A different way to build a wing

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by rtfm, Apr 10, 2019.

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  1. Apr 12, 2019 #21

    Sockmonkey

    Sockmonkey

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    I ask because it seems like a good idea at first glance to do something like this with different densities of foam to save weight.
    [​IMG]
     
  2. Apr 12, 2019 #22

    blane.c

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    You are a "REBEL"
     
  3. Apr 12, 2019 #23

    rtfm

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    Hi guys,
    OK, I overstated my position in an attempt to illustrate a point, and went too far. You're correct, and your criticisms of my over-reaching is justified.

    But I have to say, posting "novel" ideas on this forum is a scary undertaking. And I'm not the only one to think so. The knee-jerk response tends to be negative, even when folk take the time to read the OP's comments accurately - not all do. This isn't the first time an idea has been so... "attacked" is too strong a word, as is "criticised"... as to make one think "it is really worth it?" I understand the need to place safety first, and to emphasise engineering analysis front and centre. But well before we get to that point, surely there is space for spitballing, for off-the-wall ideas, and for some indulgance, as a gateway to advancing an idea.

    I am thoroughly sick and tired of the "status quo" always trumping the "novel". The "if this is such a great idea, then why isn't everyone doing it?" argument. It's bullshit. It's the essence of stagnation.

    I proposed this idea hoping that I might get some positive input. Thank you Billski and VB.

    Sigh - I'm tired of this. I am going to my workshop to build my Ranger.

    Duncan
     
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  4. Apr 12, 2019 #24

    jedi

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    And then cut 2 inch diameter lightning holes thru the low density foam and cover with Orotex if you want an ultralight.
     
  5. Apr 12, 2019 #25

    cluttonfred

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    Oratex uses a heat-activated adhesive and may not be the best choice for fabric over foam.
     
  6. Apr 12, 2019 #26

    BJC

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    I appreciate the posters who take the time to point out technical issues with, as you call them, “spitball ideas” because lots of people reading theses discussions need to be aware of the technical deficiencies. I would think that the OP would also appreciate it. It takes time and effort to educate non-technical people on a forum. Of course, doing it in a polite way is called for. That has been the norm here at HBA recently.

    Perhaps you see it differently. How would you like the more knowledgable to reply when they see an issue?
    A good approach, espoused by Steven Covey, is “First, seek to understand” the other’s position. It works in both directions, helps to avoid personal attacks, and gets better results than repeatedly trying to present one’s personal position.


    BJC
     
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  7. Apr 12, 2019 #27

    TFF

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    Different to be different makes no sense. Different because you are right, does. That is where you are jumping in without waders. This group does want something new, but you have to prove it. Burt Rutan did not ask; he showed. Bantering around little segments of what you want helps hone the details. Laying out a full plan, to this group, means you thought it all out to the end. It’s not like telling your wife what you are doing and getting a “That’s nice dear” response. This is peer review. It is a tough crowd that takes everything serious. I get shot up all the time. The doubting Thomas’s want it flown and then you teach us. I think it’s hard to see that being on your side, but telling you it’s off is how this group shows love. They don’t want people getting hurt in some idea that is not developed.
     
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  8. Apr 12, 2019 #28

    Vigilant1

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    Most people eventually recognize that some questions are not sincere requests for information ("What do you think of my little Timmy's art project?" "Does this dress make me look fat?"). But when the subject is airplane structures (whether I'm proposing the spitball idea or giving the feedback), I usually assume that a sincere and unvarnished exchange of ideas is preferred. I put forward a lot of ideas here: Like most people, I get a warm fuzzy from the replies that say "That's kewl!!!!! Build it!!!!," But the replies that say "I think you may be overlooking the torque stress on the ..." do me a lot more good. And I appreciate the time taken by those who respond--some take a few moments, some invest a half hour or more to give me information that they believe I need--and often I do need it. Being ignored is far worse.
    Anyway, I apologise here and now if I've caused hurt feelings or soured anyone's day.
     
    Last edited: Apr 12, 2019
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  9. Apr 12, 2019 #29

    Sockmonkey

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    Well the point of using the low density foam for part of it would be to not need lightening holes. That way the construction is just gluing the foam together and hot-wiring it as one big piece.
    Note that the layout shown is not necessarily the best configuration, but just to illustrate the concept.
     
  10. Apr 12, 2019 #30

    wsimpso1

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    Well, all ribs deliver the lift load collected from the skins to the spar, so the type of rib matters little to the spar within the same airloads and rib spacing. If you take one design and increase the rib spacing (use less ribs) while you have the same total wing area and air loads, then you will have higher loads at each rib mount to the spar. Usually the limitations of the skin and the airloads set rib spacing, then you design (or select) ribs suitable to the loads, then design (or select) mountings for the ribs...

