ARL/Struc. Note 424
ARL/Struc. Note 424
/2!
I
DEPARTMENT OF DEFENCE
DEFENCE SCIENCE AND TECHNOLOGY ORGANIZATION
z
AERONAUTICAL RESEARCH LABORATORIES
MELBOURNE VICTORIA
Structures Note 424
THE PERFORMANCE OF AIRCRAFT
CONTROL CABLES
UNDER SERVICE CONDITIONS
by
M. B. BENOY, R. A.
FELL and P. H. TOWNSHEND
LUL
Cm
COMMONWEALTH OF AUSTRALIA 1977
MONWEALTECHNUALI
A
INFORMATION
SERVICE
U. S. DEPARTMENT OF COMMERCE
SPRINGFIEL!D
APRIL 1976
VA. 22161
3
.!
IJ
DOCUMENT CONTROL DATA
I. Security Grading/Release Limitation
(a)
Document Content
Unclassified
(b)
This Page
Unclassified
2.
Document Type/Number
Structures Note 424
3. Dociment Date
April 1976
4. Title and Sub-Title
The Performance of Aircraft Control Cables under Service Conditions
h
5. Personal Authois: M.B. Benoy, R.A. Fell & P.H. Townshend
6. Corporate Author(s): AERONAUTICAL RESEARCH LABORATORIES
Abstract
Investigations w.ere conducted on the adverse effects of service conditions on the
per/forman' o/flexihle steel wire ropes used in aircraftcontrolsysterms with particular
reference to the A uster aircraft.
i
S8. Computer Prugr:mn(s)
--
Titles and Language
9. l)escriptors
I1. Cosati Classifications
Aircraft
Control Equipment
Wire Ropes
Service Life
0103, 1402, 1106, 1309
RD 73 50-4
10. L.ibrarv Distribution (Defence Group)
Central Library
STIB
.110
ARI.
CS IMRI.., Vic, NSW, South Aust.
WRI-
Air Office. ARDU, AMTS
Army Office, EDE, Bridges Library
13.
Sponsoring Agency Reference
14.
Cost Code
7100
15. Imprint
MELBOURNE AERONAUTICAL RESEARCH LABORATORIES 1976
"i L--:':"/-•
"...¥
' •••- '' .. . .
..
I
DEPARTMENT OF DEFENCE
DEFENCE SCIENCE AND TECHNOLOGY ORGANIZATION
AERONAUTICAL RESEARCH LABORATORIES
-
.!~~~/ .
....
.....
......
.....
!
",." ..
•
.'
STRUCTURES NOTE:424
THE PERFORMANCE OF AIRCRAFT
CONTROL CABLES
UNDER SERVICE CONDITIONS,
SM. BBENOY, RAFELL 0 P. H. TOWNSHEND
•
....../"SUM M A R )Y
,4iter e.xaminationa
oaircraft control cahles broken or wornt in service together
with mechanical tests antd theoretical analysis the.fillowing /actors were found to
contribute,to f.ailure:
(a)
Abrasion ofthe cahles onpullevs orfairleadscausinga much more serious loss of
strength than is apparent from surface wear.
"(B)lBrinelling"
oft he individual wires in the strands due to.fluctuatingcable tension
inducing high contact stresses between helicall' wound wires. This causesstrainhardening fdiohwed hi brittleness in the hard-drawn wires leading to the
formation.0/' sur/ace cracks.
(c)
(d)
F'atigue of wires due to flexing around pulehys and fairleads.
('nobserved/failure of'the core stranddue to the combined effects of(h) and (c).
Subsequent to the investigations a review was made of the published work on
aircrafl control cables and covers the development oJfstandardsand air-worthiness
requirements. A bibliography, which includes wvire ropes in general engineering has
also been added.
/
._
(ONTENTS
Page No.
1.
INTRODUCTION
3
2.
INVESTIGATION OF SERVICE CONDITIONS AND CABLE DAMAGE
3
2.1
Examination of Cables Damaged in Service
3
2.2
Inspection of control cable installations
4
2.3
Measurement of cable loads in service
4
3.
4.
5.
6.
7.
INVESTIGATION OF THE FACTORS AFFECTING CABLE STRENGTH
3.1
Determination of the mechanical properties of cables
5
5
3.2
Effect of cable wear on ultimate strength
5
3.3
Evaluation of nylon covered cable
5
ANALYSIS OF CABLE STRESSES
6
DISCUSSICN
7
5.1
Historical
7
5.2
(ab'ts
7
5.3
5.4
Pulleys
(able Standards
8
8
('ON('ICUSIONS
9
6.1
Cables
9
6.2
Nylon covered cables
9
6.3
Pulleys
9
6.4
Rudder control circuit
9
6.4.1
Rudder limit stop
9
6.4.2
Circuit tensioning
9
6.4.3
Fairleads
9
RI:VIEW OF STANI)ARi)S AND RELATED INVESTIGATIONS
7.1
(able Standards
7.1.1
7.2
Nylhr. covered cables
EM ENTS
RE F ER EN(CE;S
APPENI)IX I: Analysis of stresses in a 7 x 7 cable
DISTRIBUTION
DOCUMENT CONTROL DATA
TABI.ES
FIGU!R ES
IBBIIOGRAPlHY
=,j~
II
Pulley groove profile
A(KNOWLEDI)
*
10
Pulleys
7.2.1
I,
9
I
12
A
I.
INTRODU
nTION
Following the occurrence of service failures in rudder cables of Auster aircraft an
investigation was undertaken at the request of ).C.A. asi rer'rted in Reference 1. As a result of
this work the problem of progressive deterioration of aircraft control cables in service was
considered to be of sufficient importance to warrant extending the A.R. I.. investigations.
i
Examination of worn control cables froim small civil and military ,ircraft and gliders
indicated that a substantial reduction in strength could occur with little or no visible evidence of
deterioration. An experimental programme, including sonic theoretical analysis, was therefore
planned
investigate the Iactors contributing to the reduction of ultimate strength of control
cables if)toservice.
[he following threc types of cables were considercd:
(i)
7 x 7 aircraft cable (i.e. 7 strands of 7 wires) miinimutmn breaking load I(cwt., (4,98kN)
made to an Auster specification (Relf. 2)
(ii)
7 x 14 aircraft cable (i.e. 7 strands of 14 wires) nurnimtum breaking iad 10 cwt., (4.98
kN) made to British Standard Spciciication W9. (Ref. 3).
7 x 19 aircraft cable (i.e. 7 strands of I) wires) minimtumn breaking load 17601bf (7.83
kN) made to American Military Specilication MIi.-I'-5424. (Rel. 4).
Mhe layout of the wires in the cables is illustr;tcd in Ifigs. I ald 2.
(iii)
Fatigue tests were also carried out on samples of galvanised steel cable (5 cwt. (2.49kN)
minimumil breaking lo0ad, 7 x 7 construction to B..S. W.90) s.peci; lly prepared with i a nylon covering
to resist abasion.
ork %%
hii'h was carried out in t lie early 1960's
It should be noted that t his report is based onl %%
and was tie
1CSubject oflt wo i nternal A. R. I.. report s. I'cause of delays in vetting etc. due to higher
priority tasks, it has only now been consolidated into one report with the addition of later
deye lopmernts.
The investigatiooIS, experimental %olrk aind restilts are as presented in (le earlier reports but in
the I )iscussioi allItusions aer made to t lie i nfl tie ices of ad v,•lices iii cable design, current technology
ard standards. An up-to-date hibliography is included at the end of the report.
2.
I VESTI(CATION OF SERVICE (CONI)ITIONS ANI) (CABLE DAMAGE
hi order to detcrmnine thlie factorS that cauSetd dteLrioratiOll Of Control cables a number of0
cables damaged in service were examined and the control cable installation was inspected in a
number of Auster aircraft.
