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Thread: Ducted Fan propulsion

  1. #1
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    Ducted Fan propulsion

    Well, since some of you provided such thoughtful feedback on the question of fuel injection, I may as well put this one to you as well:

    Why do ducted fan configurations seem to be entirely absent from LSA's and homebuilt's? Without having actually tried constructing such a propulsion system, I wouldn't imagine that cowling the fan with reasonable precision would be exceedingly difficult. Also, the challenges of weight and reliability of the cowling and drive mechanisms should be feasible too, given the indirect-drive configurations of many motorgliders.

    The disadvantages that come to mind are potential fan balancing complications, and a drive mechanism that can handle the higher rpm's of ducted fans.

    The benefits, aside from efficiency, would be smaller diameters and greater safety (given enclosure).

    Just curious, really.

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    Registered User bmcj's Avatar
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    Re: Ducted Fan propulsion

    Hi YC,

    If you do a search, you'll find that this has been discussed on this forum in the past. If I recall correctly, I think that the bottom line was that, in addition to being difficult to build a precise, structurally sound duct, a duct may benefit you for a static thrust design such as a hovercraft, but is not as efficient as a well designed unducted propeller for general aircraft use. There is also a distinction between a true ducted fan (with a long duct) versus a shrouded fan with a very short ring around the prop to control tip losses. In the case of hovercrafts, you will find that they use the shrouded fan. Long ducts almost always come with great penalties and very little gain.

    Bruce

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    Registered User Lucrum's Avatar
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    Re: Ducted Fan propulsion

    I'm sure there are other threads but here is one of them.

    http://www.homebuiltairplanes.com/fo...-builders.html

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    Re: Ducted Fan propulsion

    I once considered this system only briefly. It struck me that there are far too many failure points for catastrophe. It was the close tolerances for proper effect combined with a structure that was safe and didn't restrict the shroud. Way out of my league and tooling capabilities.

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    Re: Ducted Fan propulsion

    Probably it's too finicky, but this guy Goldschmied came up with some neat tricks for simultaneous propulsion and flow control on axisymmetric bodies. Perhaps these could have some application to aircraft. See following URL:

    CAFE Foundation & The GAT Challenge

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    Re: Ducted Fan propulsion



    Mick Duckt N7XR

    Most of the detail is down the bottom, now does better with a regular prop.

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    Re: Ducted Fan propulsion

    I think the reason that Varieze was too slow is because the drag caused by the prop shroud wasn't put to any use. By putting it to use I mean optimizing the other surfaces to account for the large increase in tail volume added by the duct. For example it probably doesn't need any vertical tails due to the side area of the duct, the rear wing could be made quite a bit smaller due to the added horizontal area (times 2) of the duct, and the landing gear could have been shortened. After doing that sort of thing the plane would probably have about the same overall drag as a standard version.

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    Re: Ducted Fan propulsion

    Jim Miller had a shroud on his formula racer (aka, Miller Special, Gem & Pushy Galore), but it was later removed and replaced with a small elevator atop the rudder. Not sure why, but might be worth looking into.

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    Re: Ducted Fan propulsion

    It wasn't giving him the stability and control that he expected. He changed it to 'conventional' fin and tail (if you can call it that, on his latter airplanes!) to improve the stability and control effectiveness.
    "If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them." - Henry James Thoreau

    Design Project: Conceptual Design of an "Inexpensive" Single-Seat Motorglider
    Discussion Thread for the Project: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

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    Re: Ducted Fan propulsion

    It also went faster without the shroud. Several other pushers were built and flown at Reno based on this design. None had ducted props. One was destroyed in a fatal accident when a propeller failure took off the tail. I saw that happen. Very sad.

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    Re: Ducted Fan propulsion

    Well obviously, they're doing it all wrong They're using props and comparing them to props when they could be using fans and comparing them to jets

    If you, for example, were to take the fan off a modern turbofan engine used for something like a Learjet and power that fan with a 1000 HP engine then you've got the same thrust as a single Lear jet turbofan because they make about 1000 HP (My info could be 20 years outdated) I think you can get better fuel efficiency with a piston engine making 1000HP than a turbojet core doing the same thing.

