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The Cold Jet (ducted fan/propeller)

  1. Aug 19, 2007 #1
    I had an idea once which I'd like to go into a little. It involves how a fan or propeller draws in air verses only providing thrust.

    The idea is/was to cut down on aerodynamic drag and resistance a propeller imposes once the craft is up to speed. By having a remote fan/propeller air intake like a jet or even a NACA inlet; in-line tandem fans which are located down a serpentine duct would provide thrust.

    The first fan/propeller would act to drawn air into the air intake plenum feed duct; the second fan would kick it up a notch. Much like a compressor.

    The second fan might rotate in the same direction or be counter-rotating; it does not matter at this point. I would also use additional duct length to straighten the flow in lieu of stator vanes AFTER THE LAST FAN.

    All this together is aerodynamic and quiet if not the last word in weight control or efficiency margins.

    I'm not claiming this is a new idea or invention, do a search on "cold jet" or "cold thrust", pre-WWII Italy had an example or two, and of course there was Henri COANDA in 1910.

    Example the Davis Wing:

    Note: The inlet may not always face the direction of motion in my example, it could be at a right angle. This might also be used in a hovercraft, not just an aircraft.

    What do I need to know or reference to pull off such a configuration?

    Attached Files:

    Last edited: Aug 20, 2007
  2. jcsd
  3. Aug 19, 2007 #2


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    Sounds like a more complex way of driving a turbofan.
  4. Aug 19, 2007 #3


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    Not a turbofan; it's just a basic ducted fan. Nothing special there. The machine would probably be more efficient if that same engine power was directed through a propeller.
  5. Aug 20, 2007 #4
    There is a hovercraft design booklet from the 1960's which shows a diagram of one fan in front of the other. It claimed that it would be able to absorb twice as much power as a single fan, not that it would be more efficient than a single larger fan.

    There are also examples in light hovercraft design in which two ducted fans are used side by side (not in tandem) in lieu of a single large fan to lower the C of G.

    Many ways to skin a cat, not looking for the best here, just looking for what I need to match an engine (4-stroke automobile) to a ducted fan (in series) to another another fan. Would you call this a two stage compressor?
    Many people have matched engines to fans/propellers, what do I need to match fan to fan.

    The question "why" can be only answered by "because I can".

    NOTE: maybe the word "quest" is similar to the word "question" for a good reason.

    EDIT: This is different.

    Attached Files:

    Last edited: Aug 20, 2007
  6. Aug 20, 2007 #5


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    If you can't see why this won't work with fans rotating in the same direction (at the same speed) with no stator vanes, I think you need to learn some basic fluid mechanics of turbomachinery.

    Contra-rotating two stage fans, both ducted and unducted, are nothing new of course.
  7. Aug 20, 2007 #6
    Okay, lets say we have stator vanes in between the fans.

    Now what?

    As I understand the concept the first fan is of shallow pitch, the second fan greater pitch. To keep it simple I want to stay away from Contra-rotating fans.

    In essence the first fan is feeding the second fan higher velocity air or is it higher pressure air? Why is this a good or bad thing? The first fan in the scenario I've outlined is also acting to draw air in which is part of the work its doing.

    Assume I know nothing of the basic fluid mechanics of turbomachinery (which I don't), where do we go next?

    EDIT: Found this...................click to see figures
    Last edited: Aug 20, 2007
  8. Aug 19, 2008 #7
    I have been working on this particular type of model for about------ oh my goodness--12 YEARS!!------ I have developed the prototype and computations within an auto cad program----- however have yet to build the darn thing...

    These Models account for many different things----- first off.... TURBULENCE....

    Having two fans with separate angles of attack within a tube---- and then adding the drag properties of the fan disc--- with the increase of inlet wind velocity speed (while in flight)---- you are looking at a major catastrophe within the distance separating the two fans.....

    You will at minimum create huge deficiencies in laminar wind velocity..... and at maximum create a pressure/turbulence bubble that would cause your fans to stall or even cavitate your duct (If not make it blow up)----

    Using Fans and Stators close together is the best thing to do in terms of creating a huge velocity difference (the real goal) on each side of the Fan(s) Disc

    With Stators in a Jet engine.... they are used to compress the air for combustion with fuel mixtures....... If you reverse the airfoil in the stator and decompress the air ..... You remove pressure and build velocity inversely (as stated--- the goal is velocity).....

