Bulent Aliev wrote:

From: TSParker99@aol.com
Reply-To: TSParker99@aol.com
Date: Wed, 09 Oct 2002 23:35:46 -0400
To: cozy_builders@canard.com
Subject: Re: COZY: Re: Heating System

When I became interested in the Cozy, one of the first things I investigated
was the cooling aspect of a pusher aircraft.  My investigations into this
via the NACA report server led me to read a significant number of reports on
the naca scoop design.  I will relate of few of the things I distilled from
the research.

1. The drag from a NACA scoop is actually higher than a straight sided scoop
of equal area.

2. The pressure recovery of the NACA scoop is better than any other flush
mounted scoop.

3. The drag on the NACA scoop when the inflow and outflow are not properly
balanced is lower over a wider range than with other scoops especially when
compared to ram scoops.

4. assuming that you have an efficient internal path with low losses, the
net drag from a NACA scoop can be lower than other scoops.

5. NACA scoops work best with sharp edges, actually pressure recovery is the
highest with small fences about midway along the profile, but there is a
drag penalty.

6. NACA scoops are very very sensitive to boundary layer thickness.  The
NACA reports emphasize this fact.

7. The approach angle of the scoop can vary between 5 and 15 degrees with
little effect, but optimal is around 7 degrees.

8. The angle of attack and shape of the scoop entrance can greatly change
the pressure recovery in the scoop.

9. Improper diffusion of the flow within the scoop can easily drop the
efficiency of the airflow (momentum loss), but will not greatly influence
the pressure recovery.

REFERENCES:
NACA technical note 2323 August 1951
NACA ACR No. 5120  October 1945
NACA technical note 3125 April 1958
NACA RM No. A7I30 January 1948

In addition to this work, I have been researching exit designs to minimize
the drag and increase momentum recovery.  This is a work in progress,
however, the research to date cleary suggests that the cooling exit as
designed on the Cozy is not in the best location for reduction of drag or
pressure recovery.  The research suggests that louvers or rectangular rear
facing ramps with appropriate loss reducing internal flow paths with the
exits opening on the wing roots, fuselage, or cowling as far upstream from
the the engine as possible would reduce the drag and increase the momentum
recovery.  This path would also allow the the cowling to be moved further
from the propellor which would have it's own benefits.

At some later time I will send my views on the lower cowling shape which I
think also needs some attention.

I hope the information above is helpful and if you want to verify my
statements I have given you the references to look for on the NACA server.

Marc, The turbulators or VG's or whatever you want to call them are the
right answer for thinning the boundary layer.

Of course, if you make any of the changes I have suggested you may not be
flying a Cozy MkIV any more.  ;-)

Todd Parker

I will now step down off my soap box.

This is useless for our liquid cooled purposes. 
This says absolutely nothing about using a NACA duct to feed
a radiator.  The NACA engineers  recommend that you don't use
them for heat exchangers.

Furthermore there are no numbers stated here. Without numbers statements
like this are meaningless.

Example:

1. The drag from a NACA scoop is actually higher than a straight sided scoop
of equal area.

How much higher? 1%, 10% or 50%?

I do commend this person for sending the paper reference numbers.

Paul Lamar
 
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