2.1
Examination of (aibles
I)amuiged in serfie
A numnber of cables worn or broken in servicc \k as exanmined byhnicroscope. The rudder cable
failures frot tnlhe Atustcr aircralt (reported in Ref. I) arc includcd. The aircraft have been given an
arbitrary designation f1or the purposes of this repor'. i.e. VIIA. VII II. VfI(., V\IE.
I tit lie A uster rudder cables most of t lhe kkOr iard distorted sect ions of ca hle occurred at 254305mrm (10 - 12 i nches) from t lie rudder pedal eve attachnient and 356mmni (14 inches) from the
horn turnbuckle attachment (Fig. 3). [he weartor the flormner condition was at a change of
direction pulley and the liater where the cable rubbed on the [abric fairlcad oruemerged from the
rear fuselage. It was at this latter location where sonic ol t ecables \ere corroded and the position
where VIIC had severe internal corrosion which was not detected by external visual inspection.
[he wires of the 7 x 14 cable from VIIA were heavilv abraded, the abrasion marks being
clearly visible and laying approximately at right angles to the cable. This abrasion had occurred on
one side of the cable only (approximiately halt the circu~mferen~ce) and extended about 57 mm (2'i4
inches) each side of the failure. rhe individual wires were each examined at the point of fracture by
microscope and this revealed no clear evidence olf atigue failure but showed that most of the wires
had failed by static-tensile load. A photograph of the failure showing the extent of abrasion is
shown in Fig. 4.
In the 7 x 7 cable VHB the wires showed no marked external abrasion but were heavily
damaged consistent with high pressure between wires. There was considerable deformation and in
some strands the core wire had assumed a hexagonal cross-section tinder pressure, which was
3
A
!
probably comihined wit h rubbing, fromni fhe surrounding wir es. Fig. 5 shows ,r photo of the failure
in which the damage to the ssrres call be ckcarly seen,
The difficult v of* detect ing inter nalI wire damage was cnmphasised whe n thle operat or of' V HEL
removed both 7 x'14 rudder cables two Iliing hoursa Iter a ('ertificate of A irworthiness beca use of'
a broken wire f'ound in one of* the cables. 'Ihese cables were examined in the laboratory and one
was f'ound to have a completely hroken core strand. each side of- the f'racture was corroded a
distance of' about 12.7 mim 1'., inch) and indications were that f'ailure had existed f'or some time
prior to the inspection.
[he 7 x 7 cable hrorm V IIC had worn approximately 0.051 mim (0.W02 inch) where it had
rubbed onl the fabric but had no othter abnormal change in appearance. However, extremely close
inspect ion revealed a sli, ilt discolouration at the point of the strands. Cutting of' the cable and
subsequent inrspect ion with atmicroscope showed that a considera ble area of the strand in contact
with the core strand had( corroded and bri nell ing oft the wires of adjacent strands bad also
occurred. [his damage was at tribhuted to the continualI haminmering and rubbing between the wires
ofItihecahle under Ii rictuating tension arising fromn cable vibrationi due to rudder buffetting f'rom
the propeller slipstream. Ilioweser. thre samelI State Of Corrosion and brinelling was l'ourd in a 7 x 14
cable rcnmosed from aIgl ider where vi bration could occur dute to self excitation or running over
tinevenl ground surf'aces. It wais, interestinrigto note that this glider cable. while showing outer wear
of appnrox. (1.05 1 rum1
(t).t002 j ich,) had seseral core strand wires failed as shown in Fig. 7. In another
glider cable wires III seseratl strands had failed in ftilgue at %arious locations but little or no wear
wais x
lveident
C\
( Fig. 8), It ssais only alrci scveralIinspect ions of the cable that these wires
were ribsers ed beca rise II, r ctiics rc of stuch atnature that the "'clothI method"' of detection
(Ref. (f did lnot indicate airs tailc
IL' ires.
2..,
Inspection of control err hie inistallations
Ifrere arc four f'act ors I n conitrol cable IIrst ilIl~ritIor[is whItich cani be recogrii/ed as contri buti ng
to cable damrrage. I liesc arc.
(I) (abl
v\i hn'lrt Ion irgarils puilleys, and tairleads. I Iris is,particularly inmportarit in rudder
carhcs nrrr sinallIllropeller driseiiaircraft due fto tire effects of slipstream on the rudder.
(iii
Cilt~hlc' bendingp Stres-ses (fle to f'Clexig of1tIre cable ar rmund pulleIYS.
liii) D ariiage ito urnproteecite cables.
6\s ) (orrosrori, dute in) r'eirrisal of lirntecfis couitirngs by rubhinig oft wires against each
of hier.
I Ire COIrIrIrl Lir'cirirt tfIre Aristc is described ii soriemcdriila~s it is tv (ica Iof small propeller
drisr'ri aircrirtt arnd illustratres Minnie: ot the ardscisc conditions III which'cables operate.
In some nIl Ilie Auster airicrfAt Cx~airiiited I lie ca bles were slarck rithIle unloaded condition while
ri ni thers file.\ were tenrs~ioned fri 2.27 kg (5 lb.) hby brrimpcccord. I lie cable was linked directly to the
ruidder pedal. f ire load f rrir which iNsubject fit a lcser rat it)of 1.5: 1. [hle cable is deflected through
approix. IS- bya 3.Xmi1.2ri)daii'rprlc
sir uated at I lie forward lower corner of' the
cabin entry door. I Iilsis thIre unv pulleyv (iii the pr srde. \k hrill onr the starboard side there are two
puilleys oneif)1. .X iran 11.25 in. lnliaiietcu iiu
anritl oif57 - miii (2.25 in. Idiamecter. the targer- pulley
appearinig no be piIMs L lii
1d
0 aecrirurndate a Ii
IIgCIr an1gularl deflectionr.
lIn the A.1. series aircraft thle cable runs arc uiritriulcened thirmnughout the cockpit area, while in
the .1,5 series, shiroudinig is provided aft of* thre pulleys. However the pulleys are in all cases
vulnerable fto dam
rage [rorm personnelri ernterinrg oir Iea\ irg thre crick pit and to contamination. F'rom
the pulley thle cable runs af't via two block and tw
wi
nirg fairleads down the fuiselage and emerges
through a rei riforced h ole in thle la brie intthe side of thle fuselage. thle tinral 610 mm (24 in.) of the
cable to the rttdder being exposed. [hle cable %%as subject tor about 3" deflection at the aft fairlead
and the exit hole [~rnin thre fuselage. at the former filie airlead was worn to a bout 3014 of the cable
diameter arid slight wear arid corrorsiomn was evident at tlie exit hole. The adjustment and stop
limiting rudder miovemlent were located inrthe rudder honii.
2.3 Measurement oIf cable loads In service
[he majority of cables exmirninc were from Auster .15 aircraft and to investigate the reasons
f~or cable fa3ilures in service typical cable generating loads were measured.
4
LI
A type .156 Auster was modified to enable the insertion of an A. R,.. loop transducer of 2.23
kN (500 lIl.) capacity. Ihe aircraft was hiown under various conditions, and the loads at the
transducer were rceorded automatically by a S. F.I.M. recorder. A series of ground manoeuvre
loads were also recorded using this equipment, These conditions and results (shown in Tables I
and II) where the manoeuvre of full "left" and -'right" rudder at 80 knots straight and level
produced a roacasured load of 0.98 kN (2201bf*), while a maximum toad of 1.95 kN (440 Ibf) was
recorded for a ground manoeuvre when the instruct or appl'ed opposite rudder control to over-ride
that being applied by the pupil (Rcf. 5 relates to a cable failure during dual instruction).
The ultimate design load requirements for the various types of Auster aircraft are set out in
Table III to give an indication oftypical control forces that can be expected in small aircraft olth.s
C
INVESTIGATION OF THE FACTORS AFFECTING CABLE STRENGTH
A number of investigations were carried out to determine the causes of cable damage
observed in FPara. 2.
3.
