Laminar Flow

1
Laminar Flow

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

2
Introduction

• Why laminar flow?
• Less skin friction Lower drag

3
Natural Laminar Flow

• NACA 6-Series Airfoils
• Developed by conformal transformations, 30 50
  laminar flow
• Advantages Low drag over small operating range,
  high Clmax
• 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 Cl range
• Minimum pressure at transition location

NACA Report No. 824
4
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
5
Natural Laminar Flow
XFOIL

• NACA 64-012 xtrupper 0.5932, xtrlower 0.5932
• NACA 64-012A xtrupper 0.6214, xtrlower 0.6215

6
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.
7
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 Clmax
• Docile stall characteristics
• Insensitivity to LE contamination

Fujino et al, Natural-Laminar-Flow Airfoil
Development for the Honda Jet.
8
Natural Laminar Flow Case Study (Continued)

• Requirements
• Clmax 1.6 for Re 4.8x106, M 0.134
• Loss of Cl less than 7 due to contamination
• Cm gt -0.04 at Cl 0.38, Re 7.93x106, M 0.7
• Airfoil thickness 15
• MDD gt 0.70 at Cl 0.38
• Low drag at cruise

Fujino et al, Natural-Laminar-Flow Airfoil
Development for the Honda Jet.
9
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 Clmax and high speed characteristics
• Transition-location study
• Shock formation
• Drag divergence

Fujino et al, Natural-Laminar-Flow Airfoil
Development for the Honda Jet.
10
Natural Laminar Flow Case Study (Continued)

• Resulting SHM-1 airfoil
• Favorable pressure gradient to 42c upper
  surface, 63c lower surface
• Concave pressure recovery (compromise between
  Clmax, Cm, and MDD)
• LE such that at high a, transition near LE
  (roughness sensitivity)
• Short, shallow separation near TE for Cm

Fujino et al, Natural-Laminar-Flow Airfoil
Development for the Honda Jet.
11
Natural Laminar Flow Case Study (Continued)

• Specifications
• Clmax 1.66 for Re 4.8x106, M 0.134
• 5.6 loss in Clmax due to LE contamination (WT)
• Cm -0.03 at Cl 0.2, Re 16.7x106 (Flight)
• Cm -0.025 at Cl 0.4, Re 8x106 (TWT)
• MDD 0.718 at Cl 0.30 (TWT)
• MDD 0.707 at Cl 0.40 (TWT)
• Cd 0.0051 at Cl 0.26, Re 13.2x106 (TWT)
• Cd 0.0049 at Cl 0.35, Re 10.3x106 (WT)

Fujino et al, Natural-Laminar-Flow Airfoil
Development for the Honda Jet.
12
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
  CLmax to be achieved

Ref McCormick, Aerodynamics, Aeronautics and
Flight Mechanics, pg. 202.
13
Notable Laminar Flow Control Flight Test Programs
Date Aircraft Test Configuration LF Result Comments
1940 Douglas B-18 (NACA) 2-engine prop bomber NACA 35-215 10x17 wing glove section suction slots first 45 chord LF to 45 chord (LF to min Cp) RC 30x106 Engine/prop noise effected LF surface quality issues
1955 Vampire (RAE) single engine jet upper surface wing glove suction - porous surface full chord suction full chord LF M0.7 / RC30x106 Monel/Nylon cloth 0.007 perforations
1954- 1957 F-94 (Northrup/USAF) jet fighter NACA 63-213 upper surface wing glove suction 12, 69, 81 slots Full chord LF 0.6 lt M lt 0.7 RC 36x106 at Mlocalgt1.09 shocks caused loss of LF
1963-1965 X-21 (Northrup/USAF) jet bomber 30 sweep new LF wings for program suction through nearly full span slots both wings full chord LF RC 47x106 effects of sweep on LF encountered
1985-1986 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.
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Why Does LFC Reduces Drag?

• removes turbulent boundary layer

XFOIL output
15
Why Does LFC Reduce Drag?

• turbulent boundary layer has a higher skin
  friction coefficient

upper surface
lower surface
XFOIL Output
16
Why Does LFC Increases CLMAX?

• move boundary layer separation point aft

Ref A.M.O. Smith, High Lift Aerodynamics,
Journal of Aircraft, Vol. 12, No. 6, June 1975
17
Raspet Flight Research Laboratory Powered Lift
Aircraft

• Piper L-21 Super Cub (1954)
• distributed suction - perforated skins
• CLMAX 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
• CLMAX 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
18
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)
0.0025 dia

• 45 x 12 grid 439,470 holes
• Preq .00318 Hp / foot of span
• assumes
• use highest vw and ?p in calculation
• discharge coefficient of 0.5
• pump efficiency of 60

12 span
0.035
45 chord
19
Laminar Flow Control Approaches
2). Distributed Suction (perforated skin or
slots)
1). Leading Edge Protection
Ref Applied Aerodynamic Drag Reduction Short
Course Notes, .Williamsburg,VA 1990.
3). Hybrid Laminar Flow Control
20
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
21
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 ? 7.5x106
• Cruise CL ? 0.2

22
Drag Benefit of Laminar Flow
23
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

24
Centuria Design Details

• Cruise altitude 10,000ft
• Cruise Speed 185kts
• Range 770nm
• Take-off run 1575ft
• Aspect Ratio 9.0
• Wing Area 12.3m2/132.39ft2
• Thrust 2.877kN/647lbs
• MTOW 1360kg/2998lb
• Fuel Volume 773 litres/194 USG
• Stall Speed 68kts (Clean) 55kts (Flap)

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Drawing by Anne Ocheltree Nick Smalley
26
Calculating Laminar Flow
Wing Tail
60
100
Laminar
Turbulent
Fuselage
40
100
Laminar
Turbulent
27
V-Tail 60 LM flow upper and lower surface
Fuselage Laminar to max thickness
Wing 60 LM flow upper and lower surface
28
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29
Centuria NLF Manufacturing Tolerances
Rh,crit hcrit (in.)
900 0.0072 inches
1800 0.0143 inches
2700 0.0215 inches
15,000 0.1195 inches
Carmichaels waviness 0.0139
inch/inch criteria
h
?
Ref A.L. Braslow, Applied Aspects of
Laminar-Flow Technology, AIAA 1990
30
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

31
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, 2nd 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.