Sailplane & Gliding 1966 - Lakes Gliding Club

NEW FINNISH UTU
W
ORK on the Utu was begun in
1961. After sever.al tests on structural
applications of FRP Shells stiffened
with different foam plastics, a
prototype was flown for the the first time
in August, 1964. Since then, five other
prototyJlCs have been built with various
techniques and structures at the factories
of Oy FIBERA Ab, HelsinkL The
oldest ones. with one or two flight
seasons behind them. have been used
during 1966 for continued structural tests.
As a result the superiority of plastic
materials, especially as compared with
wood, has become evident, both in use
and in production technique.
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11-1.
UTU
During evaluation work the aerodynamic
concepts and outlook of the Utu
have remained the same, except for some
minor modifications based on test flight
results and the opinions of test pilots.
Structure, however, is another story, as
production techniques for metal and
wood aircraft structures are neither as
a whole nor in part directly applicable
to plastic.
The wing structure is an FRP sandwich
shell with a foam plastic core,
single "I" spar and no ribs.. The fuselage
consists. of a load-carrying outer skin,
the rear part of which is also a sandwich,
and separately produced and
bonded fittings for controls and seat, and
a detachable instrument panel. Because
of the big local loads from landing
wheel and wing-fuselage fittings, there
still remains a rib behind the pilot'!> seat.
The structure as a whole is thus open
and easy to inspect. The attachment of
wings is done with two conical bolts.
Some traditional metal parts in control
mechanisms have been replaced with
injection-moulded nylon parts.
Several specimens of alt main parts
have been used in proof loadings. Several
wings with the same amount of
fibre in spars but spread in different
widths and ways on the wing shell were
proof-loaded to clear up the question of
buckling strengths. The resuhs varied
from load factors of + '5.7 to + 14. The
lowest values were obtained with wings
in which a substantial part of the fibre
pattern was spread over a rather broad
surface. This produces a thick and hence
dimensionally stable and accurate wing
contour. However, the cQnstruction of
the wing main fittings needs the fibre
pattern to be concentrated in the middle
at the wing root. This again produces
instability on the wing shell. Even very
small buckling which may come up in
normal use, either in the leading or
trailing edges. showed a tendency to
widen into the middle of the wing with
the addition of 10adin2- The result was
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