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

Jets are not constant thrust devices. it is just that their working area is
so small, and the delta V
so high that they do not drop off with forward speed as fast as a classic
prop. Fans are also not
constant shaft power devices, the input power varies related to the amount
of pump work.
It is possible that your divergent area does add some mass flow by ejector
action on the
clearance between your fan tips and the duct inner wall, The big increase
with the duct
is at slow/near static operation where they pump down a lifting surface (the
inlet bell)
This advantage goes away pretty rapidly with forward speed. I think you
would improve
the overall peformance by terminating the duct very shortly behind the prop,
and permit
free expansion instead of forcing the expansion which is only optimum at
single,
fairly narrow point. As you indicated this might require a different size
prop to
match engine power. So far as I know, there is no specific best point for
jet operation
(they stink everywhere, which is why they are almost extinct) - Thrust is MV
power is MVsqared/2 which is why for max thust per hp - move a lot of mass
a little bit for best use of hp - props are only 100 percent efficient with
mass
infinate and delta V zero. For most EAA class airplanes ducts are a waste of
time
unless you want to look like a jet. The design goal of the fanjet trainer
was to acquaint
student pilots with the disadvantages of the jets at lower cost. - Vance

The AirCraft Rotary Engine NewsLetter.  Powered by Linux.
ACRE NL web site. http://home.earthlink.net/~rotaryeng/            
Copyright 1998-2003 All world wide rights reserved.