GRP Primary Structure Helicop

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G.R.P. FOR HELICOPTER PRIMARY STRUCTURES

51

curves are related to a certain resin cured in a certain way and may no t be a feature of all

the resins in a group.

The glass reinforcement carries the direct loading in a laminate as it is twenty times

stiffer than the resins. Large strains in the resin must be avoided as the ductility is com-

paratively low (3 to 4 elongation at failure) in resins for high temperature performance.

The strength of resins is poor and consequently the interlaminar strength of a laminate is

always low and falls with increasing temperature. Improved properties at high temperature

can be achieved by optimizing the cure of the resin for maxim um strength at the tem-

perature required. The resin stabilizes the reinforcement in compression and therefore

its strength and stiffness and adhesion to the reinforcement determines the strength of

the laminate. The reinforcement is protected by the resin from abrasion, ingress of

moisture, and weathering, which otherwise would reduce its strength. Fire resistance of

the resin is in many cases not compatible with high wet strength and a compromise must

be made for the component concerned.

Resin content within a laminate is reduced to a minim um for high tensile strength but

some workers have suggested that, for optimum compressive properties, the resin content

should be increased. Recent research has shown that a 15 sapphire whisker content

can increase the streng th and stiffness of epoxy resins three o r four times (I).

2.2. GLASS

M ost glass reinforcement is of high alkali E glass, although new high strength and

stiffness glasses are being produced to meet the need for greater tensile strength in rocket

motor cases. The filaments contain numerous surface cracks which form after the glass

is drawn. These flaws weaken the glass appreciably and it is within these flaws that

moisture attack takes place and is partly responsible for the reduced wet strength of a

laminate. To com bat this effect, an d to provide the best bond between the glass and the

resin, the glass surface can be given one of many chemical surface treatments and sig-

nificant improvements have been made. F lawless glass has been made in the laboratory

and demonstrates the great improvement in strength which was expected. However, at

present, practical difficulties make it unlikely that it can be produced commercially 2).

Large diam eter and hollow glass fibres have been made to improve compressive proper-

ties, but this development is unlikely to help structural laminates until resin strengths at

high temperature have been improved.

2.3. GLASS REINFORCEM ENT

The drawn glass filaments are spun or collected into bundles to form rovings and

used to reinforce the laminating resins. T he spun fibres can be woven into fabric and these

fabrics may have a variety of weaves an d be balanced (as much warp m aterial as weft) or

arranged in any proportion which in the limit becomes

unidirectional , when nearly all

the material is in one direction. In filament winding, the spun fibre is wound around a

rotating mandrel under uniform tension from a moving carriage which can be controlled

to vary the helix angle. This material can be used for tubular components or slit along its

length and opened out to form a flat sheet. Non-woven materials are made w ith the rovings

laid flat and pre-tensioned before curing the resin. The pre-tensioning of the rovings does

little to improve the strength properties and the small gain that there is disappears as the

fibre relaxes after three months or so. Due to their straight unwoven reinforcements,

these materials show gains in strength and stiffness over those with woven fabric. T he

high streng th and stiffness of all lamin ates occurs only in the directions of the reinforce-

ment. The properties normal to the laminate and interlaminar shear are properties of the

resin or the resin/glass bond.

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J. S. WILSON

2.4. THE LAMINATE

The glass reinforcement is impregnated with resin and cured using various proportions

of suitable hardener and accelerator at room temperature and pressure, or elevated

temperature and pressure, depending upon the resin concerned. The strength of the

laminate, and especially that at high temperature, will depend upon the resin and its cure

cycle and therefore this mu st be optimized for the design temperature. Practical con-

siderations of tooling and manufacture time and ease of repair under service conditions

will also affect the choice of cure cycle. Mechaniza tion of the man ufacturing process ,

such as in filament winding and pre-tensioned non-woven material, should reduce the

scatter of results found with materials manufactured by hand lay-up methods.