3.1 Determination of the mechanical properties of cables
As previously stated some cables removed from service were found to have a broken core
strand while the outer strands appeared intact. Tensile tests were therefore made on cables, core
strands from the cable and wires from the core strand.
Ile specification and test data for the three types of cable are shown in Table IV. Tensile
properties and dimensions of single wires from the cable are shown in Table V. Curves of direct
tensile stress versus extension are shown in Fig. 9. for the centre wire and outer wire of the core
strand of a 7 x 7 cable. Since the wires were preformed it was not possible toexpress theextension
in terms of strain since a large part of the extension for an outer wire occurs in straightening of the
helical twist. Slight crimping of the core w;re may also be present due to the effect of the contact
forces of the outer wires.
However. these graphs give the average tensile stress and an indication of the plastic
elongation at failure. It is interesting to note that both the ultimate stress and the plastic
deformation of the outer wire appear to be significantly less than the centre wire, It is possible that
this effect is due to the superimposed bending stresses applied in straightening the preformed outer
wire. hn such very hard drawn wires these stresses are likely to be quite significant.
3.2 Effect of cable wear on ultimate strength
To investigate the effect of wear in service, the minimum diameter of the worn cables taken
from service was measured and the failing load determined in a tension test. New cables were also
abraded using fine ema-rv cloth and rubbing the cable by hand in a longitudinal direction over a
length of three inches and around one third of the circumference. This corresponded to the extent
of the worn area observed on cables fromn service. The failing load of the cable was then determined
in a testing machine, the overall length of the cable being 610 mm (24 inches).
By microscopical examination of each cable tested the area of the failed section of each wire
was measured to within an estimated 50'i. This enabled the failing load to be plotted against
percentage reduction of area in Fig. 10. and against reduction of cable diameter in Fig. II. There is
a considerable variation in results but curves have been drawn through average values. The
percentage reduction of area was estimated by the microscopic examination of abraded cabies
that had failed in service and an estimate of the expected failing load could be obtained from Fig.
10.
3.3 Evaluation of nylon covered cable
Wear and abrasion had been cited as one of the principle causes of cable failure, Ref. I.
Therefore, in conjunction witf, D.C.A. it was decided to carry out fatigue tests to evaluate the
performance of nylon jacketed cable in comparison with standard cable.
Standard 5 cwt. (2,49kN) 7 x 7 cable to B.S.S. W9covered with nylon was available in only
limited quantity so only four test specimens were available. Two specimens of nylon covered cable
"As the max. speed for this type ofaircrai ..- 120 knots it was estimated that flight loads may exceed
I. Ilk N (250 INf) for a similar manoeuvre.
,
5
M'
4.
A,
and two specimens of standard cable were tested on hardened steel and "Micarta' pulleys (at a
temperature of 53'C ( 1200 F.) Although, as expected, the nylon jacketed cable on the steel pulley
was far superior to the standard cable the results using a Micarta Pulley were inconclusive. The
results of these tests are summarised in Table VI.
ANALYSIS OF ('ABLE STRESSES
Some of the wires of the core strand under contact from the wires of the outer strands showed
evidence of rlastic deformation. Evidence of freuting was also present. In the tests to failure on new
"cable,:described in Para. 3. I there was also evidence of the wires in the core strand having been
indented by wires from the outer strands, In a cable the outer strands are arranged in a helix
around the core strand. When tension is applied to the cable the outer strands close on the core
strand resulting in pressure between adjacent strands in the rope. In a similar way contact forces
occur between wires within the core strand. Forces at contact between strands also arise due to the
cable passing around a pulley, the most heavily loaded strand being the inner strand bearing
against the pulley,
A theoretical analysis ofthe contact forces between wires ofa 7 x 7 cable was therefore carried
out by applying the theory of Ref. 7.
The following equations for this case are derived in Appendix L.
In a 7 wire strand the load in the strand is given by:
I - I, (I f 6cos I3)
...
(I)/I'
Whcre I, is the tensile load in the core wire and I, is the helix angle.
4.
Similarly lotr the cable the axial load is given by:
P l), 0-: f10 Cos AB,)
... (2)
Whete 11,is the load in the core strand and /3, is the lay angle.
Considering now the radial force where the outer strands contact the core strand we have:
trr I cos 13 sin (3
9 ( - 6 Cos 1/3)
Applyiog thc generl equations in Ref. 8 for elastic contact stresses between two cylinders, an
expression for conta.t stresses between the wires of' adjoining strands has been obtained in
Appendix I. !he contact stresses as a function of cable load have been calculated and the results
are presented in Table VI.
Considering the core stiond the radial loading on the core wire exerted by an outer wire is:
, 2 1,, sin' 3, in the st:,ind
d
where d is the wire diameter and L, the load in the outer wire
and, .
2 R,. sin' (, in thiý cable
I)
where 1) is the strand diameter and P.. is the. load in the outer strand -.'ith the assumptions of'
Appen:d::.: 1.
This leads to an expression for the contact stresses on the core wire of:
where A is a function oft he wire diameter and contact angle and may be obtained graphically from
Ref. 8.
The experimentally determined failing loads of a 7 wire strand have been compared with those
predicted from the strength of a single wire using equation (i) which assumes that all wires behave
elastically up to failure. In the same way the ultimate strength of each of two cables has been
predicted from the measured failing load for a single strand. All these results are presented in Table
Villa which shows that there is reasonabhe agreement between predicted and measured values of
ultimate strength.
6!
I
i
"
lests were also conducted on cables in which the core strand had been cut. The predicted
failing load in this case is:
P = 6 V,, Cos ,,
assuming that all the outer strands carry equal tensile loads. The test results are compared with the
predicted values in fable VIlIb.
12
5.
DISCUSSION
5.1
Historical
With the limited documentation available an attempt has been made to piece together the
history of the Auster rudder control circuit.
[he Auster was the product of laylorcraft Aeroplanes (England) L.td. a company formed
about 1940 to manufact Lire the American Tavlorcraft light cabin monoplane under licence. The
aircraft was used during t he war as a light utility and observation aeroplane. The company later
became Auster Aircraft L.td. It is presumed that the rudder control circuit was originally designed
7x 7
cable
diameter
in.)was
mmlbf).
(3,32
a 2.38
probably
to MII.-C-1511,
an American
with
Belgian
the of
the cable
(This
N (920
h of 4.09k
breaking strengt
and ancable
ultimate
construction
authorities used as a replacement for the British cable in 1949 and reported no further defects in
1961). [he nearest British cable would have been the 10cwt. (4,98kN) 7 x 14cable of 3.05 (0.120
in.) diameter to BS.S. W2 (not preformed: W9 did not appear till 1946).
In 1945 duc to failure of cables then in use (presumably the 7 x 14 Britishcable) British Ropes
carried otit a series of testS using an oscillator to reproduce the hammering and conse luent
abrasioi and fraying of the cable on pulleys. The hammering was due to the slipstream over the
rudder. (para 2.2). he conclusions reached in these tests was that a 7 x 7cable was more resistant
to abrasion than the 7 x 14 due to the larger diameter of the wire. 7 x 7 cables were therefore
adopted. In 1952 anot her "crop of troubles" was experienced resulting in a further series of tests by
British Ropes, in this case comparative fatigue tests. I he existing 7 x 7cable was compared with "a
coniparable American' cable" and a 7 x 7 cable "manufactured to a new technique", (probably
pref.rnmed as in W9), [he new 7 x 7 cable had a life 2'.: times the American cable and 5 times the
existing 7 x 7 cable. This new 7 x 7 cable subsequently became Auster Standard .Il 264 and was
adopted for all Austers and is one of the cables which is the subject of this investigation.
One of t lie ot her cables investigated was of 7 x 14construction and this would appear to be the
result of I).C.A. Air Navigation Order which reqiestLd replacement of all 7 x 7 cables with 7x 14
or 7 x 19 construction, (Ref. 9).
