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Wing calculations 11 Dec 2013 23:48 #691

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
To: This email address is being protected from spambots. You need JavaScript enabled to view it.
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.>
To: This email address is being protected from spambots. You need JavaScript enabled to view it.
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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