AvP 970 requires an additional strength factor for thermosetting plastics of 1.5 as this

has been shown over many years to assure satisfactory components. The adv ent of the use

of structural plastics on a larger scale shows a need to review this value an d perhaps put it

on a more rational ba sis. American workers have suggested that different factors should

be used for different classes of mem ber or joint. This philosophy takes into account the

difference to be expected in the scatter of strength results, for example, a continuous

laminate and a lap joint under peal loading. A great deal of research work will be required

to determine these factors and there m ay be difficulties in definition of the various classe s

of joints, etc.

2.5. SANDWICH CORE MATERIALS AND PROTECTIVE FOAM

The mo st successful lightweight core materials for sandw ich constructions are of

honeycomb form. These materials are available in aluminium alloy, glass fibre and

resinated papers. Glass fibre is used for its electrical properties in radomes but is heavier

and far more costly than the lightest aluminium alloy and paper honeycombs. Resinated

paper w as, for a time, lighter and stronger than aluminium , but recent improvements in

the alloy have reversed the original position. The long-term quality of paper has been

questioned, since, even when new, the shear failure is accompanied by cracking of the

brittle material. Consequently, the failure is catastrophic in the sense that virtually no

strength remains after failure.

Alum inium honeycomb distorts m arkedly after a static shear failure but retains much

of its strength. Joints between blocks of honeycombs are lighter than w ith paper and the

material can be readily fitted to any shape of single curvature. Metal honeycomb cells used

to be pierced to allow solvent, given off in the cure of the adhesive, to escape. This is no

longer necessary and so, if the sandwich should be damaged , water cannot penetrate

through out. Adh esives to form the skin to core, and core to core bonds formerly were of

low strength at 9oC. Now film and liquid adhesives are available which exceed the

strength of the honeycomb at this temperature and are sufficiently ductile to hold the skins

firmly to the core even when it is severely distorted after failure.

Foam materials of polyurethane and polyether are also used as core materials because

they can be lighter than honeycomb and can be moulded to double curvature. They are,

however, weaker and less stiff than honeycomb and although suitable for stabilizing shear

pane ls they are not stiff enough to stabilize a compression panel efficiently. Where the

sandwich structure is to be immersed in water during service, an outer protective layer

of foam m ay be required to seal the sandw ich. This foam is then covered with a thin

laminate which, being non-structural, can have a greater fire resistance than the structural

skin. Increased impact resistance and noise insulation can be achieved w ith this construc-

tion.


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J. S. WILSO N

modu lus, the secondary being about 10 lower than the primary. A balanced weave

fabric laminate should have a mean modu lus of about 3 x 10~ lb/in2 and some non-woven

materials may achieve 5 x 10~ lb/in

2. Unidirectional materials can be expected to give

5-6.5 x

10~

lb/in2. At 45 the stiffness is reduced to about 66 of the above values in

directions of reinforcement.

3.3.

COMPRESSION IN THE PLANE OF THE LAMINATE

Although the glass reinforcement supports the load as in tension, the resin stabilizes it

when compressive forces are applied. Failure usually takes place following a breakdown

of the resin or the resin/glass bond. This mode of failure is similar to that in interlaminar

shear an d, in fact, a direct relationship has been show n to exist between the compressive

and interlaminar shear strengths of a laminate.

The failing stress of a balanced weave fabric laminate is about 30-4 0,000 lb/in2 in

the directions of reinforcement. A marked reduction has been found at Faireys in

laminate strength when tested at 70 and go

C

due to the reduction in stiffness and

strength of a heat resisting polyester resin. Development of the resin cure system h as

increased the strength from 1 3,000-24,000 lb/in2, which is sufficient for our purpose.

A laminate of unwoven material with an epoxy resin system which showed a failing

stress of over 50,000 lb/in2 at room temperature, dropped to 40,000 lb/in2 at 70~ C, and

between 10,000 and 20,000 lb/in2 at go C. These problems were not expected at such low

temperatures with these resin systems and this illustrates the need for more compressive

testing of thin lam inates at elevated temperatures.