    There are some articles out there on this theory.....

    The Idea of a ducted fan is a great one..... but must have the focus of its creator on creating high air velocities--- w/o Pressurizing the air mass.... as well as creating laminar (non turbulent) flow (you need stators to accomplish this)

    Its a love/hate game in efficiency----

    and someday....... I will get the time to put a working example together......

    Congrats on your theory though........ It is where I started until I progressed the Model once I became a FAA certified Mechanic and majored in aeronautics....

    The way to do it----- is out there... and it is no longer imagination--- but, the person who builds it first that will reap the benifits.....
  9. Aug 19, 2008 #8
    OH..... and counter rotation of the fans is the only way to maintain efficency---

    Look at Propeler efficency theory!!! or even look up dual rotating propeller test that have been studied by NASA
  10. Aug 19, 2008 #9
    This appears to be quite similar to the drive system for jet-boat. The material being moved is quite different in density, but the principle is the same.

    In either case (water or air) the intake duct introduces frictional losses that decrease the efficiency of the system, regardless of the direction of flow.

    A similar application (kort nozzles) is used on slow moving watercraft to gain thrust. These are ineffective above a threshold speed as they induce more and more drag on the vessel.

  11. Feb 7, 2009 #10
    1. I thought that it was the difference in pressure which moves the body (craft) though it's medium.

    2. And the greater the exiting velocity of air (or water when compared to body speed) the more losses in energy conversion (sub-sonic conditions).

    3. The concept as stated mentions hovercraft because I really meant to say that I am willing to give up some efficiency for safety and noise considerations. I mean how many surface based vehicles have open spinning propeller blades? You can put guards on propellers/fans, you can put them in a duct, and you can locate them high on the roof and out of the way of curious hands. However the potential danger will always be there. By enclosing the fan/compressor/propeller deep in the body of any craft, a certain element of safety is bound to be added. At least that's what I was thinking.
    Last edited: Feb 7, 2009
  12. Feb 7, 2009 #11
    Someday I would like to measure the thrust coming out the back of my two person hovercraft (6ft W x 10ft L). I also need a way of metering/measuring the mass air flow being drawn into the ducted fan.

    Any ideas on how to do that?

    I'm assuming both sides an equation to confirm the results would have to equal, so that I could calculate the work being done (and energy lost).

    The second phase of the experiment would be to build and install a rigid foam board inlet which would allow air to be drawn from overhead (or from the sides) in lieu of straight in.

    As crazy as that sounds (don't know where I would sit) several hovercraft do something similar for their lift fans already. Nobody I know of has tried it for their thrust fan/propeller for reasons all to obvious (except to nutty guys like me).
  13. Feb 7, 2009 #12
    1) the body moves forward because of the momentum the engine imparts on the air (and vice versa) Momentum is mass times velocity, so high velocity is good. In turbofans, the annular flow through the fan is high velocity, high mass flow of air to achieve the bulk of the thrust. The flow through the core is there to power the jet engine which drives the fan. As the core flow goes through the compressor, it slows down as pressure goes up. Increased pressure is desired so the air has better combustion properties. After blowing up the air with fuel, it accelerates and drives the turbine, which drives the compressor and fan.

    2) This is like saying the faster you go the more drag you feel.

    3) Since the success of a new technology depends on whether or not it meets the needs of the customers, I think it would be good to get some numbers on the safety issue. Ex- how many people are injured every year due the engines? How can you quantify the improved safety? You need the baseline data. Also the noise issue is important, and any improvements in this area would be well received. They key is to show that your up front design requirements (related to safety and noise) are actually desired by the aerospace industry.
  14. Feb 7, 2009 #13
  15. Feb 7, 2009 #14
  16. Feb 8, 2009 #15
    Yes! There are still people who are talking here!

    Ok, for those who don't know... I trying to make something similar to a cold jet. Problem is, I haven't an inkling about physics.

    Can anyone help or show me to look for a calculation? I need to find a out what size fan would be efficient with a motor with 306 g/cm of torque.
  17. Nov 1, 2009 #16
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