Alpha Computer Systems wrote:

Paul,
What's the story on the moveable teardrop in the airflow exit area?
Does it reduce drag by streamlining outflow?  Inquiring minds
(non-engineering types) want to know! ;-)

Jim Brewer
Albemarle, NC

When you don't need all the cooling capacity the plug moves backwards and
partially closes off the exit air. This causes the intake air flow to go
around the outside of the duct in what is known as external diffusion
thereby reducing the drag. It also speeds up what hot air is left
to provide a bit of thrust that overcomes some of the basic drag
of the cowling. Same idea with cowl flaps.

Paul Lamar

BTW come to think of it this diagram also explains why a thin rad in oblique
flow is better. The vertical axis on the left of the data plot is efficiency.

The flow velocity through the rad is w. The forward velocity of the airplane is V
so the ratio w/V is sort of like the mass air flow through the rad.
This is also proportional to the restriction the rad presents
to the air flow through it. Thicker rad, more restriction. Thinner
rad, less restriction. According to this data as the rad gets thinner 
the w/V ratio increases and the efficiency goes up. In other
words the velocity of the air flow through the rad gets closer to the
speed of the airplane. 

Now for a given rad volume or cooling capacity h * w * t the height and 
width must increase as the thickness is decreased to keep the volume
constant.

That would, with a perpendicular rad as shown here, increase the frontal
area of the cowl or duct. Obviously not a good idea aerodynamically
speaking. So we use a tilted thin rad with a small frontal area duct
or no separate duct at all or place the tilted rad inside the cowl.

The remaining challenge is to make the airflow through the tilted
rad uniform to maximize the heat transfer. That has been researched
by Kuchemann & Weber and London and others.

If all this were taken to its logical conclusion the entire
surface area of the airplane would be radiator and the velocity
ratio would be one and the drag minimal. That was proven not to be
practical back in the 30's and 40's.

I hope all this is clear.

Paul Lamar 
 
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