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Thread: Rib construction comparisons

  1. #1
    Moderator Dana's Avatar
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    Rib construction comparisons

    I came across this comparison of wing rib construction (various built up vs. capstripped foam) on the Vintage Ultralight forum, though it's originally from a sailplane publication. Interesting...

    -Dana

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    Registered User GESchwarz's Avatar
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    Re: Rib construction comparisons

    Great post Dana. I'm building my design now which includes the utilization of PVC foam ribs. I have not done any testing yet but I do have sample materials of 4 different densities. The test results you provided is very interesting.

    As with aluminum, other materials like foam and adhesives are all vulnerable to stress risers. My testing of adhesives has taught me that failures tend to initiate where there are stress concentrations and where deformation begins under load, in tension or compression. Maximizing moment arms and using stiffer materials that resist deflection reduce the stress risers that initiate failure.

    If anyone has any thoughts on what foam/adhesive tests I should perform, please let me know. I haven't really begun to plan my testing yet so I'm soliciting your thoughts.
    If you see something, say something.

  3. #3
    Registered User rtfm's Avatar
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    Re: Rib construction comparisons

    Hi,
    I've been pondering how to build my wings also. I've followed Billski's excellent comparison between different building techniques (solid foam core vs ribs). In post http://www.homebuiltairplanes.com/fo...html#post42507
    Bill said:
    "With a constant chord wing, at 15% thick, the wing will be 7.38" thick, and the foam will be 72ft^2*(7.38/12ft)*2lb/ft^3*0.62 = 55 pounds of foam". Using numbers more representative of my plane, this equates to something like:
    43ft^2*(4.3/12ft)*2lb/ft^3*0.62 = 19lbs of foam That's still quite a lot for a small wing. Add to that the .18lbs/ft^2 for the glass which Bill calculates and the wing will now weigh 19lbs+18lbs (approx) = 37lbs. Add the weight of the spars ( I seem to recall a weight in the region of about 6 lbs). Total weight of wings: 43lbs

    I have also read with great interest how the SD-1 is built and how much its wings weigh. And I have to say - the SD-1 numbers are also most impressive. From their web site:
    Wing Two part wing uses GA 37U-A315 airfoil. It consists of composite main spar with carbon caps on which are glued ribs made of extruded polystyren. It is covered with 1 mm plywood. Wingtips are made of extruded polystyren with fiberglass layup or premolded are vacuum bagged sandwich. The weight of painted wing half is 11 kg (24 lbs). So their entire wing weighs about 48lbs, ready to fly.

    Granted, the SD-1 has small wings (64.6 sq ft), but mine are going to be even smaller (43 sq ft) If the SD-1 numbers are to be believed, and if they have been sized correctly, I must assume they are adequate to the task, and so if I build using the same method (marske-type spar, polystyrene ribs, 1mm plywood skins), I must end up with wings which will weigh almost a third less than theirs (32lbs approx).

    There's not a lot in it, but the SD-1 method does seem to produce a lighter wing.

    But, I don't trust my maths, so please feel free to correct me.

    Duncan

  4. #4
    Registered User wsimpso1's Avatar
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    Re: Rib construction comparisons

    rtfm,

    I don't doubt that a lighter wing can be built, nor do I doubt that it can be adequate for a fairly low speed craft with low wing loadings. We know that the foam cored, 3 UNI skinned wing design is good in Long EZ (and its derivatives), Defiant, and Catbird level aircraft.

    Forces trying to destroy wing skins and pull them off the ribs go with wing loading times g factor, with V^2, and with panel size (between ribs, spars, etc) squared. Forces trying to break the ribs goes with wing loading times g factor and with panel size squared. If your wing loading will be higher and/or your dive speed will be higher than in the Markse gliders (I think both will be much higher) you need to analyze it to see if it will stay together.

    Billski

  5. #5
    Registered User Jan Carlsson's Avatar
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    Re: Rib construction comparisons

    The SD1 is a very light airplane, using a light engine and is slower then you will be with the BMW so i think it will be hard to make a lighter wing then that.

    look also at the Gazaile2 it is 2 place and have a slightly heavier engine then the BMW
    Accueil

    Jan

  6. #6
    Registered User rtfm's Avatar
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    Re: Rib construction comparisons

    Hi guys,
    Thanks for the input. And Jan, fabulous photos of the Accueil construction.

