Laminar Flow Rodney Bajnath, Beverly Beasley, Mike Cavanaugh AOE 4124 March 29, 2004

Introduction Why laminar flow? Less skin friction Lower drag

Why laminar flow?

Less skin friction Lower drag

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

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

Natural Laminar Flow NACA 6A-Series 30 - 50% laminar flow Eliminated TE cusp Essentially same lift and drag characteristics as 6-series NACA Report No. 903

NACA 6A-Series

30 - 50% laminar flow

Eliminated TE cusp

Essentially same lift and drag characteristics as 6-series

Natural Laminar Flow NACA 64-012: x tr upper = 0.5932, x tr lower = 0.5932 NACA 64-012A: x tr upper = 0.6214, x tr lower = 0.6215 XFOIL

NACA 64-012: x tr upper = 0.5932, x tr lower = 0.5932

NACA 64-012A: x tr upper = 0.6214, x tr lower = 0.6215

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. 201686, 1997. 2. Lutz, “Airfoil Design and Optimization,” 2000. 3. Garrison, “Shape of Wings to Come,” Flying 1984. 4. NASA Technical Memorandum 85788, 1984.

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

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.”

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

Natural Laminar Flow: Case Study (Continued) Requirements C lmax = 1.6 for Re = 4.8x10 6 , M = 0.134 Loss of C l less than 7% due to contamination C m > -0.04 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.”

Requirements

C lmax = 1.6 for Re = 4.8x10 6 , M = 0.134

Loss of C l less than 7% due to contamination

C m > -0.04 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

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.”

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

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.”

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

Natural Laminar Flow: Case Study (Continued) Specifications: C lmax = 1.66 for Re = 4.8x10 6 , M = 0.134 5.6% loss in C lmax due to LE contamination (WT) C m = -0.03 at C l = 0.2, Re = 16.7x10 6 (Flight) C m = -0.025 at C l = 0.4, Re = 8x10 6 (TWT) M DD = 0.718 at C l = 0.30 (TWT) M DD = 0.707 at C l = 0.40 (TWT) C d = 0.0051 at C l = 0.26, Re = 13.2x10 6 (TWT) C d = 0.0049 at C l = 0.35, Re = 10.3x10 6 (WT) Fujino et al, “Natural-Laminar-Flow Airfoil Development for the Honda Jet.”

Specifications:

C lmax = 1.66 for Re = 4.8x10 6 , M = 0.134

5.6% loss in C lmax due to LE contamination (WT)

C m = -0.03 at C l = 0.2, Re = 16.7x10 6 (Flight)

C m = -0.025 at C l = 0.4, Re = 8x10 6 (TWT)

M DD = 0.718 at C l = 0.30 (TWT)

M DD = 0.707 at C l = 0.40 (TWT)

C d = 0.0051 at C l = 0.26, Re = 13.2x10 6 (TWT)

C d = 0.0049 at C l = 0.35, Re = 10.3x10 6 (WT)

Laminar Flow Control stabilize laminar boundary using distributed suction through a perforated surface or thin transverse slots 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. plenum chamber outer skin inner skin Boundary layer thins and becomes fuller across slot

stabilize laminar boundary using distributed suction through a perforated surface or thin transverse slots

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

Notable Laminar Flow Control Flight Test Programs Ref: Applied Aerodynamic Drag Reduction Short Course Notes, Williamsburg,VA 1990. effects of sweep on LF encountered full chord LF R C = 47x10 6 new LF wings for program suction through nearly full span slots – both wings X-21 (Northrup/USAF) jet bomber 30 ° sweep 1963-1965 no special maintenance required lost LF in clouds & during icing LE protection effective LF maintained to front spar through two years of simulated airline service two leading edge gloves Lockheed – slot suction & liquid leading edge protection McDD – perforated skin & and bug deflector JetStar (NASA) 4-engine business jet 1985-1986 at M local >1.09 shocks caused loss of LF Full chord LF 0.6 < M < 0.7 R C = 36x10 6 NACA 63-213 upper surface wing glove suction – 12, 69, 81 slots F-94 (Northrup/USAF) jet fighter 1954- 1957 Monel/Nylon cloth 0.007” perforations full chord LF M~0.7 / R C =30x10 6 upper surface wing glove suction - porous surface full chord suction Vampire (RAE) single engine jet 1955 Engine/prop noise effected LF surface quality issues LF to 45% chord (LF to min C p ) R C = 30x10 6 NACA 35-215 10’x17’ wing glove section suction slots first 45% chord Douglas B-18 (NACA) 2-engine prop bomber 1940 Comments LF Result Test Configuration Aircraft Date

Why Does LFC Reduces Drag? removes turbulent boundary layer XFOIL output

removes turbulent boundary layer

Why Does LFC Reduce Drag? turbulent boundary layer has a higher skin friction coefficient XFOIL Output upper surface lower surface

turbulent boundary layer has a higher skin friction coefficient

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

move boundary layer separation point aft

Raspet Flight Research Laboratory Powered Lift Aircraft Piper L-21 Super Cub (1954) distributed suction - perforated skins C L MAX = 2.16 ->4.0 2.0 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 ->5.0 7.0 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

Piper L-21 Super Cub (1954)

distributed suction - perforated skins

C L MAX = 2.16 ->4.0

2.0 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 ->5.0

7.0 Hp required for suction

(Ref: Joseph Cornish, “A Summary of the Present State of the Art in Low Speed Aerodynamics,” MSU Aerophysics Dept., 1963.)

