With the 38in dia at 5500 the tip speed is a tad above 900 fps, a

bit

high

for a typical thick wood prop but modest for metal or composite

sections.

A search on net suggests that at least at one time the soob was

running

about
50 inch dia and 60 in pitch - looking for 4000 where the Soob

shoud be

just

a tad over 100hp. I was suprised to hear that your exit area is

greater than

the
fan area. This means that you are diffusing the flow velocity down

before

exit. This cannot be good. Treating your whole plane as a controll

volume,

thrust is the total air weght flow times the difference between

entrance

velocity and exit velocity, so diffusing that exit flow is thrust

loss.

Only sonic throat flow (ie rockets and turbojets) benifits from a
diverging exit. Vance

Basically the reason it works Vance is it increases mass flow. It is
more efficient to have high mass flow and low delta-v, than to have

high

delta-v and low mass flow. The fan is truly working in a 200 mph
environment, while the aircraft is in a 160 mph environment.

Perry

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

I don't think it is 200 MPH at the fan. I think the advance ratio is

beyond

one. BTW Vance does your prop program optimize blade area. Perhaps you
could run some numbers for Perry on an optimum fan blade width and pitch
for his duct?

Perry, You could check the fan exit velocity with a pito tube back there.
Better yet a velocity rake.

Paul Lamar

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

 
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