber outer skin inner skin Boundary layer thins and becomes fuller across slot Benefits A laminar b.l. has a lower skin friction coefficient (and thus lower drag) A thin b.l. delays separation and allows a higher C L max to be achieved Ref: McCormick, “Aerodynamics, Aeronautics and Flight Mechanics,” pg. 202.
Notable Laminar Flow Control Flight Test Programs DateAircraftTest ConfigurationLF ResultComments 1940Douglas B-18 (NACA) 2-engine prop bomber NACA ’x17’ wing glove section suction slots first 45% chord LF to 45% chord (LF to min C p ) R C = 30x10 6 Engine/prop noise effected LF surface quality issues 1955Vampire (RAE) single engine jet upper surface wing glove suction - porous surface full chord suction full chord LF M~0.7 / R C =30x10 6 Monel/Nylon cloth 0.007” perforations F-94 (Northrup/USAF) jet fighter NACA upper surface wing glove suction – 12, 69, 81 slots Full chord LF 0.6 < M < 0.7 R C = 36x10 6 at M local >1.09 shocks caused loss of LF X-21 (Northrup/USAF) jet bomber 30° sweep new LF wings for program suction through nearly full span slots – both wings full chord LF R C = 47x10 6 effects of sweep on LF encountered JetStar (NASA) 4-engine business jet two leading edge gloves Lockheed – slot suction & liquid leading edge protection McDD – perforated skin & and bug deflector LF maintained to front spar through two years of simulated airline service no special maintenance required lost LF in clouds & during icing LE protection effective Ref: Applied Aerodynamic Drag Reduction Short Course Notes, Williamsburg,VA 1990.
Why Does LFC Reduce Drag? turbulent boundary layer has a higher skin friction coefficient XFOIL Output upper surface lower surface
Why Does LFC Increases C L MAX ? move boundary layer separation point aft Ref: A.M.O. Smith, “High Lift Aerodynamics,” Journal of Aircraft, Vol. 12, No. 6, June 1975
Raspet Flight Research Laboratory Powered Lift Aircraft Piper L-21 Super Cub (1954) distributed suction - perforated skins C L MAX = 2.16 → Hp required for suction (Ref: Joseph Cornish, “A Summary of the Present State of the Art in Low Speed Aerodynamics,” MSU Aerophysics Dept., 1963.) Cessna L-19 Birddog (1956) distributed suction - perforated skins C L MAX = 2.5 → Hp required for suction (Ref: Joseph Cornish, “A Summary of the Present State of the Art in Low Speed Aerodynamics,” MSU Aerophysics Dept., 1963.) Photographs Courtesy of the Raspet Flight Research Laboratory
Suction Power Required for Cruise Condition Suction velocity required to maintain incipient separation of the laminar b.l and prevent flow reversal is given by: Joseph Schetz, “Boundary Layer Analysis,” Equation (2-37) 0.035” ” dia 45” chord 12” span 45” x 12” grid – 439,470 holes P req = Hp / foot of span* *assumes: use highest v w and Δp in calculation discharge coefficient of 0.5 pump efficiency of 60%
Laminar Flow Control Approaches 1). Leading Edge Protection 2). Distributed Suction (perforated skin or slots) 3). Hybrid Laminar Flow Control Ref: Applied Aerodynamic Drag Reduction Short Course Notes, …….Williamsburg,VA 1990.
Laminar Flow Control Problems/Obstacles Sweep –Attachment line contamination (fuselage boundary layer) –Crossflow instabilities (boundary layer crossflow vortices) Manufacturing tolerances / structure –Steps, gaps, waviness –Structural deformations in flight System complexity –Ducting and plenums –Hole quantity and individual hole finish Surface contamination –Bypass transition (3-D roughness) –Insects, dirt, erosion, rain, ice crystals Ref: Applied Aerodynamic Drag Reduction Short Course Notes, Williamsburg,VA Ref: Mark Drela, “XFOIL 6.9 User Guide”, MIT Aero & Astro, 2001
Boundary Layer Transition Flight Tests on GlasAir Oil flow tests on GlasAir (N189WB) Raspet Flight Research Laboratory August KIAS 5500 ft pressure altitude Airfoil: LS(1)-0413mod →GAW(2) Mean aerodynamic chord: 44.1 in. Re 7.5x10 6 Cruise C L 0.2
Drag Benefit of Laminar Flow
CENTURIA 4 Passenger Single Jet Engine GA Aircraft Competition Cirrus SR22 Cessna 182 Targets existing General Aviation pilots Cost ~ $750,000 International Senior Design Project Virginia Tech and Loughborough University
Fuselage Laminar to max thickness Wing 60% LM flow upper and lower surface V-Tail 60% LM flow upper and lower surface
Centuria NLF Manufacturing Tolerances R h,crit h crit (in.) inches inches inches 15, inches Carmichael’s waviness inch/inch criteria Ref: A.L. Braslow, “Applied Aspects of Laminar-Flow Technology,” AIAA 1990 h
Conclusions Natural Laminar Flow –Improvement of materials and computational methods allows inverse airfoil design for desired characteristics or specific configurations Laminar Flow Control –LFC is a mature technology that has yet to become commercially viable Drag Benefit on Centuria –61% reduction in skin friction drag due use of laminar flow on wings, tail and fuselage
References Abbott, I.,H., Von Doenhoff, A.,E., Stivers, L.,S., “Summary of Airfoil Data,” NACA Report 824, Loftin, L., K., “Theoretical and Experimental Data for a Number of NACA 6A-Series Airfoil Sections,” NACA Report 903, Drela, M., “XFOIL 6.9 User Guide,” MIT Aero & Astro, Green, Bradford, “An Approach to the Constrained Design of Natural Laminar Flow Airfoils,” NASA Contractor Report No , Lutz, Th.,”Airfoil Design and Optimization”, Institute of Aerodynamics and Gas Dynamics, University of Stuttgart, Garrison, P., “The Shape of Wings to Come,” Flying Magazine, November McGhee,R.,J., Viken, J.,K., Pfenninger, W., Beasley, W.,D., Harvey, W.,D., “Experimental Results for a Flapped Natural-Laminar-Flow Airfoil with High Lift/Drag Ratio,” NASA TM 85788, Fujino, M., Yoshizaki, Y., Kawamura, Y., “Natural-Laminar-Flow Airfoil Development for the Honda Jet,” AIAA , McCormick, B.,W., Aerodynamics, Aeronautics and Flight Mechanics, 2 nd Edition, John Wiley & Sons, New York, “Applied Aerodynamic Drag Reduction Short Course,” University of Kansas Division of Continuing Education, Williamsburg, VA Smith, A.,M.,O., “High-Lift Aerodynamics,” Journal of Aircraft, Volume 12, Number 6, June Schetz, J.,A., Boundary Layer Analysis, Prentice Hall, Upper Saddle River, New Jersey, Cornish, J.,J., “A Summary of the Present State of the Art in Low Speed Aerodynamics,” Mississippi State University Aerophysics Department Internal Memorandum, Raymer, D.,P., Aircraft Design: A Conceptual Approach, AIAA Education Series, Braslow, A.,L., Maddalon, D.,V., Bartlett, D.,W., Wagner, R.,D., Collier, F.,S., “Applied Aspects of Laminar-Flow Technology,” Appears in Viscous Drag Reduction in Boundary Layers, AIAA Progress in Astronautics and Aeronautics, Volume 123, 1990.