5.2
Cables
('able of 7 x 14constrtiction is more flexible than 7 x 7cable and hence has higher resistance to
hatigue. It follows that the cable of 7 x 19 construction is superior to both the 7 x 14 and the 7 x 7
giver similar conditions and materials. The 7 x 19 cable. to MI 1.-C5425 is 50% stronger (static
tensile strength) for the same diameter as the 7 x 7 and 7 x 14 and is superior in fatigue to the cable
made to British Standards (See Refs. I0 and II).
[he investigation revealed that the cable of 7 x 14 construction was an inherently unstable
configuratio,; prone to internal abrasion and a tendency to ovality. l)ifficulty was experienced in
service in checking the uniform diameter of the cable witha micrometer because of ovality of up to
0. 15 mm (0.(),6 in.). The tendency to instability is well illustrated in Fig. 2, where a length of 7 x 14
cable has been set in Araldite and the ends of 'he cable polished to show the wire layout in the
strands. thc mobile four wire core is adequately illustrated leading to a variety of other than
circalar strand shapes. ['he comparatively orderly arrangement of 7 x 7 and 7 x 19construction is
shown in the sketches in Fig. I.
It may be of interest to note that the daughtsman who prepared these sketches was unable to
find any illuNiration of 7 x 14 cable despite a thorough search of references and the literature on
aircraft and commercial cables. [here was also no mention of 7 x 14 cable except in the British
Standards rciated to aircraft cables.
The comparative fatigue tests on standard and nylon covered cable (5 cwt. (2.49kN)capacity
7 x 7 construction) on steel pulleys demonstrated the superiority of the covered cable, 2.7 times the
endurance of the standard cable. However, the results using an aircraft pulley (Micarta material)
7
I
were inconclusive due to the limited number of specimens available at that time. It was noted that
the nylon jacket tended to interlock the broken strands o, the .;.bleand thus prevent total failure
unless the failure was entirely at one cross section. One se-rious disadvantage of the nylon covered
cable is the difficulty in detecting broken wires, an important azpect of maintenance inspection in
service.
Other characteristics noted were the tendency to local bending' the cable at the site of cracks
in the nylon cover and the increased resistance to bending ca):- u by the nylon cover on small
diameter cable.
5.3 Pulleys
The existing "sheave ratio" (Pulley diameter, cable diameter) for the Auster rudder circuit is
11.8 for the 31.75 mm (1.25 in.) diameter pulley and the 7 x 7 cable. If this pulley was the same
diameter in the American design then the sheave ratio using the MIL-C-151I cable of 2.38 mm
(3:32 in.) diameter would have been 13.3. For a satisfactory fatigue performance for 7 x 19
galvanised cable loaded to 20% of its breaking strength Ref. 12 recommends a minimum sheave
ratio of' 16, and states "less than 16 rope diameters falls within a critical region in which cable life is
relatively low". Reference 13 recommends a sheave ratio of 18 for 7 x 19 galvanised and stainless
,ables and a ratio of 28 for 7 x 7 cables.
In Ref 14 F.T. Hill states "A minimum pulley diameter is suggested by the British
Airworthiness Authorities as not less than 20 times the diameter of the cable".
It would seem that sufficient information was available at that time to enable reasonable
sheave ratios to he specified, although the figure of 11.8 could possibly have been condoned on the
basis of the small wrap angle of 18'. Again, in the elevator circuit a 57.15mm (2.25 in.) diameter
pulley is fitted apparently to accommodate a larger wrap angle. There seems to be little
justification for this as Ref. 15 finds there is no significant varation in fatigue life above a wrap
angle of 200.
5.4 ('able Standards
The relevant British Standards for aircraft cables at the time were: 6W2 (Sept. 1946) Steel wire rope and straining cords (not preformed)
W9 (Sept. 1946) Preformed steel wire rope.
WIO
Non corrodible steel wire rope (not preformed)
W II (1946) Preformed non corrodible steel wire rope.
The American standards were:
MIIL-C-151 I (Nov. (1949) ('able: steel (carbon) flexible, preformed.
MII.-C-5424 (Aug. 1948) Cable: steel (corrosion resisting) flexible preformed for aeronautical
use).
The specification drawn up by Auster Aircraft for 10 cwt. (4.98 kN) cable called for wire in
accordance with BSS W9.
In assessing the relative merits of the various cables it is difficult to make a direct comparison
because of the differences of so-called equivalent cables. The British method of designation of the
cable is by ultimate breaking load whereas the American system is by cable diameter. If the
American and British cables happen to have the samr' breaking load then the cor struction may be
different. For example Reference 10 finds that the cable to MIL-C-5424 is superior to the British
cable but this finding is qual;fied by the fact that the tests were carried out over the same diameter
pulleys for cables for cables of different diameters, viz.:...18mm (0.125in.)and 3.64mm(0.143
in.) ard 15 cwt. (7.47 k N) capacity. Generally, the only obvious differences between the American
an i Brii;sh specifications are:
(i) The American specification calls for an endurance test.
(ii) The American specification calls for lubrication of the cable during manufacture.
The Auster specification, incidentally, called for an endurance test of 2.400 cycles with the
cable loaded to I 3 of the breaking load and a sheave ratio of over 20.
If we now examine the Standards for cables in the Auster category we find the following
comparison:
8
N
Construction
Diameter
mm (inches)
Min.
Breaking Load kN (Ib)
7x 7
4.09(920)
(MIL-C-1511
(M IL-C-5424
4.98 (1,120)
Auster JB 264
*7 x 14
2.38
(0.094)
2.69
(0.106)
3.05
(0.120)
4.98 (1,120)
(BS W9
(as W1II
7 x 19
3.18
(0.125)
7.83 (1,760)
MIL-C-5424 (1957)
7 x 19
(0.125)
8.90 (2,000)
MIL-C-1511 (1948)
*7 x 19
3.81
(0.150)
7.48 (1,680)
B.S.W9- WII
7x 7
!
* Also
6.
Specifications
available not preformed BS W2 and WIO.
CONCLUSIONS
6.1
Cables
The performance of the 3.18 mm ('I/ in.) diameter cable of 7 x 19construction to MIL-C-1511
or MI1.-C-5424 is superior to cables of 7 x 7 or 7 x 14 construction.
6.2 Nylon covered cables
No conclusion is possible on the performance of the nylon covered cable due to the limited
number of specimens available. However, inspection for broken wires is likely to be a major
problem for this type'of cable.
6.3 Pulleys
Pulley diameters should be as large as practicable and at least 20 times the cable diameters.
6.4 Rudder control circuit
6.4.1 Rudder limit stop
The rudder limit stop should be situated at the pedal rather than at the rudder horn. Many
aircraft later than the Auster feature stops at the pedal as well as on the control surface itself.
6.4.2 Circuit tensioning
The rudder control circuit needs to be "closed" and tensioned. (Ref. 16)
6.4.3
Fairleads
More direct routing of the cable (particularly at the fuselage exit) is required to avoid
abrasion due to fairleads. Protective covering over exposed cables in the cockpit area are
desirable.
7.
REVIEW OF STANDARDS AND RELATED INVESTIGATIONS
With the passage of time some of the findings and recommendations from this investigation
have been over'aken by the updating of standards in the light of experience, increasingseverity of
operating conditions and improvements in manufacturing techniques. For example the
International Organisation for Standardisation (ISO) has played an increasingly important role in
the formulation of standards through Committee ISO/TC 20 (Aircraft and Space Vehicles).
In 1957 ISO/TC 20 had asked the Italian Member Body to draw up a proposal on the
international standardization of pulleys for flight controls. This proposal was based on a revision
of the American standards for pulleys at that time, AN 219. AN 220 and AN 221 (now MS 20219,
MS 20220 and MS 20221) where it was permitted to use several cable diameters for a given pulley
diameter and vice versa e.g. M.S. 20220 (Ref. 17) for cable diameters of 3.18 mm( /Kin.). 3.97mm
(5/32 in.) and 4.76 mm (3/16 in.) permits the use of pulley diameters 31.88 mm (1.255 in), 63.63
9
-L
..