    Soo, with one of those you could go high subsonic in the right plane (hey, I just realized, I'm designing the right plane now =)

    But yes, good info, but bad news, about the prop failure taking the tail off, something for me to think about and design for, thanks.

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    Re: Ducted Fan propulsion

    In these discussions it is important to discern between a shrouded propeller and a ducted fan. There is a vast difference between these two ideas however the distinction between them tends to be a bit gray. For the purpose of a general discussion a shrouded prop is generally defined as a shroud configuration whose chord is one half the prop diameter long. Furthermore, a shroud provides not only a sealing surface for the prop tips but if shaped properly, it can also produce a surprising amount of thrust, especially near the lower end of the speed spectrum.

    A ducted fan on the other hand is generally characterized by an inlet duct, a working section and an exhaust section. Some can be quite short but others are designed to be look-alikes to a jet engine installation, most likely to be buried in a fuselage. This design is configured primarily for the fan but a very slight flow improvement can be gained through proper shaping of the exhaust, although this must be done very carefully since it does tend to be much more a point design than any other alternative.

    Ducted fans do have a tendency to have substantial penalties since the small diameter fans do tend to be rather inefficient and the added wetted area of the ducting can introduce sizable friction losses to the whole. The classic design case (back in college) is where the goal is to configure a Cessna 150 type airplane but one that is designed about a ducted fan propulsor. The goal of the exercise is to determine the amount of horsepower needed in order to provide it with identical performance to that of the small Cessna. The answer is always surprising since it turns out that the ducted fan configuration needs almost three times as much horsepower as the conventionally powered Cessna in order to achieve the same speed and climb (although these two values can be traded a bit, one against the other).

    And it turns out when you look at power to weight ratios and use the proper scaling factors for the modern RC aircraft, you'll find that the ratios for the small planes do tend to be even higher. But RC engine technology has come far so the specialized two-strokes can deliver the high power levels the scale planes need for the performance. Too bad we can't make reliable two strokes of this size for the big toys - but then that would really suck down the fuel - probably close to that of a more conventional jet.
    "To live is to learn; to learn is to live" (author unknown)

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    Re: Ducted Fan propulsion

    I think the high power ducted fan would only be advantageous for going over 400mph since the ability of open props falls drastically at that point.

    There are fairly high powered two stokes available. The Kawasaki H2 750cc could put out a lot of power and have done less than eight second quarter miles on a bike. You could chain gang three or four of them together to get 1000hp. It might be good for setting records, but wouldn't go far.

    Since were on this subject, has anyone seen any of these Learjet size fans for sale cheap, like maybe one that came off a broken engine? Or say, how much for a broken Lear turbofan engine but with a good fan on it?

    edit. I think the best way for something like that would be an in fuselage duct that is optimized to slow the air down at the fan and then speed it up at the exhaust.

    I wonder if you could add and afterburner to something like that?
    Last edited by Starman; September 13th, 2009 at 12:49 AM.

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    Re: Ducted Fan propulsion

    A few years back there was a company in Florida that was developing a fan type propulsor designed for use in a ducted fan configuration. Their goal was to provide these fans to the market in a way similar to that of propeller manufacturers - each fan had some adjustability so that it could be fitted to a slight variation of application. The pictures I saw looked very impressive in that it looked to be a very professionally designed and built product. But the main problem with these types of things is that these units have to be specifically designed for, something that your average homebuilder is generally incapable of nor I'm sure is willing to pay for. The developer disappeared fairly quickly.

    Regarding the recip powered fan with after-burning, that too has been proposed a number of times and I think I recall there was a patent on the thing at one time. Interesting idea but far from practical since the single stage fan will most likely be incapable of producing enough flow and pressure ratio to make that configuration feasible.
    "To live is to learn; to learn is to live" (author unknown)

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    Re: Ducted Fan propulsion

    Quote Originally Posted by Starman View Post
    Since were on this subject, has anyone seen any of these Learjet size fans for sale cheap, like maybe one that came off a broken engine? Or say, how much for a broken Lear turbofan engine but with a good fan on it?
    20 - 25 years ago just one blade from a Garrett 731 fan was at least $1,000. There are over 30 blades per fan. I believe they're Titanium.
    Something like that is going to be hard to find "cheap" I'm afraid.

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