composite helicopter

Composite Helicopter Structures

Current and Future Challenges

Mike Overd

AW Head of Structures Design and Development

Copyright 2011 Agusta SpA, Westland Helicopters Ltd, Westland Transmissions Ltd and AgustaWestland International Ltd. (Collectively known as AgustaWestland)

Copyright and all other rights in this document are vested in AgustaWestland.This document is supplied on the express condition that it may not be disclosed, reproduced in whole or in part, or used for any purpose other than for which it is supplied, without the written consent of

AgustaWestland.

Composite Helicopter Structures Current and Future

Challenges - Presentation Outline

Helicopter market

Product Competitiveness

Airframe Construction material and architecture choices

Helicopter Airframe Design Challenges Vibration,

Crashworthiness and Cost of Ownership

NCC and Supply Chain Priorities

Questions

Copyright and all other rights in this document are vested in AgustaWestland.See title page for conditions on use.

Global helicopter market is for approximately 1000 1450 airframes per

year

Aircraft prices range from $50k (small civil) to $30M+ (large military with

complex weapon systems)

UK Market ~1000M/yr total value (new build and support)

Airframes account for ~5-10% of the product cost

NH90 500 in the order backlog

AW139 >400 a/c delivered since 2003

Very competitive market


Challenges The Market


Helicopter market also has significant value in the secondary structures

nearly all cowls and doors are composite


Challenges The Market



Challenges - Product Competitiveness

Productivity Payload/range

Drag

Vibration suppression

Hover performance

Effectiveness/Availability DNAW

Reduced maintenance

Reduced crew workload

Cost of Ownership Initial purchase cost

Corrosion

Vibration Structural fatigue

Crew fatigue

Equipment reliability

Safety Cat A Performance

Crashworthiness

Reduced empty weight fraction

Optimised structural design/new materials


Helicopter Airframe

Construction Material and

Architecture choices


AW do not see that future airframe structures will all adopt a common

material or architecture choice

Use of composite materials has increased - now be levelling off slightly?

Metallic fuselage frames combined with carbon composite skins (a hybrid

architecture) is currently seen as a good choice by AW


Challenges - Material and Architecture Choices

The choice between sheet

metal, composite, monolithic

and hybrid structures depends

on a number of factors: Weight

Cost

Environment

OffsetCopyright and all other rights in this document are vested in AgustaWestland.

See title page for conditions on use.

Helicopter airframes generally feature frame and panel structures

Widely pitched frames and large panels are more weight

efficient

Sandwich panel required for stability but problems: progressive facing ply delamination (for example caused by hot

exhaust gas impingement)

progressive core tearing failures (caused by water vapour

expansion following moisture ingress into the cells of the core)

failures must be detected cost of ownership penalty




Composite monolithic construction not widely used

weight is only competitive if designed as post-buckled

difficult to analyse

difficult to guarantee bonds (also peel stresses) so AAs require

fasteners

Thermoplastic skins/welded integral stiffeners in a post-buckled design

could be the answer




Our ability to predict the failure strength of composite structures has

improved

This allows less conservative designs and hence improved empty weight

fraction




Conventional sheet metal+ Tooling and raw material

cheap

+ Easy to modify and repair

+ Post buckled design

structures are very light-

weight

Monolithic machinings+ Tooling cheap

+ Low parts count

+ Efficient design possible at

high load

High parts count

High direct labour content

Maximum sheet gauge -

becomes inefficient at high

loads

Minimum machining

thicknesses may cost wt

High material wastage




Composite plus points

+ Large components with complex surfaces

+ Reduced parts count can be achieved through single piece

mouldings

+ Reduced cost if production volumes exceed the break-even

point

+ Weight saving

+ Tailored stiffness can be designed if required

+ Increased fatigue resistance

+ Increased corrosion resistance

+ Increased damage tolerance

+ Improved damping - so noise and vibration are potentially

lower




Composite negative points

Need to account for manufacturing variability

Raw material - costs are high, and epoxy materials have limited shelf lives and require

refrigerated storage

Material strengths incur moisture, temperature and impact damage degradation penalties

There is a higher risk of material obsolescence (requiring requalification) than with metals

Tooling and processing costs are higher (high break-even point)

Costly manufacturing development (spring-back/distortion)

Costly to make variant changes (cost of design and tooling changes)

Quality assurance requires strict process controls with associated cost

Poor electrical conductivity gives a poor ground plane for avionic systems and poor

lightning strike dissipation thus requiring the use of bonding leads or conductive layers

which are effectively parasitic weight and are themselves prone to corrosion

Metallic components, in contact at joints, are at risk of galvanic corrosion (cost of

prevention)

Fire hazard toxic fumes in the event of a fire

Difficult final disposal (no recycling)

NDI techniques for the detection of in-service damage can be difficult and costly for the

operator.




