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Wing calculations 11 Dec 2013 23:39 #686

The other big problem with West Systems epoxy is it starts to soften at 120 F. Aeropoxy is closer to 200 F.



From: Philip Lardner <This email address is being protected from spambots. You need JavaScript enabled to view it.>
To: This email address is being protected from spambots. You need JavaScript enabled to view it.
Sent: Tuesday, September 6, 2011 9:40 AM
Subject: RE: [Carbondragonbuildersandpilots] Wing calculations



Hi Russ,

The original CD wing attachment system transfers the cap loads to the centre bolts using strips of 6061T6 Alu either side of and bolted through the spruce spar caps. As you cannot drill holes through the carbon rod caps without totally compromising their strength, the Marsk manual goes into some detail on how to design and calculate a safe way to transfer the cap loads via the carbon caps and shear web using fiberglass or CF spar root end fittings. I haven't done those calculations yet (I wanted some feedback on my CF spar design first) but I will try and get that done this week if I have a spare evening, and post it up. In destructive load testing of his Genisis wing the spar web/caps/root-end fittings survived 20,000 cyclic loadings to +/-g design load before being static loaded to destruction at an equivalent of 19g - so I guess his numbers are pretty sound! I know Steve Arndt used titanium fittings on his spar root-ends but unless I can get my hands on a small enough quantity of this then I will probably use 6061T6 Alu which I know I can get locally.

As composite fabrication is a new field for me I am being guided by Jim Marske's comments in his design manual when it comes to which epoxy system to use. Aeropoxy is tried and tested in the aerospace industry and comes with a range of pot-life times (18-20 / 60-65 / 120-140min) depending on which hardener is used. West System epoxies (also covered in the manual) have considerably shorter pot life times (between 12 & 51 minutes only.) I do not want to be rushed when it comes to laying up large critical pieces!

I haven't really looked in detail into the different foam-boards available (just seeing what's on the market locally) but as ever, the devil's in the detail and the spec sheets will need to be studied closely. Ultimately I will almost certainly order a tried and tested foam from the likes of AircraftSpruce.com or similar. Just curious - what do you mean by 'cotton sheet'?

Phil.



From: This email address is being protected from spambots. You need JavaScript enabled to view it. [This email address is being protected from spambots. You need JavaScript enabled to view it.] On Behalf Of russell wilson
Sent: 06 September 2011 13:28
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Subject: Re: [Carbondragonbuildersandpilots] Wing calculations



hi phill
will the weakest point in the wing be the attachement of the extruded rods to the fitting hard ware?i guess you will use jim marsk's hardware and extruded rod to hardware joining method?
why aroepoxy?..i think i remember reading it was fire resistant...if its costing you moor money and this is the only advantage ...then you might not need it on a glider...or does it have a high T,G or a very low density perhaps?

theres a structual foam called rema that has the surfaces sealed so it does not absorb resin....i have samples here but i have not tested it.

that blue foam used for insulation will be a big miss match of materials with carbon laminated to it.1 layer of your thinnest oldest cotton sheet would maybe be a match..cheeper, lighter and the similar strenth.. as the failure will be in the core material.

if you alow the cotton/blue foam to be the same weight as the carbon blue foam by increasing the thickness of the core material then the blue foam cotton will be stronger..

the pelican uses blue foam.



russ.


On Sat, Sep 3, 2011 at 11:18 PM, Philip Lardner <This email address is being protected from spambots. You need JavaScript enabled to view it.> wrote:


Hi Russ,

Meant to reply sooner - apologies!

The example wings in Jim Marske's manual are all two-part - that is to say, each full wing is comprised of two half wings joined in the centre, the same as the Crbon Dragon and most sailplanes.

I'm still on the hunt for a suitable fine and closed cell foam board. The samples I have so far have, as you rightly point out, too large pores. I have come across a very fine pored insulation board (Kingspan) at my local builders merchants but the thinnest it comes in is 50mm - a bit thick! A thin carbon-plywood-carbon sandwich may indeed be the answer... as long as the plywood can be sealed against atmospheric moisture ingress - the whole point of the exercise.

I'm still trying to get my hands on a batch of Aeropoxy (PR2032/PH3665) to make up test pieces... so far the shipping charges for this 'haz-mat' substance is more expensive than the epoxy/hardner itself - looks like I'll have to order a large quantity straight just to justify the shipping :-(

Phil.



From: This email address is being protected from spambots. You need JavaScript enabled to view it. [This email address is being protected from spambots. You need JavaScript enabled to view it.] On Behalf Of russell wilson
Sent: 31 August 2011 01:07

To: This email address is being protected from spambots. You need JavaScript enabled to view it.
Subject: Re: [Carbondragonbuildersandpilots] Wing calculations




hi phil

does mask;s wing have a join in the middle/at the root?or does the spar on the mask wings continue from tip to tip with out a join?

also on using foam cores with carbon skins for ribs.

in an atempt to be as light; the foam you will search for will be light but the bigger pores will carry moor resin....so the weight saving will not be there.

also...with the very thin layer of carbon....these light core materials fail under compresion...the compression side of the pannel buckles in and squshes the core.

a thin laminate acts like high heals on the foam....thick laminates act as snow shoes on the foam.