....
mm (2.505 in.), 95.38 mm (3.755 in.) and 127.13mm (5.005 in.). The design parameters are the
pulley maximum operating loads and the cable maximum operating loads and no reference is
made to the minimum sheave ratio. Thus it is possible to specify a pulley /cable combination with a
sheave ratio of less than 10.
Meanwhile, in 1966. British Civil Airworthiness Requirements (Ref. 18) laid down rules for
control system installations and gave recommendations for pulley sizes. This was presented in
graphical form with the number of wires in the cable as abscissa and the sheave ratio as the
ordinate and a family of curves with the sheave ratio increasing for a given cable according to the
load (expressed as a percentage of the breaking strength) in the cable. Within practical limits this
standard has set a sheave ratio of 20 or above.
In Reference 19(1971) the U.K.. commenting on the Italian proposals and other international
investigations, pointed out that the U.S. were still discussing pulley standardisation and planned
to conduct additiona! tests. The U.K. recommended that one cable should be used for two pulley
sizes, one with a minimum sheave ratio of 25 to be used where space was limited and the other with
an ideal sheave ratio of 40 for use wherever possible. The Italian Member Body was again invited
to recommence a study of aeronautical pulleys and so far as can be ascertained no ISO standard
[-as yet been agreed upon.
7.1 ('able Stmndards
Of the British Standards issued in 1946. viz. 6W2, W9, WI0. and WI I, 6WE and WI0 were
rendered obsolescent in 1964 because the S.B.A.C. indicated a preference for preformed cables.
W9 was replaced by 2W9 in 1965 and 2WI I superseded WI I in l;67. These two specifications
recognised that "there has been a marked increase in the severity of conditions under which
flexible wire ropes operate in aircraft flying control systems", and introduced corresponding
improvements. Lubrication during manufacture was introduced (U.S. practice), the wire from
which the rope was made was more fully specified, dimensional tolerances and ranges of tensile
strength were specified. The issue of British Standard W12 in 1968 showed the effects of I.S.O.
with :he in.roduction of the specification of the cable by diameter (U.S. practice) instead of by
breaking strength. lVhp cable of 7 x 14 construction was discontinued.
W 12 was followed by W 13 in 1973 specifying the corrosion resisting equivalent of W12. The
situation can be well summarised by quoting from the Foreword to W13 viz.:
"This British Standard specifies the requirements for preformed corrosion-resisting steel wire
rope to comply with ISO Recommendation R 564 'Designations, diameters and brea4king
strengths of preformed stranded steel cables for aircraft controls' and International Standard ISO
2020 'Technical Specification for flexible steel wire rope for aircraft controls'.
ISO, R 564 specifies the dian!,ter, construction and minimum breaking strength of the rope,
all of which are in accordance with RgCommendation Nu,. 3310ofl'Associatiou Internationale des
Constructeurs de Materiel Aerospatia!e .'A.I.C.M.A.) and with the United States Military
Specification MII.-C-5424A. Sizes of rope. larger than 6.4 mm (,4 in.) diameter, though included
in M I LC-5424A. are seldom used in mode: n aircraft, and have been omitted from this standard".
Recapitulating. it is int&estingto note that in 1965 in 2W9 a sentence inthe Foreword stated
"An endurance test is being formulated and will be added to the specification by amendment as
soon as possible". In 1973, eight years later, the same statement appears in the F-oreword of W13.
Evidently, some difficulty was being experienced in the formulation of an endnirance test.
Moreover, in 1969, at the ISO meeting, the U.K. and French delegates were charged with the
responsibility of producing recommendations for an endurance test. Investigations on this matter
had been in progress for some time and a test programme designed to compare the scatter ofresults
between a proposed U.K. method and the U.S. MI I.-Spec. method showed a considerable scatter
between three nominally identical machines built to the MIL-Spec (Ref. 20).
The issue of the ISO Technical Specification for flexible steel wire ropes for aircraft controls
ISO 2020 shows that the problem of the endurance test has still not been resolved. A footnote to
the specification states: p
"While awaiting the publication of an ISO test ,rocedure --- an endurance test can be carriedd
out in accordance with a relevant national specification". The United States, incidentally, has
expressed its disapproval of ISO 2020.
If we now turn to the American scene we find that the U.S. Military Specifications MIL-C10
-
.
-
X
151 1(1949 and M IL-C-5424 (1949) have continued through to 1973 witsh a few minor amendments
such as specification of the chemical composition of the steel, wire tolerances and tensile strengths.
In IS73 Mli,-W-83420 was issued zuperseding MIL-C-1511 aii, 5424 and combining in one
specification carbon steel and corrosion resisting steel cables and their nylon jacketed equivalents.
I-is specification is now entitled "wire rope", flexible instead of "cable"
flexible.
7.1.1 Nylon covered cables
Because of'reports of excessive wear on control cables in F-5 and T-38 aircraft extensive tests
on nylon covered cables were carried out by Wright Patterson Air Force Base in 1967-68. The
nylon cover, or jacketing, is extruded over the cable to completely enclose the cable with a
protective coating. The cables tested were nylon covered stainless steel cables to MIL-C-5424 and
tests were made at normal temperature and ai -40°C(-4.00 F). (Ref. 2i). The results of these tests
"showedthat the life of the cables was significantly increased by the wear protection afforded by the
nylon covering and in addition assisted in vibration damping, protection against sand, grit, fluids
and otner harmful elements. However Ref. 22. commenting on the tests, again emphasised the
serious problem of cable wear detection on this nylon covered cable and states that, to date, no
satisfactory solutions have been found to solve this problem. Reference 22 also i idicates that in the
11.S. carbon steel cables are being replaced by stainless steel cables. Results o' endurance tests at
low temperature show that the stainless steel cables are far superior t,; the carbon steel product.
(Ref. 23).
7.2 Pulleys
l)uring the course of the ISO sponsored investigati, ,is already mc:ntioned in Paras 7 and 7.1
other significant factors in cable wear have been discovtr-d. (Ref. 23). One of these is the
considerable w'ear sustained by a cable where it passes over a pulley. at a small wrap angle. * (This is
relevant to the Auster see paras. 2.2 and 5.3). There was a marked difference in the wire wear and
failure at pulleys in which the angle of wrap was only 15' compared with those of a wrap angle of
over 90'. It appears that the rope with the small wrap angle, due to the lack of support from the
pulley groove, has a transverse scrubbing action imposed upon the wires in contact with the pulley
which leads to abrasiont and early fatigue failure, However these findings do not appear to agrec
with the investigations in Italy (lab. Centre FIAT. Ref. 24) where it was found that cable life
decreases with increase of wrap angle from 0" to 450.
7.2.1
Pulley grooe profile
In 1971 the United Kingdam ISO l)ocument (Ref. 19) still indicates some disagreement with
I SO proposals on groove profiles and favours a groove to give maximum support to a cable and to
provide for regular proportional increases in pulley groove dimensions as the cable size increases.
Some U.S. investigations in this area are described in Ref. 26.
In conclusion it seems there is still much work to be done to arrive at agreement on standards
and international acceptance of design criteria to ensure the satisfactory performance of aircraft
control svstems and cables.
ACKNOWIEDGEMENTS
Ihe advice and assistance of I)r. A.0. Payne in the planning of the investigation and during
compilation of the report is gratefully acknowledged.
The author wishes to express his appreciation for the willing co-operation of A.R.L. Library
staff in the procurement of extensive reference material.
"Thanks are also due to the Department of Transport, Air Transport Group ('ormerly
Department of Civil Aviation) for discussions and assistance with reference material.
*1The angle subtended, at the pulley centre, by the arc of contact of the cable.
