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Glass reinforced plastics for helicopter primary structures
J. Sound I% (1966) 3 (3),
510-520
GLASS REINFORCED
PLASTICS
PRIMARY J. s.
FOR
HELICOPTER
STRUCTURES WILSON
Faire-y Aeriation Division, Westland Aircraft Limited, Hayes, Middlesex, England (Received I June 1965) Glass reinforced plastics are described from the viewpoint of a structural engineer engaged on the development of helicopter structures of superior resistance to fatigue loading, free from corrosion problems, and with consequently reduced maintenance costs. Emphasis is placed on the wide variety of G.R.P. materials, the dependence of their mechanical properties upon the manufacturing technique, and on test methods. Successful primary structures can be developed with these materials provided their properties are used to advantage and their limitations are understood. I. INTRODUCTION
Many of today’s aircraft structural engineers have worked with metallic materials exclusively. Isotropic material properties are taken for granted to an extent that the previous generation, used to wooden aircraft, would never have done. Glass reinforced structures must be designed to load the reinforcing material to the best advantage and reduce the load on the resin. This requirement can be readily appreciated when it is realized that 180,000 lb/in2 can be carried in tension by a laminate which will fail in interlaminar shear at 1*5~/~ of this value and not 60% as in most isotropic materials. This factor explains the disappointing performance of some structures which have been designed using the same techniques used for metallic structures. With structures correctly designed for the materials very satisfactory results can be achieved. The cantilevered side lobes and fuselage attachments of the large Gannet AEW Mk. 3 radome can be taken as an example. The heavily loaded areas of this radome are exposed to a marine atmosphere, liberal doses of engine oil, and exhaust gases from the jet pipes. It is placed in a vulnerable position and is consequently exposed to handling damage on the ground and continual buffeting from the turbulent airflow in flight. During seven years of operation there have been no structural problems in spite of very severe worldwide service which has caused several minor problems due to vibration and corrosion on the metallic aircraft itself. 2.
2. I.
MATERIALS
RESINS
The function of the resin is to protect, stabilize and seal the laminate. The favourite thermosetting plastics for use with glass reinforcement are the polyester, epoxy and phenolic groups of resins. Broadly, polyesters are used for ease of manufacture and repair and consequently reduce costs; epoxys are used for high strength and weather resistance, and phenolics for very high temperature applications. It must be appreciated that within each group widely different properties may be achieved. The common practice of annotating one curve on a graph “epoxy ” and another as “polyester ” can be meaningless as the 5’0
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curves are related to a certain resin cured in a certain way and may not 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 comparatively 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 maximum strength at the temperature 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 minimum 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 strength and stiffness of epoxy resins three or four times (I). 2.2.
GLASS
Most 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 combat this effect, and to provide the best bond between the glass and the resin, the glass surface can be given one of many chemical surface treatments and significant improvements have been made. Flawless 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 diameter and hollow glass fibres have been made to improve compressive properties, but this development is unlikely to help structural laminates until resin strengths at high temperature have been improved. 2.3.
GLASS REINFORCEMENT
The drawn glass filaments are spun or collected into bundles to form “rovings ” and used to reinforce the laminating resins. The spun fibres can be woven into fabric and these fabrics may have a variety of weaves and be balanced (as much warp material 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 with 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. The high strength and stiffness of all laminates occurs only in the directions of the reinforcement. The properties normal to the laminate and interlaminar shear are properties of the resin or the resin/glass bond. 34”
J. S. WILSON
512 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 must be optimized for the design temperature. Practical considerations of tooling and manufacture time and ease of repair under service conditions will also affect the choice of cure cycle. Mechanization of the manufacturing 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 advent of the use of structural plastics on a larger scale shows a need to review this value and perhaps put it on a more rational basis. American workers have suggested that different factors should be used for different classes of member 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 may be difficulties in definition of the various classes of joints, etc. 2.5.
SANDWICH
CORE MATERIALS
AND PROTECTIVE FOAM
The most successful lightweight core materials for sandwich 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 was, 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. Aluminium honeycomb distorts markedly after a static shear failure but retains much of its strength. Joints between blocks of honeycombs are lighter than with 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 throughout. Adhesives to form the skin to core, and core to core bonds formerly were of low strength at 9o’C. 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 panels 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 may be required to seal the sandwich. 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 with this construction.