    The foams selected in massive core wings and their spars usually have a lot to do with if fuel is adjacent, fabrication, and convenience to the associated manufacturing process.

    The airfoil shape is hotwire cut from styrene foam slabs because styrene is the only foam we can both hotwire cut and produce reasonably safe fumes. Other common and relatively inexpensive foams either do not hotwire well or make highly poisonous gases in the process.

    Then there are foams that are flat stock and make nice box sections when that is needed. You can laminate the inside of the pieces, glue it up into a nice sturdy box section, then build all of the structure on the outside of the box.

    If you have fuel in the vicinity of a spar, you must use foams that are unaffected by fuel vapors and liquids. Styrene foams practically disappear in the presence of gasoline - yes, gasoline vapors diffuse through fiberglass and destroy styrene foams - I have seen it during a post-accident teardown. So if the spar forms part of a fuel tank, you should use no styrene foam in it. Between the need for a box of high torsional stiffness and fuel in the adjacent structure, this had to drive the design decision in the center section of the Vari-Ez and Long Ez.

    In my spars I have a several core materials. PU foam or Divinycel for most of the shear web core, and laminated plywood or phenolic-cotton plate at hard points.

    The thing about most foams and other core materials in sandwich structures is that the foams have strain at failure that are much higher than glass-epoxy and graphite epoxy we use for carrying loads. In properly designed and built structures, the foam is not part of the load path, it just deforms with the composite structure, which will then reach its failure point long before the foam or other cores are threatened.

    Billski
     
  11. Apr 12, 2019 #31

    wsimpso1

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    Bob's Akafliegs look like a lot of fun, and they build really cool stuff. Funny, his part timing sounds just like mine to build wing skins and spars and stuff.
     
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  12. Apr 12, 2019 #32

    Victor Bravo

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    ^^^ This hits the nail on the head. Most of us consider most of each other to be our peers. Not necessarily equals in every way (because we have different levels of experience and expertise), but most of us frequent HBA participants are peers in the sense that we are interested in experimental airplanes at a higher level than just an occasional passing fancy. The amount of time and energy and thought that many of us put into this forum is some sort of a threshold, or street cred, or rite of passage.

    Formal "Peer Review" in a scientific journal is of course not what we do here. But the scientific journals also do not represent much real-world hands-on experience either.

    I cannot speak for how it works in Australia, but here in my corner of the world pilots sit around for hours in a hangar explaining loudly why our airplane is 100 times better than everyone else's, and how each of us is smarter and better looking than everyone else. We throw around an awfully large amount of BS, laughter, tall tales, and unsolicited advice. Mixed in with all of that are serious moments where we keep each other from getting carried away with ideas that won't work. Also. our hangar BS sessions at my airport are overflowing with comments that would seem insulting and completely offensive if they weren't being made by our peers who actually love us like brothers... "You f****ng moron, you can't carry a motorcycle under the wing, how the **** would you balance out the weight on the other side, with a refrrigerator you a**hole?!?"

    In any other place this would be the start of fist fight, or a deep insult, but in our hangars it's met with roaring laughter and a pat on the back, because we're looking out for each other.
     
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  13. Apr 12, 2019 #33

    blane.c

    blane.c

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    Why would you balance the weight of the motorcycle? Just fly Wun Wing Low.
     
  14. Apr 12, 2019 #34

    jedi

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    I thought that is what ailerons are for! Just put a bungee cord on the stick. Might need one on the rudder cables also if it's not a streamlined crotch rocket. :)
     
  15. Apr 13, 2019 #35

    Sockmonkey

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    Yeah, all ribs take load stress, but what I was getting at is that the zigzag ones in a warren truss are doing extra duty compared to regular ones.

    I was under the impression that the foam used in hot-wired wings was a structural component to a greater extent than it apparently is.
     
  16. Apr 13, 2019 #36

    Vigilant1

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    Probably true. The XPS foams typically used by homebuilders for hot-wired solid cores are about as lightweight (approx 2.0 - 2.2 lbs/cu ft) as can be used while remaining enough "toughness" to survive the building process. Incorporating foams of higher density in this type of construction would increase weight for little benefit.
     
  17. Apr 13, 2019 #37

    wsimpso1

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    Usually ALL of the foam in a massive core is 2 lb/ft^3 polystyrene flotation billet from Dow. Way sturdy on thousands of VariEze's, LongEz's and their many derivatives for wings, canards, tip sails, and control surfaces.
     
