Wing's Ribs , how many, and space between them ?

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by lightyagami, Sep 23, 2014.

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  1. Sep 24, 2014 #21

    Topaz

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    Completely outside my expertise, honestly. I know the basics of structural work, the gross theory, but my detailed learning to-date has focused more on the aero side of things. I would feel confident helping you with the aero loads, but not with the question you're asking. Hopefully Wsimpso1 will chime in - based on posts I've seen, he has a good grasp on the subject. You might consider sending him a PM. He sometimes visits HBA only sporadically.
     
    Last edited: Sep 24, 2014
  2. Sep 24, 2014 #22

    nickec

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    As you can see above, some RC aircraft fly very fast.

    What is the goal of the airframe? Need it just fly? If so you can use a rule of thumb - space ribs on third of their length apart. Nose ribs can be added between all ribs to better support the wing skin or nose sheeting of the D-cell.

    If the flight goal involves competition then more analysis may be warranted.

    Some RC aircraft space ribs one half their length apart. Such wings are lightly stressed.

    Anything between one third and one half rib length apart is very very common.

    The airframe in the video has a molded composite wing with virtually no ribs. Great attention to detail and expensive materials make the high stresses surmountable.
     
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  3. Sep 24, 2014 #23

    nickec

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    RC DS Records

    At the link above you will see that 500 mph is long overdue - as an RC Dynamic Soaring record.

    Breaking the 500 mph limit is very likely very soon.
     
  4. Sep 24, 2014 #24

    autoreply

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    I got that ;)

    But if you look at all the accidents, none of them leads to damage further than the local impact, in other words, skin won't separate if manufactured normally, even at very high dive speeds.
     
  5. Sep 24, 2014 #25

    bmcj

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    You're probably seeing the plexiglass above the plane and the canopy frame still attached to the plane.



    :gig:
     
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  6. Sep 24, 2014 #26

    lightyagami

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    No problem man, thank you for all the help , I'll try send him a PM. I will ask you one more thing , is about the aero loads, I've found the critical loads using schrenk's approximation ,but my wing is both rectangular and trapezoidal , then what i've done to use this approximation was break the calculation in two parts, one for the rectangular and one for the trapezoidal. To do it I've used the taper ratio of the rectangular portion as 1 because the chord is the same in this portion of the wing, then where the wing transitions from rectangular to trapezoidal i've used the ratio of the wing tips length to the wing root's length. This was how i've found the rectangular lift distribution , to use schrenk I also need the elliptical distribution. But then for the elliptical i've done just one calculation because the only variable in the elliptical equation was the position in the spanwise that varies from 0 to b/2 , where b is the total spanwise. Is this approach right ?
     
  7. Sep 24, 2014 #27

    wsimpso1

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    I used Roark's Formulas for Stress and Strain, chapter 11, Flat Plates.

    For fabric covered airplanes, the sections on membrane stresses and large deflections are applicable.

    For aluminum skins and composite sandwich skins, the tables on flat plates are applicable, but with a little composite plate theory will be needed to correct from the existing equations for homogenous material to composite sandwich plates. Provided are conditions for fixed edges, uniform loading, linear increasing loading, etc. By superposition, you can add up the stresses and deflections to get your loadings.

    But if I were building something of this scale, I would skip the molds and build with hotwired cores and Rutan methods. Much simpler to analyze and design, much simpler to build and lighter at any strength or stiffness. SpaceShipOne flew to to 112 km and Mach 3.09 with hotwire cut solid cored control surfaces. It will work for your build.

    Billski
     
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  8. Sep 25, 2014 #28

    Topaz

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    I'm not sure I'm understanding you correctly. You shouldn't need to break the wing planform contribution into two parts, just because the planform isn't constant-taper. The planform contribution just needs the local chord at the same spanwise station for which you're calculating the elliptical contribution. For the constant-chord part of the wing, that will remain the same number. Once you get into the tapering part of the wing, that local chord value will be getting smaller until you reach the tip, at which point it'll be the same value as the tip chord. It would be convenient if the start of the tapering section fell at one of the spanwise stations, but not critical.
     
  9. Sep 25, 2014 #29

    Autodidact

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    There are two ways to do Schrenk's; perform the integration, or just draw a quarter ellipse of the same area as the planform, and put it on the same graph that the actual planform is drawn on, and then just measure the height of each and find the arithmetical mean. The integration would need to be broken into two parts, because the taper ratio is different; same problem, but each with a different taper ratio, and each with different limits of integration - the first with limits from 0 to where the break is, and the second with limits from the break to b/2. Clear as mud?
     
