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
Yes it is 200 mph air in the fan plane, I already calculated that in a
previous message as 38" pitch * 5500 RPM gives around 197 mph.
I have 3 datapoints:
1. duct2 divergent duct exit, flown for 460 hours in 5 years, performs
as well or better in takeoff/climb, 20mph cruise penalty vs.
conventional engine/prop (160 mph vs. 180 mph).
2. duct3, convergent duct exit, bogs engine down, no faster, reduced
takeoff performance (thus no reason to use instead of duct2)
3. unducted, terrible thrust, won't fly
You are the resident scientist here Vance. It's up to you to take real
world data (observations) and devise theory to fit the facts - not the
reverse, that is what us engineers do. My background is not aeronautics
and I don't have a clear idea why it works, but would like to know so I
can make some realistic predictions for improvements. I am presenting
real experience. I would also like to see you read Marc's book and write
a review of it here, since my experience is that it agrees with the real
world data, and so far your assertions do not.
Then once you have it figured out why it works you will be much better
at explaining it to the rest of us than I am. You like a challenge, correct?
FMI:
http://www.bridgingworlds.com/duct.htm
http://www.bridgingworlds.com/fan.htm
http://www89.pair.com/techinfo/MassFlow/ductbook.htm
Perry
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