The laminates in our test programme have been the skins of a sandwich with the

stabilizing core of aluminium alloy honeycomb. Other test methods are in use and may

well give different, and sometimes erroneous, results. One rig uses steel guide blocks which

locate the thin specimen at intervals along the gauge length. The faces of these blocks

are lubricated , but we believe tha t the results are still affected by friction.

3.4. MO DULU S F ELASTICITYN COMPRESSION

Primary and secondary moduli are apparent as in the case of tension. Somew hat

surprisingly, in view of the role of the resin in compression, the compressive modulus is

very similar to that in tension at room temperature. The original polyester compression

columns mentioned in the last section showed a 66 loss in stiffness from room tem-

perature to 90 C. After m odifying the resin cure to increase the hot strength the stiffness

regained its room temperature value. A 30 drop in stiffness at 90 C has also been noted

with the non-woven reinforced epoxy material. It is hoped that this drop will be reduced

or eliminated when the high temperature performance of the resin has been improved.

3.5. SHEAR N THE PLANEOF THE LAMINATE

Tests on thick lam inates have shown a balanced weave fabric will fail at 9-10,000

lb/in2 and posses s a modu lus of rigidity of 0.5 x IO

6. Our

0.02

in. thick laminates stabilized

by a foam core in a panel 34 in. x 20 in. achieve a nominal shear stress of 12,000 lb/in2.

There are some reservations about this result a s resin cracking took place at about 6000

lb/in2 on one specimen. These results apply to a shear applied p arallel to one of the

reinforced directions. Tests w ith shears applied at 45 show double the stiffness and a

failing stress of 16,000 lb/in2 (again on a panel 34 in. x

20

in. with foam stabilized 0.02 in.

thick skins). The primary failures are believed to be in the foam and we shall not be able to

establish the ultimate strength of the skins until the panels with honeycomb core are

tested.


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3.6. INTERLAMINAR SHEAR

The interlaminar properties of the laminate are largely controlled by the resin although

the type of reinforceme nt has som e effect. Care must be exercised to limit this loading to

a low safe value and testing to determ ine this must be representative of the application.

Testing for interlaminar shear provides the classic case of the impo rtance of the test

method. It is interesting to note that a non-woven material tested in II different ways

to determ ine its interlaminar shear strength gave results which varied from

2200 to

I I ,700 lb/in2 (4).

3.7.

FATIGUE IN THE PLANE OF THE LAMINATE

An outstand ing property of glass reinforced material is its resistance to fatigue loading .

Woven materials show a considerable improvement over metallic m aterials and non-

wov en are better still. Our axial tension specimen s wh ich are of sandw ich construction

t

P+

lb

St ra in gauge l i nks

Stee l l oad ing blocks

Sk in end w raps

Upper and l ower l oad ing l i nks

gr ipped i n j aws o f 20 U.H.S.

Losenhausen mach ine

Figure

I. Axial tension fatigue specimens tested in 20

U H S Losenhausen machine

and include representative wrapped en d attachments show an endurance limit of + 12

ultimate with a mean load of 24% ultimate. A loading arrangement is shown in Figure

I,

and test results in Figure 2. The allowable skin stress (1.25 scatter factor) is 7200 + 3600

lb/in2.

Surprisingly, our bolted an d riveted specim ens show a similar endurance limit and

these can be compared with allowables of + 4500 lb/in2 in 85 ton steel and rt:1400 b/in2

in high grade aluminium alloy. The glass laminate has a specific gravity o f

1.8

and dividing

the allowa bles by the specific gravity w ill give a mea sure of fatigue perform ance in relation

to we ight. W oven glass reinforced lugs have then a specific fatigue limit of _t 2000 lb/in2,

aluminium alloy & 480 lb/in2 and steel 5 30 lb/in2.