  10. Sep 25, 2014 #30

    Matt G.

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    He must be doing it the graphical way where you use the average of the elliptical area and the wing planform area as is explained in Hiscocks. As long as he isn't restarting the ellipse where the wing transitions from straight to tapered, it should come out right.

    Side note- This graphical method appears easier, but the math needed to find the centroid of some arbitrary segment of a quarter-ellipse is not trivial. I've made a spreadsheet for each, someday I need to re-figure out the integration one and compare it to the graphical one...
     
  11. Sep 25, 2014 #31

    Autodidact

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    Just to make things more confusing (I don't want to, but my skill at explaning things is not that great); for the outer, trapezoidal part of the wing, if the integration is being done, I think you would have to go ahead and extend the leading and trailing edges of the outer tapered panel all the way back to the root (in a mathematical sense, only), and then do the integration with limits from the break to b/2. But use the 1/4 ellipse that has the same area as the whole wing, and not the area of the outer panel plus its extended leading/trailing edges, which would be quite a bit larger than the actual wing area.

    With the lift distribution curve sliced up into ten or twelve pieces, the mid-point of each piece is just slightly outboard of the centroid location, so that makes it conservative, doesn't it? With the error getting smaller as the slices get thinner and more numerous.

    Hiscocks' is the best explanation of the graphic way I've read.

    It would take some digging to come up with that, if it wasn't just right at the front of your mind or you don't have a photographic memory...
     
  12. Sep 25, 2014 #32

    Matt G.

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    Yeah, an integral that requires trig substitution to solve. I was able to recreate the table in the book, so I'm fairly sure I did it right, although the numbers don't exactly match to 3 decimal places. I think that is probably due to some round-off error in the book since it was written before the time of calculators...:lick:
     
  13. Sep 25, 2014 #33

    Highplains

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    Typically models that size have the ribs about every 75mm (3 inches). The rib thickness is 3/32" (for ease of handling while building, often with full sheeting at the leading edge of at least 1/16" from the spar forward to build a "D" tube, Behind the D, 1/4" wide cap strips on the ribs. Balsa density in the 8-12 lb/ft^3 range for the ribs.

    Plastic films have no torsional strength so the wing structure must.
     
  14. Sep 25, 2014 #34

    Norman

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    You guys can do trig by hand if you want to but it doesn't have to be that hard, or fun depending on your personality. The centroid of a 1/4 ellipse is always at 42% +/- <1% from the origin. Any solid modeling CAD program can find the centroid of any arbitrary 2 dimensional shape by turning that shape into a region and checking the mass properties.

    ellipse.PNG
     
  15. Sep 25, 2014 #35

    Norman

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    That's only partially true. While films don't have any torsional strength they do have tensile strength and that does add to the torsional resistance of the structure by closing out the cells between ribs. This effect is hardly noticeable in a structure with a proper D-tube but on an open framework it does help.
     
  16. Sep 25, 2014 #36

    Highplains

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    The effect is hardly noticeable in any open framework structure. Back before the US Nationals became a week long local contest, I watch a C class free flight start it's vertical climb. About 2 seconds in, the wing started a violent flutter and it lost lift from the wing. Film covered wing with no D tube or diagonal structure. The design was fine with silkspan or silk, but not plastic film.
     
  17. Sep 25, 2014 #37

    Matt G.

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    The centroid of the quarter-ellipse isn't the issue, the centroid of some portion of it is. This is needed to get the shear and moment variation across the span. Putting each strip area into a CAD program does not lend itself well to spreadsheet analysis, hence the need for the integral solution.
     
  18. Sep 25, 2014 #38

    Victor Bravo

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    SOME of the plastic model films contribute some amount of torsional strength to the wings, and some don't.

    The Monokote brand is Mylar-based, and I believe some of the other brands are polyester based.

    Using Monokote is not a substitute for leading edge sheeting by any means. But an experienced modeler can tell you that the Mylar film results in a noticeably stiffer wing in torsion, without without a D-tube, than the polyester films.

    When you are using Monokote, the material "feels" more crisp and springy in your hands. The other films feel more like Saran Wrap, no spring.

    Again, this is not a substitute for putting a D-tube on an airplane that is fast enough to need a D-tube.
     
  19. Sep 25, 2014 #39

    BBerson

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  20. Sep 25, 2014 #40

    autoreply

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    Incorrect. The torsional contribution of even conventional dope and fabric is very significant.

    The only reasons not to take them into account are that it's impossible to load-test without making the structure invisible and that a simple tear will ruin your wing structure.
     

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