II
S.
.......
.. '=•--• ."
..dz~. ii.. • • . . .• h .. ...
14
R EFER ENCES
I,3.
1. R.A. Vell.
C.A. patching and
A.O. Payne
Preliminary report or the f'ailure of -Atster" rudder cables.
Aero. Research Labs.
Structures and Materials Tech. Merril 88 July 1960.
2.
Specificat ion f'or 7 x 7. 10 cwt. cont rol ce bes X' type) AJ.!3 264.
Auster Aircraft L~td.
Rearsbv L~eicester. Jan. 1959
Pref'ormed stec! wire rope. British Standard Specification W9.
Brit ishi Standards Institution. Sept. 1946,
C'able: Steel (corrosion resisting). Flexible. Preformed (f'or
acrona iiiical Lise).
MIilitary Specification MIF.-C-5425 Anidt 1.3
4.
5.
Rudder C'ont rol Ca ble FaiIlure,
I .( 'A. Aviation Safety D)igest No. 121Dec. 1957 p. 26.
6.
7. \VI., Starkev and
N.IA. Cress
('ontrol (*ab le Inr1poct ion
I .('.A. Aviation Satetv D)igest
March 19621. 11.24.
/iAnA nlvsis of ('n-tical St resses andt M odeof F'ailure of wire rope.
I alls. A.SM.F. Nov. 1959 pp. 307 - 311.
8. I'. . Seeleyv anId
.J.. Smith
Ad va ICed MCIehariics Of M teiaS
JIohn %Wikv\ & Sons Inc, Ne\% York, N.Y. 1952 p. 342 et seq.
9.
C'ommonwealthi of Austtalia. I epartilctit of('ivil Aviation. Air
Navigation Orders, Section I105,
I. 3. o. 1.5.
A,.0N. P~art 105 21st August 1953.
1t). ( .R1.lar-ratt
II. (.14. Banra:c
12.
13,.. F{.IF
i kll'!i
14. 1.. I1.HIill
I
,
Fatigue tests of aircraft control cables.
C'ommonwealth Aircraf't Corporation Ptv. Ltd.
Report AG 410) 1961.
Long-term fat igue tests of control cables to Spec MI I-('-5424
Commonwealt h Aircraft Corporation P.' L.
Recport A6 417 196 1
"D ata fom aircraft control cables"
American C hamn anid C'able Co.
P~.6 1942.
"Wire Rope Assemblies''
MIachine IDesign Vol. 33 No. 16
p. 88 1961.
Materials Of Aircia!t ('ons(truct ion
P'itman London Ist.
Uditionl.
p.129 1933.
IS, I .1.(jod Irev
A.H~. Fleury
lDartrey Lewis
[hle corrosion fat igue of' aircralft control ca bles. Fatigue tests
under service loads.
John A. Roebling's Sons Company, for Office of Scientifie
Research and IDevelopment.
OSRI) 4543. NRC-IS M-439.
I16.
ILetter to C'ivilI Aviation Liaison Officer. Australia House from
F'. Watkin. Head of' Design D~ept., Auster Aircraf't Ltd.. Oct.
1959.
17.
Pulley, groove. flight control, aircraft U1.S. Military Standard
MS 20220 Mar. 1965.
is.
British C'ivil Airworthiness Requirements Chapter 1)4-8.
Appendix Sub Section D)4 para. 5. Control System Installations
Feb. 1966.
1
BIBIOG(RAPHV
I lie preparat ion al' this report has entailed a Illa'rly extensive literature search and has revealed
at number of publications which, while not directly associated with this investigation may.
nevertheless, be of' incidental interest to those involvecd with wire ropes.
Ihlis bibliography with the refecrences cited and the references f'rom this report should form
the basis of a reasonably comprehensive int'ormation source on wire ropes.
I.
Hlaigh. HI.P
Wire Rope Problems. S. 'ale~s Insvituth' a//igitteers., Provs.
Vol. 52 No. 5. 1937 pp. 327 - 357.
2. Cables and ('able Terminals f'or Aircraf't, American Chain and (Cable Co. C'onn. U.S.A.
1940.
3 1-114t
..
Calculation of stresses inwr rpsIir
1;:'
Wmire Prodlucts. Vol, 26 Sept.'
Iell. R.A.. and JIohnstone W.W, Tensile and E~ndurance Tests onl preformed
6.ole A..
stanlssstelcables. Aero Research Labs. Melbourne. Structures and Materials Note 206
I
7. Aircraft Inspection and Repair A('43 - 13 - I Chapt. 4. Control cablesand terminals Federal
Aviation Agency 1965.
8. lDang, R.(i. and Steidel., R+ . Contact Stresses in Stranded C'able, Ev~perimentaI Alec/junktis.
Vol. 5 May 1965 pp. 142 - 147,
9. Senior Ii .A. and Spear, .1A.N, [rndUranee! tests onl flexible wire rope for aircraft flying
cont rols, Report All) I) 000002 68 Part 1.
Aer-onautical1 Inrspection D irectorate Minist ry oflTechnology U.K. Jan. 1968.
It). (ianihrell. S.C. St uidv low cycle Fatigue otf wire rope. Wiire andi Wire Products Oct. 1969 pp.
127 - 130 (Commnnctts oin effects of pulley material onl fatigue life of'cables).
11. Wood IHerbert I ., A surve * of* publications dealing with corrosion in wire rope. Report 71-3
I.O.S, . I hie (Cat holic University of' Amnerica Washing I .C, June 1971.
(A collect ion oft bothIiJournal and report literature dealing with corrosion and prevention of*
corrosionl of wkire ropes. Forty-one references are cited including other suirveys).
12. Starr. I .J. ( orrosion prevention and Control in Steel wire ropes in the mineral industry.
.111strala.sian Co rrosion hDrgin'erinrg Nov. 1971 pp. 7 - 12.
13. Inspect ion of' Coant rol C'ables. A1irccorihim'sAN ,'11'sor[ C'icular NVo. 61. D ept, of' Civil
Aviation Australia Feb. 1972.
14, Hecker. Karl. On the latigue strength of steel wires. Stahldawi l:'iven Vol 18. August 1972 pp.
87.3 - 88(0 (In G erma n).
I5. M achida. S., and I 11urelli. A.J Response of a strand to axial and torsional displacements.
JIournal of Mechanical Frngineering Science. Vol. 15 No. 4 1973 pp. 241 - 251.
l6. Wire rape lubrication and corrosion Protect ion. Indlustrial ILuhri'afion tiinI riholiogy
17.
18.
Ian I-eb. 1973 pp. 7
- 12.
P1hillips. lamies W., and Costello. George A. Contact stresses in twisted wire cables. .ournal
of Engineering Mechanics D~ivision. Proc. of Amer. Soc. of* C. Engrs. April 1973 pp. 331 341.
Australian Wire Ropes. Australian Wire Industries Newcastle n~d. (A comprehensive and up
to date handbook prepared by A.Wl. f'or the guidance of' engineers, operators and rope
users.)
19.
Co~mments by the United Kingdom on D~ocument 20 N 1276 -.
Control Pulleys
ISO TIC 20 (Secretariat 807) 1305 Nov. 1971.
A Technical Report covering the exarninat ion ol'threc American
type flexible steel wire cable testing mnachines.
Report No. A.Wl) 2 70 AQI)
ILaboratory Ministry of' Technology Mar. 1970.
Evaluation of' nvo and polyolefin Jacketed aircraft conirol
ca bles
20. FI.A. Senior
J.F. Stoddart
V
21. Perry L.. Smith
Technical Report ASD-TR-68-30 W-P.A.F.B Ohio. August
1968
W hv contitrol cabhies of* stain less steel give America n aircraft great
reliabhility.
Wirc and Wire Products June 1969
pp. 62-64
22.
213.Perry I., Smith
24V
.Sno
~Endurance
R. Everard
25.
