lml@lancaironline.net Mailing List Archive

In a message dated 4/1/2004 6:16:42 PM Central Standard Time, REHBINC@aol.com writes:

If you mean by this that one will stick to the other and nothing more, then I completely agree with you.

If you are suggesting that adding 50 lbs of tensile strength in carbon to 50 lbs of tensile strength in glass will result in a combined tensile strength of 100 lbs then you are dangerously mistaken.

Rob,

Of course not! But it ain't 50 lbs either. Perhaps there is over focus on "tensile" strength. Think of the caps operating in opposite directions, say compression for the top and tension for the bottom in positive G's plus enough wing stiffness to distribute the span determined lifting load culminating by support of the fuselage, my seat and ultimately, my slightly overstuffed body.

<<<

While I can't speak for the engineer who designed the spar to which you refer, I think it entirely possible that the contribution of the glass wrap on the spar cap was completely ignored in calculting the capacity of the spar cap. Certainly its contribution is minimal.>>>

Wrong. The carbon spar cap is part of the integral spar. The material used in the web, glass, continues to wrap and encapsulate the carbon cap and contributes to the synergistic combination of both - just like an I-beam (C-beam?) overlayed with sheets of aluminum and rivets, only better. How the hell else would you do it? I don't know what you are flying, but you might want to look at your spar construction if it was made by Lancair. In any event, whoever flys this type of spar need not worry, they only seem to break when meeting the ground at a bad angle and speed. If you really think the contribution is minimal, perhaps you ought to talk to those flying the various EZE's. In some of that class of airplane, a "spar" doesn't even go to the wingtip (the skin takes the load). In others, it's wet layup glass and foam that forms the spar - no carbon at all - and yet they don't come apart either. Of course, their strakes are a bit thick and draggy for my liking.

<<<<<

While you are correct that the carbon spar cap will fail before the the glass wrap, the time difference will be measured in milliseconds.>>>>>

And, your point is? Do you really think that the breaking point defines the work that was successfully done before the break?

<<<<<

This perfectly illustrates the danger in mixing high modulus materials with low modulus materials in a composite structure. If one hasn't taken the difference in modulus into account, the component may fail at 2/3's or less of what was expected. If you normaly operate at up to 50% of the expected failure, then I wish you extremely smooth air and extremely uniform materials.>>>>>>

Excuse me? Let's see, Lancair claims at gross weight that I should be able to handle 4.5 Gs and the wing was tested to 9 Gs. I do not normally operate at 4.5 Gs, the wing usually stalls first because of the envelope in which I operate - you know it as the Yellow arc just beneath the red line.

<<<<<<<My point is simply that one needs to have his thinking cap on when he starts modifying structural components with materials having significantly different modulii>>>>>

I have my carbon cap on. What do you fly anyway? I have never modified a structural component, only the cosmetic speed related stuff - Besides that I have been accused of holding everything together with an overuse of tie wraps.

Scott Krueger AKA Grayhawk
Sky2high@aol.com
II-P N92EX IO320 Aurora, IL (KARR)

LML, where ideas collide and you decide!