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Thread: Shrouded prop for static lift?

  1. #16
    Super Moderator orion's Avatar
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    Re: Shrouded prop for static lift?

    Shrouded props do work and the increase in static thrust can be significant. The critical aspect of the configuration is that it must be designed for the condition you anticipate in its operations. As such, for a vertical lift platform or similar, the design must be optimized for a static condition or at least, for very low speeds.

    The additional performance comes from three areas of design: One, the higher efficiency the prop can operate under with the tip seal the shroud provides; two, the suction gained off the leading edge and three, the surface of the expansion nozzle aft of the prop disk.

    I've been involved in a couple of shrouded prop developments - on the first one (hydrofoil with two C-130 shrouded props), we showed that forty to sixty precent of the operational thrust came from the shroud. The second was a new shroud/prop design for the military LACV vehicles (hovercraft) - there we were able to show over 100% more thrust over the standard vehicle's propulsion with a prop/shroud that was about 70% of the original prop's diameter.

    And currently I'm working on a six to eight place amphib that will also use a shrouded prop and here too we're seeing an increase in thrust of over 40% for the take-off and climb conditions.
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  2. #17
    Registered User BBerson's Avatar
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    Re: Shrouded prop for static lift?

    Thanks orion,
    Sounds like you generally agree that the results shown in the Dr. August Raspet paper* are possible.

    Do you have a drawing of the "cambered elliptic nose" shape of the inlet that Raspet designed? I could not understand the description.

    Thanks
    BB


    *Raspet paper:http://4wings.com.phtemp.com/lib/files/Raspet_c.pdf

  3. #18
    Super Moderator orion's Avatar
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    Re: Shrouded prop for static lift?

    I see where the confusion might lie. Unfortunately I really don't have any data specific to his paper and as for our stuff, much of the design work was done by David Lednicer of AMI and my ex-partner, Robert Gornstein, so I personally don't have all that much general data to share since that wasn't my realm of responsibility. The only place I might suggest you look is at the NACA technical paper site - I remember quit a few of the shrouded-prop and ducted-fan design papers being available there and several of them did include the studied geometries and the applicable results.
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  4. #19
    Registered User BBerson's Avatar
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    Re: Shrouded prop for static lift?

    This site has a drawing that gives some insight about Raspets "cambered elliptic nose".

    Ducted Fan Design for a Cozy Mark IV

  5. #20
    Moderator Dana's Avatar
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    Re: Shrouded prop for static lift?

    Quote Originally Posted by orion View Post
    Shrouded props do work and the increase in static thrust can be significant. The critical aspect of the configuration is that it must be designed for the condition you anticipate in its operations...
    Meaning that the off design performance would be pretty poor, no? Of course for a VTOL there's not a wide range of operating conditions...

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  6. #21
    Registered User leviterande's Avatar
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    Re: Shrouded prop for static lift?

    In any case where you want static thrust, diamter wins but a shroud will increase the thrust up to doubling but I am not sure about it. the hiller platform is a working example of gaining 40%

    Lippisch duct I think it is called got huge thrust amounts from the lip intake. From what I know the size of the "lip intake" is the vital part of getting more thrust per hp. I read alot about ducted propellers and I remember i read they made a test where they simply modified everything in a 20inch test duct. they tested the exit area, clearence etc.. and the most affecting factor was the diameter of the intake lip.

    so we should ask how big can the intake be, is there a limit, (neglating the weight factor)


    Kalle

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    Re: Shrouded prop for static lift?

    Issue 26 of Contact Magazine has an article about building a scale A-4 Skyhawk using a Chevy 350 and a ducted fan. The article discusses the search for information about ducted fans. The report the author's used was written by Rose Worobel of Hamilton Standard. I believe it is on NASA's reports server. The title had "Q-fans" in it.

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    Re: Shrouded prop for static lift?


  9. #24
    Super Moderator orion's Avatar
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    Re: Shrouded prop for static lift?

    Quote Originally Posted by Dana View Post
    Meaning that the off design performance would be pretty poor, no?
    That's the general line of thought however in our programs we've found that that is not necessarily accurate, but then it's all in how you design the shroud. For instance, historically it's been thought that although a shroud will certainly help a lot on the low end of the speed spectrum, by the time you hit about 120 mph the benefits will be overcome by the losses of the increased whetted area and interference flow penalties.

