JEC COMPOSITES MAGAZINE - Issue #80 - April 2013 - 42

structure
tion, fewer individual
parts and fasteners
during assembly and
a reduced usage of
sealants.

Design and
manufacturing

Fig. 3: Cocured bottom integrated skin (5.5mx1m)

Fig. 4: C scan plot of bottom integrated skin’

tolerances, especially the radii tolerances and an outer contour tolerance
of +/- 0.5 mm. The innovative tooling
concepts included the development of
tools showing the required stiffness at
the required location, so as to ensure
uniform consolidation. An innovative
vacuum bag technology was also developed for this project.

Cost and weight
The main challenge in the development of any aircraft structure is to reduce cost and weight. Compared to an
equivalent aluminium box, the weight
saving was about 25%. The integral
construction not only reduced the
number of individual parts, but also
eliminated 2500 fasteners and resulted
in a cleaner aerodynamic surface.
This innovative technology reduced
manufacturing time and cost from
many different aspects, including
reduced inspection and documentaTab. 1: Comparison between Metal &
Composite parts

Metal

Composites

Weight

33.5 kg

25.2 kg
(24.8% reduction)

No. of parts

No of
fasteners

3400

0 (zero)

In any composite
development, design
and manufacturing
have to be done
concurrently. The
design team came out
with an innovative
ply development design where the entire
sub-structure, consisting of bottom skin,
front spar, rear spar,
ribs and stringers, could be developed
in one single shot. The structure was
conceived to ensure the continuity
of spars, skin and stringers across the
left-hand (LH) and right-hand (RH)
sides. The primary load-carrying plies
in the skin and spars were made continuous across the LH and RH sides.
This not only helped to maintain a
smooth load path from outboard to
inboard, but also avoided a heavy mechanically fastened joint between the
LH and RH skins and spars. The development phases included: conceptual design, detail design, structural
analysis, and design for manufacturing
including CFC tool design, reinforced
flexible silicon rubber intensifiers
and assembly sequence. The tooling
developed was extremely challenging
for co-curing the ribs (with large web
depth) while meeting dimensional
accuracy requirements, as the co-cured top skin has to be mechanically
fastened to these rib flanges.
The very first prototype parts were
qualified for the final assembly on
the aircraft. The innovative approach
adopted to develop the horizontal tail
bottom skin using composite materials thus saved weight and accelerated
the structure production time.

42 jec composites magazine / No80 April 2013

More information
The Advanced Composites Division (ACD) is one
of the major divisions of the National Aerospace
Laboratory (NAL) and a centre of excellence
in composite structures. ACD contributed
significantly to the design and development
of composite structures for both military and
civil aircrafts. The division, with a judicious mix
of basic research and applied R&D, strives to
identify future trends and stay at the forefront
of technology. It has a high level of expertise in
the area of design, fabrication, non-destructive
evaluation, repair and structural testing. ACD has
developed primary structures (wing, horizontal
tail, fin, rear bulkhead, control surfaces) with
the co-curing technology for both military and
civil aircraft, which have resulted in substantial
reductions in part count, weight and cost.

Military and civil applications
There is a huge market potential for
this innovation as it has demonstrated
that it is possible to explore co-curing
technology using composite materials
to produce large structures such as the
bottom skin of an aircraft horizontal
tail. This innovative approach can be
extended to larger structural parts
for both military and civil aircrafts.
This will provide even greater benefits with respect to weight and cost.
The additional benefits, when used
for wet wings, will be the reduced
maintenance costs as all leaks will
be eliminated due to the co-curing
technology.

Fuel and fatigue savings
The baseline statistic available from
literature is that 1kg of airframe mass
reduction implies a 3-tonne fuel
savings over the lifetime of a typical
large civil aircraft. When considering
a fleet, it is an enormous benefit. The
weight of the structure was reduced
by 25% as compared to a conventional
metallic structure. The reduced number of holes increases the fatigue life
of the structure and this innovation
also results in significant manufacturing and assembly cost savings. n
More information:
www.nal.res.in

http://www.nal.res.in

JEC COMPOSITES MAGAZINE - Issue #80 - April 2013

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