This excerpt from Hoerner confirms what the Long
Ranger (Jeff?) was illustrating with his hand drawn sketches.

Namely; if the external diffuser is over driven you
will have higher external drag. Hoerner has not yet mentioned
the term "external diffusion" but he does indeed deal with
it. The paragraph and figure 12 were in another part of the
book. Not in the section on ducted rads.

The illustration is pretty self explanatory.
The over driven part is the left side of the curve.
The drag jumps up 50% over that of a solid body!

The bottom line is; as Jeff says, we need some data on
the flow rates and heat transfer capabilties of
different radiators.

IMHO lacking that data we need flaps on both the inlet 
and the outlet.

PL

"Streamline Cowling. 

The engine nacelles and cowlings investigated in (10), 
represent modest aircraft installations as around 1937. 
Cowlings and nacelles have been developed since, so that 
they are suitable for higher speeds, having higher 
critical Mach numbers (l0, e) and (12,c). 

The arrangement in figure 12 can be considered as an 
extreme example, with the powerplant (whatever type it 
may be) assumed to be located near the maximum diameter 
of the streamline nacelle or fuselage shape. After 
subtracting the internal drag (which is not at issue in 
the investigation quoted), the external drag is 
obtained.  Both the streamline body without openings, 
and the ducted body at flow ratios (based on frontal 
area of the body) between 0.3 and 0.6, have some laminar 
flow along the forebody. The drag is equally increased 
in both conditions, after fixing transition near the 
nose by means of a carborundum strip. 

At any rate, the external or parasite drag caused by 
inlet and outlet of the cooling flow, is practically 
reduced to zero in these conditions. 

However, at small rates of flow and without stimulation 
of turbulence, the ducted configuration shows higher 
drag coefficients than the solid body. The inlet opening 
is evidently "over-flowing" in this case; and the flow 
around the rim of the opening produces boundary layer 
turbulence in a manner similar to that as caused by the 
carborundum strip. - As an alternative for the 
discharge through the rear end of the body, an annular 
outlet slot (at 63% of the body length) has also been 
investigated (see in figure 12). The external drag of 
this configuration is somewhat higher than with the tail 
outlet (Delta C sub da is between 0.005 and 0.010). 

With either outlet, the external drag coefficient (0.04 
to 0. 06) is considerably smaller than those of the 
old-type nacelle installations quoted later, in Chapter 
XIII, (having total drag coefficients between 0.15 and 0.
21)."


 
The Aircraft Rotary Engine Newsletter.              Powered by Linux.

http://home.earthlink.net/~rotaryeng/            http://www.linux.org