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PROPERTIES
In section 2 on materials, the many different constructions and process variables which affect the properties of a laminate were outlined. For an optimum design in these circumstances one must develop what is considered to be the best material to suit the design requirements of a particular component. Using this technique, absolute figures for material properties are not available until the particular development has taken place. Results from the many “standard ” test methods used for material development are intended to be used for comparison of materials and are of little use for design. Testing to determine the design properties must be performed in a manner as representative of the service of the component as possible. The order of properties to be expected, and a knowledge of the important factors affecting them, are what is required for preliminary design.
3. I.
TENSION
IN THE PLANE
OF THE LAMINATE
A balanced weave woven fabric should fail between 40,000 and 50,000 lb/in2 and an unwoven material at 75,000 lb/in2 in the directions of reinforcement. When loaded at 45’ to the reinforcement, narrow specimens show these figures to be roughly halved. Wide specimens, in which some of the fibres are continuous between the loading points, are stronger and may achieve 80% of the strength in the fibre directions. Resin cracking, due to high tensile strain, has been noted in one case and this started at about the failing stress for the narrow specimens. Until further tests have been carried out the lower figures from the narrow specimens are therefore being used in the work at Fairey’s. When the reinforcement is almost all in one direction 85,000 lb/in2 can be expected from a fabric and 160,000 to 180,000 lb/in2 from unwoven, unidirectional material. The transverse properties are down to 10,000 lb/in2 for the fabric and to the resin properties for the non-woven. The wet strength of the materials is lower than the above but this depends upon the resin and the glass finish. Generally, laminates using epoxy resins show less reduction than polyesters but, recently, great advances have been made in glass finishes to increase the wet strength. Laminates tested hot after soaking at IOO’C may lose some of their strength at room temperature but this effect is dependent upon the resin and its cure cycle. At low temperature, - 50’ C, the strength may be slightly greater than at room temperature. Creep strain, at both room temperature and high temperature, is a relatively small percentage of the total deformation except when loads are applied at 45’ to the reinforcement when it may be important. Creep rupture, on the other hand, is important, especially when the glass is highly stressed as in filament wound pressure vessels. This mode of failure is sometimes called “ static fatigue ” and test results from pressure vessels (extrapolated from failures after two months) indicate failure at 60% of the normal burst pressure after holding this load for four months (3). The effect is still present, but to a much reduced degree, in fabric reinforced laminates when a failure can be expected after holding 600/ of the normal ultimate for 12 years. Scale effect has also been noted in cases of creep rupture of filament wound structures. Larger specimens fail at lower stresses than small ones. These creep rupture effects are attributed to moisture trapped in small cracks in the glass and improvements in glass finishes may well have reduced them. If very high preloads (80% ult.) are applied during proof tests a loss of ultimate strength will result. 3.2. TENSILEMODULUSOF ELASTICITY Most laminates with both woven and non-woven reinforcement exhibit a distinct kink in the stress/strain curve. Strictly, there are, therefore, primary and secondary values ofthe
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modulus, the secondary being about 10% lower than the primary. A balanced weave fabric laminate should have a mean modulus 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 and, in fact, a direct relationship has been shown to exist between the compressive and interlaminar shear strengths of a laminate. The failing stress of a balanced weave fabric laminate is about 30-40,000 lb/in2 in the directions of reinforcement. A marked reduction has been found at Fairey’s 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 has increased the strength from 13,000-24,000 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 laminates 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 that the results are still affected by friction. 3.4. MODULUSOF ELASTICITYIN COMPRESSION Primary and secondary moduli are apparent as in the case of tension. Somewhat 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 temperature to 90’ C. After modifying 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. SHEARIN THE PLANEOF THE LAMINATE Tests on thick laminates have shown a balanced weave fabric will fail at 9-10,000 lb/in2 and possess a modulus of rigidity of 0.5 x IO6. 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 as resin cracking took place at about 6000 lb/in2 on one specimen. These results apply to a shear applied parallel to one of the reinforced directions. Tests with 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.
G.R.P.
3.6.
