The grain type does not make any significant difference in bowing or bending stiffness. Having c-grain on the sides rather than the top and bottom does allow for bit more "bonding" between the top and bottom layers of a motor stick but the bowing or bending stiffness is largely determined by the stiffness of the longitudinal fibers. However, one does not want to glue to a c-grain surface as the interlayer bonding is weak perpendicular to a c-grain surface. This weak interlayer bonding also causes failures when bending along the grain for c-grain sheets. That is why most people use a-grain or b-grain for making formed tips and other such parts.
The torsional stiffness, again, does not depend on the grain orientation.
Rolled motor sticks are another case. Again, the bending stiffness does not depend on the grain of the sheet used. The same goes for the torsional stiffness. However, c-grain should be used for rolled motor sticks rather than a-grain in our lightest planes. Sheets of c-grain balsa have a great deal more cross grain stiffness than do a-grain sheets. In fact, earlier in the history of indoor flying, a-grain sheets were used for rolled tubes as a-grain sheets rolled easier and with less splitting. However, as indoor flyier began using thinner sheets, rolling and splitting were less of a problem. For these thinner sheets, the cross grain stiffness of c-grain sheets is important because tubes rolled from thin c-grain sheets are much less apt to dent and buckle. This makes for easier handling for tubes that also held up better in use. Tubes that are easily dented or buckled cannot standup to compression. This is not because such sheets are weaker but because the dent or buckle is the point of failure.
One more thing to remember is that a-grain sheets usually have more consistent properties across the sheets so spars cut from one side have about the same properties as spars cut from the other side. This is less so for c-grain sheets. A-grain sheets are cut from wood in about the same growth layer while c-grain sheets are cut radially and include many growth layers.
LP
--- In Indoor_Construction_at_yahoogroups.com, Warthodson@... wrote:
>
>
> A debate is raging in our local club. So far the vote is tied. We need your input.
> Imagine two identical motor sticks, same dimensions, same weight, etc. When viewing the motor stick from the side, one is made from "A" grain & the other from "C" grain wood. Which will resist bowing better, which will resist twisting better?
> Thanks,
> Gary
>
Received on Fri Feb 22 2013 - 09:42:17 CET
This archive was generated by Yannick on Sat Dec 14 2019 - 19:13:47 CET