Foam wings - spar sizing, cutout questions

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When making foam wings (as opposed to built-up wings) for a 2M sized ship I just realized I might need to consider adding a spar. This was not an issue with the 1/32 balsa skinned handlaunch glider I am building.

So how do I cut out the indentations in the foam cores for the spar? Do I make it part of the template?

How about spar materials? I have CF arrowshafts, but they are only 36" long and probably difficult to join. Would I be better off just going to the LHS and buying a spruce spar with plywood dihedral braces like on my Highlander?

TIA

[Ollie]

Before you deside on the construction method you should design the spar. The vertical height of the spar should match the thickest part of the airfoil. The top spar cap should be sized to carry the compression associated with the maximum bending load. The bottom sparcap should be sized to carry the tension associated with the maximum bending load. The shear web should be sized to carry the shear associated with the maximum bending load and to resist the crushing load associated with buckling in the top spar cap.

Take the maximum tow line tension and divide it by two, half for each wing half.

For ease of computation and a built in safety factor, distribute the load uniformly along the semi span.

The resultant shear at the center of the wing will be equal to half the tow line tension and zero at the wing tip. The shear will decline in a straight line between the root and tip of the wing.

The bending moment at the center of the wing will be half the maximum tow line force times half the semi span of the wing. The bending moment along the semi span will decrease in a parabola that is zero at the tip.

The compression force in the top spar cap at the wing center will be the bending moment there divided by the vertical dimension of the spar. The tension in the bottom spar cap will be equal to the compression in the top spar cap. If the spar caps are linearly tapered in both width and thickness their strength will match the load all along the span.

If the shear web material and width are selected to match the shear load at the center of the wing and tapered linearly to zero at the tip, the shear web strength will match the shear load all along the span.

A spar that is of uniform crossection will weigh about three times more than a spar that is tapered to match the bending load along the span.

In the case of a wood spar the strength in compression is only about half the strength in tension. Therefore, the bottom spar cap can be about half the crossection of the top spar cap.

In the case of pultruded carbon rod and precured, tapered carbon spar caps, the compression strength is about 86% of the strength in tension. Therefore the bottom spar cap can be 14% smaller in crossection than the top spar cap. In the case of tapered carbon spar caps, the tapering not only saves weight but cost.

Go to:
http://groups.yahoo.com/group/Allegro-Lite/?yguid=108420033
Join the group for access to the files section. In the files section you will find plans for several foam core wings and some ideas of how to build them. Study Dr. Mark Drela's designs for properly designed spars.

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Wow, thanks Ollie. After reading all that and Dr. Drela's comments on spar sizing I realize it isn't so simple as walking into the store and asking for a 1/4" spruce spar and gluing into a foam channel.

I think I'll go back to my Sovereign and dreaming about a Big Bird... :)