Perry, Have you seen this web site?
Paul Lamar
http://www.gdsys.net/WWWmembers/unicorn/DUCTED_FAN/ducted_fan.htm
-- The UNICORN fan.
When I decided to design and build a ducted fan for the gyro
copter, I knew that I had to do better then existing fan designs to
make the gyro fly with 40hp. The first problem I ran into was the
airfoil for the blades. I was unable to match a propeller airfoil, as
recommended in the books, with the flow condition in the fan.
The answer I found in high performance industrial fans and the
confirmation in turbojet books. The airfoil should be very thin
and have a high camber. The major difference to a propeller, is
the much higher velocity increase over the blade. If using a prop
airfoil, it would be necessary to use two rotors with progressive
pitch. Secondly, the shroud has
to be long enough to prevent an outside vortex at no forward
speed. A one third diameter rule is a good start, however, the
power level is also to be considered. I found that the air on the
outside surface of the shroud is flowing forward from 10? behind
the inlet. I was glad that the shroud is 12? long. The 10? is 38% of
the 26? diameter. No book I read said anything about that.
Having a rough feeling about the required blade area, I realized
that I had to provide for more then three blades if I wanted
flexibility for experimenting. I settled on a maximum of 9 blades,
pitch ground adjustable. This gives me the option of using 3/6/9
blades depending on the engine size. Structurally, this design was
limited to 3700 RPM, which determined the total blade are.
Performance was not overwhelming, but it was sufficient to do
the flight testing and some learning, as far as this particular gyro
was concerned. With all the flight testing done in 1999, I was
ready to look into putting a bigger engine to the fan. The first
thing I did, was to convert the 40 hp setup to direct drive with
6500RPM max. This required, new blades for strength and power
match. What a disappointment. Thrust efficiency went down by
20%. and the noise level went up to a point where it was
unbearable. Conclusion? The essentially lower
total blade area used compared to the first, slow running
configuration resulted in a blade density low enough to cause a
measurable change. The blade shown on the
right in the picture was
used in the first, slow
running configuration, it
was molded fiberglass in
epoxy. The one on the left
is the new configuration in
carbon fiber for up to
7500RPM on a 8? hub.
These blades do not have
any twist, because of the
low air speed the fan is
used at.
Blade twist is a very peculiar issue in a fan. Because of the high
inherent velocity gradient over the fan diameter, a twist is not
applicable until really high forward airspeed is to be considered. I
do not know at this time what this speed may be, and I have not
been able to locate any flow field analysis answering the
questions. This issue is one of the first items to be investigated
with the new fan configuration. This investigation will also give
information on the airspeed at which a high speed shroud would
become an advantage. The only piece of data I have obtained
from my tests so far, is that at static running, the air flow on the
outside surface of the shroud is forward, as mentioned above.
With increasing airspeed, this point is moving forward until it
reaches the inlet lip. at this point, a high speed shroud becomes
essential.
The next configuration fan will be a 9-Blade rotor direct driven
by a ROTAX 583 engine. The shroud is the same as the one used
on the previous configuration.
The ultimate goal is a high speed fan competing with a same
power propeller in the same type of air plane.
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