Materials for Turbine Power Blades, Reinforcements, and Resins - Hybrid Anisotropic Materials for Wind Power Turbine Blades - page 71

TABLE 3.12
A.Comparison.of.Leading.Blade.Core.Materials
CoreMaterial Advantages Disadvantages

Low.density.(60-70.kg/m 3 ..
Rigid.form.PVC,.SAN

Light.weight
Affordable.at.low.densities

Low.stiffness,.more.expensive.
than.balsa

Medium.density.(155.kg/
m3)..End.grain.balsa

Low.cost
High.stiffness

Heavier.than.foam,.subject.to.
processing.irregularities,.
availability.problems

Source: . Webcore.Inc.. 21 .With.permission.

Other.options.use.impregnated.prepreg.as.a.honeycomb.material.
The. axes. of. honeycomb. cells. are. always. quasi-horizontal,. and. the. non-
angled. rows. of. honeycomb. cells. are. always. horizontally. (not. vertically).
aligned..Thus,.each.cell.has.two.vertical.walls.with.“loors”.and.“ceilings”.
composed.of.two.angled.walls..The.cells.slope.slightly.upwards,.between.9.
and.14°.toward.the.open.ends. 22
Two. researchers. of. the. geometry. of. honeycombs. offer. possible. expla-
nations. for. the. reason. that. a. honeycomb. is. composed. of. hexagons. rather.
than.any.other.shape..One,.given.by.Jan.Brożek,.is.that.the.hexagon.tiles.the.
plane.with.minimal.surface.area..Thus,.a.hexagonal.structure.uses.the.least.
material. to. create. a. lattice. of. cells. within. a. given. volume.. Another,. given.
by.D'Arcy.Wentworth.Thompson,.is.that.the.shape.simply.results.from.the.
process. of. individual. bees. putting. cells. together—somewhat. analogous. to.
the.boundary.shapes.created.in.a.ield.of.soap.bubbles..In.support.of.this,.he.
notes.that.queen.cells,.which.are.constructed.singly,.are.irregular.and.lumpy.
with.no.apparent.attempt.at.eficiency. 22
The.closed.ends.of.the.honeycomb.cells.are.also.an.example.of.geometric.
eficiency,. albeit. three. dimensional. and. little. noticed.. The. ends. are. tri-
hedral. (i.e.,. composed. of. three. planes). sections. of. rhombic. dodecahedra,.
with.the.dihedral.angles.of.all.adjacent.surfaces.measuring.120°,.the.angle.
that. minimizes. surface. area. for. a. given. volume.. (The. angle. formed. by.
the. edges. at. the. pyramidal. apex. is. approximately. 109°. 28'. 16”. (=. 180°. -.
arccos.(1/3))..Three-dimension.geometry.of.a.honeycomb.cell.is.shown.in.
Figure 3.5.
The. shape. of. the. cells. is. such. that. two. opposing. honeycomb. layers. nest.
into.each.other,.with.each.facet.of.the.closed.ends.being.shared.by.opposing.
cells.(Figure 3.6).
DuPont's. aramid. iber. honeycomb. core. PN2. aerospace. honeycomb. uses.
include. aircraft. galleys,. looring,. partitions,. aircraft. leading. and. trailing.
edges,.missile.wings,.radomes,.antennas,.military.shelters,.fuel.tanks,.heli-
copter.rotor.blades,.and.navy.bulkhead.joiner.panels. 23
PN2.honeycomb.is.available.in.sheets,.blocks,.or.cut-to-size.pieces.in.both.
regular.hexagonal.and.over-expanded.(OV).cell.conigurations.