INTERLAMINAR
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SHEAR
The interlaminar properties of the laminate are largely controlled by the resin although the type of reinforcement has some effect. Care must be exercised to limit this loading to a low safe value and testing to determine this must be representative of the application. Testing for interlaminar shear provides the classic case of the importance of the test method. It is interesting to note that a non-woven material tested in II different ways to determine 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 outstanding property of glass reinforced material is its resistance to fatigue loading. Woven materials show a considerable improvement over metallic materials and nonwoven are better still. Our axial tension specimens which are of sandwich construction P+$ lb t
Strain gauge links Steel loading
blocks
Skin end wraps
Upper and lower loading links gripped in jaws of 20 U.H.S. Losenhausen machine
Figure
I.
Axial tension fatigue specimens tested in 20 U.H.S.
Losenhausen machine.
and include representative wrapped end 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 and riveted specimens show a similar endurance limit and these can be compared with allowables of + 4500 lb/in2 in 85 ton steel and rt: 1400 lb/in2 in high grade aluminium alloy. The glass laminate has a specific gravity of 1.8 and dividing the allowables by the specific gravity will give a measure of fatigue performance in relation to weight. Woven 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 problems were centred upon 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 attachments 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 Alternating
stress = ztz50%
mean stress
Failures ore of the face skin away from the end wrap
\
I
-.
--+__
+
I
105
+++
IO6
N-cycles
I
1 I
104
I
10’
108
to failure
Figure 2. G.R.P. axial tension fatigue test. 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 blade construction the lower stiffness of a glass reinforced blade can be exploited to reduce the loading. Blade deformations in flight being similar with a variety of blade stiffnesses the lowest stiffness blade has the lowest stress. This is an added
Alternating
I
I
stress = +50%
t
I
mean stress
Spec: DRG. no. 101985/9 Specimen type X9
cles to 0.01
d
I
thickness 0.05 in.
elongation
I04
105
106
IO'
N-cycles
Figure
3.
G.R.P. lug fatigue test.
attraction for glass fibre blade construction (6). Flexible couplings 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 honeycomb 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 honeycomb.
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Fatigue testing of glass reinforced specimens can be time consuming as there is a need to keep the rate of load cycling down, especially at high strains. If tested too rapidly the specimens heat up due to the material’s low conductivity. The test failing stress will be somewhat lower than that at ambient. For this reason we restrict the rate of load cycling in our tests to a maximum of 1400 c/min. We are also testing at 2800 c/min to establish the magnitude of this effect in an attempt to speed up testing at low stresses.
50
2 ;
45
E zl
40
08
35
::
30
2
a5
EIn
20
F ‘Z z t = cl
ernaling Failures
are
stress = + 50% of
the
alum.
mean all
honeycomb
15 10 5 ’ 103
’ ’ “‘1”
104
’
N-cycles
’
’ “11’1’
’ “1111’
106
105
to
IO’
’
’ ’ “‘111 IO8
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 damping energy per unit volume per cycle was ten times greater than that for aluminium and steel (8). The total damping of a plastic structure is unlikely to increase in this proportion, as most riveted joints will be replaced by bonded ones of reduced damping 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 maintenance and manufacturing costs. 4. I.
FUSELAGE
The helicopter fuselage can vary in size from an 8 ft diameter shell, containing passengers and equipment, to a tail boom of I ft diameter, with little more than control cables within 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.
5’8
J. S. WILSON
If stringers and frames were added, as in a conventional aircraft, the vast number of separate bonding operations on joints might lead to joints of doubtful quality as well as a multitude of stress concentrations. A sandwich construction using aluminium foil honeycomb is more attractive for this size of structure. Inherent sandwich panel compressive stability is illustrated in Figure _c,. The shell can be reinforced with a gridwork of frames and longitudinal sandwich members under the floor to carry the loads from freight lashing points. Sponson or stub wing attachments are likely to be most effective if they are designed to distribute their loading
2600
2400
in. thick 5052
foil 0.0016
50 in. wide ------_-_-
=G?%._l~-i
-2 ‘;
g ,600
\ I UY \\ I I
‘-’
material
’
I
Full lines shear stiffness CD Dotted lines shear stiffness G=30x
IO3 lb/in*
600 ‘. 400
lOO\n. wide
4
I
200
01
0
I
I
1
I
I
I
I
I
20
40
60
80
100
120
140
160
Panel l,ength (L in.)
