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Thread: Ducted Fan Research

  1. #1
    More than 10 posts! Club Member
    Join Date
    Aug 2009

    Default Ducted Fan Research

    Hi Guys,

    Interesting information is coming out of research into ducted fans for light aircraft applications.

    Some of this research at present is into the properties of stators positioned either before or after the fan and how much drag is related to each type. Other research will be made on noise, overall efficiencies etc. At the moment this is in the mathematics stage using every scrap of physics known to man that is available and relevant to the subject.

    I will post pieces as they become available, I am not interested in having a discussion or argument here on the merits of any information posted or for the application in hovercraft, it is merely being put here for information purposes only. Make of it whatever you want, some of it may even be out of logical order, C'est la vie....

    If admin doesn't want the information posted, just say so.


    The guide vanes will create a little drag but it is meaningless. The reason is that inlet guide vanes are like nozzles in that they accelerate the flow, as opposed to slowing it down which is what the outlet stators do.

    Anytime you are accelerating a flow you have a very stable boundary layer, so that is the main thing that keeps drag in check.

    Inlet guide vanes are actually like turbine vanes, as opposed to compressor vanes in that they force the air through an opening (between the blades) that gets smaller as you go from leading edge to trailing edge.

    Compressor and fan blades and stators do the opposite, the opening widens as you go from LE to TE and that slows down the flow and causes the pressure rise. This sounds counterintuitive because we know that the fan speeds up the flow and this is what makes the thrust.

    But this is the flow in the axial direction that speeds up. This will make a lot more sense when I put up the velocity triangles.

    What happens is that when the fan hits the incoming flow the speed at the fan LE is very high due to the rotation of the fan blade which is spinning very quickly. But from there to the trailing edge that initial speed actually slows down and as a result pressure rise is created.

    Now the net result is that the flow is still going much faster than when it came in (double approximately) but it is much slower than the initial speed of the fan hitting the flow with the leading edge. Think of it like a speeding truck hitting a parked car. that car will get tossed so its speed increases quite a bit, but that truck will slow down from its initial impact speed.

    Anyway, things are really coming together. There will be some very exciting news here in the coming weeks. As soon as I get the initial design nailed down I will build a model for testing. That will tell us everything.

    But now I see we have a real shot at this. If that fan is even partially stalled at static and slow speed it is a disaster.

    As my initial calcs show we have a real shot at making 500 lb of thrust from 200 hp, maybe even a bit more. Now when we stop and think that a fixed pitch prop and the same 200 hp engine on a 200 mph airplane is only going to give you maybe 550 lb of thrust then we are really in business.

    There is another variable that I have been using in the calcs and that is the inlet geometry. I talked previously about a Kort nozzle where the inlet accelerates the flow, as well as the standard turbofan diffuser inlet which slows it down.

    This is a very important variable. What I have been doing is trying all kinds of different combinations of inlet geometry, fan size, hub size, guide vanes, etc and trying to find the best combination. It's a huge amount of basically plugging in all kinds of combinations.

    Some of the results are really surprising. More to come.

  2. #2
    More than 30 posts! Club Member
    Join Date
    Jun 2009

    Default Re: Ducted Fan Research

    Id be more worried about keeping the airflow straight coming out the back of the duct. Inlet guide vanes are used on very few aircraft these days, rolls spey and conway and some other military types, but commercially cant think of any! A good inlet design on large aircraft is designed to slow air velocity down to mach 0.4-0.5 and also straighten it!

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