X-Mozilla-Keys:                                                                                 
Paul wrote:

fluiddy1.gif

I would like to comment on this figure lest someone gets
the idea the duct shape shown on the lower half of the
figure is ideal. Unfortunately it is not. The top,
most important part  of the drawing is absolutely correct.

After World War II improvements were discovered
in duct shapes for cooling radiators. Much of what Hoerner wrote
about was based on prewar and war work in Germany. A more modern
duct would be bell shaped with the rad located at the large
end of the duct.

A drawing of such a duct can be found on the ACRE NL web site
http://home.earthlink.net/~rotaryeng/  titled; "Excellent duct design
from "Aerodynamics of Propulsion, Kuchemann & Weber book chapter 12."

Paul Lamar

Fredmoreno@aol.com wrote:

Good observations on the Horner figure.  Also worth noting is that a modern
design would have an inlet on the leading edge of the scoop standing a bit
away from the fuselage to keep the boundary layer from getting ingested into
the diffuser where it causes separation and losses.  See the inlet of the
P-51.  The original XP-51 did not have such a boundary layer separator and
suffered problems in the diffuser.

 In order to estimate the distance that the lip should be located away from
the fuselage skin, figure (very roughly) that the boundary layer thickness
(distance from the surface to outer edge of the boundary layer) is roughly 1%
of the distance from the leading edge.  Thus if you are 100 inches back from
the spinner, make the inner edge of the scoop about 1 inch from the fuselage
skin.  You can crowd this a bit (3/4 inch?) but there is no benefit in going
out much farther.

Fred

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