Figure 5. Flat sandwich panels, elastic stability in compression.
over the greatest lengths possible. The major attachments 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 diameter structures, such as a tail boom of a light helicopter, a greater variety of structures seems possible. If the diameter is small enough, a solid laminate of high stiffness, non-woven material of the right weight may be stable in its own right. Some results from strut stability tests are shown 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 manufacture would necessitate it being split into halves. A foam-filled sandwich providing transverse stability with longerons carrying the primary bending loads is also a possibility. Research is required to determine the best of these methods. Other influences, such as the need to provide a folding joint at either end
G.R.P.
FOR HELICOPTER
PRIMARY
STRUCTURES
5’9
on a particular boom, could well swing the balance in favour of one of the possible constructions. The stiffness of these constructions may 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 damping 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
uiral
axis
y
to
skin
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 (ends pinned).
4.2.
ROTOR
BLADES
Several successful blades have been developed in the U.S.A. using non-woven glass reinforced plastic materials (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 design is made of reinforced plastic (7). The susceptibility of the material 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 damping 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.
J. S. WILSON
520 4.4. TRANSMISSION
Glass reinforced plastic seems unlikely to be successful in this application where highspeed shafts are designed by whirling limitations. A filament wound drive shaft might at least damp out some of the torque fluctuations of a piston engine power 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 materials is not without its share of thorns. It has, however, developed sufficiently to be a serious alternative to orthodox aluminium alloy construction where economical corrosion resistant 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 somewhat obliquely at his exhortation : “Long live wood, long live steel ! ” Long live glass re-ihfmed plastics ! ACKNOWLEDGMENTS
The research described herein was conducted under Ministry of Aviation contract and my thanks are due to them and the Directors of the Westland 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 development, especially to Mr. R. Harvie and Mr. Y. V. Badri Nath. REFERENCES I. J. V. MILEWSKI and J. J. SHYNE 195.5 Proc. 20th Ann. Mtg of Reinforced Plastics Div. of the Sot. of the Plastics Industry. Whiskers make reinforced plastics better than metals. 2. G. M. BARTENEV1964 (October) Chem. Engr. The nature of high strength and new qualities of glass fibres. 3. J. 0. OUTWATER1964 (October) Chem. Engr. The promise and reality in filament wound laminates. 4. J. W. DAVIS 1964 PYOC.19th Ann. Mtg of Reinforced Plastics Div. of the Sot. for the Plastics Industry. Interlaminar shear testing of filament winding materials. 5. J. A. ANDERSONand J. A. MCCARTHY 1963 Proc. 18th Ann. Mtg of the Reinforced Plastics Div. of the Sot. of the Plastics Industry. Prepreg reinforced plastics in fatigue applications. 6. F. L. STULEN Ig6I Proc. 16th Ann. Mtg of Reinforced Plastics Div. of the Sot. for the Plastics Industry. Reinforced plastic helicopter blades. 7. U. HUTTJZR1961 Proc. 16th Ann. Mtg of the Reinforced Plastics Div. of the Sot. of the Plastics Industry. Glass fiber reinforced plastics as structural material for the aircraft industry. 8. B. J. LAZAN 1954 WADC Tech. Report 54-20 1954. Fatigue failure under resonant vibration conditions. 9. R. ABELIN and E. A. WOHLBERGI962 Proc. 17th Ann. Mtg of the Reinforced Plastics Div. of the Sot. of the Plastics Industry. Glassfiber reinforced plastics in highly stressed aircraft parts. IO. B. POSNIAKI962 Proc. 17th Ann. Mtg of the Reinforced Plastics Div. of the Sot. for the Plastics Industry. Development of a directed fiber F.R.P. helicopter rotor blade. II. V. R. DAVIS1964 Proc. Ann. Conf. of the Reinforced Plastics Div. of Plastics Federation, London. A helicopter blade-a composite plastics construction.
This paper was first presented at the Symposium on the Noise and Loading Actions of Helicopters, V/Stol Aircraft and Ground Effect Machines, September 1965 at the Institute of Sound and Vibration Research of the University of Southampton.