Multiple structural materials are selected

Dependent on design case, failure mode and manufacturing




Thermoplastics

chemically inert lower environmental degradation

good mechanical properties at significantly higher temperatures

than most conventional epoxy resins

tougher - higher compression after impact strength

very difficult to bond using conventional adhesives without the use

of expensive surface treatments such as corona discharge

press or stamp formed from pre-heated flat sheets specific

tooling and techniques required to avoid cracking or fibre wrinkling

resistance and induction welding techniques developed to join

thermoplastic components together. For example, internal ribs

can now be joined to skins to form integrated structures without

the need for fasteners.




Helicopter Airframe Design Challenges

- Vibration and Crashworthiness



Challenges - Vibration

High frequency vibration is the number one technology challenge

Determines cost of ownership

Crew fatigue

Passenger comfort

Equipment reliability

May limit the flight envelope

Vibration suppression can account for significant proportions of the

empty weight fraction




High frequency fatigue loads very difficult to predict

Elastic response of

fuselage to all the

different forcings

Large amplitude high

frequency unsteady

aerodynamics varying

around the azimuth

Transmission

fatigue at gear

meshing

frequencies

Elastic response of blades

long flexible cantilevers,

rotating, stiffened by CF,

subject to unsteady

distributed loads

Large Mach

Number range

around the

azimuth in forward

flight

Significant rotor/rotor

and rotor/airframe

interactions




The dynamic response of the structure is also very hard to predict

Non-linear, varies with load, multiple modes, modal interaction, damping

Can the wider adoption of composites help with modal frequency

placement optimisation and improved structural damping?



Challenges - Crashworthiness

A crashworthy structure maximises the occupants chances of survival in a given

crash scenario

Becoming a key design requirement

Fuselage provides a protective shell

Interior components do not detach and harm occupants

No penetration of occupied areas by heavy masses (engines, gearboxes etc)

No inward buckling to minimise injury risk to occupants

Seats & harnesses restrain and decelerate occupants within human tolerance

The undercarriage,

undercarriage attachments,

structure and the occupant

seating and harnesses all need

to operate as a system of

systems optimised to the same

level



Challenges - Crashworthiness

Crashworthiness requires the structure to deform to absorb energy

Composite materials are generally brittle not well suited to this reqmt

Frangible (progressive crush) composite elements have been designed

but at extra weight, complexity and cost

Good crash design costs weight and requires fundamental configuration

choices

Crushable under-floor structures (generally metallic)

Stroking seats (complex mechanisms)

Strong but relatively ductile frames (generally metallic) supporting

the heavy mass items

Strong attachments so that heavy mass items do not break lose

Long stroke undercarriages with features to optimise crash

performance (such as crash blow-off damping valves)

Fore/aft oriented under-floor members to resist ground ploughingCopyright and all other rights in this document are vested in AgustaWestland.

See title page for conditions on use.

Helicopter Airframes - Cost of

Ownership



Challenges - Cost of Ownership

Corrosion of metallic structures

Breakdown of surface protections,

water traps, dissimilar metals

No 1 cost of ownership driver (US

Army)

Composites have a clear advantage

De-bonding of bonded structures

Hot gas impingement/high temps,

manufacturing defects, growth from

impact sites

Both composite and metallic are

susceptible

skin debonding




Fatigue cracking

Global and local vibration, pre-stress

during manufacture, usage increases,

in-service abuse, errors or omissions in

qualification

Composites have a clear advantage

Damage

Discrete source damage greater than

the threat allowed for at the design

stage

Composites require particular attention

esp BVID and lightning strike

Location 6.14-5r

Lower Right Fitting P/N 3G5350A01853

pad location

Location 6.14-5rBIS


pad location

Location 6.14-5rABIS


pad location


NCC/Supply Chain Priorities



Challenges Future Priorities

Supply chain

We want the impossible low weight, high quality and low cost

Risk sharing look for suppliers with a technology offering esp weight

Generally make to print but with the manufacturer pre-selected and involved at the

design stage

NCC

Must attack the composite negative points

Lower cost tooling and processing

Simulation of finished shape and processing

New matls must have better degraded allowables at the same cost

Thermoplastics a priority

RTM jury still out

Ultra high temperature (carbon/carbon) materials of interest

Reduce operator NDI through reliable/certified SHM



Challenges Future Priorities

Use of

thermoplastics

to resist

exhaust

impingement

temps and

reduce empty

weight fraction

post buckled

design

Simulation of

manufacturing

processes to

achieve right

first time

Advanced

manufacturing

technology fully

digital, high

speed

machining, rapid

tooling etc

Embedded SHM

to detect defect

growth

Cabin design optimised for

minimum vibration response

architecture/component

design/active damping

Ultra high temp

carbon/carbon

exhausts


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