Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Tue, 08 Oct 2002 15:23:36 -0400
Message-ID: <redirect-1800496@logan.com>
From: StarAerospace@aol.com
Subject: Cooling and Stagnation
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In a message dated 10/5/2002 8:16:10 AM Pacific Daylight Time, 
glcasey@adelphia.net writes:

<< A properly designed inlet doesn't reduce efficiency by much if it
 is too large as the pressure in front of it will just push air around the
 sides and if they are generously radiused there will be very little
 associated drag.  The Lancair cowl is a good example of this.  One of the
 fastest piston engined planes built at the time was the Corsair and look at
 the huge cooling inlet.  The cowling was radiused to provide for a smooth
 flow around the outside and that is what mattered.  The prop hub was then in
 a relatively stagnant area and thus didn't need a spinner. >>

Whoaaaaa, let's be VERY careful of the assumption that stagnation does not 
create drag.

First the theoretical side.  Take a flat plate of plywood and force it to fly 
normal to the air.  Half of the drag can be quantified by separation of the 
airflow on the back side of the plywood.  The other half (the so-called 
"pressure drag") is caused by the stagnation of the air forward of the 
plywood.  In this area, air at a given p and v has to give up that v and 
create p.  

On to practical drag reduction.  On an aircraft, oil streak and tuft testing 
can reveal areas of stagnation and separation.  Properly designed aerodynamic 
improvements can clean these areas up and enable attached air flow aft and 
decrease stagnation forward.  But how much drag?  How do we quantify the 
improvement and how does that relate to the theory?

On three radically different configurations we have quantified this.  In 
finding the separation area and curing it, we were able to account to less 
than 0.5% of the total drag by simply looking forward from the tail at areas 
of separation (picture this as the reverse perspective of "frontal area") and 
calculating the square footage of flat plate drag equivalent as half of the 
square feet of separated cross sectional area;  then subtracting the wetted 
area drag added by the fairings considering the boundary layer to be fully 
turbulent at the test Rn.  Worked perfectly.

On to stagnation.  Other areas exhibited stagnated flow and in some cases 
reversed flow.  Theses areas were quantified as equivalent flat plate drag 
area by looking at their cross section from the front and dividing by 2.  
Fairings were added and the wetted area drag of the fairings subtracted from 
the calculated drag improvement.  Partially laminar boundary layers existed 
in some areas and we had to also add in some extra drag equivalent on the 
baseline to account for reversed flow (even worse than stagnation, this is 
when the air actually turns 180 degrees and goes forward on the aircraft).  
After all this, the drag reduction from reduction of stagnation was still 
greater than we could directly account for.  What else happened?

Turns out that since stagnation disrupts airflow forward on the airframe, it 
is capable of really messing things up downstream as well.  Some downstream 
separation, vortex flows, and disruptions of spanwise flow creating 
discontinuities in the spanwise lift distribution (higher induced drag) that 
were all improved by reducing stagnation, accounted for the extra drag 
reduction.

Be very wary of oversize inlets.  On two aircraft that I am now working on, 
they create other problems that have increased the total airframe drag by 
over 10%.  And this is with radii and fillets that anyone would consider 
generous.  As for big cowlings and little spinners making no difference, I 
would point to the large number of Unlimited air racers that have achieved 
large drag reductions through the use of highly oversized spinners vs. their 
stock configurations.

Eric Ahlstrom