510-520
GLASS REINFORCED
PLASTICS
PRIMARY J. s.
FOR
HELICOPTER
STRUCTURES WILSON
Faire-y Aeriation Division, Westland Aircraft Limited, Hayes, Middlesex, England (Received I June 1965) Glass reinforced plastics are described from the viewpoint of a structural engineer engaged on the development of helicopter structures of superior resistance to fatigue loading, free from corrosion problems, and with consequently reduced maintenance costs. Emphasis is placed on the wide variety of G.R.P. materials, the dependence of their mechanical properties upon the manufacturing technique, and on test methods. Successful primary structures can be developed with these materials provided their properties are used to advantage and their limitations are understood. I. INTRODUCTION
Many of today’s aircraft structural engineers have worked with metallic materials exclusively. Isotropic material properties are taken for granted to an extent that the previous generation, used to wooden aircraft, would never have done. Glass reinforced structures must be designed to load the reinforcing material to the best advantage and reduce the load on the resin. This requirement can be readily appreciated when it is realized that 180,000 lb/in2 can be carried in tension by a laminate which will fail in interlaminar shear at 1*5~/~ of this value and not 60% as in most isotropic materials. This factor explains the disappointing performance of some structures which have been designed using the same techniques used for metallic structures. With structures correctly designed for the materials very satisfactory results can be achieved. The cantilevered side lobes and fuselage attachments of the large Gannet AEW Mk. 3 radome can be taken as an example. The heavily loaded areas of this radome are exposed to a marine atmosphere, liberal doses of engine oil, and exhaust gases from the jet pipes. It is placed in a vulnerable position and is consequently exposed to handling damage on the ground and continual buffeting from the turbulent airflow in flight. During seven years of operation there have been no structural problems in spite of very severe worldwide service which has caused several minor problems due to vibration and corrosion on the metallic aircraft itself. 2.
2. I.
MATERIALS
RESINS
The function of the resin is to protect, stabilize and seal the laminate. The favourite thermosetting plastics for use with glass reinforcement are the polyester, epoxy and phenolic groups of resins. Broadly, polyesters are used for ease of manufacture and repair and consequently reduce costs; epoxys are used for high strength and weather resistance, and phenolics for very high temperature applications. It must be appreciated that within each group widely different properties may be achieved. The common practice of annotating one curve on a graph “epoxy ” and another as “polyester ” can be meaningless as the 5’0
G.R.P.
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51’
curves are related to a certain resin cured in a certain way and may not 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 comparatively 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 maximum strength at the temperature 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 minimum 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 strength and stiffness of epoxy resins three or four times (I). 2.2.
GLASS
Most 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 combat this effect, and to provide the best bond between the glass and the resin, the glass surface can be given one of many chemical surface treatments and significant improvements have been made. Flawless 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 diameter and hollow glass fibres have been made to improve compressive properties, but this development is unlikely to help structural laminates until resin strengths at high temperature have been improved. 2.3.
GLASS REINFORCEMENT
The drawn glass filaments are spun or collected into bundles to form “rovings ” and used to reinforce the laminating resins. The spun fibres can be woven into fabric and these fabrics may have a variety of weaves and be balanced (as much warp material 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 with 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. The high strength and stiffness of all laminates occurs only in the directions of the reinforcement. The properties normal to the laminate and interlaminar shear are properties of the resin or the resin/glass bond. 34”
J. S. WILSON
512 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 must be optimized for the design temperature. Practical considerations of tooling and manufacture time and ease of repair under service conditions will also affect the choice of cure cycle. Mechanization of the manufacturing 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 advent of the use of structural plastics on a larger scale shows a need to review this value and perhaps put it on a more rational basis. American workers have suggested that different factors should be used for different classes of member 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 may be difficulties in definition of the various classes of joints, etc. 2.5.
SANDWICH
CORE MATERIALS
AND PROTECTIVE FOAM
The most successful lightweight core materials for sandwich 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 was, 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. Aluminium honeycomb distorts markedly after a static shear failure but retains much of its strength. Joints between blocks of honeycombs are lighter than with 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 throughout. Adhesives to form the skin to core, and core to core bonds formerly were of low strength at 9o’C. 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 panels 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 may be required to seal the sandwich. 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 with this construction.