    But in our current work we are seeing a measurable and significant benefit of the shroud to just past 200 mph, although by that point the increase in net thrust is only about ten to fifteen percent. And outside of an initial estimate, we did not go into the design's detail parameters enough to determine whether at that point the duct is still a "net" benefit or not. (In our program's case the duct is actually part of the customer's patent and so is being utilized as an inherent part of the configuration - whether it is a net benefit at 200 mph or not is not really an issue so we didn't go into it any deeper.)
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  10. #25
    Registered User BBerson's Avatar
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    Re: Shrouded prop for static lift?

    Quote Originally Posted by leviterande View Post
    so we should ask how big can the intake be, is there a limit, (neglating the weight factor)


    Kalle
    The simple bell mouth inlet (used by Hiller) is described in my industrial fan book under "duct inlet orifice design". A simple cutoff pipe has a pressure loss of .47, for example. The bell mouth has a loss of just .05, and "can be neglected for ordinary ventilation work"
    The drawing shows a bell mouth about the same size as the pipe radius. The Hiller inlet is probably less than ideal, but it may not matter much.
    BB

  11. #26
    Registered User leviterande's Avatar
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    Re: Shrouded prop for static lift?

    BB, can you perhaps reveal a little what you intend to build? or do you want just to increase the static thrust of airplanes

    if you are on the same road as me looking for info to design a vtol, one could consider having a coaxial system inside a ductlip. I am not sure wether the coaxial whirl of air will put any efficiencies away regarding tip clearence. but having a huge intake lip on a coaxial can provide a good start for a vtol

    Kalle

  12. #27
    Registered User BBerson's Avatar
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    Re: Shrouded prop for static lift?

    Quote Originally Posted by leviterande View Post
    BB, can you perhaps reveal a little what you intend to build? or do you want just to increase the static thrust of airplanes


    Kalle
    I am working on a lift fan to add vertical lift on an airplane to allow flight below the normal stall speed for takeoff and landing. I am hoping to get 50% of takeoff weight with direct lift. This would reduce the landing speed about 25%, I think.
    A future design may have more thrust, but I think true VTOL would be difficult with a piston engine.

    So I am working on an a lift fan to get experience with direct lift.
    BB

  13. #28
    Registered User Tom Nalevanko's Avatar
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    Re: Shrouded prop for static lift?

    Like this one?
    Attached Thumbnails Attached Thumbnails Shrouded prop for static lift?-img_0645.jpg   Shrouded prop for static lift?-img_0647.jpg   Shrouded prop for static lift?-img_0646.jpg  


  14. #29
    Registered User BBerson's Avatar
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    Re: Shrouded prop for static lift?

    Nope.

    I think one lift prop at the CG would be safest. In this case, a failure of the lift system would simply result in a higher sink rate but no loss of control. The pilot would apply forward stick to maintain a higher landing speed with the wing without the lift augmentation. There would be a band of altitude near the ground that a failure could be a problem. But I think it would be superior to a helicopters dead man curve problem and the need to drop the collective in less than 1.5 seconds as required in a R-22 helicopter (or be killed).

    The issue of failure is my primary concern.
    BB

  15. #30
    Registered User leviterande's Avatar
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    Re: Shrouded prop for static lift?

    Quote Originally Posted by BBerson View Post
    Nope.

    I think one lift prop at the CG would be safest. In this case, a failure of the lift system would simply result in a higher sink rate but no loss of control. The pilot would apply forward stick to maintain a higher landing speed with the wing without the lift augmentation. There would be a band of altitude near the ground that a failure could be a problem. But I think it would be superior to a helicopters dead man curve problem and the need to drop the collective in less than 1.5 seconds as required in a R-22 helicopter (or be killed).

    The issue of failure is my primary concern.
    BB

    We are on the same lines BB, We want as slow landing and take off speed as possible. as you said the lift MUST be at the C.G or it lll be dangerous.

    but i didnt get you, you mean you want to have a central liftfan in a cruize airplane where it has a standard propeller at the front. unless you want to tilt the central lift fan it will be HUGE dead weight in cruize or maybe you could have a small sketch to show us?

    it is always intersting to unlock new doors
    good luck

    kalle

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