From: tostan@dumbo.lerc.nasa.gov (Stan Mohler)
Subject: Re: Frost accumulation
Organization: NASA Lewis Research Center
Date: Thu, 31 Dec 1992 16:16:12 GMT

In article <23DEC199213460702@venus.lerc.nasa.gov>, fskevin@venus.lerc.nasa.gov 
(Kevin R. Kirtley (SVER)) writes:

> In article <2566@usna.NAVY.MIL>, dfr@usna.navy.mil (PROF D. Rogers (EAS FAC)) 
writes...

> >You now have extreme roughness right at the leading edge.
> >Upon rotation for lift-off there is a very good possibility
> >that the airfoil and hence a good part of the wing will
> >stall. 
> 
> IMHO the wing will not stall.  The flow near the leading edge of the wing
> is dominated by inertial forces, i.e., inviscid. Such a flow is not strongly
> influenced by surface roughness.  ...  But a rough, even very
> rough leading edge will only cause the boundary layer to transition to
> turbulent earlier or if already turbulent, more turbulent.  The result may
> actually be beneficial since turbulent boundary layers can sustain stronger
> adverse pressure gradients than laminar or transitional boundary layers and so
> can inhibit boundary layer separation near the trailing edge.



[ Hi Kev! :-) ]

A recent paper by Frank T. Lynch at Douglas Aircraft, titled "The Adverse
Aerodynamic Impact of Very Small Leading-edge Ice (Roughness) Buildups on Wings
and Tails" will put some quantitative information into this picture.  Experi-
ments were carried out at Langley Research Center to see how badly wing 
performance suffered when sand-paper-type roughness was applied to the leading 
edges of wings.  It turned out that having roughness confined approximately to
the first 7 percent chord was about as bad as having it extend to the trailing
edge!  

Using the data in the paper, here's a quick prediction for a small plane with a 
5-foot chord wing.  Assume frost or ice with a roughness height of 0.3mm.  Most 
of the data in the paper is for Reynolds no. greater than 5*10^6.  For those
Reynold no's., CLmax would drop by 25%!  Stall angle would drop by about 5 
degrees!  The data is disturbing for large transport aircraft.  But we want to
examine a small plane.  What's the Reynolds no. for the small plane?  At
a takeoff speed of 60 knots, it would be about Re=3.6*10^6.  This happens to be 
below 5*10^6 where performance is not degraded nearly as badly.  However, the
small plane could still lose about 15 percent of its maximum lift.  I can't
predict stall for this Reynolds no. from the data. 

The point is, small roughness at the leading edge IS very dangerous, but
light planes get somewhat of a break because they are at lower Reynolds no's.
In my example, I assumed the roughness features were 0.3mm in size.  Make them 
larger (i.e., rougher) and performance suffers more.

The paper points out that if a plane's stall angle is decreased, the stall 
warning device may not give a warning, as it is designed to signal a 
predeterined angle of attack!

-- 
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Stan Mohler
NASA Lewis Research Center     	|    
Cleveland, Ohio  44135         	|     email: tostan@dumbo.lerc.nasa.gov 
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