Hi Kenny,

I don't think I contributed much (anything!) when you were posting about FEM because I have to confess I'm not entirely up to speed on what a FEM analysis could do, or how to exploit it - but I *am* interested!!! Please don't abandon it! A thorough analysis of the wing loads, structure and flight surfaces will be very valuable in designing the Next Generation of CD.

I like your idea for stiffening just the top half (or less?) of the D-section - an extra layer of cloth down the length of the top surface would indeed stiffen that section and minimise the weight gain.

I am curious to discover if the false ribs, made from 5mm paper covered foam board that go between the main load bearing ribs of the D-section, are strictly necessary if we are using moulded carbon fibre D-skins, which should be significantly stiffer than the original 0.8mm ply skins. The original plans call for two foam false D-ribs to be inserted between each pair of plywood ribs from Rib #1 to Rib #4, and a single foam false rib between each pair of plywood ribs from Rib #4 out to the tip. I can't see such thin foam ribs adding much to the strength of the D-section, and I imagine that their primary purpose was in helping to shape the 0.8mm plywood skins when they were being fitted. Would anyone care to comment?

Happy Christmas,

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 Kenny Andersen
Sent: 24 December 2012 14:23
To: This email address is being protected from spambots. You need JavaScript enabled to view it.
Subject: RE: [Carbondragonbuildersandpilots] ad First Flight, reserve parachute and 3 piece wing



I haven't posted much since I've been super busy and no one seemed particularly interested in the wing FEM, so I quit working on it due to the demands of work and the low apparent interest.


Anyway, one other option would be to bond in a small hat stiffener rather than a whole additional layer of skin, which would add considerable weight. The forward curved portion of the leading edge is quite stiff due to its shape, the low part is in tension so it is stable, you only need more stiffness in the upper surface of the D-tube. So, you could add material there, or add some type of stiffener.
Kenny



Hi Rick,

I'm interested to read about the wing's behaviour when you flew it at 70mph. I'm guessing the 'boink' sound you describe was coming from the upper surface of the leading edge D-section as the compression forces built up with speed.

I am vaguely hoping that I might be able to tow my CD behind the local sailplane club's Super Cub tug plane, which has a Best Rate of Climb Speed (Vy) of around 60mph (though I suspect sailplane tug pilots tend to fly faster on tow - 70-80mph?)

My existing plans are to use just two laminates of CF cloth (in +45deg & -45deg strips) in the leading edges, but I wonder now if I shouldn't take the weight penalty and use three layers for extra stiffness. How many layers of cloth did you use in yours? I've read that Steve Arndt has towed up at speeds in excess of 70mph without wing flutter or any other issues. Do you know how many layers of cloth Steve used?

Happy Christmas,

Phil.