It is not surprising,that our initial testing prob lems were centred upo n the metallic

plates connecting these by specimens to the test machine. In our bolted specimens we


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J, S. WILSON

have inspected the bolt hole diameter at intervals and have plotted an SN curve for 1

increase in hole diameter as well as for failure. This shows that these attachm ents can be

used without breaking the holes. We may get a reduction with temperature and wet

conditions but neither of these factors should be appreciable.

Spec. DRG. no. 102014

Al te rna t i ng s t ress = z tz50 mean s t ress

Fa i l u res o re o f the face sk in

\

away f rom the end w rap

-.

I I I

I

--+__

+

1

I

+++

1 4

1 5 IO6 1

N-cycles to fai lure

Figure 2. G.R.P. axial tension fatigue test.

108

Judging from material development test pieces the properties of the non-woven

materials are better than the woven and show a 30 improvement in strength at IO

cycles (5).

For main rotor b lade construction the lower stiffness of a glass reinforced blade can be

exploited to reduce the loading. Blade deformations in flight being similar w ith a variety

of blade stiffnesses the lowest stiffness blade h as the lowest stress. This is an added

I I

t I

Al te rna t i ng s t ress = +50 mean s t ress Spec: DRG. no. 101985/9 I

Spec imen type X9 th i ckness 0 .05 i n .

c les to 0 .01 d e longa t i on

I04 105 106 IO

N-cyc les

Figure

3.

G.R.P. lug fatigue test.

attraction for glass fibre blade construction (6). Flexible coup lings for rotating shafts are

another application where the reduced stiffness and superior fatigue properties can make

a lighter design than steel (7).

Our axial tension specimens show the effect of in-plane stresses on honeycomb filled

sandwich specimens and we have recently started loading the honeycomb core and core

to skin bond in shear fatigue. Results of a shear beam fatigue test are shown in Figure 4.

The failures so far are all as a result of repeated buckling of the aluminium alloy honey-

comb cell walls and in terms of the ultimate strength the results are similar to those in

axial tension (core shear stress 34 &

17

lb/in2 at

10

cycles). An improvement could

obviously be made in areas which demand better properties by using glass fabric honey-

comb.


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Fatigue testing of glass reinforced specimens can be time consum ing as there is a need

to keep the rate of load cycling down , especially at high strain s. If tested too rapidly the

specimens heat up due to the materials low conductivity. The test failing stress will be

somew hat lower than that at ambient. For this reason we restrict the rate of load cycling

in our tests to a maximu m of

1400

c/min. We are also testing at 2800 c/min to establish

the magn itude of this effect in an attempt to speed up testing at low stresses.

2

50

; 45

E

zl

40

ernaling stress = + 50 mean

08 35

:: 30

Failures are of the alum. all honeycom b

2 a5

E

In 20

F 15

Z

z 10

t

= 5

cl

1 111

1111 111

1 3 1 4 1 5

1 6 IO IO8

N-cycles to failure

Figure

4.

Shear beam fatigue test.

3.8. DAMPING PROPERTIES

Glass laminates have been shown to possess greater internal damping than structural

metals. Quantitatively, the laminate tested showed that the damp ing energy per unit

volume per cycle was ten times greater than that for aluminium and steel (8). The total

damp ing of a plastic structure is unlikely to increase in this proportion, as most riveted

joints w ill be replaced by bonded ones of reduced damp ing capacity. The response of

individual components when being vibrated at a critical frequency should certainly be

reduced. This could be important, as all critical frequencies of various parts cannot be

avoided in helicopter design.

Undercarriages for light aircraft have been made in high strength, non-woven materials,

to replace steel springs loaded in bending (9). It should be possible to make light helicopter

skid undercarriages lighter and with greater energy absorption with these materials.

Telescopic undercarriages containing conical glass reinforced springs have been built and

show greater energy absorption and are lighter than the customary design using rubber

blocks in compression (7).

4. DESIGN

The previous sections of this paper have described glass reinforced plastic materials

and their structural properties. The design problem is to find successful ways of making

reliable, light, aerodynamically clean structures which have reduced m aintenance and

manufacturing costs.