    Duncan

  7. #7
    Registered User Mac790's Avatar
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    Re: Rib construction comparisons

    Quote Originally Posted by Jan Carlsson View Post
    look also at the Gazaile2 it is 2 place and have a slightly heavier engine then the BMW
    Jan
    Jan thanks for this link, I haven't heard about that little plane before, looks nice.

    btw here is a direct link to the official Gazaile2 page, in English, (they also sell plans for 230 Euro)
    Translation result for http://gazaile2.free.fr/communaute.php

    Seb
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    Registered User WonderousMountain's Avatar
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    Re: Rib construction comparisons

    Wow, you guys are pretty obsessive.

    rtfm

    You have 150 lbs of engine, 250 lbs for the pilot, since you intend to sell this plane right? Landing gear control surfaces and every other manner of essential thing on the airplane. Assuming 500 lbs weight on the wing, you're trying to build a wing that weighs 43 lbs; roughly 1/12 the planes weight.

    Unless you're going around the world or have already pushed yourself to the edge of the microlight catagory and need to save the weight. I don't see the point. Splurg for a few extra pounds and rest easy in the air.


    Wonderous Mountain

  9. #9
    Moderator addaon's Avatar
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    Re: Rib construction comparisons

    Having a wing at about 8% of MTOW isn't particularly unusual; the current design I'm looking at is right at that point.

    "Splurging" on pounds doesn't make one rest easier; besides increasing cost, it potentially decreases safety both in structural influence (higher weight and inertial loads) and in reduced performance (climb rate, roll rate, stall speed).

  10. #10
    Registered User WonderousMountain's Avatar
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    Re: Rib construction comparisons

    There wasn't any intentions of insulting either the airworthiness or frugality of the Strongback designer. The roll rate might be the biggest reason for extreme lightness in the wings. Perhaps splurg on materials, splurg on research, splurg on testing, and splug on refreshments, but splurging on pounds is probably my worst advice yet.

    Well, I guess nothing's stopping me from making a wing as heavy as I like on my own Plane!


    Wonderous Mountain


    Future builder of the flying pig!

  11. #11
    Registered User rtfm's Avatar
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    Re: Rib construction comparisons

    G'day mate,
    Hey - ease up on the worry... The guys have been plenty honest in their opinion of my designs over the time I've been posting here. So definitely no thoughts of your post being insulting. If we don't all question each others' designs, we're all in the crap come initial flight time...

    But, the Razorback with its carbon fibre strongback taking most of the flight loads is good to go into production, I think. Just waiting for some funds to come in so that production can begin.

    As you've said in the past - those who wish to disagree with me have my full support (or something like that...)

    Cheers,
    Duncan
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  12. #12
    Moderator Dana's Avatar
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    Re: Rib construction comparisons

    Quote Originally Posted by WonderousMountain View Post
    Unless you're going around the world or have already pushed yourself to the edge of the microlight catagory and need to save the weight. I don't see the point. Splurg for a few extra pounds and rest easy in the air.
    But most ultralights are right at the edge of the legal weight (and usually, on the wrong side of that edge). In ultralight design, ounces count.

    -Dana

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    Re: Rib construction comparisons

    saw that someone mentioned the sd-1 wing. I am planning on building a sd-1 but would like to have more carbon/ glass in the plane instead of wood. anyone think that the foam ribs could be laminated with carbon sheet with carbon tape on the top and bottom of the rib, and a carbon wing skin bonded to the top and bottom of the wing instead of a plywood skin. with this construction the ribs could even have lightening holes cut in them since sheeted with carbon they will not have the compression forces on them and will serve just to keep the sheets of carbon apart.

  14. #14
    Moderator Dana's Avatar
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    Re: Rib construction comparisons

    With a solid wing skin, I don't think you'd need the carbon tape capstrips on the ribs; you could just bond the wing skins directly to the foam.

    -Dana

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    Re: Rib construction comparisons

    If foam weight is a concern why not cut lightening holes in 1" foam with 2" capstrips or, use 1/2" with 1" capstrips or, 1/4" folded foam 4'x4' folded 50' long with 1/4" sq. with 1" capstrips and vertical bracing on both sides. Using 1/4" bracing you'd eliminate cutting and gluing all those gussets. For drooping between widely spaced ribs use stringers like models do. Even Zenith 750 builders added a stringer to reduce oil-canning.

    Looking a many aluminum ultralights with only the upper camber out of bent 3/8" tubing it scares me. Sure it's engineered but that does mean I like it. Have you ever seen the WWII carrier plane skid into the tower, bet the pilot would kiss the engineer for rounding up on the slide rule. You won't see the same with computer calculations 3% or less is the maximum overage today. Over designed is a thing of the past so is repair-ability with hi-priced service calls and parts. As the head of UTA engineering department said; too bad the Romans overbuilt everything, most of which are still standing.

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