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3. MECHANICAL
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PROPERTIES
In section 2 on materials, the many different constructions and process variables which affect the properties of a laminate were outlined. For an optimum design in these circumstances one must develop what is considered to be the best material to suit the design requirements of a particular component. Using this technique, absolute figures for material properties are not available until the particular development has taken place. Results from the many “standard ” test methods used for material development are intended to be used for comparison of materials and are of little use for design. Testing to determine the design properties must be performed in a manner as representative of the service of the component as possible. The order of properties to be expected, and a knowledge of the important factors affecting them, are what is required for preliminary design.
3. I.
TENSION
IN THE PLANE
OF THE LAMINATE
A balanced weave woven fabric should fail between 40,000 and 50,000 lb/in2 and an unwoven material at 75,000 lb/in2 in the directions of reinforcement. When loaded at 45’ to the reinforcement, narrow specimens show these figures to be roughly halved. Wide specimens, in which some of the fibres are continuous between the loading points, are stronger and may achieve 80% of the strength in the fibre directions. Resin cracking, due to high tensile strain, has been noted in one case and this started at about the failing stress for the narrow specimens. Until further tests have been carried out the lower figures from the narrow specimens are therefore being used in the work at Fairey’s. When the reinforcement is almost all in one direction 85,000 lb/in2 can be expected from a fabric and 160,000 to 180,000 lb/in2 from unwoven, unidirectional material. The transverse properties are down to 10,000 lb/in2 for the fabric and to the resin properties for the non-woven. The wet strength of the materials is lower than the above but this depends upon the resin and the glass finish. Generally, laminates using epoxy resins show less reduction than polyesters but, recently, great advances have been made in glass finishes to increase the wet strength. Laminates tested hot after soaking at IOO’C may lose some of their strength at room temperature but this effect is dependent upon the resin and its cure cycle. At low temperature, - 50’ C, the strength may be slightly greater than at room temperature. Creep strain, at both room temperature and high temperature, is a relatively small percentage of the total deformation except when loads are applied at 45’ to the reinforcement when it may be important. Creep rupture, on the other hand, is important, especially when the glass is highly stressed as in filament wound pressure vessels. This mode of failure is sometimes called “ static fatigue ” and test results from pressure vessels (extrapolated from failures after two months) indicate failure at 60% of the normal burst pressure after holding this load for four months (3). The effect is still present, but to a much reduced degree, in fabric reinforced laminates when a failure can be expected after holding 600/ of the normal ultimate for 12 years. Scale effect has also been noted in cases of creep rupture of filament wound structures. Larger specimens fail at lower stresses than small ones. These creep rupture effects are attributed to moisture trapped in small cracks in the glass and improvements in glass finishes may well have reduced them. If very high preloads (80% ult.) are applied during proof tests a loss of ultimate strength will result. 3.2. TENSILEMODULUSOF ELASTICITY Most laminates with both woven and non-woven reinforcement exhibit a distinct kink in the stress/strain curve. Strictly, there are, therefore, primary and secondary values ofthe
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J. S. WILSON
modulus, the secondary being about 10% lower than the primary. A balanced weave fabric laminate should have a mean modulus 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 and, in fact, a direct relationship has been shown to exist between the compressive and interlaminar shear strengths of a laminate. The failing stress of a balanced weave fabric laminate is about 30-40,000 lb/in2 in the directions of reinforcement. A marked reduction has been found at Fairey’s 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 has increased the strength from 13,000-24,000 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 laminates 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 that the results are still affected by friction. 3.4. MODULUSOF ELASTICITYIN COMPRESSION Primary and secondary moduli are apparent as in the case of tension. Somewhat 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 temperature to 90’ C. After modifying 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. SHEARIN THE PLANEOF THE LAMINATE Tests on thick laminates have shown a balanced weave fabric will fail at 9-10,000 lb/in2 and possess a modulus of rigidity of 0.5 x IO6. 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 as resin cracking took place at about 6000 lb/in2 on one specimen. These results apply to a shear applied parallel to one of the reinforced directions. Tests with 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.
G.R.P.
3.6.
