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A DUCTED FAN IS NOT A SHROUDED PROPELLER |
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When is a ducted fan a shrouded propeller?
Defining this difference with numbers, it boils down to the blade density, which is the ratio between total blade area divided by the area produced by the shroud diameter or the propeller diameter in the case of an open propeller. A high-power open propeller may have a blade density of up to .4 . A ducted fan should always have a density of close to 1.0 or better for achieving an efficiency equal to an open propeller of the same power. My fan with the 40hp engine was more like a shrouded propeller. Some history. Ducted Fans have been a curiosity for the aviation community including air plane home builders and experimenters, for a long time. many
experimental planes have been build and flown, but non, except for one, ever went in production, as far as I know. It was a military trainer built by the German company Rhein Flugzeugbau. Did the Fan Trainer have a ducted fan? According to my above definition it did not. This fan had a three blade propeller running in a shroud. Blade density was approximately .2 . The performance was not up to expectations. The same is true for an air plane by the name of OMNI, built in Britain. It had a 3 blade Hoffmann propeller in the shroud. In addition, both fans had a variable pitch propeller, which is totally useless in a true ducted fan. |
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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.
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unicorn@aic-fl.com |
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This company joined in a bid in the 80s for the US Air Force Next Generation Trainer, with LTV in Dallas. The picture shows
their design.
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 t
wist 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. |
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[default] [GYRO] [DUCTED FAN] [PRAGMA] [WANKEL] [UNICORN PLACE] |