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Thread: Strut braced vs. cantilever wings

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    Strut braced vs. cantilever wings

    Curious to know how a clean strut-braced plane, ex- Wittman W-10, compares to a cantilever winged plane like an RV in efficiency.

    Properly done for a sub 200kt plane, could strut bracing and it's benefits (lighter structure, thinner wing), outweigh the negatives (drag from strut). Assume all landing gear is fuselage mounted, and all fuel is held in the wings.

    Thanks

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    Super Moderator orion's Avatar
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    Re: Strut braced vs. cantilever wings

    For proper comparison you really need to look at apples and apples - there are several differences between a high wing and a low wing so a direct comparative study would not yield really useful results. But what you can do is look at airplanes like a Cessna 172 and maybe a Cardinal. There are still differences but if you back out the coefficients you should be able to get an inkling of what's going on.

    But in general, the struts are not very penalizing and as you indicated, you could save a substantial amount of weight versus a cantilevered design, which could be used for payload or simply, a lighter airframe.

    And BTW, a thinner wing is not necessarily going to allow you to go faster. The highest L/D values for most section families lie between 12% and 15% thickness, tending to the higher side of that range.
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    Re: Strut braced vs. cantilever wings

    http://cafefoundation.org/v2/pdf_cafe_apr/WittTail.pdf
    http://cafefoundation.org/v2/pdf_caf...inal%20APR.pdf
    CAFE has some reports of instrumented homebuilts. Might not be exact apples to apples but they are at least all fruit

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    Registered User Dan Thomas's Avatar
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    Re: Strut braced vs. cantilever wings

    Quote Originally Posted by orion View Post
    For proper comparison you really need to look at apples and apples - there are several differences between a high wing and a low wing so a direct comparative study would not yield really useful results. But what you can do is look at airplanes like a Cessna 172 and maybe a Cardinal. There are still differences but if you back out the coefficients you should be able to get an inkling of what's going on.
    The Cardinal has a laminar wing, the 172 cambered. Wouldn't the laminar have less drag in cruise anyway?

    The cantilevered wing needs far more structure within it to carry those loads at small spacing between upper and lower attachments. The Cardinal's root bolts are only about 7 or 8 inches apart, IIRC , while the 172's wing root and strut attach bolts are more like three feet apart. The size of the cardinal's spar structure (and carrythough) is huge compared to the 172's and the fuselage has to be stronger since none of the lift is being transferred to its bottom; it's all at the top, so doorposts and the like are much heavier. And all of this weight hurts takeoff and climb performance a lot and cruise will suffer somewhat less. And all that material costs more, too.

    Strut bracing, then, tends to be light and cheap and ugly. Cantilever is all the opposite. At least we have choices.

    Dan

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    Super Moderator orion's Avatar
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    Re: Strut braced vs. cantilever wings

    Quote Originally Posted by Dan Thomas View Post
    The Cardinal has a laminar wing, the 172 cambered. Wouldn't the laminar have less drag in cruise anyway?
    They both have cambered airfoils in that both airplanes utilize sections that are built around a particular camber line. The C-172 uses the classic 2412 section while the vast majority of Cardinals use sort of a mixed section that consists of a 2415 and a 64a215 at the root and a 2412 and a 64a212 at the tip. The 1967 Cardinal used the two laminar sections net but if I recall the story, that was considered too radical (I think I also heard that there were tip stall issues but i don't know if that was accurate or not) for low time pilots so the mixed sections were used in 1968 and on. Also keep in mind that the term "laminar section" is more a name than a physical description of the flow - the flow trips turbulent anyway, a bit further aft than on the "turbulent" type section (which also has a laminar component) but for typical riveted wings, the difference is not that great.

    Both planes use cambered sections as well as those that might fall more into the turbulent category, although the Cardinal might have laminar flow over a slightly larger percentage of the chord. The drag coefficient difference between the two planes is most likely not greatly affected by the difference in sections. As such, the two airplanes are not all that different since they also utilize a virtually identical fuselage width and form and so can be used with reasonable results for this type of comparison.
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    Re: Strut braced vs. cantilever wings

    Wing struts allow a lighter structure. I have compared the wing structural weight for a light sport airplane. There are many components involved.

    Cantilever wing weight: 124 lbs, 17 lbs spar cap, 18 lbs shear web
    Strut braced wing weight: 108 lbs, 6 lbs spar cap, 11 lbs shear web, 5 lbs strut
    Other items change weight: wing attach fittings, fuselage to wing joint, control connections.
    I found a total weight change of 34 lbs for my design.

    The strut is a drag item. Large wing attach fittings can influence the fuselage and result in higher drag. Weight affects performance differently in all phases of flight.

    Struts are structurally efficient but are perceived as old fashioned.

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    Re: Strut braced vs. cantilever wings

    Thank for all of the replies guys!

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    Re: Strut braced vs. cantilever wings

    The drag of struts should be analysed together with other drag components. A strut itself is very small portion of the total wetted surface and also equivalent drag area. It is usually a symmetruical airfoil-shaped body with friction drag coefficient quite comparable with that for the whole airframe. Some relevant data can be found here: cyberiad
    The independent problem is the interfernce drag at the wing and fuselage junctions? but there are known solutions to make it almost undetectable.

    Interestingly, sometimes it is possible to compensate the strut drag by reducing the drag of other airfame components, thanks to strutted layout. Such conditions result in pure positive effect of struts, which can manifest itself not only by reduction of structural weight, but also total wetted area. The use of reduced airfoil thickness was already discussed here. Let me note, that the optimal thickness for most airfoil series is indeed 12%, and sometimes even less. The elimination of massive carrythrough from the fuselage can result in much better use of the internal space and more optimized cabin layout.

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    Registered User cluttonfred's Avatar
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    Re: Strut braced vs. cantilever wings

    French company Hurel Dubois created many designs and some limited-production aircraft with long, strut-braced, glider-like wings and showed that they provided higher speeds and lower fuel consumption for a given weight. They never really caught on, though, perhaps because of the ground handling issues--75-foot wingspan in a converted 12-seat Mies Aerovan, for example.

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    Re: Strut braced vs. cantilever wings

    If I recall correctly, the Tecnam P92 and the P2004 are identical from an aerodynamic point of view, except for the wing (P92 has struts, the 2004 has cantilever). The cantilever wing is just over 10 kts faster.
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    Re: Strut braced vs. cantilever wings

    The infamous Parasol FRED has that strut problem solved nicely and it has several other advances too..no flaps..now how make that modern with just three struts with aerodynamical shape ? I bet it could solve a problem how to make PART 103 "legal" Parasol in Finland for instance ( 75 kg max empty weight ).

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    Re: Strut braced vs. cantilever wings


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