INTERLAMINAR
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515
SHEAR
The interlaminar properties of the laminate are largely controlled by the resin although the type of reinforcement has some effect. Care must be exercised to limit this loading to a low safe value and testing to determine this must be representative of the application. Testing for interlaminar shear provides the classic case of the importance of the test method. It is interesting to note that a non-woven material tested in II different ways to determine 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 outstanding property of glass reinforced material is its resistance to fatigue loading. Woven materials show a considerable improvement over metallic materials and nonwoven are better still. Our axial tension specimens which are of sandwich construction P+$ lb t
Strain gauge links Steel loading
blocks
Skin end wraps
Upper and lower loading links gripped in jaws of 20 U.H.S. Losenhausen machine
Figure
I.
Axial tension fatigue specimens tested in 20 U.H.S.
Losenhausen machine.
and include representative wrapped end 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 and riveted specimens show a similar endurance limit and these can be compared with allowables of + 4500 lb/in2 in 85 ton steel and rt: 1400 lb/in2 in high grade aluminium alloy. The glass laminate has a specific gravity of 1.8 and dividing the allowables by the specific gravity will give a measure of fatigue performance in relation to weight. Woven 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 problems were centred upon 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 attachments 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 Alternating
stress = ztz50%
mean stress
Failures ore of the face skin away from the end wrap
\
I
-.
--+__
+
I
105
+++
IO6
N-cycles
I
1 I
104
I
10’
108
to failure
Figure 2. G.R.P. axial tension fatigue test. 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 blade construction the lower stiffness of a glass reinforced blade can be exploited to reduce the loading. Blade deformations in flight being similar with a variety of blade stiffnesses the lowest stiffness blade has the lowest stress. This is an added
Alternating
I
I
stress = +50%
t
I
mean stress
Spec: DRG. no. 101985/9 Specimen type X9
cles to 0.01
d
I
thickness 0.05 in.
elongation
I04
105
106
IO'
N-cycles
Figure
3.
G.R.P. lug fatigue test.
attraction for glass fibre blade construction (6). Flexible couplings 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 honeycomb 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 honeycomb.
G.R.P.
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517
Fatigue testing of glass reinforced specimens can be time consuming as there is a need to keep the rate of load cycling down, especially at high strains. If tested too rapidly the specimens heat up due to the material’s low conductivity. The test failing stress will be somewhat lower than that at ambient. For this reason we restrict the rate of load cycling in our tests to a maximum of 1400 c/min. We are also testing at 2800 c/min to establish the magnitude of this effect in an attempt to speed up testing at low stresses.
50
2 ;
45
E zl
40
08
35
::
30
2
a5
EIn
20
F ‘Z z t = cl
ernaling Failures
are
stress = + 50% of
the
alum.
mean all
honeycomb
15 10 5 ’ 103
’ ’ “‘1”
104
’
N-cycles
’
’ “11’1’
’ “1111’
106
105
to
IO’
’
’ ’ “‘111 IO8
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 damping energy per unit volume per cycle was ten times greater than that for aluminium and steel (8). The total damping of a plastic structure is unlikely to increase in this proportion, as most riveted joints will be replaced by bonded ones of reduced damping 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 maintenance and manufacturing costs. 4. I.
FUSELAGE
The helicopter fuselage can vary in size from an 8 ft diameter shell, containing passengers and equipment, to a tail boom of I ft diameter, with little more than control cables within 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.
5’8
J. S. WILSON
If stringers and frames were added, as in a conventional aircraft, the vast number of separate bonding operations on joints might lead to joints of doubtful quality as well as a multitude of stress concentrations. A sandwich construction using aluminium foil honeycomb is more attractive for this size of structure. Inherent sandwich panel compressive stability is illustrated in Figure _c,. The shell can be reinforced with a gridwork of frames and longitudinal sandwich members under the floor to carry the loads from freight lashing points. Sponson or stub wing attachments are likely to be most effective if they are designed to distribute their loading
2600
2400
in. thick 5052
foil 0.0016
50 in. wide ------_-_-
=G?%._l~-i
-2 ‘;
g ,600
\ I UY \\ I I
‘-’
material
’
I
Full lines shear stiffness CD Dotted lines shear stiffness G=30x
IO3 lb/in*
600 ‘. 400
lOO\n. wide
4
I
200
01
0
I
I
1
I
I
I
I
I
20
40
60
80
100
120
140
160
Panel l,ength (L in.)
