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From: scw@ucla-cs.UUCP
Newsgroups: net.aviation
Subject: Re: High lift vs. laminar air foils
Message-ID: <6537@ucla-cs.ARPA>
Date: Thu, 8-Aug-85 12:05:24 EDT
Article-I.D.: ucla-cs.6537
Posted: Thu Aug  8 12:05:24 1985
Date-Received: Tue, 13-Aug-85 02:24:30 EDT
References: <3460@decwrl.UUCP>
Reply-To: scw@ucla-cs.UUCP (Stephen C. Woods)
Organization: UCLA Computer Science Department
Lines: 53

In article <3460@decwrl.UUCP> savage@rainbo.DEC (Dennis DTN 282-2614) writes:
>
>One of the replies came [...] uses a high lift airfoil and the others use
>laminar airfoils.

>My question here is perhaps silly but I thought all airfoils had to rely on
>a laminar flow of air across the upper surface to fly.

This is true,but not silly (see below).

>What is the difference here... and why would anyone want to have a wing that
>would let them drop like a rock?

The difference between a 'laminar' flow wing and a 'high lift' wing is simple
a 'laminar flow' (LF) wing has the thickest point well back (usually in the
40-50% zone), a normal airfoil typically has the thickest point near the 30%
zone (25-35 %).  The net effect of having the thickest point 'far' back
on the wing is that the wing has laminar flow over more of the wing
surface. This produces a dip in the l/d curve (called a bucket). (pardon
the poor graphics):



   Regular		    'Laminar'

.                   .  .                   .  
 .                 .    .                 .   
  .               .      .               .    
   .            .         .            .      
     .       ..             .       ..        
       ......                 ..  ..          
                                ..            

                                ^^
			    Bucket

One of the first production aircraft to take advantage of this was the
P-51 (Mustang) with it's phenomenal range.  A major DISadvantage of this
type of airfoil is its poor to nasty stall charisterics (some can really
get quite harry as the thick part moves aft), another weak point is a
requirement that the surface of the arifoil must be *VERY* smooth  (no
bugs) or you loose the 'bucket' (while retaining the nasty stall), also
some of these airfoils have poor pitch moments (pitch moment is the
tendancy of the airfoil to pitch (up or down) as the angle of attack
changes, large abs(pitch_moment) are a NO-NO). Moderate (40-45%) LF
airfoils have all of these features in moderate amounts.

For a full (detailed) discussion of this there is a nifty little (well
not so little) book called ,as nearly as I can remember, 'The Theory of
Wing Sections' (or words to that effect) which should be available in
any Eng. Library, and most Public Libraries, it's also available from
the SSA and probably from the EAA.