Notes from Roger Partington re mainspar laminations in
answer to a question as to how much wood is needed. (March 2007)
I am sure you are well past this now but I kept meaning to
have a proper look at the question and only now have I found a spare few
minutes.
(If you believe that...)
Anyway, your measurement sounds about right. I reckon that the laminations need
to be of the following lengths (in mm) :
Upper Cap :
1330
1680
2080
2480
3280
4080
5680
7280
Lower cap :
1680
2450
3250
4880
7280
Giving a total length of timber of 47,430mm
Obviously, you would be a bit optimistic to be too exact on those measurements
as you are bound to to get a bit of slippage in the clamping, defects you have
to cut out and if necessary, allowance for scarf joints.
The plans give the required scarf joint lengths and spacing although of course,
it is ideal if you can do it without joints.
I ended up putting a total of 4 scarf joints in the complete spar and in a
laminated member such as this, the reduction in strength is reckoned to be
virtually nil. But of course it is more work.
Also, I built mine out of Douglas Fir which was easier and cheaper to source. I
also like the 10% additional strength as I always feel that it makes up for any
hidden slight defects within the timber and also gives me a little confidence
while rattling through the mountain wave rotor that we get here.
Hope this helps
Roger