after testing many samples the heavier core materials(when using thin laminates are best in terms of weight and strenth.
...ie 200kg m2 foam and balsa(not end grain as it soaks up resin)...you can also get very good results using ply with thin laminates.

on a 200kgm2 foam i had to go down to one layer of 6 ounce glass before i got the panell to breack under tension...all other failures were under compresion of the foam...it is counter productive in term of weight to cary any thing that is heavier and stronger under tention.

you will find that with laminated foam cores you will be heavier or waker...when you reach a happy medium you will find that the wood out performs them.

i hope this saves you time and money.

pure carbon in a shape;ie squre tubing might be your answer for ribs..iv made a set of tail boom ribs that are lighter and stronger than standard..i cant break them with my hands...but vert time consuming to make..also jute/hemp/natural fibers are as strong under tension as e glass but lighter than kevlar.

also a tip.....laminates that are straight not woven zig zag bent are stronger under compresion.

russ.





On Tue, Aug 30, 2011 at 11:18 PM, Philip Lardner <This email address is being protected from spambots. You need JavaScript enabled to view it.> wrote:


Yes, Jim Marske does describe the prototype CD as 'appearing fragile' when compared to other designs. That's one of the big reasons I want to replace all the spruce & plywood with stronger carbon components. I have been gathering together a selection of different square section pultruded CF tubes to compare their properties and perhaps use in the construction of the ribs and stringers. Fabricating the main ribs out of CF-foam-CF sandwich offers the possibility of much stronger components with little (or no) weight penalty. The pdf on rib testing in the files area makes for interesting reading and suggests several possibilities for maximising rib strength... but that's still ahead of me or now!

Phil.



From: This email address is being protected from spambots. You need JavaScript enabled to view it. [This email address is being protected from spambots. You need JavaScript enabled to view it.] On Behalf Of Rick Mullins
Sent: 30 August 2011 13:22

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Subject: Re: [Carbondragonbuildersandpilots] Wing calculations




I bought one of Jim's books and attended one of his composite seminars. He is a sharp guy and a very talented builder, but he does consider the Carbon Dragon a weak design. Shows you that even the really smart guys can't be right all the time :)


I used his book as the basis for all my wing calculations, and I put the spreadsheet with all my calculations in the file section here. I entered the formulas into the spreadsheet so you can change different numbers and it will recalculate the entire spreadsheet. When I get a chance I'll look over what you posted and see how it compared to mine



From: Phil <This email address is being protected from spambots. You need JavaScript enabled to view it.>
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Sent: Monday, August 29, 2011 2:08 PM
Subject: [Carbondragonbuildersandpilots] Wing calculations



Hi Folks,

I recently purchased a copy of Jim Marske's Composite Design Manual and have been using it to calculate the stresses and loads on the Carbon Dragon wing so that I can build using pultruded carbon rods. I have uploaded a summery of my calculations to my folder (Phil Lardner) in this Yahoo Group along with photographs of my long-hand calculations, referenced back to the various steps in Jim Marske's manual. As I am by no means an aeronautical engineer(!) I'd be very grateful if someone could cast an eye over my numbers (and workings... pretty please!) and let me know if I am on the right track.

Interestingly, while working through the example wings in Jim's manual, I discovered a number of errors in the numbers on the diagrams (though the formulae are correct) on pages D3 & D4. I emailed Jim Marske who confirmed my findings and added that no one else had ever spotted them in all the years the manual has been in print. I was quite chuffed at that!

For the purposes of my calculations I derived the shear web depth dimensions by directly measuring them off a full-scale drawing which I drew out on my work bench. The measurments are made at each of the 13 stations along the wing from the root-rib to the tip-rib. Wing cords at each of the 13 stations were calculated trignometrically - I assumed the wing to be a truncated isoceles triangle 60" at the base and 22" at the top and 256.5" tall. The areas of the semi-spans (the area from each station out to the tip) were also calculated using trig. I have treated the entire wing and flaperon as a single piece for the purposes of my calculations - I don't know if this is ok or if I should not include the flaperon area... but seeing as it forms part of the lifting surface, I included it.

One quantity that causes me to raise an eyebrow is the angle at which the wing tapers along its plan form. Measuring the drawings directly I get an angle of 3degrees. However, when I use this figure in conjunction with the other given wing dimensions (root cord, tip cord, wing length) The calculated wing area is nowhere near the published figure in the builders manual. When I work out the angle using trig I get 4.24degrees and the area works out almost perfectly. I have used an angle of 4.24degrees for the purposes of my calculations.

I am aware of the different numbers thrown up on page-10 (step 3) of my workings and later pages in calculating the number of rods required per cap (pages D11 and F1 of the Marske manual.) I haven't spoken to Jim about this yet and don't fully understand the anomaly.

Note that although carbon rod is capable of taking 280,000psi in compression, Jim Marske recommends using a figure of 200,000psi to prevent compressive buckeling.

Please let me know what you think.

Thanks,

Phil.

PS - I am currently building a test rig (based around a 4-ton bottle jack) to measure the shear strength of various materials (0.8mm plywood V CF-twill cloth.) I'll post up photos as soon as I have it finished.

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