26. Werner Gi K roggel
Postal Addrless:
-lif
Breaking Stre:,gth aind Endurance lest ing of' Aircraft control
cables
Technical Report SF.(-*FR-67-l9
SEFIl Wright-Patterson Al-KB. Ohio May 1907.
tests on flexible wire ropes f'or aircralt flying controls
Report No, All) L)EV 92 66. Aeronautical Inspection
D~irectorate, Ministry of' Aviation August 1966.
Second lDraf't proposcd by Italian Member Body f'or
Standardisation of' Aircrafýit Control Pulleys
ISO IC 20 (Italy-16) 1276 Oct. 1971
Evaluation of' Aircraft Puilley Groove Effects on Jacketed
control cable life.
I ccl. Menmo ASDI) N [I ;rM-74-l
Wright-Patterson A.l'.lt Ohio, August 1974,
Chief Superintendent,
Aeronlkwtical Research labor101atories.
P.O. Box 4.31.1
MEII LB)I RN I. Victoria 3)001
APPENDIX I
by
Patricia D)iamond & Sue Vanderhorght
(i) Notation
#I
0B.
V
Angle of lay of the axis of an outer wire of a strand to the core wire.
Argle of lay of the axis of an outer strand of a cable to the core strand.
Angle between the line of contact of outer wire and axis of core wire
D. d
Diameter of strand and wire respectively, inches
T,,, P
T,, P,
Load in outer wire and outer strand respectively. lb.
Load in core wire and core strand respectively, lb.
T. P
FI,
Load in strand and cable respectivespcl lb.
Radial loading between wires at parallel wire critical region lb/in.
F,
Radial force between wires at crossed wire critical region, lb.
b.
x
y
Semi minor axis of ellipse of contact.
Axial co-ordinate, inches.
Tangential co-ordinate, inches
SCo-ordinate radially inward from cylindrical surface, inches.
A. B
Terms dependent in geometry of contacting bodies.
4
Term dependent on material and geometry of.contacting bodies.
"E
Modulus of elasticity, p.s.i.
M
(,
Poisson's ratio.
0,.
Normal stress in axial direction in outer wire of strand, p.s.i.
(,,
Critical normal stress, psi.
Normal stress in axial direction in core wire of strand, p.s.i.
•
(i11) Analysis of stresses in a 7 x 7 cable
Consider six wires wound round a centre wire at helix angle , to form a strand and six strands
wound round a core strand at a lay angle of `8..
The load T in a strand is given by:
T = T, + 6 T,, cos 13,
(i)
...
where T, and 1,. are tensile loads in core and outer wires respectively.
If wires are elastic, compatibility of strain gives:
cos (3,
.1.
(2)
r Cos3,
I +6 cos' 3,
and
.
T= T, (I + 6 cos' fl)
. . (3)
..
(4)
Similarly the load in the outer strands of the cable is given by:
P"Cos (,
1+ 6 cos' (3,
''(5)
P P, ( + 6cos' ,)
and
where P. and P,, are loads in the core strand and outer strands respectively and P is the load in the
cable.
"!
'5
......
4
There are two important critical regions of contact stress in a cable subjected to a tensile force.
The first of these, called the parallel wire critical region is located at the line ofcontact between the
wires of the strand. Whether the regiot, is between an outer wire and core wire of a strand or
between two outer wires is dependent on the diameter of the core wire. The critical stresses are
higher when the contact is between the outer wire and core wire of a strand (Fig. 13 (al). The
second critical region, called the crossed wire critical region, is located between the wires of
adjacent strands.
ta)
Parallel wire critical region
The most serious case of :n outer wire wrapped round the core wire results in a radial loading
... (7)
= 2 T,, sin' , 3
•'Fp
where .
(L
(Fig. 1.3(N)
However the outer wire makes contact with the core wire at an angle iwhere tan =,s=,½tanf(Fig.
13 (C))
Hence the contact loading per inch is given by:
F,, =-2 I, sin' 13 cos (/13- 0.
6Sin'
1 Cos (/30-
IT Cos*
.
')
(8)
(9)
d1
f Co
6 cos' fl)
where v
artan (
tan /1)
Contact Stresses
From Ref. H
,,.-2
Q I l( L].. . 1
h
(10)o.
A
J h
,,A
.....
where h is one hall tihe width of the rectangle area ot|contact and A is a function of the geometry of
the contact region.
h-- v' 22 1 ,, Arr
...
(13)
d(
and
-
. . (14)
)
F
The principal stresses o,,
,, :.. have their maximum Valuies at
0,
0 that is, at the surface of
contact.
(ritical Stresses
The critical normal stress, n,,, is given by substituting equations (9), (13) and (14) into
equatifn (12).
"A
(b)
'
b
~ The radial force, F_, between two wires of' adjacent strands is given by:
3tan(?
A
. •5
411! c,,s_,:/ si.- J3 cos (1.- v)]":.
ti--d' (I +-6cos-'• /3"1)
,•
( --,
Crossed Wire Critical Region
K --FI dr
-
L1
To
sin: /
9 tanf,
(16)
"
,
Critical Stresses
The critical normal stress in the crossed wire critical rvgion is given by:
e,.,= - c,
(h: A)
. . . (17)
where
b = ch
(F, A)"
(Ref. 7)
... (18)
where'Ch and ca are functions ot, the angle fP and are presented graphically on pages 356 and 357 of
Ref. 8. plotted against a quantity Bi A where
B1A
=
(I +sin" P) + (I -sin'fi)
3
3
(l+n
+ j
-( l-si-n.6)
3
3
cos2/
cos2f3
(19)
.,
1 ,
.. .
~~~~~~~~~~~~~~.
•o,• ,•............,:
:,,.••,
.
..
••
,••.,•,
•
.
.
..
.
...
..
TABLE I
CABLE LOADS RESULTING FROM FLIGHT MANOEUVRES
Speed - Knots
Manoeuvre
_
_
Cable Load
Pedal
kN (lb.)
Force kN (lb.)
Straight &
Level
Full L. & R.
Rudder
Rudder
Rudder
Estimated
50
60
70
80
120
Steep Turns
L. & R,
Side Slip R
1,
'.evel Medium
Turns
Taxying with
application
of brake
-.
--
0.712(160)
0.890 (200)
0,935(210)
0.9,9 (220)
1.1 (250)1
0.472
0.592
0.623
0.654
0.743
(106)
(133)
(140)
(147)
(167)
0.312 ( 70)
0.209 ( 47)
0.579 (130)
0.668 (150)
0.383 ( 86)
0.445 (100)
0.312 ( 70)
0.209 ( 47)
0.445 (100)
0.298 ( 67)
*The load for the manoeuvre at the m,,ximum design cruising speed of 120 knots was obtained by
extrapolation.
I
I
t
:
TABLE I!
CABLE LOADS RESULTING FROM STATIC GROUND MANOEUVRES
j..B"
T]
Manoeuvre
"A"
kN (lb.)
Average
Average Pedal
Force kN (lb.f)
kN (lb.)
Instructor applying
opposite rudder to
pupil
1.56
(350)
1.96
(440)
1.70
(385)
1.099
(247)
Applying parking
brake
0.445
(100)
0.623
(140)
0.534
(120)
0.356
(80)
Releasing parking
brake
0.579
(130)
0.623
(140)
0.600
(135)
0.401
(90)
Shifting position
in seat
0.890
(200)
0.592
(133)
Full rudder to
limit stop against
horn
1.56
(350)
1.037
233
"A" and "B" were different combinations of "Instructor and Pupil"
*For safety reasons this test was not made during flight.
TABLE III
AUSTER RUDDER CABLE DESIGN PROOF LOADS
IType
.5F
Pilot Effort
kN (lb.)
Cable Load
kN (lbf.)
