I am very familiar with the relationship of mass flow and velocity
in the momentum equation - however, the divergent section cannot
increase the mass flow (basic continuity). If the velocity at the
fan plane is 200 mph then it would seem that terminating the duct
at that point would be the best answer  - you would have that
mass flow at the higher exit velocity. - Vance

Vance,

The problem is the fan performance curve. You are dealing with a shaft power
(constant HP) problem. Unlike a jet (constant thrust).  If you wish to
discharge the same mass flow rate at 200 mph, you will have to raise the
pressure ratio on the fan. This will increase power required. To maintain
the same power absorption and get a 200 mph discharge would require reducing
mass flow. No way around this. It is possible to increase thrust by reducing
the discharge speed as long as it moves the fan down the curve to a place
where it flows more air while consuming the same power. This works
theoretically until the discharge speed is the same as the aircraft speed
Ve-Va>1.  If you increase mass flow while decreasing discharge speed you
increase propulsion efficiency (turbo-jet vs. turbo-prop). For jets, the
most efficient cruise is found for Ve=2Va (for constant mass flow). My
prediction is a shaft powered ducted fan is similar to a prop in that the
propulsion efficiency is higher for greater mass flow and smaller difference
between Ve and Va.

Monty

I was going to include this on my last message but forgot. An
unscientific layman's explanation for how it works:

http://www.bridgingworlds.com/raspet.htm

Perry

 
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