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Thread: Strut speed fairings.

  1. #21
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    Default Re: Strut speed fairings.

    General rule is the chord of the fairing should be aporoximately 4.25 times the thickness of the tube you want streamlined.
    I'm going expound on this statement. The only one I know of that put a "number" to it was Hoerner in one of his aerodynamics books. For struts that number was a c/t ratio of 3.7 or t/c of 27%. That is based on fairing thickness NOT tube diameter or member being faired thickness.

    I thouroghly respect Mr. Hoerner and all he has done for aviation. But, some of his 1960's experiments and modelling have been vastly expanded upon by the likes of NASA (as a major example).

    It has been found that more elongated shapes are more effective (better laninar flow) - the widest point more in the range of 40-50% from the leading edge (think more like a cigar). And streamlined symetrical airfoils with a length of up to t/c 9% are also more effective (diminishing returns beyond that). 9% probably not very practical and elongated streamlined shapes more difficult to build and heavier, so may not be worth the effort for likely minimal gain. So as a guide, I would say min. 27% to max. 9% t/c, keeping weight to a minimum.

    I do agree with Lowell that the fuselage (door) to wing intersection could benefit a lot from streamlining that intersection.

  2. #22
    Senior Member av8rps's Avatar
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    Default Re: Strut speed fairings.

    Danzer1,

    Thank you for expounding on that. Excellent job I may add. Yes, there is a lot of unproven theory out there, and then there is just a bunch of guys experimenting who sometimes luck out, and other times just plain give up. I guess that's probably why many of us enjoy playing around with these experimentals.

    Lowell,

    I love your horizontal stabilizer strut fairings. Mine don't cover up the rod ends, nor do they fit flush against the fuselage skin like yours. But yet they work well. I really do think if we could clean up all the small areas (like all the rod ends as just one example) it would provide us with a nice drag reduction when they are all added up.

    Oh, and btw, your latest Model 4 looks really nice in those pics, with an exceptional attention to final fit and finish. I liked it so much I just finished looking through all your photos in your photo albums.

    Here's the million dollar question? Did you think your speed mods were worth the effort?

    Oh, and I agree if we could fair out the wing to fuselage area I agree it could be a good thing to try. But for what its worth, many Lake amphib owners spent a lot of $$ to install a really slick set of leading edge wing root fairings, but unfortunately no one but the company selling the fairings saw any improvements. I like to look what the old air racers did to their airplanes for drag reduction ideas. I think they figured a lot of that out through trial and error.

    Paul
    Last edited by av8rps; 12-28-2016 at 05:38 PM.

  3. #23
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    Default Re: Strut speed fairings.

    Lowell, Great job on those fairings , I suspect more pictures of your plane would reveal some more of your nice workmanship, lets keep them coming! Bruce N199CL

  4. #24
    Senior Member PapuaPilot's Avatar
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    Default Re: Strut speed fairings.

    I have been thinking about this area too. It's too bad there isn't a carbon fiber airfoil extrusion that could go over the wing lift struts and horizontal stab support.

    Even better yet would be someone who could design some CAD cuffs for these fairings. If someone has the ability to draw these they could be made with a 3D printer out of nylon, ABS or even carbon fiber material. Is there anybody in the KF group that is good with CAD (I don't have that skill)?
    Phil Nelson
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  5. #25
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    Default Re: Strut speed fairings.

    Quote Originally Posted by jiott View Post
    That's really interesting! I wonder if the rest of us who have our fairings in a fixed position could gain some more speed by angling them down. From the photo it looks so extreme, I wouldn't have believed it. Does both sides of the airplane do the same? Wonder if it is a spiraling propwash effect?
    I would like to pick up on Jim's comment and Jan's original comment about the strut fairing. I too was surprised by that photo Jan. If the fairings have given you about +9mph and they naturally adopt that angle, then that would appear to be telling us something about the airflow in that region. I suspect that the follow-on discussion about the area between the doors and the wings may relate to this phenomenon? However, if Jan is getting +9mph with the fairings at that angle, are those of us who have aligned the fairings 'straight' actually not gaining the streamlining we assumed? I have only flown with them on so have no comparison. I know some here have added them after flying with the exposed struts so would be interesting to hear their results. If I remember rightly, most people who add them after report similar gains to those of Jan, with them fixed. If this is the case I struggle to understand what is going on from an aerodynamic viewpoint??

    thoughts??

    r
    Ross
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    Sold to Richard and Scott Taubman in OZ, 2019. Kitfox SS7,Rotax 912is Sport, Airmaster CSP 75" blades.
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  6. #26
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    Default Re: Strut speed fairings.

    The optimum angle will vary somewhat depending on attitude (aircraft not yours), prop direction of rotation and what axis' you trim and how much. So it might vary a bit between flight phases and between aircraft.

    Assuming the longest duration of your type of flight is straight and level - I'd put tuft streamers along the length of the struts, video tape them and see if the angles match your fairings. You'll probably never get them perfect anyway as I'd bet the angle changes between the top at the wing and the bottom at the fuselage anyway. Likely will be a compromise somewhere.

  7. #27
    Senior Member jrevens's Avatar
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    Default Re: Strut speed fairings.

    I believe that builders have reported similar increases with the struts installed "Conventionally". Even if it did give you, say, an additional 1 or 2 mph, I'd opt for installing them so that they looked good... I think they'd look strange to me drooped like that. That's just me.
    John Evens
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  8. #28
    Super Moderator Av8r3400's Avatar
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    Default Re: Strut speed fairings.

    +1 to that, John.
    Av8r3400
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  9. #29
    Senior Member av8rps's Avatar
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    Default Re: Strut speed fairings.

    Mine are not set down like that and yet my Kitfox is pretty fast, even on floats. I agree with doing some tuft testing to know where they really should be set.

    Is it possible that the heavy weight of the plastic fairings pulled them down in the photo rather than being moved by the airstream?

  10. #30
    Senior Member jiott's Avatar
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    Default Re: Strut speed fairings.

    Yes, I also agree that Greg's suggestion of tuft testing off the trailing edges of the fairings (both sides and fore and aft fairings) would answer a lot of questions. It would still leave the "why" unanswered. If its spiraling airstream from the prop then the right hand side of the plane should see the tufts deflected opposite. In Jan's photo they are deflected down, which indicates a LH turning engine maybe?
    Jim Ott
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