Suction Power Required for 23012 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” 0.0025” dia 45” chord 12” span 45” x 12” grid – 439,470 holes P req = .00318 Hp / foot of span* *assumes: use highest v w and Δ p in calculation discharge coefficient of 0.5 pump efficiency of 60%

Suction velocity required to maintain incipient separation of the laminar b.l and prevent flow reversal is given by:

45” x 12” grid – 439,470 holes

P req = .00318 Hp / foot of span*

*assumes:

use highest v w and Δ p 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 1990. Ref: Mark Drela, “XFOIL 6.9 User Guide”, MIT Aero & Astro, 2001

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

Boundary Layer Transition Flight Tests on GlasAir Oil flow tests on GlasAir (N189WB) Raspet Flight Research Laboratory August 1995 200 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

Oil flow tests on GlasAir (N189WB)

Raspet Flight Research Laboratory

August 1995

200 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

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

Centuria Design Details Cruise altitude 10,000ft Cruise Speed 185kts Range 770nm Take-off run 1575ft Aspect Ratio 9.0 Wing Area 12.3m 2 /132.39ft 2 Thrust 2.877kN/647lbs MTOW 1360kg/2998lb Fuel Volume 773 litres/194 USG Stall Speed 68kts (Clean) 55kts (Flap)

Cruise altitude 10,000ft

Cruise Speed 185kts

Range 770nm

Take-off run 1575ft

Aspect Ratio 9.0

Wing Area 12.3m 2 /132.39ft 2

Thrust 2.877kN/647lbs

MTOW 1360kg/2998lb

Fuel Volume 773 litres/194 USG

Stall Speed 68kts (Clean) 55kts (Flap)

Drawing by Anne Ocheltree & Nick Smalley

Calculating Laminar Flow 60% 100% Laminar Turbulent Laminar Turbulent Wing & Tail Fuselage 40% 100%

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.) 900 0.0072 inches 1800 0.0143 inches 2700 0.0215 inches 15,000 0.1195 inches Carmichael’s waviness 0.0139 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

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, 1945. Loftin, L., K., “Theoretical and Experimental Data for a Number of NACA 6A-Series Airfoil Sections,” NACA Report 903, 1948. Drela, M., “XFOIL 6.9 User Guide,” MIT Aero & Astro, 2001. Green, Bradford, “An Approach to the Constrained Design of Natural Laminar Flow Airfoils,” NASA Contractor Report No. 201686, 1997. Lutz, Th.,”Airfoil Design and Optimization”, Institute of Aerodynamics and Gas Dynamics, University of Stuttgart, 2000. Garrison, P., “The Shape of Wings to Come,” Flying Magazine, November 1984. 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, 1984. Fujino, M., Yoshizaki, Y., Kawamura, Y., “Natural-Laminar-Flow Airfoil Development for the Honda Jet,” AIAA 2003-2530, 2003. McCormick, B.,W., Aerodynamics, Aeronautics and Flight Mechanics , 2 nd Edition, John Wiley & Sons, New York, 1995. “ Applied Aerodynamic Drag Reduction Short Course,” University of Kansas Division of Continuing Education, Williamsburg, VA 1990. Smith, A.,M.,O., “High-Lift Aerodynamics,” Journal of Aircraft, Volume 12, Number 6, June 1975. Schetz, J.,A., Boundary Layer Analysis, Prentice Hall, Upper Saddle River, New Jersey, 1993. Cornish, J.,J., “A Summary of the Present State of the Art in Low Speed Aerodynamics,” Mississippi State University Aerophysics Department Internal Memorandum, 1963. Raymer, D.,P., Aircraft Design: A Conceptual Approach , AIAA Education Series, 1989. 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.

Abbott, I.,H., Von Doenhoff, A.,E., Stivers, L.,S., “Summary of Airfoil Data,” NACA Report 824, 1945.

Loftin, L., K., “Theoretical and Experimental Data for a Number of NACA 6A-Series Airfoil Sections,” NACA Report 903, 1948.

Drela, M., “XFOIL 6.9 User Guide,” MIT Aero & Astro, 2001.

Green, Bradford, “An Approach to the Constrained Design of Natural Laminar Flow Airfoils,” NASA Contractor Report No. 201686, 1997.

Lutz, Th.,”Airfoil Design and Optimization”, Institute of Aerodynamics and Gas Dynamics, University of Stuttgart, 2000.

Garrison, P., “The Shape of Wings to Come,” Flying Magazine, November 1984.

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, 1984.

Fujino, M., Yoshizaki, Y., Kawamura, Y., “Natural-Laminar-Flow Airfoil Development for the Honda Jet,” AIAA 2003-2530, 2003.

McCormick, B.,W., Aerodynamics, Aeronautics and Flight Mechanics , 2 nd Edition, John Wiley & Sons, New York, 1995.

“ Applied Aerodynamic Drag Reduction Short Course,” University of Kansas Division of Continuing Education, Williamsburg, VA 1990.

Smith, A.,M.,O., “High-Lift Aerodynamics,” Journal of Aircraft, Volume 12, Number 6, June 1975.

Schetz, J.,A., Boundary Layer Analysis, Prentice Hall, Upper Saddle River, New Jersey, 1993.

Cornish, J.,J., “A Summary of the Present State of the Art in Low Speed Aerodynamics,” Mississippi State University Aerophysics Department Internal Memorandum, 1963.

Raymer, D.,P., Aircraft Design: A Conceptual Approach , AIAA Education Series, 1989.

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.