Direct Drive Fans

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HeliDev

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This quetion is maninly to orion, however anyone else feel free to post.
In the thread on re-drives in the auto conversion section of the forum, orion mentioned that a 40 inch fan, without ducts or shrouds would make a good cantidate for an aircraft without a re-drive.
Could you expand on this a little further?
Why no duct or shroud? Are you of the opinion that the reduction of the tip losses are outweighed by the drag of the duct or shroud?
In a earlier post on the same thread mention was made reguarding prop efficiency in relation to diameter. You responded that this isnt nessecarily true. That if properly designed, the diameter is not such a big issue.
I have been toying with smaller diameter drives (RC), with larger chords, and getting reasonable results so far. Is this the sort of idea you were thinking of?
If you could also expand on this area as well I would appreciate it.
Sorry for the 30 quetions, its just that I have been looking into this are for a while, and everyone that I speak to thinks Im nuts.
 

orion

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The fan I mentioned would be a good candidate however it is important to remember that the design should ideally be matched to the particular installation. As such, you would be defining a fan and an airframe within the same design effort.

As far as the propulsor configuration is concerned, first we should define the difference between the ducted fan and a shrouded prop. I've done this in another thread within this board but just real quick, a shrouded prop is anything that has a shroud that is about half the prop's diameter or less in chord length. However when looking at specific designs, ducted fan applications are those that ususally try to immitate a jet fighter configuration of some sort and thus bury the fan within the fuselage.

For the sake of this thread, I'll ignore the latter as it is driven by other requirements and requires relatively high power levels.

Most of the efficiency gained in the use of a shroud evaporates by the time you hit about 120 mph. Doing a specific point design may improve the range of operation but I tend to use this number as a rule of thumb. By the time you reach that particular speed the surface area of the shroud becomes a larger factor than the efficiency gained by sealing the tips of the prop blades.

There is a bit of further efficiency you can gain by installing flow straighteners in the shroud, thus removing the vorticity generated by the prop and converting all the flow into straight thrust. However this too works best at lower speeds and the added wetted are becomes a penalty at higher speeds so one must be able to optimize for the goals of the design.

NASA has done some pretty good work with unducted fans, the only probelm is that I've been having a hard time finding the papers that resulted from the work. Several years ago there was an article published in Av Week that showed a NASA development program where they built a small (36" dia) multi-bladed fan that was installed on a 700 hp turbine. In the test phase of the program they were able to determine that the fan was able to operate nearly to Mach .8 at nearly 80% efficiency.

My then partner, who is much more of an aero propulsion expert than I am, indicated that the database could be used to generate a family of fans capable of providing excellent performance for virtually the entire gamut of general aviation applications. I however was never able to find the reports dealing with the specific NASA development so at the time I gave up and figured I could get to it another day.

Regarding issues of diameter and such: Designing a prop requires dealing with numerous variables, all of which can be massaged, within reason, to meet a wide variety of needs. As an example, several years ago we worked on a prop that was to go into service for a gentleman who flew competition SuperCubs. His best performance was based on a huge 92" dia. fixed pitch prop. At the time he was looking to increase the performance at low speed.

We designed a new prop for him that was substantially shorter (~75" if I recall right) but was able to deliver about 10% more thrust on the low end and about four to five percent better for cruise. The characteristics of the new prop were a combination of new sections, a wider chord and a different twist distribution.

The opinions of those that think you are nuts is based on conventional thinking, which does base much of the efficiency on the prop's diameter and rpm. This reasoning is anchored on the standard database of propeller design, which today covers virtually all of the off-the-shelf props you can buy for your airplane application. However most of that database was developed fifty to sixty years ago and for the most part, was never really updated to account for newer modes of analysis, data, powerplant and aircraft type.

I've been proposing a direct drive powered fan unit for general aviation application for years however since there are really no aircraft that could utilize such a configuration effectively, the propulsor package would have to be combined with a new airframe development effort. This does not need to be expensive but it would require a finite investment for it to become a reality. The key to the technology then is to find the right person with the interest and the necessary funding capacity.
 

HeliDev

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Thanx orion, bit busy now, I got a couple more questions, Ill get back to you.
 

pylon500

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[COLOR]I know there are some operational UnDucted Fans out there that appear to be efficient, I thought the main problem was with noise?
Arthur[/COLOR]
 

orion

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There are no operational unducted fans in any type of regular service that I know of. There has been quite a bit of work done by NASA within the past ten or fifteen years, along with companies such as Boeing, Mc Donnel Douglas, and others. The Russians have also been involved in this type of development however as of late, most of their work seems to be concentrated on the larger, counter-rotating prop/fan designs for the applications to large commercial transports.

As far as I know, there has been no work in this field for application to the general aviation sector. There have been a few developments of smaller, higher-turning propellers for point designs and racing application, but no actual unducted fans.

My company's Arrow development hopes to develop this application however, at this point we have not located a firm funding commitment. There has been actually quite a bit of interest in this airplane but most folks are interested in a finished, ready to assemble kit, not the actual research and development. Hopefully next year.

Regarding noise, you are right - the high rpm fans do generate a bit more noise than a conventional, slower turning prop. This however is dependent on several factors and is, as one might expect, a function of power, rpm and size. At this point, I don't think that the fan will be too different from a prop for the general aviation application (probably within 5 to 10 db), but I will admit that right now this is a guesstimate.
 
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