4. I. FUSELAGE

The helicopter fuselage can vary in size from an 8 ft diameter shell, containing passen-

gers and equipment, to a tail boom of I ft diameter, with little more than control cables

with in it. No one type of plastic construction is likely to be the most efficient for this wide

range of structures. In the case of the large structure subjected to bending, it would

clearly be unstable in compression if a solid laminated tube of a sensible weight were used.


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J. S. WILSON

If stringers and frames were added , as in a conve ntional aircraft, the vast num ber of

separate bond ing operations on joints migh t lead to joints of dou btful quality as well as

a multitude of stress con centrations.

A sandwich construction using aluminium foil honeycom b is more attractive for this

size of structure. Inheren t sandw ich pan el comp ressive stability is illustrated in Figure _c,.

The shell can be reinforced with a gridwork of frames and longitudinal sandwich mem bers

under the floor to carry the loads from freight lashing points. Sponson or stub wing

attachm ents are likely to be mo st effective if they are designed to distribute their loading

2600

2400

fo i l 0.0016 in. th ick

5052 mater ial

=G?%._l~--

50 i n . w ide

--- ----_-_-

-2

i -

;

\

g ,600

I UY\ I

I

Fu l l l i nes shear s t i f fness CD

I Dotted l ines shear st i f fnes s G=30x IO3 lb/ in*

600

400

lOO\n . w ide

4

I

200

1

I I

1 I I

I I

I

0 20 40 60 80

100 120 140 160

Panel l ,ength (L in.)

Figure 5. Flat sandw ich panels, elastic stability in compression.

over the greatest lengths possible. The major attach men ts are designed to feed the loading

into the sandwich skins and will be of the wrapped form developed from the axial tension

specimens.

With small diam eter structures, such as a tail boo m o f a light helicopter, a greater

variety of structures seems possible. If the diameter is small enough, a solid laminate of

high stiffness, non-w oven material of the right w eight m ay be stable in its ow n right.

Som e results from strut stability tests are show n in Figure 6. If stringers and frames are

required, the number of joints would be less than in larger constructions but the problem

of interior access for man ufacture wo uld necessitate it being split into halves.

A foam-filled sandw ich providing transve rse stability w ith longeron s carrying the

primary bend ing loads is also a possibility. Research is required to determ ine the best of

these methods. Other influences, such as the need to provide a folding joint at either end


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on a particular boom, could well swing the balance in favour of one of the possible con-

structions. The stiffness of these construc tions

ma y

be lower than the equivalent metal

in the case of the sandwich constructions and could be higher with stringers or a solid

laminate. In either event it should be possible to adjust the rotor dam ping to avoid overall

ground resonance problems. The need to avoid exciting a natural mode of the tail boom

in its own right is a problem that can only be assessed for a particular design. The improved

damping characteristics of the plastic material must also be taken into account.

ia

eccentrlcl y

u i r a l a x i s t o s k i n

8 Spec .nos . X3 /1 /1 and

2

8 Specnos . X7 /1 /1 and

2

Spec.DRG. :- 101980

Figure 6. Stability of G.R.P. struts en ds pinned).

4.2. ROTOR BLADES

Several successful blades have been developed in the U.S.A. using non-woven glass

reinforced plastic mate rials (6,

IO, I I).

It has been found that the design can be prepared

and developed more quickly than a metal blade. The material can be graded and orientated

to match applied stresses more efficiently. In Germany the blade root attachment in a

rigid rotor de sign is mad e of reinforced plastic (7). The susceptibility of the ma terial

to static fatigue must not be overlooked in this application but there have been enough

successful designs in the world to show that they can be made to work.

4.3.

UNDERCARRIAGE

A light aircraft undercarriage has been made with high strength filament wound

reinforcement. A lighter spring, with improved damp ing characteristics, has been

achieved and provided the static fatigue and compressive strength at high temperature

requirements are satisfied, a successful skid undercarriage for light helicopters is feasible.


