[aerohead]

The topic of 'laminar' aircraft surfaces from time to time as pertaining to EcoModding and I thought it deserved its own thread.
I will let the guys with the PhDs speak for themselves as is practical,and will list sources for comments I make.
By numbering we may make quick references to a particular passage if need be.
(1) Aircraft aerodynamic drag is ruled by surface friction drag and designers are obsessed with laminar flow.Aerodynamacists engaged primarily in the design of the aircraft don't even concern themselves with flow separation as flow attachment is a premise of the computational tools they employ.(Theory of Wing Sections,Abbott and von Doenhoff,Ch.13.10.Pg.534,also Fluid Mechanics with Engineering Applications,Daugherty and Franzini,Pg.291.)

(2)Road vehicle aerodynamic drag is governed by flow separation and its attendent pressure drag.Surface drag is virtually insignificant.Above 4-MPH ( 7 Km/h ) a laminar boundary layer cannot exist and no amount of smoothing or polishing can improve the surface drag situation,except by limiting the wetted area to the 'crossover' point as is governed under the rules of streamlining.( Daugherty et al,CH.10,Example 10.2.pg. 288/Elementary Fluid Mechanics,Rouse,pg.248-249/Hucho pg. 54,215,and 535.)

(3) In aircraft,a laminar boundary layer is beneficial.In road vehicles a laminar boundary layer would be a detriment as a turbulent boundary layer can better maintain flow attachment against an adverse pressure gradient,reducing the all-important wake.Consider golf ball dimpling,dimple tape on aircraft,and turbulators ( vortex-generators ).( Daugherty pg.294/Rouse CH.8,Pg.234,246,and PLATE XVI.)

(4) For both 'laminar' and turbulent' boundary layers the flow outside these in the free stream 'inviscid'/'ideal fluid' layers is laminar,unless 'stall' or 'separation' is occurring.( Daugherty,pg. 280.)

(5) 'Laminar' aircraft are designed for 'flight conditions' and 'flight values of Reynolds number' far away from the ground with free air above and below,lower air density,and freedom from turbulence.( Daugherty pg.541 TABLE A.3a,IACO standard atmosphere.)

(6) "the flow around actual road vehicles is mainly turbulent.... random eddy fluctuations..... unsteady macro-structures which may be present." ( Hucho,CH.13.9,pg. 528,pg. 534.)

(7) "seemingly insignificant surface details can trigger major changes in the overall vehicle flow field." ( Hucho,Ch.13,pg. 529.)

(8) In ground proximity the transition from laminar to turbulent boundary layer occurs at Reynolds number = 500,000,which say,for a Toyota Prius will take place below 4-MPH ( 7 km/h ).( Daugherty/Hucho ).

(9) Describing an aircraft as 'laminar' can be a misnomer,as unless suction is provided along the chord,the laminar boundary layer will only last up to approximately the first location of minimum pressure,separate,then re-attach at flight Reynolds numbers as a turbulent boundary layer extending to the trailing edge.
*********** "if the airstream is turbulent..... transition from laminar to turbulent flow may occur anywhere upstream of the calculated laminar separation point." ( Abbott et al ).

(10) Abbott et al: Ch.1 pg. 28,"APPLICABILITY of SECTION DATA"
----- "simplifying assumptions"
---- " Strict compliance with this assumption would require 2-dimensional flow"( road vehicles operate within 3-D flow,hence the Navier-Stokes equations of 3-D in spherical coordinate system to solve,especially their wakes ) ".....no lift along the span..... no crossflows in the boundary layer.... (otherwise) wing characteristics may depart seriously from the calculated ones........... conditions are obviously not satisfied near the wing tips,near the extremities of partial span flaps or deflected ailerons,near cutouts and large fillets,or if the wing is partially stalled.Since the assumed conditions are not satisfied near the wing tips,it is obvious that section data are not applicable to wings of low aspect ratio.In fact,an entirely different theory applies to wings of very low aspect ratio."

(11) Abbott cites Reference#57,Jones,Robert T.: Properties of Low-aspect Ratio Pointed Wings at Speeds below and above the Speed of Sound,NACA TN No. 1032,1946.).

(12) Example:
Aerospace engineer,Matt Van Leeuwan designed his 3-wheeled "Sylph" around the architecture of a NACA 66-Series airfoil of 6% thickness.
From Abbott,this wing section demonstrates a drag minimum of Cd 0.003 'clean',and Cd 0.0085'rough'.
NOTE: The airfoil Cd is based on surface drag of the wings planform area ( both sides ),a function of chord length and a given span.As Patrick was kind enough to point out,to convert to frontal area Cd as used in road vehicle aerodynamics,the surface drag coefficient is multiplied by the inverse of the airfoils thickness percentage,which in the case of Sylph would be 1/0.06=16.666 X 0.003 = Cd 0.05.
Section thickness for this airfoil occurs at 45% of chord behind the leading edge.
Allowing the shoulder room of a Piper Cub,Sylph would require a length of over 33-feet to respect the NACA thickness ratio.
Also,as Abbott points out,Sylph might embody two 'wingtips' of the 66-Series section joined together but it can never satisfy the criteria for section data "APPLICABILITY".
Additionally,the Sylph is operating in ground proximity,which Jaray found to double a body's drag,as well as introducing turbulence effects as mentioned by Hucho.
Consequently,Sylph registered only Cd 0.103 at California Institute of Technology's Guggenheim Aeronautical Laboratory wind tunnel ( a 206 % drag increase over the 66-Series section at flight value ).

(13) I hope the above discussion will illustrate that while 'laminar' aircraft are to be highly desired as aircraft,their attributes may offer little when in the context of a road vehicle operating in ground proximity,in turbulent air,and 3-Dimensional flow; none of the parameters for which aircraft designs are executed,nor in which their efficiency relies.