Figure 5. Flat sandwich panels, elastic stability in compression.
over the greatest lengths possible. The major attachments 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 diameter structures, such as a tail boom of a light helicopter, a greater variety of structures seems possible. If the diameter is small enough, a solid laminate of high stiffness, non-woven material of the right weight may be stable in its own right. Some results from strut stability tests are shown 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 manufacture would necessitate it being split into halves. A foam-filled sandwich providing transverse stability with longerons carrying the primary bending loads is also a possibility. Research is required to determine the best of these methods. Other influences, such as the need to provide a folding joint at either end
G.R.P.
FOR HELICOPTER
PRIMARY
STRUCTURES
5’9
on a particular boom, could well swing the balance in favour of one of the possible constructions. The stiffness of these constructions may 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 damping 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
uiral
axis
y
to
skin
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 (ends pinned).
4.2.
ROTOR
BLADES
Several successful blades have been developed in the U.S.A. using non-woven glass reinforced plastic materials (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 design is made of reinforced plastic (7). The susceptibility of the material 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 damping 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.
J. S. WILSON
520 4.4. TRANSMISSION
Glass reinforced plastic seems unlikely to be successful in this application where highspeed shafts are designed by whirling limitations. A filament wound drive shaft might at least damp out some of the torque fluctuations of a piston engine power 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 materials is not without its share of thorns. It has, however, developed sufficiently to be a serious alternative to orthodox aluminium alloy construction where economical corrosion resistant 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 somewhat obliquely at his exhortation : “Long live wood, long live steel ! ” Long live glass re-ihfmed plastics ! ACKNOWLEDGMENTS
The research described herein was conducted under Ministry of Aviation contract and my thanks are due to them and the Directors of the Westland 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 development, especially to Mr. R. Harvie and Mr. Y. V. Badri Nath. REFERENCES I. J. V. MILEWSKI and J. J. SHYNE 195.5 Proc. 20th Ann. Mtg of Reinforced Plastics Div. of the Sot. of the Plastics Industry. Whiskers make reinforced plastics better than metals. 2. G. M. BARTENEV1964 (October) Chem. Engr. The nature of high strength and new qualities of glass fibres. 3. J. 0. OUTWATER1964 (October) Chem. Engr. The promise and reality in filament wound laminates. 4. J. W. DAVIS 1964 PYOC.19th Ann. Mtg of Reinforced Plastics Div. of the Sot. for the Plastics Industry. Interlaminar shear testing of filament winding materials. 5. J. A. ANDERSONand J. A. MCCARTHY 1963 Proc. 18th Ann. Mtg of the Reinforced Plastics Div. of the Sot. of the Plastics Industry. Prepreg reinforced plastics in fatigue applications. 6. F. L. STULEN Ig6I Proc. 16th Ann. Mtg of Reinforced Plastics Div. of the Sot. for the Plastics Industry. Reinforced plastic helicopter blades. 7. U. HUTTJZR1961 Proc. 16th Ann. Mtg of the Reinforced Plastics Div. of the Sot. of the Plastics Industry. Glass fiber reinforced plastics as structural material for the aircraft industry. 8. B. J. LAZAN 1954 WADC Tech. Report 54-20 1954. Fatigue failure under resonant vibration conditions. 9. R. ABELIN and E. A. WOHLBERGI962 Proc. 17th Ann. Mtg of the Reinforced Plastics Div. of the Sot. of the Plastics Industry. Glassfiber reinforced plastics in highly stressed aircraft parts. IO. B. POSNIAKI962 Proc. 17th Ann. Mtg of the Reinforced Plastics Div. of the Sot. for the Plastics Industry. Development of a directed fiber F.R.P. helicopter rotor blade. II. V. R. DAVIS1964 Proc. Ann. Conf. of the Reinforced Plastics Div. of Plastics Federation, London. A helicopter blade-a composite plastics construction.
This paper was first presented at the Symposium on the Noise and Loading Actions of Helicopters, V/Stol Aircraft and Ground Effect Machines, September 1965 at the Institute of Sound and Vibration Research of the University of Southampton.