  18. Apr 13, 2019 #38

    wsimpso1

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    What does any one rib do? The top skin is being pulled up by the moving air outside and stationary air inside the wing. The skin from half-way to the next rib on the inside to half way to the next rib on the outside collects this airload and delivers it to the rib, trying to pull the top strip up. The bottom skin does similar things downward to the bottom strip. The difference between the top skin upward force and the bottom skin downward force is the net lift for that piece of the wing. The rib has to keep both skins from leaving the airplane and deliver the difference between top and bottom to the spar. If the wing chord, rib spacing, airspeeds, and lift generated are the same, the load in the rib and into the spar are the same... what truss you use and how heavily stressed the parts are does not change the load delivered to the spar, nor the separating loads within the rib.

    Where the truss design comes into things is in how heavy the ribs are when they have adequate strength. There may be weight advantages to certain truss designs when built to strength, but that does not change the loads at the spar unless you change one or more of wing chord, airspeeds, lift developed, or rib spacing.

    The foam in a massive core sandwich structure has low strength but is lightly loaded compared to its strength. These structures have sometimes been called ribless, when indeed what they are is one long low density distributed rib.

    The foam core does carry the forces trying to separate the skins but even sea level air density and up around Mach level speeds, these forces are only in the 10 psi range, so the foam is not breathing hard over that...

    The lifting forces (the difference between upper and lower skin forces) are reacted to the spar through three paths - the wing skins' are bonded to the spars, the foam cores are bonded to the spars, and any close-out ribs at the ends of each piece of wing will distribute lift from foam and skins to spars. You have so much foam and so much bond area of foam to wing that a significant fraction of the lift and moments can find their way to the spar through foam.

    The single most important task of the foam is to give the skin shape and keep that shape. If the foam is not well laminated to the spar and to the skins, the skins can buckle locally and otherwise get the wing shape all messed up.

    So make no doubt about it, the foam does stuff. Done right, it is lightly loaded, but it holds the shape of the foil, keeps the skins from buckling, keeps loads on the skins below their shear strengths, helps to carry loads between top and bottom skins, and helps to carry lift to the spars.

    As long as you are below chords of 6 feet or so, that solid core makes for a lighter wing than any "hollow" structure of the same strength and same cloths, as the stuff you have to put in when you remove the massive foam core (skin cores and inner facings, ribs and rib flanges, attaching tapes, adhesives, and other stuff) all weigh something. Solid foam cored pieces can be very efficient aero structures...

    Billski
     
    Last edited: Apr 14, 2019
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  19. Apr 14, 2019 #39

    Sockmonkey

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    That's about what I figured. The spar is usually carbon fiber or something yes? Is it viable to use some sections of higher density foam to function as the spar?
     
  20. Apr 14, 2019 #40

    wsimpso1

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    Foam is generally fully encased in fibers and resin that contribute the vast majority of the stiffness and strength of the structures. The foam then moves with the rest of the structures. The foam is great for getting the shape you want for the composites that go around it, and then helping them stay in that shape. Beyond that, they contribute measurable but small load carrying capacity.

    Use foam for carrying shear and bending (what spars do)by itself? Only for really small loads... Holding foam in your hand, it is only a little better than wet noodles...

    Let's get a simple concept in place and then do some things with it. Axial load is easier to get, we will expand to bending and torsion further down.

    If you have a foam and fiberglass structure and you load it, the loads will be distributed per the stiffness of each of the pieces. Stiffness in tension or compression is the sum of EA for any cross section. Imagine a strut of foam, wrapped in fiberglass and put in tension. If it is an ellipse 1"x3" and the fiberglass is 0.021" thick, the cross section will have 6.28" perimeter of glass, 0.132 in^2 of glass and 2.356 In^2 of foam. Your glass cloth and epoxy will have E of about 2.2Mpsi and the foam has an E of about 500 psi. EA of the glass is 290000 lb, while EA for the foam is about 1178 lb, for a total stiffness of 291000. The foam is around 0.4% of the total. Apply 1000 pounds to that strut and the glass carries 996 pounds, while foam carries 4 pounds.

    We could look at bending stiffness as that combined with the inverse of length squared is what determines when this strut will buckle under compressive load. In bending, EI is the stiffness. Fiberglass I is about 0.010, foam I is about 0.137. EI of the fiberglass is about 22110 lb*in^2, while the foam is about 69 lb*in^2, for a total 22180 lb*in^2. Foam is about 0.3% of the bending stiffness. Torsion will be similar.

    Billski
     
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