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J S WILSON

4.4.

TRANSMISSION

Glass reinforced plastic seem s unlikely to be successful in this application where high-

speed shafts are designed by whirling limitations. A filament w ound drive shaft might at

least damp out some of the torque fluctuations of a piston engine p ower plant. Flexible

couplings for shafts have been successfully made and here the reduced stiffness and

improved fatigue properties make reinforced plastic very attractive (7 ).

5. CONCLUSIONS

The huge family of glass reinforced plastic m aterials is not without its share of thorns.

It has, however, developed sufficiently to be a serious alternative to orthodox aluminium

alloy construction where econom ical corrosion resistan t structures are required.

It may be as well to remember that not very many years ago the great French amateur

aircraft designer, Henri Mignet described the treacherous and unreliable aluminium

alloys as compressed earth . Times have changed and perhaps now we look back

somew hat obliquely at his exhortation : Long

l i v e w ood , l o ng l i v e s t eel

Lo ng l i v e g lass re - ih fm ed p la s t i cs

ACKNOWLEDGMENTS

The research described herein was conducted under Ministry of Aviation contract and

my thanks are due to them and the Directors of the Wes tland Aircraft Company for

permission to publish this paper. I should also like to thank Dr. Winny and Dr. Roberts

for their help and guidance and to pay tribute to the work of my colleagues in this develop-

ment, especially to M r. R . Harvie and Mr. Y . V. Badri N ath.

I.

2.

3.

4.

5.

6.

7.

8.

9.

IO

II

REFERENCES

J. V. MILEW SKI and J. J. SHYNE 195.5 Proc. 20th Ann. M tg of Reinf orced Plastics D iv. of the

Sot. of the Plastics I ndustry.

Whiskers make reinforced plastics better than metals.

G. M. BARTEN EV 964 October)

Chem. Engr.

The nature of high strength an d new qualities

of glass fibres.

J. 0. OUTWATER1964 October) Chem. Engr. The promise and reality in filament wound

laminates.

J. W . DAVIS 1964 PYOC

19th Ann. M tg of Reinforced Plastics Div. of the Sot. for the Plastics

I ndustry.

Interlaminar shear testing of filament winding m aterials.

J. A . AND ERSON nd J. A. MCC ARTHY 1963

Proc. 18th Ann. M tg of the Reinf orced Plastics Div.

of the Sot. of the Plastics Industry. Prepreg reinforced plastics in fatigue applications.

F. L. STUL EN Ig6I

Proc. 16th Ann. M tg of Reinforced Plastics Div. of the Sot. for the Plastics

I ndustry.

Reinforced plastic helicopter blades.

U. HUTTJZR 961

Proc. 16th Ann. M tg of the Reinf orced Plastics Div. of the Sot. of the Plastics

I ndustry. Glass fiber reinforced plastics as structural material for the aircraft industry.

B. J. LAZAN 1954 WADC Tech. Report 54-20 1954. Fatigue failure under resonant vibration

conditions.

R. A BELIN and E. A. W OHLBERG 962

Proc. 17th

Ann.

Mtg of the Reinf orced Plastics D iv. of

the Sot. of the Plastics I ndustry.

Glassfiber reinforced plastics in highly stressed aircraft parts.

B. POSNIAK 962

Proc. 17th Ann. M tg of the Rein for ced Plastics Di v. of the Sot. for the Plastics

I ndustry.

Developm ent of a directed fiber F.R.P. helicopter rotor blade.

V. R. DAVIS1964 Proc. Ann. Conf. of the Rein for ced Pl astics Div. of Plastics Federation, London.

A helicopter blade-a composite plastics construction.

This pa per was first presented at the Sym posium on the Noise and Loading Actions of Helicopters,

V/Stol Aircraft and Ground Effect Machines, Septem ber 1965 at the Institute of Sound and Vibration

Research of the University of Southampton.

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