Reserve
factor
1.335
(300)
1.962 (441)
1.9
1.780 (400)
2.621 (589)
1.43
J5K
.151,
All others
Incl. JSA
J5R1
The above information was obtained from the Auster Type Record and assumes an ultimate
load for the cable of 840 lb. based on a splice efficiency of 75% for the 10 cwt. cable.
tI
CC
ViI
ci.
-
-t
KA-
--
OW,
N-
I
TABLE V
TENSILE PROPERTIES AND DIMENSIONS OF
SINGLE WIRES
Cable
7 x7
7 x 14
7 x 19
I
Wire Location
in strand
Meas. l)ia.
mm (inch)
Max. Load
kN (Ib)
Approx,
0.2% P.S
kPa (ps.i.)
Max. Stress
kPa (p.s.i.)
Spec.
Wire
Dia.
('ore of 'entre
(Core of outer
Outer of centre
Outer of outer
0.325 (0.0128)
0.300 (0.0118)
0.305 (0.012)
0.279 (0.011)
0.167
(37.5)
0.125
(28.0(
1.722,560
(250.000)
1,894,750
(275,000)
2.039,440
(296,000)
2,032.550
(295.000)
0.33 (0.013)
0.305 (0.012)
0.305 (0.12)
0.2K5 (0.0112)
Core of centre
Core of outer
Outer o1. centre
Outer of outer
0.216 (t).(Xis)
0.216 (0.0085)
0.203 (0.00q)
0.203 (0(.0))
0.076
(17.0)
0.065
(14.5)
1.722.500
(250,000)
1,825.850
(265.00))
2,067,000
(300,000)
1,99H,100
(290.000)
0.203
(0.(X)8)
0.229
(u. '%Y)
Core of centre
(ore of outer
2nd row. of centre
2nd row of ot;.er
Outer of centre
Outer of outer
0.295 (0.0116)
0.229 (0.(X)9)
0.229 (0.(X)9)
0.216 tO.tX)85)
0.216 (0A()85)
0.203 ((.(X)95)
0.142
(32.0)
1,79140X)
(260.000)
2.067,000
(300.000)
Not
Specified
0.076
(17.0)
2.135.9(X)
(01 ),0()00
2.342.600
(340,0(X))
-
:
-
r-'.
IIN
-
C
O*• -
,,.1
u
t=
V3~
00*
00
-~
__<
IP
0
TABLE VII
LOADS AND STRESSES
(a) PARALLEL WIRE CRITICAL REGION
o cr
Pascal (psi)
-6.59 x 10'
(-9.57 x 10')
-1.52 x 10'
(-2.2 x I10)
-1.65 x 10'
(-2.4 x 10)
T
N. (Ib)
P
N (Ib)
962.0 (216,09)
6008(1350)
8.18(1.8385)
51.1 (11.486)
8.55 (1.9203)
53.4 (11.997)
(b) CROSSED WIRE CRITICAL REEGION
o cr
Pascal (psi)
-1.79
(-2.6
-1.52
(-2.2
x IOW
x IO()
x 10'
x 10)
- 1.65 x 10'
(-2.4 x IOL)
T
N (Ib)
P
N (Ib)
8793 (197.6)
6008(1350)
0.549 0.1233
3.748 (0.8423)
0,713 0.1602
4,870 (1.0943)
00
000
0
00
0o
STRUC. NOTE 424
FIG. 1
Ih
'
7x7
I.i
7 19
I
AE
I ,
L~.
O4
ARRANGEMENT OF WIRES IN 7 X 7 CABLE AND 7 X 19 CABLE
__________________..-
STRUC. NOTE 424
FIG. 2
(ýI AS ul
t 01c
n
(b) Polished at the other cnd
ARRANGEMENT OF WI RES IN A 4D8 kN (10 CWT) 7 x 14 CABLE TO BSW 9
(Set inaraldite)
STRUC. NOTE 424
FIG. 3
Cable deformations
6.4 mm
[~C~F
5~---.-~%~'Stbd
cable
attachment end
366mm51mmHorn,
Abrasion Starts
27 m
Cable JA 2393X
Port cable
m
279mm6
Horn. attachment end
(11(1346m
6.4 mm
(1/4
267 mm
-
Port cable
pedal attachment end
(10.5')Rudder
318 mm
(12.5")
CABLES ON VHD
Corrosion 38 mm (1.5"1) long.
ii
Cable JA 2393X
(14")
'orrosion 25.4 mm (1.0"1) long
~~
305mmCable
__254
mm
318 mm(12.5")
-
Mx. abrasion measured 0.05 mm (0.002") on dial
CABLES FROM VHC.
1L.
JA 2394X
i
STRUC. NOTE 424
FIGS. 4 & 5
FIG. 4 CABLE FAILURE IN VHA SHOWING ABRASION OF STRANDS.
FS
FIG. 5 STRAND FAILURE IN VHB SHOWING DAMAGE BETWEEN WIRES.
1.o
STRUC. NOTE 424
FIG. 6
iL
:1I
Neg. no. 5409
INTERNAL WIRE DAMAGE IN CORE STRAND VHE
STRUC. NOTE 424
FIG. 7 FAILURE OF CORE- sTF'8AND WIRES.
(a)
"--tom
Worn and fretted cable 7 x 14
removed two fiymig hours after a C.of A. inspection
FIG. 8 (VISIBLE DAMAGED WIRES PRIOR TO UNRAVELLING)
1
STRUC. NOTE 424
FIG. 9
1'
2.07
(0.3)
E
E2
1.72
....
01
(0.25)
-
1.03
(10.15)
.. . ...
. ...
/
C4-,
,-
~E~
0.69
(0.1)
.
E
..-
0.34
(0.05)
0
.01
.02
.03
.04
.05
Strain
STRESS-STRAIN CURVES FOR TWO WIRES OF A 10 CWT (498 kN)
7 X 7 GALVANIZED CABLE.
3/
STRUC. NOTE 424
FIG. 10
Abraded in laboratory
u 7 x 14 Aircraft cables
A 7 x 7 Aircraft cables
6.23
_ I
11400)
6.34
L
_
11200)
4.45
(1000)
z
.
___
__
3.5$
(800)
Measured dia reduction
of 0.203 mm (0.008")
2.67
(600)
...
--
Core strand load + max
manoeuvre load
A
_
'1
_
1.78
(400)
Core strand failing load
0.89
_
_,
(200)
0
20
40
60
80
100
% Reduction of area by abrasion
VARIATION OF FAILING LOAD WITH REDUCTION IN AREA OF CABLE.
(Extracted from ref. 1)
m, m *tt..
•
• - ft
~
m
•
.. ..
32-
...
. ..
-..-
•
,
-. .
STRUC. NOTE 424
IOFIG. 11
(8.00)
Abraded in lab.
(100o
Abraded in service
nil 9 x. 9aircraft cables
a
0
7 x14
(1200)
S(1000)
71
(1200)0
(6000)X
_7x1
(200)7x7
6.893
-
*,
(200)
0.0(2
(0104
0.5s
.0()-2()03(2
to
STRUC. NOTE 424
FIG. 12
0. 15 mm (0.006") wear line
wire dia. 0.28 mm (0,011
cable dia. 2.59 mm (0,102")
0.15 mm (0.006") wear line.
7 x 14
wire dia. 0.23 mm (0.009")
cable dia. 2.90 mm (0.114")
0.15 mm (0.006") wear line
7 x19
wire dia. 0.20 mm (0.008")
cable dia. 3.35 mm (0.132")
WEAR ON INDIVIDUAL WIRES OF VARIOUS CABLES.
H
STRUC. NOTE 424
FIG. 13
Point of wire contact
Radial interstrand loading
Core strand wire
b)d
Ilei
/
*-.
*
______.
Lay of extreme fibre of a wire
Line of contact between
outer and core strands
Inside
wire
w,,o '
•",,,
SDevelopment of wires
'and strands
d 7.1-5d
GEOMETRY OF 7 X 7 CABLE.
*
U A6,
-
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